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coterra 321 3rd Avenue South, Suite 406 Seattle, Washington 98104 206.596.7115 TECHNICAL INFORMATION REPORT Project: Homestead Willowcrest Townhomes Renton, WA 98056 Prepared For: Homestead 412 Maynard Ave S #201 Seattle, WA 98104 Prepared By: Max Berde, PE Reviewed By: Peter Apostol, PE Date: March 14, 2019 ENGINEERING PLLC TABLE OF CONTENTS Section No. Subject Page No. SECTION I PROJECT OVERVIEW.....................................................................1 SECTION II CONDITIONS AND REQUIREMENTS SUMMARY...................15 SECTION III OFFSITE ANALYSIS......................................................................19 SECTION IV FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN..........................................23 SECTION V CONVEYANCE SYSTEM ANALYSIS AND DESIGN................27 SECTION VI SPECIAL REPORTS AND STUDIES.............................................27 SECTION VII OTHER PERMITS...........................................................................27 SECTION VIII EROSION AND SEDIMENT CONTROL ......................................27 SECTION IX BOND QUANTITIES AND DECLARATION OF COVENANTS...............................................28 SECTION X OPERATIONS AND MAINTENANCE MANUAL.......................28 LIST OF FIGURES FIGURE 1 VICINITY MAP.................................................................................4 FIGURE 2 TIR WORKSHEET ............................................................................5 FIGURE 3 EXISTING SITE SOILS...................................................................12 FIGURE 4 EXISTING LAND COVER .............................................................13 FIGURE 5 DEVELOPED LAND COVER ........................................................14 FIGURE 6 OFF-SITE DRAINAGE SYSTEM...................................................21 FIGURE 7 WATER QUALITY TREATMENT AREA ....................................27 LIST OF APPENDICIES APPENDIX A CIVIL PLANS APPENDIX B FLOW CONTROL/DETENTION CALCULATIONS APPENDIX C WATER QUALITY CALCULATIONS APPENDIX D GEOTECHNICAL REPORT Willowcrest Townhomes Project Page 1 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION I – PROJECT OVERVIEW General Description: The proposed Homestead Willowcrest Townhomes Project includes the construction of three townhome buildings, consisting of four townhomes each, on a proposed 0.76 acre lot located off of Edmonds Ave NE in the City of Renton (see Figure 1 for vicinity map). The site is bordered by developed parcels to the east (RHA Glennwood Townhomes), Edmonds Ave NE to the west, and developed parcels to the north and south. The site is proposed to be subdivided for this townhomes project and later developed in Phase 2 by Renton Housing Authority which would include a new commercial/apartment structure, extension of the emergency access driveway, and Edmonds Ave NE frontage improvements. The total area of the project site: the Willow Crest Townhomes comprising Phase 1; the future RHA Phase 2 lot; and the front improvements total 1.76 acres. The future Phase 2 frontage improvements will include construction of new curb, gutter, and sidewalk. 270 feet of right-of-way will be dedicated along the site’s frontage on Edmonds Ave NE. In addition, a City of Renton high pressure 12” water main will be extended to the project to loops it to Edmonds Ave NE. For the purposes of the permitting we have been requested by the City of Renton to provide a preliminary design of Phase 2 improvements so that the short plat or subdivision of the property can occur. The proposed project has been designed to meet the requirements of the 2017 City of Renton Surface Water Design Manual (SWDM). A summary of the project data is provided in the TIR worksheet (see Figure 2). Site Soils: The NRCS map (see Figure 3) of the site shows “Ragnar-Indianola” soils located within the project area. Additional information on the site’s soils is provided in a geotechnical report prepared by GeoEngineers, Inc., and dated Nov 15, 2018. The findings of the report are based on soil samples taken from test pits that were excavated on-site. In general, the results of the test pit data uncovered about one foot of topsoil mixed with loose fill underlain by a 2-3.5 foot thick layer of silty sand that was underlain by impermeable glacial till soil. For more information, see the geotechnical report included in Appendix D. Predeveloped Conditions: The existing site is undeveloped with vegetation consisting of areas of tall grasses; brush; and portions of trees covering most of the site and sloping from east to west towards Edmonds Ave NE. The future Phase 2 development lot consists of vegetation sloping to the west towards Edmonds Ave NE as well, though historic aerials indicate there was a single-family home on the lot previously. A steep asphalt driveway accessing Edmonds Ave NE as well as an existing rockery wall ranging from 5’ to 15’ is still present onsite in the future Phase 2 property. The limit of work in the current phase as well as the future Phase 2 lot is located within one Threshold Discharge Area which is defined by the City of Renton Surface Water Design Manual as:“an onsite area draining to a single natural discharge location, or multiple natural discharge locations that combine within one- quarter –mile downstream.” Willowcrest Townhomes Project Page 2 Project No. 18011 Renton, WA Coterra Engineering PLLC Existing grades on the project site are generally 3% - 10% with stormwater runoff draining overland from east to west towards Edmonds Ave NE. On the west margin of the site, the previously improved street, Edmonds Ave NE, slopes from north to south at approximately 7% slope along the frontage of the project site. The existing land coverage of each section of the project site is presented in the table below. The existing site is presented graphically in Figure 4 which follows this section. The downstream path of stormwater runoff is described in the offsite analysis in Section III. Table 1 – Existing Site Land Coverage (Acres) Rooftop (Acres) Cem. Conc. (Acres) Gravel (Acres) ACP (Acres) Pervious (Acres) Total (Acres) Basin A – Project Site - - - - 0.758 0.758 Basin B – Phase 2 Future RHA Development 0.023 - - 0.020 0.836 0.879 Basin C – Phase 2 RHA Off-Site ROW Area - 0.042 - - 0.079 0.121 Total 0.023 0.042 - 0.020 0.978 1.758 Developed Conditions: The area of proposed improvements in Phase 1 include three townhouse buildings – consisting of four connected townhomes in each building; an access drive with hammerhead turnaround; residential parking stalls; pedestrian walkways; and associated storm drainage, utility, and landscape improvements. The following table quantifies the areas of the proposed land coverage. The proposed site is presented graphically in Figure 5 which follows this section. Willowcrest Townhomes Project Page 3 Project No. 18011 Renton, WA Coterra Engineering PLLC Table 2 – Developed Site Land Coverage (Acres) Rooftop (Acres) Cem. Conc. (Acres) Gravel (Acres) ACP (Acres) Grass/ Landscape (Acres) Total (Acres) Basin A – Project Site 0.165 - - 0.300 0.293 0.758 Basin B – Phase 2 Future RHA Development 0.445 - - 0.063 0.371 0.878 Basin C – Phase 2 RHA Off-Site ROW Area - 0.060 - - 0.061 0.121 Total 0.610 0.060 - 0.363 0.725 1.757 The existing site topography will not be dramatically altered in the proposed condition. In general, the site will continue to drain from east to west towards the Edmonds Ave NE right-of-way as it currently does in the existing condition. The proposed improvements within the project site (Basin A) described above and delineated in Table 2 trigger both flow control and water quality mitigation per the City of Renton Surface Water Design Manual. Stormwater runoff will be collected by roof drains, catch basins, and area drains. Per Core Requirement #4 of the 2017 City of Renton Surface Water Design Manual, the proposed conveyance system will be designed to convey runoff resulting from the peak rates resulting from the 25-year storm event. The proposed flow control facility is a detention vault located at the west edge of the project site within a Tract, which will discharge to the existing storm drainage main in Edmonds Ave NE to the west. A StormFilter CatchBasin filtration system is proposed to provide water quality treatment for the proposed improvements in Basin A and will be located upstream of the detention vault. See Section IV for a detailed description of the design of the flow control and water quality facilities. Willowcrest Townhomes Project Page 4 Project No. 18011 Renton, WA Coterra Engineering PLLC FIGURE 1 – VICINITY MAP Willowcrest Townhomes Project Page 5 Project No. 18011 Renton, WA Coterra Engineering PLLC FIGURE 2 CITY OF RENTON TECHNICAL INFORMATION REPORT (TIR) WORKSHEET CITY OF RENTON SURFACE WATER DESIGN MANUAL 2017 City of Renton Surface Water Design Manual 12/12/2016 8-A-1 REFERENCE 8-A TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 1 PROJECT OWNER AND PROJECT ENGINEER Part 2 PROJECT LOCATION AND DESCRIPTION Project Owner _____________________________ Phone ___________________________________ Address __________________________________ _________________________________________ Project Engineer ___________________________ Company _________________________________ Phone ___________________________________ Project Name __________________________ CED Permit # ________________________ Location Township ________________ Range __________________ Section _________________ Site Address __________________________ _____________________________________ Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS Land Use (e.g., Subdivision / Short Subd.) Building (e.g., M/F / Commercial / SFR) Grading Right-of-Way Use Other _______________________ DFW HPA COE 404 DOE Dam Safety FEMA Floodplain COE Wetlands Other ________ Shoreline Management Structural Rockery/Vault/_____ ESA Section 7 Part 5 PLAN AND REPORT INFORMATION Technical Information Report Site Improvement Plan (Engr. Plans) Type of Drainage Review (check one): Date (include revision dates): Date of Final: Full Targeted Simplified Large Project Directed __________________ __________________ __________________ Plan Type (check one): Date (include revision dates): Date of Final: Full Modified Simplified __________________ __________________ __________________ REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-2 Part 6 SWDM ADJUSTMENT APPROVALS Type (circle one): Standard / Blanket Description: (include conditions in TIR Section 2) ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Approved Adjustment No. ______________________ Date of Approval: _______________________ Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: _______________________ Completion Date: _______________________ Describe: _________________________________ _________________________________________ _________________________________________ Re: SWDM Adjustment No. ________________ Part 8 SITE COMMUNITY AND DRAINAGE BASIN Community Plan: ____________________________________________________________________ Special District Overlays: ______________________________________________________________ Drainage Basin: _____________________________________________________________________ Stormwater Requirements: _____________________________________________________________ Part 9 ONSITE AND ADJACENT SENSITIVE AREAS River/Stream ________________________ Lake ______________________________ Wetlands ____________________________ Closed Depression ____________________ Floodplain ___________________________ Other _______________________________ _______________________________ Steep Slope __________________________ Erosion Hazard _______________________ Landslide Hazard ______________________ Coal Mine Hazard ______________________ Seismic Hazard _______________________ Habitat Protection ______________________ _____________________________________ REFERENCE 8-A: TECHNICAL INFORMATION REPORT (TIR) WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2017 City of Renton Surface Water Design Manual 12/12/2016 Ref 8-A-3 Part 10 SOILS Soil Type ______________________ ______________________ ______________________ ______________________ Slopes ________________________ ________________________ ________________________ ________________________ Erosion Potential _________________________ _________________________ _________________________ _________________________ High Groundwater Table (within 5 feet) Other ________________________________ Sole Source Aquifer Seeps/Springs Additional Sheets Attached Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE Core 2 – Offsite Analysis_________________ Sensitive/Critical Areas__________________ SEPA________________________________ LID Infeasibility________________________ Other________________________________ _____________________________________ LIMITATION / SITE CONSTRAINT _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________ _______________________________________ Additional Sheets Attached Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Threshold Discharge Area: (name or description) Core Requirements (all 8 apply): Discharge at Natural Location Number of Natural Discharge Locations: Offsite Analysis Level: 1 / 2 / 3 dated:__________________ Flow Control (include facility summary sheet) Standard: _______________________________ or Exemption Number: ____________ On-site BMPs: _______________________________ Conveyance System Spill containment located at: _____________________________ Erosion and Sediment Control / Construction Stormwater Pollution Prevention CSWPP/CESCL/ESC Site Supervisor: _____________________ Contact Phone: _________________________ After Hours Phone: _________________________ REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-4 Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Maintenance and Operation Responsibility (circle one): Private / Public If Private, Maintenance Log Required: Yes / No Financial Guarantees and Liability Provided: Yes / No Water Quality (include facility summary sheet) Type (circle one): Basic / Sens. Lake / Enhanced Basic / Bog or Exemption No. _______________________ Special Requirements (as applicable): Area Specific Drainage Requirements Type: SDO / MDP / BP / Shared Fac. / None Name: ________________________ Floodplain/Floodway Delineation Type (circle one): Major / Minor / Exemption / None 100-year Base Flood Elevation (or range): _______________ Datum: Flood Protection Facilities Describe: Source Control (commercial / industrial land use) Describe land use: Describe any structural controls: Oil Control High-Use Site: Yes / No Treatment BMP: _________________________________ Maintenance Agreement: Yes / No with whom? _____________________________________ Other Drainage Structures Describe: REFERENCE 8-A: TECHNICAL INFORMATION REPORT (TIR) WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2017 City of Renton Surface Water Design Manual 12/12/2016 Ref 8-A-5 Part 13 EROSION AND SEDIMENT CONTROL REQUIREMENTS MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION Clearing Limits Cover Measures Perimeter Protection Traffic Area Stabilization Sediment Retention Surface Water Collection Dewatering Control Dust Control Flow Control Control Pollutants Protect Existing and Proposed BMPs/Facilities Maintain Protective BMPs / Manage Project MINIMUM ESC REQUIREMENTS AFTER CONSTRUCTION Stabilize exposed surfaces Remove and restore Temporary ESC Facilities Clean and remove all silt and debris, ensure operation of Permanent BMPs/Facilities, restore operation of BMPs/Facilities as necessary Flag limits of sensitive areas and open space preservation areas Other _______________________ Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch) Flow Control Type/Description Water Quality Type/Description Detention Infiltration Regional Facility Shared Facility On-site BMPs Other ________________ ________________ ________________ ________________ ________________ ________________ Vegetated Flowpath Wetpool Filtration Oil Control Spill Control On-site BMPs Other ________________ ________________ ________________ ________________ ________________ ________________ ________________ Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS Drainage Easement Covenant Native Growth Protection Covenant Tract Other ____________________________ Cast in Place Vault Retaining Wall Rockery > 4′ High Structural on Steep Slope Other _______________________________ REFERENCE 8: PLAN REVIEW FORMS AND WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 12/12/2016 2017 City of Renton Surface Water Design Manual 8-A-6 Part 17 SIGNATURE OF PROFESSIONAL ENGINEER I, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were incorporated into this worksheet and the attached Technical Information Report. To the best of my knowledge the information provided here is accurate. ____________________________________________________________________________________ Signed/Date Willowcrest Townhomes Project Page 12 Project No. 18011 Renton, WA Coterra Engineering PLLC FIGURE 3 – EXISTING SITE SOIL CONDITIONS coterra 321 3rd Avenue South, Suite 406 Seattle, Washington 98104 206.596.7115 coterraengineering.com ENGINEERING PLLC coterra 321 3rd Avenue South, Suite 406 Seattle, Washington 98104 206.596.7115 coterraengineering.com ENGINEERING PLLC Willowcrest Townhomes Project Page 15 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION II – CONDITIONS AND REQUIREMENTS SUMMARY The proposed project is subject to a Full Drainage Review per the 2017 City of Renton Surface Water Design Manual and is therefore subject to all nine core requirements and all six special requirements. These requirements are listed below along with a discussion of their applicability to this project. Core Requirements: Req. #1 Discharge at Natural Location: Existing discharge locations will be maintained. Req. #2 Offsite Analysis: See Section III below. Req. #3 Flow Control: The project is required to meet the Peak Rate Flow Control Standard. The proposed improvements within Phase 1 exceed the threshold of 5,000 sf of new and replaced impervious surface, therefore triggering the requirement of a flow control facility and on-site flow control BMPs. A detention vault is proposed to meet the Peak Rate Flow Control Standard requirement. Onsite BMPs are also required as part of the project and are discussed below in Core Requirement #9. For further details see Section IV. Req. #4 Conveyance System: The new conveyance system has been designed to convey the 25-year peak flow from the developed site conditions. A backwater analysis, if required by the City of Renton, will be included for civil construction permit submittal. Req. #5 Erosion and Sediment Control: Construction erosion and sediment control systems will be designed and provided for review at civil construction permit submittal. Willowcrest Townhomes Project Page 16 Project No. 18011 Renton, WA Coterra Engineering PLLC Req. #6 Maintenance and Operations: A Declaration of Covenant is required for this project and will be submitted for review by City of Renton staff before recording. The proposed facilities will be owned and maintained by Homestead. Req. #7 Financial Guarantees: A financial guarantee will be necessary for this project. Bonding will be required for the construction of improvements and will be obtained prior to construction. Req. #8 Water Quality: The proposed improvements located on the project site exceed 5,000 sf of new and replaced Pollution-Generating Impervious Surface (PGIS), therefore triggering the requirement of a water quality facility. Based on the proposed site usage, multi-family housing, Enhanced Basic Water Quality treatment is required per Core Requirement #8 of the 2017 City of Renton Surface Water Design Manual. Per Exception #3 of section 1.2.8.1 of the SWDM, the project site, which is located within the Basic WQ Treatment Area per City of Renton mapping, qualifies for the Water Quality Treatment requirement to be reduced from the Enhanced Basic Treatment Menu to the Basic Treatment Menu based upon the stormwater from the project site discharging directly to the City of Renton municipal storm drainage system via piped conveyance completely to its ultimate discharge point downstream – which is one of the City of Renton’s designated major receiving waters. Therefore, Basic Water Quality Treatment is provided on-site for targeted PGIS. A two cartridge StormFilter catch basin is proposed to meet the requirements of the Basic Water Quality Menu within the SWDM. For further details see Section IV. Req. #9 On-Site BMPs: The proposed improvements exceed the threshold of 5,000 sf of new and replaced impervious surface, therefore triggering the requirements of Core Requirement #9 to implement on-site flow control facilities to the maximum extent feasible. Willowcrest Townhomes Project Page 17 Project No. 18011 Renton, WA Coterra Engineering PLLC Following the City of Renton SWDM required list method for implementing on- site BMPs to comply with Core Requirement #9, all listed BMPs were determined to be infeasibly for Phase 1 of the proposed project site. Despite infeasibility determinations for for listed on-site BMP facilities, the proposed project plans to implement perforated pipe on all roof drain connections from the three proposed townhouse buildings per the recommendation of the City of Renton SWDM. For further details see Section IV. Special Requirements: Req. #1 Area Specific Requirements: No area specific requirements are applicable to this project. Req. #2: Floodplain/Floodway Delineation This project is not adjacent to any floodplains or floodways. Therefore no delineation is necessary. Req. #3 Flood Protection Facilities: This project is not adjacent to any applicable areas and will not affect any applicable facility. Req. #4 Source Controls: This project is a multi-family project and will provide appropriate source controls. Req. #5 Oil Control: No oil control requirements are applicable to this project. Req. #6 Aquifer Protection Area The proposed project is within Zone 2 of the APA. The contractor will be required to obtain imported fill material from a Washington Department of Transportation approved source. There are no open flow control or water quality Willowcrest Townhomes Project Page 18 Project No. 18011 Renton, WA Coterra Engineering PLLC facilities which will introduce stormwater to ground water given the project soil conditions. Willowcrest Townhomes Project Page 19 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION III – OFFSITE ANALYSIS Task 1 – Study Area Definition and Maps Maps of the project site and surrounding area were obtained from the King County GIS website. Topographical site information is from the topographical survey for the project. Storm drainage system maps were compiled from City of Renton GIS maps and as-built plans. Aerial images were obtained from Google Earth. Task 2 – Resource Review The City of Renton Public Works Department was contacted regarding the resources listed in section 2.3.1.1 of the 2017 City of Renton Surface Water Design Manual. The following is a summary of the resource review: Adopted Basin Plans o King County Basin Name: East Lake Washington (per King County iMap) Basin Reconnaissance Summary Reports o None completed to our knowledge. Floodplain/Floodway (FEMA) Maps o The site is not within a floodplain or floodway per FEMA mapping. Other Offsite Analysis Reports o Other Offsite Analysis Reports were not available. Environmentally Sensitive Areas Map o There are no Environmentally Sensitive areas mapped within the vicinity of the site. USDA Soils Survey o A geotech report has been completed and site soils are primarily silty sand that was underlain by impermeable glacial till soil. Wetlands Inventory Maps o There are no Wetlands mapped within the vicinity of the site. Willowcrest Townhomes Project Page 20 Project No. 18011 Renton, WA Coterra Engineering PLLC Task 3 – Field Inspection A Level 1 downstream analysis was performed. Weather conditions during the downstream analysis consisted of overcast skies. Periodic rainfall occurred in the week prior to the analysis. On-site landuse and topography was confirmed, and the downstream conveyance was identified. This section is complemented by the information found in Figure 6 and the Off-site Analysis Drainage System Table which follow this section. Stormwater runoff originating from the project site sheet flows to the west across the adjacent undeveloped parcel which is primarily vacant and heavily vegetated. Grades within this parcel slope to the west at approximately 10%-15%. Sheet flow patterns persist in a westerly direction until flowing onto Edmonds Ave NE. A piped conveyance system in Edmonds Ave NE collects runoff from the project site and conveys flows to the south. Due to heavy traffic conditions, the piped conveyance system was not inspected. Based on review of City of Renton GIS Utility Mapping, the piped conveyance continues to the south in Edmonds Ave NE to NE Sunset Blvd, where larger diameter conveyance piping flows west towards Lake Washington beyond ¼ mile downstream from the site. No open drainage complaints are associated with the downstream conveyance system. Task 4 – Drainage System Description and Problem Description There are no known problems with the downstream system. Task 5 – Mitigation of Existing or Potential Problems Based on the observations made during the field inspection and the requirements for the design of the proposed drainage systems, no negative impacts to the systems downstream of the project site are anticipated. Existing Upslope Drainage Areas Portions of the adjacent property to the east – the previously developed Glennwood Townhomes drain to the west based on topography to the proposed Phase 1 project site. The areas of the adjacent off-site, upstream area that drains to the Willowcrest site are fully-established pervious landscape areas or previously undeveloped pervious areas. Based on on-site observations during the Offsite Analysis, no upstream run-on was observed. Based on current landcover, site layout, and grading, minimal offsite run-on is expected onto the Willowcrest site. To mitigate any upstream overland or subsurface run-on, a perforated interreceptor storm drain line is proposed at the eastern project margin of the Phase 1 property to collect any upstream flows from the Glennwood Townhomes site and route it around the proposed project improvements on the Willowcrest site. The proposed upstream drainage collection system – consisting of a French drain – and tightline routing is intended to maintain the existing drainage patterns of any upstream runoff from the adjacent property and convey it to the existing municipal storm drainage system in Edmonds Ave NE. 18"SD18"SD24"SD24"SD24"SD30"SD30"SD36"SD36"SD36"SD36"SD36"SD36"SDCB(TYP)PROJECTSITESHALLOWCONCENTRATEDSURFACEFLOWPIPEDCONVEYANCECATCHBASINDIRECTIONOFFLOWINDICATEDBYARROWSHEETFLOWcoterra 321 3rd Avenue South, Suite 406 Seattle, Washington 98104 206.596.7115 coterraengineering.com ENGINEERING PLLC 1/1/05 OFF-SITE ANALYSIS DRAINAGE SYSTEM TABLE SURFACE WATER DESIGN MANUAL, CORE REQUIREMENT #2 Basin: Subbasin Name: Subbasin Number: Symbol Drainage Component Type, Name, and Size Drainage Component Description Slope Distance from site discharge Existing Problems Potential Problems Observations of field inspector, resource reviewer, or resident see map Type: sheet flow, swale, stream, channel, pipe, pond; Size: diameter, surface area drainage basin, vegetation, cover, depth, type of sensitive area, volume % ¼ ml = 1,320 ft. constrictions, under capacity, ponding, overtopping, flooding, habitat or organism destruction, scouring, bank sloughing, sedimentation, incision, other erosion tributary area, likelihood of problem, overflow pathways, potential impacts Willowcrest Townhomes Project Page 23 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION IV – FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Flow Control The project site is located in a Peak Flow Control Standard area per the 2017 City of Renton Surface Water Design Manual. This standard requires that the developed site discharge rates match the existing site conditions discharge rates for the 2-, 10-, and 100- year return periods. The proposed improvements will create more than 2,000 SF of new plus replaced imperious surfaces, therefore the project is required to meet the flow control standard and implement on-site flow control BMPs. Based on the engineer’s on- site visits to the proposed property and analysis of current aerial photography, the existing site conditions within the Phase 1 project site (Basin A) is assumed to be 25% forested land cover and 75% pasture land cover. MGS Flood stormwater modeling program – utilizing a continuous hydrologic model – was implemented to size a detention facility which matched the existing site discharge rates for the 2-, 10-, and 100-year return events. At this preliminary stage of design, a detention vault was sized in MGS Flood to meet the flow control requirement. As the project stormwater design is refined, the type of detention structure on the project may be revised. Based on the MGS Flood analysis, an approximately 6,000 CF detention vault is required for Basin A of the proposed project. The MGS Flood calculations are included in Appendix B of this report. The proposed detention vault was preliminarily designed per the City of Renton SWDM requirements including a design of at least 7’ clear height within the vault plus sediment storage (dead storage), and a minimum of 10’ wide within the vault structure. Preliminary sizing details are shown on the Storm Drainage and Utility Plan, sheet 4.0 of the civil plan set. Additionally, a detention vault was sized to detain the future runoff from the Phase 2 development of Basin B, adjacent to Edmonds Ave NE, as well as to mitigate for the off- site area of Edmonds Ave NE right-of-way which is unable to be feasibly collected and detained for the required street improvements required on Edmonds Ave NE. An approximately 4,550 CF detention vault is required to detain runoff from Basin B as well as the bypass area on Edmonds Ave NE. Like Phase 1, the proposed Phase 2 detention vault was sized per City of Renton SWDM criteria. The MGS Flood calculations for this detention vault including bypass area are also included in Appendix B of this report. On-Site Flow Control BMPs The proposed site is classified as a Large Lot BMP Site. Implementation of On-Site Flow Control BMPs is therefore required per Core Requirement #9. On-site BMPs are evaluated for targeted surfaces, per the list from section 1.2.9.3.1 for Small Subdivision Projects in the 2017 City of Renton Surface Water Design Manual. Willowcrest Townhomes Project Page 24 Project No. 18011 Renton, WA Coterra Engineering PLLC Analysis of feasibility of the on-site BMPs from the SWDM list follow in order per SWDM feasibility determination instructions. Full Dispersion: Infeasible. Per on-site BMP infeasibility criteria in section C.2.1.1, the total area of impervious surfaces plus non-native pervious surfaces on the project site exceed 35% of the total site area. Full Infiltration: Infeasible. Per Minimum Design Requirements for Full Infiltration in section C.2.2.2, existing soils must be “coarse sands or cobbles or medium sands.” Soils collected in on-site test pits per Geotechnical Engineering Services report prepared by GeoEngineers, Inc., dated 11/15/2018, are classified as “SM” or “silty sands, sand – silt mixtures” at the depth of typical on-site infiltration facilities or infiltration BMPs. Bioretention: Infeasible. Bioretention is considered infeasible for projects that cannot meet the minimum design requirements for bioretention. Per section C2.6, bioretention facilities receiving runoff from impervious areas over 10,000 square feet must follow setback requirements of Infiltration Ponds as outlined in section 5.2.2.1. Per section 5.2.2.1, “The toe of the exterior slope of an infiltration pond berm embankment shall be set back 5 feet from the tract, easement, or property line.” Based on the currently proposed site design, a bioretention facility cannot be feasibly located on the proposed Willow Crest Townhomes property with the required setback requirements from both proposed structures and tract, easement, and property lines. Additionally, per section C2.6, bioretention facilities are infeasible on slopes greater than 8%. Based on the proposed grading of the project site and the adjacent RHA site, it is infeasible to locate bioretention facilities on the proposed site. Permeable Pavement: Infeasible. Per section C2.7, permeable pavement shall not be installed on slopes steeper than 5% for porous asphaltic concrete. Project proposes asphalt paving for access drives. Based on existing topography and proposed site layout, majority of access drive grades exceed 5%. Proposed areas of grading less than 5% in access drive would exceed maximum run-on limit ratio for PGIS of 2:1. Basic Dispersion: o Splash blocks: Infeasible. Required 50’ vegetated flow path is infeasible based on proposed site design. o Rock pads: Infeasible. Required 50’ vegetated flow path is infeasible based on proposed site design. o Gravel-filled trenches: Infeasible. Per section C.2.4.4, setbacks of at least 10’ from buildings and 5’ from property line are required. Proposed site layout cannot accommodate 25’ flowpath with required setbacks. o Sheet flow: Infeasible. Required 10’ vegetated flow path located on the project property is infeasible based on proposed site design. Soil Amendment: Feasible. New and replaced pervious surfaces within the project limits will implement soil amendment per the soil quality and depth requirements of section C2.13 Willowcrest Townhomes Project Page 25 Project No. 18011 Renton, WA Coterra Engineering PLLC Despite infeasibility for listed on-site BMPs per the SWDM criteria, perforated pipe connections from roof drain leaders from each of the three proposed buildings, upstream of detention, are proposed. These perforated pipe connections are shown on the current Utility Plan. From the initial geotechnical investigation, it was determined that the subsurface soils on the project site are composed of several feet of fill soil laying over glacial till which inhibits any practical infiltration potential for possible stormwater mitigation on the project site. Therefore, the use of large-scale infiltrative flow control facilities and on-site BMPs are not shown on the current preliminary plans. Willowcrest Townhomes Project Page 26 Project No. 18011 Renton, WA Coterra Engineering PLLC Water Quality The proposed new and replaced pollution-generating impervious surfaces (PGIS) on the project site trigger the requirement for water quality treatment of targeted surfaces. Per the City of Renton SWDM, the project site is within the Basin Water Quality Area; but the proposed site land use of multi-family housing triggers the Enhanced Basic Water Quality Menu. However, per Exemption #3 of the Land-Use Specific Water Quality Facility Requirements within section 1.2.8.1 of the 2017 City of Renton SWDM: If the following criteria is met, the Enhanced Basic Water Quality Menu requirement is reduced to the Basic Water Quality Menu: The Enhanced Basic WQ menu as specified above for certain land uses may be reduced to the Basic WQ menu for treatment of any runoff that is discharged directly, via a non-fish-bearing conveyance system, all the way to the ordinary high water mark of a stream with a mean annual flow of 1,000 cfs or more (at the discharge point of the conveyance system) or a lake that is 300 acres or larger. This exception does not apply where the receiving water is impaired as described in the full description of major receiving water in the Definitions section or is impaired for metals according to Downstream Analysis. The project will meet the above criteria explicitly: -The proposed project stormwater runoff is conveyed completely within a piped conveyance system downstream of the site as outlined in the Offsite Analysis section of this report. -Stormwater runoff from the project site as well as downstream runoff discharges directly via conveyance pipe to Johns Creek downstream of Interstate-405 (I-405) east right-of-way, which is listed in the City of Renton SWDM as a Major Receiving Water. -Johns Creek is not listed as an impaired receiving water nor is it impaired for metals according to the Downstream Analysis of this report. Based upon meeting the above-listed criteria, the proposed project will provide treatment of targeted PGIS as outlined by the Basic Water Quality Menu. This will be provided by two ZPG cartridges within a Contech StormFilter Catch Basins which was sized using WWHM 2012 software to treat a minimum of 91% of the influent runoff from the on- site PGIS. Results from the WWHM model are provided in Appendix C. coterra321 3rd Avenue South, Suite 406Seattle, Washington 98104206.596.7115 coterraengineering.comENGINEERING PLLC Willowcrest Townhomes Project Page 27 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION V – CONVEYANCE SYSTEM DESIGN AND ANALYSIS The new conveyance system has been designed to convey at least the 25-year peak flow rate from the developed site. A backwater analysis of the proposed conveyance systems will be included with the Civil Construction Permit submittal, if required. SECTION VI – SPECIAL REPORTS AND STUDIES A geotechnical analysis of the project site was performed and is included in Appendix D. SECTION VII – OTHER PERMITS In addition to the PPUD permit, a Civil Construction Permit from the City of Renton is required, as well as an NPDES permit from the Washington State DOE for the discharge of construction stormwater from the project site. SECTION VIII – EROSION AND SEDIMENT CONTROL ESC Measures are being addressed as follows: Clearing Limits: Clearing limits are being delineated by perimeter silt fencing and chain link fencing. Cover Measures: Temporary cover shall be installed if an area is to remain unworked for more than seven days during the dry season (May 1 to September 30) or for more than two consecutive working days during the wet season (October 1 to April 30). Any area to remain unworked for more than 30 days shall be seeded or sodded, unless the City of Renton determines that winter weather makes vegetation establishment infeasible. Perimeter Protection: Perimeter protection will be implemented by silt fencing around the site perimeter where drainage paths require. Traffic Area Stabilization: A stabilized construction entrance will be built for construction traffic. Sediment Retention: Catch basin protection has been provided and is shown on the projects TESC plans. Surface Water Control: Surface water will be collected and conveyed via swales with check dams as necessary. Dust Control: Dust control, if required, will be provided through the limited use of water trucks. Willowcrest Townhomes Project Page 28 Project No. 18011 Renton, WA Coterra Engineering PLLC SECTION IX – BOND QUANTITIES AND DECLARATION OF COVENANTS A bond quantity worksheet for the proposed improvements will be included with future submittals. There are two Declaration of Covenants which are required for the proposed project; one for inspection and maintenance of proposed stormwater facilities; and one for the inspection and maintenance of the proposed on-site BMPs. A draft version of these Declaration of Covenants will be provided for review and approval by City of Renton prior to recording. They will be signed and notarized prior to recording. SECTION X – OPERATIONS AND MAINTENANCE MANUAL An operation and maintenance manual which outlines required regular maintenance necessary for the proposed stormwater facilities will be provided with the Civil Construction Permit submittal. The maintenance of the stormwater facilities will by be performed by Homestead. Appendix A Civil Plans IN COMPLIANCE WITH CITY OF RENTON STANDARDS '&/10&5#8'0'4'06109# 9+..19%4'566190*1/'5coterra 321 3rd Ave South, Suite 406 Seattle, Washington 98104 ph 206.596.7115 coterraengineering.com ENGINEERING PLLC 9+..19%4'566190*1/'5 IN COMPLIANCE WITH CITY OF RENTON STANDARDS '&/10&5#8'0'4'06109# 9+..19%4'566190*1/'5coterra 321 3rd Ave South, Suite 406 Seattle, Washington 98104 ph 206.596.7115 coterraengineering.com ENGINEERING PLLC 9+..19%4'566190*1/'5 IN COMPLIANCE WITH CITY OF RENTON STANDARDS '&/10&5#8'0'4'06109# 9+..19%4'566190*1/'5coterra 321 3rd Ave South, Suite 406 Seattle, Washington 98104 ph 206.596.7115 coterraengineering.com ENGINEERING PLLC 9+..19%4'566190*1/'5 IN COMPLIANCE WITH CITY OF RENTON STANDARDS '&/10&5#8'0'4'06109# 9+..19%4'566190*1/'5coterra 321 3rd Ave South, Suite 406 Seattle, Washington 98104 ph 206.596.7115 coterraengineering.com ENGINEERING PLLC 9+..19%4'566190*1/'5 IN COMPLIANCE WITH CITY OF RENTON STANDARDS '&/10&5#8'0'4'06109# 9+..19%4'566190*1/'5coterra 321 3rd Ave South, Suite 406 Seattle, Washington 98104 ph 206.596.7115 coterraengineering.com ENGINEERING PLLC 9+..19%4'566190*1/'5 Appendix B Flow Control/Detention Calculations ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.38 Program License Number: 201510001 Project Simulation Performed on: 01/28/2019 12:24 PM Report Generation Date: 01/28/2019 12:31 PM ————————————————————————————————— Input File Name: Homestead - Detention Size - Match Peaks Basin A 25 75 Vault.fld Project Name: Renton Homestead Analysis Title: Peak Flow - Detention Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Climatic Region Number: 13 Full Period of Record Available used for Routing Precipitation Station : 96004005 Puget East 40 in_5min 10/01/1939-10/01/2097 Evaporation Station : 961040 Puget East 40 in MAP Evaporation Scale Factor : 0.750 HSPF Parameter Region Number: 1 HSPF Parameter Region Name : USGS Default ********** Default HSPF Parameters Used (Not Modified by User) *************** ********************** WATERSHED DEFINITION *********************** Predevelopment/Post Development Tributary Area Summary Predeveloped Post Developed Total Subbasin Area (acres) 0.758 0.758 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 0.758 0.758 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Existing ---------- -------Area(Acres) -------- Till Forest 0.189 Till Pasture 0.569 Till Grass 0.000 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.000 ---------------------------------------------- Subbasin Total 0.758 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Proposed ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 Till Grass 0.293 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.465 ---------------------------------------------- Subbasin Total 0.758 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: Basin A - Detention VAULT Link Type: Structure Downstream Link: None Prismatic Pond Option Used Pond Floor Elevation (ft) : 100.00 Riser Crest Elevation (ft) : 107.00 Max Pond Elevation (ft) : 108.00 Storage Depth (ft) : 7.00 Pond Bottom Length (ft) : 48.0 Pond Bottom Width (ft) : 15.0 Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00 Bottom Area (sq-ft) : 720. Area at Riser Crest El (sq-ft) : 720. (acres) : 0.017 Volume at Riser Crest (cu-ft): 5,040. (ac-ft) : 0.116 Area at Max Elevation (sq-ft) : 720. (acres) : 0.017 Vol at Max Elevation (cu-ft): 5,904. (ac-ft) : 0.136 Massmann Infiltration Option Used Hydraulic Conductivity (in/hr) : 0.00 Depth to Water Table (ft) : 100.00 Bio-Fouling Potential : Low Maintenance : Average or Better Riser Geometry Riser Structure Type : Circular Riser Diameter (in) : 18.00 Common Length (ft) : 0.000 Riser Crest Elevation : 107.00 ft Hydraulic Structure Geometry Number of Devices: 3 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 100.50 Diameter (in) : 0.50 Orientation : Horizontal Elbow : No ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 102.20 Diameter (in) : 0.50 Orientation : Horizontal Elbow : Yes ---Device Number 3 --- Device Type : Circular Orifice Control Elevation (ft) : 104.45 Diameter (in) : 0.85 Orientation : Horizontal Elbow : Yes **********************FLOOD FREQUENCY AND DURATION STATISTICS******************* ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 1 Number of Links: 1 ********** Link: Basin A - Detention VAULT ********** Link WSEL Stats WSEL Frequency Data(ft) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) WSEL Peak (ft) ====================================== 1.05-Year 102.970 1.11-Year 103.206 1.25-Year 103.648 2.00-Year 104.397 3.33-Year 104.864 5-Year 105.414 10-Year 106.139 25-Year 107.020 50-Year 107.040 100-Year 107.043 ***********Groundwater Recharge Summary ************* Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures Total Predeveloped Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin A - Existing 132.800 _____________________________________ Total: 132.800 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin A - Proposed 35.808 Link: Basin A - Detention 0.000 _____________________________________ Total: 35.808 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 0.841 ac-ft/year, Post Developed: 0.227 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ********** Link: Basin A - Detention VAULT ********** Basic Wet Pond Volume (91% Exceedance): 2363. cu-ft Computed Large Wet Pond Volume, 1.5*Basic Volume: 3545. cu-ft Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 267.41 Inflow Volume Including PPT-Evap (ac-ft): 267.41 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 267.40 Secondary Outflow To Downstream System (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00% ***********Compliance Point Results ************* Scenario Predeveloped Compliance Subbasin: Basin A - Existing Scenario Postdeveloped Compliance Link: Basin A - Detention VAULT *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs) ---------------------------------------------------------------------------------------------------------------------- 2-Year 2.309E-02 2-Year 2.256E-02 5-Year 4.410E-02 5-Year 4.411E-02 10-Year 6.616E-02 10-Year 5.212E-02 25-Year 0.111 25-Year 0.106 50-Year 0.163 50-Year 0.192 100-Year 0.206 100-Year 0.206 200-Year 0.216 200-Year 0.237 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.38 Program License Number: 201510001 Project Simulation Performed on: 01/30/2019 8:54 AM Report Generation Date: 01/30/2019 8:57 AM ————————————————————————————————— Input File Name: Homestead - Detention Size - Match Peaks Basin B Vault w Bypass2.fld Project Name: Renton Homestead Analysis Title: Peak Flow - Detention Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Climatic Region Number: 13 Full Period of Record Available used for Routing Precipitation Station : 96004005 Puget East 40 in_5min 10/01/1939-10/01/2097 Evaporation Station : 961040 Puget East 40 in MAP Evaporation Scale Factor : 0.750 HSPF Parameter Region Number: 1 HSPF Parameter Region Name : USGS Default ********** Default HSPF Parameters Used (Not Modified by User) *************** ********************** WATERSHED DEFINITION *********************** Predevelopment/Post Development Tributary Area Summary Predeveloped Post Developed Total Subbasin Area (acres) 1.000 1.000 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 1.000 1.000 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 2 ---------- Subbasin : Basin B - Existing ---------- -------Area(Acres) -------- Till Forest 0.215 Till Pasture 0.644 Till Grass 0.000 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.020 ---------------------------------------------- Subbasin Total 0.879 ---------- Subbasin : Basin C- Existing ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 Till Grass 0.079 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.042 ---------------------------------------------- Subbasin Total 0.121 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 2 ---------- Subbasin : Basin B - Proposed ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 Till Grass 0.371 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.508 ---------------------------------------------- Subbasin Total 0.879 ---------- Subbasin : Basin C - Proposed (ByPass) ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 Till Grass 0.061 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.060 ---------------------------------------------- Subbasin Total 0.121 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: Downstream Link Type: Copy Downstream Link: None ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 2 ------------------------------------------ Link Name: Basin B - VAULT Link Type: Structure Downstream Link Name: Basin B POC with ByPass Prismatic Pond Option Used Pond Floor Elevation (ft) : 100.00 Riser Crest Elevation (ft) : 107.00 Max Pond Elevation (ft) : 108.00 Storage Depth (ft) : 7.00 Pond Bottom Length (ft) : 50.0 Pond Bottom Width (ft) : 11.0 Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00 Bottom Area (sq-ft) : 550. Area at Riser Crest El (sq-ft) : 550. (acres) : 0.013 Volume at Riser Crest (cu-ft): 3,850. (ac-ft) : 0.088 Area at Max Elevation (sq-ft) : 550. (acres) : 0.013 Vol at Max Elevation (cu-ft): 4,510. (ac-ft) : 0.104 Massmann Infiltration Option Used Hydraulic Conductivity (in/hr) : 0.00 Depth to Water Table (ft) : 100.00 Bio-Fouling Potential : Low Maintenance : Average or Better Riser Geometry Riser Structure Type : Circular Riser Diameter (in) : 18.00 Common Length (ft) : 0.000 Riser Crest Elevation : 107.00 ft Hydraulic Structure Geometry Number of Devices: 2 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 100.50 Diameter (in) : 0.70 Orientation : Horizontal Elbow : No ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 103.50 Diameter (in) : 0.80 Orientation : Vertical Elbow : No ------------------------------------------ Link Name: Basin B POC with ByPass Link Type: Copy Downstream Link: None **********************FLOOD FREQUENCY AND DURATION STATISTICS******************* ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 2 Number of Links: 1 ********** Subbasin: Basin B - Existing ********** Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 3.031E-02 5-Year 5.848E-02 10-Year 8.279E-02 25-Year 0.138 50-Year 0.203 100-Year 0.253 200-Year 0.260 ********** Subbasin: Basin C- Existing ********** Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 2.268E-02 5-Year 3.052E-02 10-Year 4.029E-02 25-Year 5.382E-02 50-Year 7.092E-02 100-Year 8.328E-02 200-Year 8.696E-02 ********** Link: Downstream ********** Link Inflow Frequency Stats Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 5.221E-02 5-Year 8.401E-02 10-Year 0.114 25-Year 0.192 50-Year 0.274 100-Year 0.332 200-Year 0.340 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 2 Number of Links: 2 ********** Subbasin: Basin B - Proposed ********** Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 0.223 5-Year 0.286 10-Year 0.356 25-Year 0.479 50-Year 0.583 100-Year 0.721 200-Year 0.728 ********** Subbasin: Basin C - Proposed (ByPass) ********** Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 2.761E-02 5-Year 3.633E-02 10-Year 4.451E-02 25-Year 6.028E-02 50-Year 7.695E-02 100-Year 9.358E-02 200-Year 9.549E-02 ********** Link: Basin B - VAULT ********** Link Inflow Frequency Stats Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 0.223 5-Year 0.286 10-Year 0.356 25-Year 0.479 50-Year 0.583 100-Year 0.721 200-Year 0.728 ********** Link: Basin B - VAULT ********** Link Outflow 1 Frequency Stats Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 4.162E-02 5-Year 5.518E-02 10-Year 6.167E-02 25-Year 0.215 50-Year 0.260 100-Year 0.288 200-Year 0.337 ********** Link: Basin B - VAULT ********** Link WSEL Stats WSEL Frequency Data(ft) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) WSEL Peak (ft) ====================================== 1.05-Year 102.958 1.11-Year 103.424 1.25-Year 103.861 2.00-Year 104.392 3.33-Year 105.048 5-Year 105.725 10-Year 106.520 25-Year 107.044 50-Year 107.053 100-Year 107.058 ********** Link: Basin B POC with ByPass ********** Link Inflow Frequency Stats Flood Frequency Data(cfs) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) Flood Peak (cfs) ====================================== 2-Year 5.315E-02 5-Year 7.274E-02 10-Year 0.107 25-Year 0.243 50-Year 0.294 100-Year 0.325 200-Year 0.381 ***********Groundwater Recharge Summary ************* Recharge is computed as input to Perlnd Groundwater Plus Infiltration in Structures Total Predeveloped Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin B - Existing 150.491 Subbasin: Basin C- Existing 9.655 Link: Downstream 0.000 _____________________________________ Total: 160.146 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin B - Proposed 45.340 Subbasin: Basin C - Proposed ( 7.455 Link: Basin B - VAULT 0.000 Link: Basin B POC with ByP 0.000 _____________________________________ Total: 52.795 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 1.014 ac-ft/year, Post Developed: 0.334 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 1 ********** Link: Downstream ********** Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 157.46 Inflow Volume Including PPT-Evap (ac-ft): 157.46 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 157.46 Secondary Outflow To Downstream System (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00% ----------------------SCENARIO: POSTDEVELOPED Number of Links: 2 ********** Link: Basin B - VAULT ********** Basic Wet Pond Volume (91% Exceedance): 2658. cu-ft Computed Large Wet Pond Volume, 1.5*Basic Volume: 3987. cu-ft Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 302.37 Inflow Volume Including PPT-Evap (ac-ft): 302.37 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 302.53 Secondary Outflow To Downstream System (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00% ********** Link: Basin B POC with ByPass ********** Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 341.70 Inflow Volume Including PPT-Evap (ac-ft): 341.70 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 0.00, 0.00% Primary Outflow To Downstream System (ac-ft): 341.70 Secondary Outflow To Downstream System (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered)/Total Volume: 0.00% ***********Compliance Point Results ************* Scenario Predeveloped Compliance Link: Downstream Scenario Postdeveloped Compliance Link: Basin B POC with ByPass *** Point of Compliance Flow Frequency Data *** Recurrence Interval Computed Using Gringorten Plotting Position Predevelopment Runoff Postdevelopment Runoff Tr (Years) Discharge (cfs) Tr (Years) Discharge (cfs) ---------------------------------------------------------------------------------------------------------------------- 2-Year 5.221E-02 2-Year 5.315E-02 5-Year 8.401E-02 5-Year 7.274E-02 10-Year 0.114 10-Year 0.107 25-Year 0.192 25-Year 0.243 50-Year 0.274 50-Year 0.294 100-Year 0.332 100-Year 0.325 200-Year 0.340 200-Year 0.381 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals Appendix C Water Quality Calculation WWHM2012 PROJECT REPORT RENTON HOMESTEAD - PRELIM WQ - 2019-02-27 2/27/2019 7:49:17 PM Page 2 General Model Information Project Name:RENTON HOMESTEAD - PRELIM WQ - 2019-02-27 Site Name:RENTON HOMESTEAD Site Address: City:RENTON, WA Report Date:2/27/2019 Gage:Seatac Data Start:1948/10/01 Data End:2009/09/30 Timestep:15 Minute Precip Scale:1.000 Version Date:2018/07/12 Version:4.2.15 POC Thresholds Low Flow Threshold for POC1:50 Percent of the 2 Year High Flow Threshold for POC1:50 Year Low Flow Threshold for POC2:50 Percent of the 2 Year High Flow Threshold for POC2:50 Year RENTON HOMESTEAD - PRELIM WQ - 2019-02-27 2/27/2019 7:49:17 PM Page 5 Mitigated Land Use BASIN A - PROPOSED WQ Bypass:No GroundWater:No Pervious Land Use acre Pervious Total 0 Impervious Land Use acre DRIVEWAYS MOD 0.25 Impervious Total 0.25 Basin Total 0.25 Element Flows To: Surface Interflow Groundwater RENTON HOMESTEAD - PRELIM WQ - 2019-02-27 2/27/2019 7:49:56 PM Page 15 Water Quality Water Quality BMP Flow and Volume for POC #1 On-line facility volume:0.0302 acre-feet On-line facility target flow:0.0453 cfs. Adjusted for 15 min:0.0453 cfs. Off-line facility target flow:0.0255 cfs. Adjusted for 15 min:0.0255 cfs. RENTON HOMESTEAD - PRELIM WQ - 2019-02-27 2/27/2019 7:51:36 PM Page 38 Disclaimer Legal Notice This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. 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Clear Creek Solutions, Inc. 6200 Capitol Blvd. Ste F Olympia, WA. 98501 Toll Free 1(866)943-0304 Local (360)943-0304 www.clearcreeksolutions.com Appendix D Geotechnical Report Geotechnical Engineering Services Willow Crest Townhomes Renton, Washington for Homestead Community Land Trust c/o Third Place Design Co-operative November 15, 2018 Geotechnical Engineering Services Willow Crest Townhomes Renton, Washington for Homestead Community Land Trust c/o Third Place Design Co-operative November 15, 2018 17425 NE Union Hill Road, Suite 250 Redmond, Washington 98052 425.861.6000 Prepared for: Homestead Community Land Trust c/o Third Place Design Co-operative 177 Western Avenue West, Suite 266 Seattle, Washington 98119 Geotechnical Engineering Services Willow Crest Townhomes Renton, Washington File No. 23656-001-00 November 15, 2018 Attention: Suzanne Davis, LEED AP, NCARB Prepared by: GeoEngineers, Inc. 17 425 NE Union Hill Road, Suite 250 Redmond, Washington 98052 425.861.6000 Michael A. Gray, PE Geotechnical Engineer R�� Principal MAG:RCM:nld Disclaimer: Any electronic form, facsimile or hard copy of the original document (email, text, table, and/or figure), if provided, and any attachments are only a copy of the original document. The original document is stored by Geo Engineers, Inc. and will serve as the official document of record. GEoENGINEER� November 15, 2018 | Page i File No. 23656-001-00 Table of Contents INTRODUCTION ..................................................................................................................................................... 1 Project Description ........................................................................................................................................... 1 Purpose and Scope .......................................................................................................................................... 1 FIELD EXPLORATIONS AND LABORATORY TESTING ......................................................................................... 1 Field Explorations ............................................................................................................................................. 1 Laboratory Testing ........................................................................................................................................... 1 Previous Studies .............................................................................................................................................. 2 SITE DESCRIPTIONS............................................................................................................................................. 2 Site Geology ..................................................................................................................................................... 2 Surface Conditions........................................................................................................................................... 2 Subsurface Conditions .................................................................................................................................... 2 Groundwater Conditions .................................................................................................................................. 3 CONCLUSIONS AND RECOMMENDATIONS ........................................................................................................ 3 Summary .......................................................................................................................................................... 3 Earthquake Engineering .................................................................................................................................. 4 2015 IBC Seismic Design Information .................................................................................................... 4 Liquefaction Potential ............................................................................................................................... 4 Ground Rupture ......................................................................................................................................... 4 Landslides ................................................................................................................................................. 5 Foundations ..................................................................................................................................................... 5 Foundation Design .................................................................................................................................... 5 Foundation Settlement ............................................................................................................................. 5 Lateral Resistance .................................................................................................................................... 6 Construction Considerations .................................................................................................................... 6 Footing Drains ........................................................................................................................................... 6 Slab-on-Grade Floors ....................................................................................................................................... 6 Pavement Recommendations ......................................................................................................................... 7 Subgrade Preparation ............................................................................................................................... 7 New Hot Mix Asphalt Pavement ............................................................................................................... 7 Portland Cement Concrete Pavement ..................................................................................................... 8 Asphalt-Treated Base ................................................................................................................................ 8 Earthwork ......................................................................................................................................................... 8 Clearing and Site Preparation .................................................................................................................. 9 Subgrade Preparation ............................................................................................................................... 9 Structural Fill ........................................................................................................................................... 10 Permanent Cut and Fill Slopes ............................................................................................................... 12 Utility Trenches ........................................................................................................................................ 12 Sedimentation and Erosion Control ....................................................................................................... 13 Excavations .................................................................................................................................................... 13 Temporary Cut Slopes ............................................................................................................................. 13 Drainage Considerations ............................................................................................................................... 14 November 15, 2018 | Page ii File No. 23656-001-00 Infiltration Considerations ............................................................................................................................. 14 RECOMMENDED ADDITIONAL GEOTECHNICAL SERVICES ............................................................................. 15 LIMITATIONS ....................................................................................................................................................... 15 REFERENCES ..................................................................................................................................................... 15 LIST OF FIGURES Figure 1. Vicinity Map Figure 2. Site Plan – Proposed Development Figure 3. Compaction Criteria for Trench Backfill APPENDICES Appendix A. Field Explorations Figure A-1. Key to Exploration Logs Figures A-2 through A-7. Logs of Test Pits Appendix B. Laboratory Testing Figures B-1 and B-2. Sieve Analysis Results Figure B-3. Compaction Test Results (TP-3) Appendix C. Boring and Test Pit Logs from Previous Studies Appendix D. Report Limitations and Guidelines for Use November 15, 2018 | Page 1 File No. 23656-001-00 INTRODUCTION This report presents the results of GeoEngineers, Inc.’s (GeoEngineers) geotechnical engineering services for the Willow Crest Townhomes project located in Renton, Washington. The site is shown relative to surrounding physical features on Figure 1, Vicinity Map and Figure 2, Site Plan. Project Description We understand that the proposed development will consist of three four-unit wood-frame townhomes (12 total townhomes) constructed at existing grade. The development will also include new underground utilities, access drive, sidewalks, and landscaping. Minor site grading is anticipated and no underground structures are planned for the project. Purpose and Scope The purpose of our geotechnical engineering services is to evaluate soil and groundwater conditions as a basis for developing design criteria for the geotechnical aspects of the proposed development. Field explorations and laboratory testing were performed to identify and evaluate subsurface conditions at the sites to develop engineering recommendations for use in design of the project. Our services were performed in general accordance with our scope of services outlined in our Proposal dated October 2, 2018. FIELD EXPLORATIONS AND LABORATORY TESTING Field Explorations The subsurface soil and groundwater conditions were evaluated by reviewing existing geotechnical information and completing six test pits (GEI-TP-1 through GEI-TP-6) across the planned building locations. The test pits were completed to depths of approximately 8 feet below the ground surface (bgs). The test pits were completed on October 22, 2018 using a rubber-tracked backhoe. The approximate locations of the explorations are shown on Figure 2. Descriptions of the field exploration program and the explorations logs are presented in Appendix A. Laboratory Testing Soil samples were obtained during the exploration program and taken to our laboratory for further evaluation. Selected samples were tested for the determination of moisture content, fines content, grain size distribution (sieve analysis), and modified Proctor compaction tests. A description of the laboratory testing and the test results are presented in Appendix B. November 15, 2018 | Page 2 File No. 23656-001-00 Previous Studies The exploration logs (borings and test pits) of selected explorations from previous subsurface studies in vicinity of the proposed project were reviewed. Logs of relevant explorations reviewed for this study are presented in Appendix C and approximate locations are shown on Figure 2. The existing subsurface information includes: ■ “Report of Geotechnical Investigation, Proposed Residential Development, Edmonds and Glennwood Ave NE, Renton, Washington,” prepared by Soil and Environmental Engineers, Inc., dated January 10, 2011. SITE DESCRIPTIONS Site Geology Our review of the geologic map for the area (Mullineaux 1965) and previous geotechnical reports completed in the vicinity of the project site indicate that the proposed development is underlain by dense to very dense glacial till at relatively shallow depths. Dense to very dense native glacial till was observed in all the test pits completed at the site. Glacial till commonly consists of a very compact, poorly sorted, non-stratified mixture of clay, silt, sand, gravel and cobbles. Glacial till commonly appears gray or blue on a fresh surface, while weathered glacial till may be brown to yellow in color. The till was observed to contain cobbles in some areas and commonly also includes large boulders. Additionally, test pit GEI-TP-4 encountered a boulder at approximately 3.5 feet. Surface Conditions The location of the proposed townhomes consists of an undeveloped lot covered with vegetation. The lot is relatively flat and slopes down from approximate Elevation 343 feet in the northeast corner to about Elevation 330 feet in the southwest corner. Vegetation generally consists of blackberry brambles with various deciduous trees spread across the site. An existing storm drain outfall is present in the southeast corner of the site. Subsurface Conditions Shallow fill overlying loose to medium dense weathered glacial till and dense to very dense relatively unweathered native glacial till was observed in all of our test pits completed at the planned townhome locations. In general, the soils encountered in the test pits consisted of the following: Fill: Fill consisting of loose to medium dense sand with silt and gravel exists across the site. The fill is relatively thin and is typically less than 1 foot deep in the test pits completed at the project site. Till: Glacial till was observed to the depths explored in all of the test pits. The glacial till is relatively unweathered at a depth ranging from about 2.5 to 3.5 feet bgs and consists of gray dense to very dense silty fine to medium sand with variable gravel content and occasional cobbles. Weathered glacial till exists near the ground surface below the fill and above the relatively unweathered parent material and is typically brown and loose to medium dense. The weathered till may be 1 to 3 feet thick across the site. November 15, 2018 | Page 3 File No. 23656-001-00 Groundwater Conditions No groundwater seepage was observed in the test pits completed on site. Additionally, previous explorations completed to depths of 30 feet below site grades did not encounter groundwater. Groundwater observations represent conditions observed during exploration and may not represent the groundwater conditions throughout the year. We anticipate that perched groundwater will exist at the contact between the glacial till and the overlying looser fill and weathered till, and within more permeable layers within the native glacial till. Groundwater seepage is expected to fluctuate as a result of season, precipitation, and other factors. CONCLUSIONS AND RECOMMENDATIONS Based on the results of our field exploration program, laboratory testing, and engineering analysis, we conclude that development of the proposed development can be accomplished as planned. A summary of primary geotechnical considerations for the site development and design of the proposed development is provided in the subsequent sections. Summary ■ The planned townhomes site is classified as Site Class C, in accordance with the 2015 International Building Code (IBC). ■ The planned townhomes may be supported on conventional spread footings bearing on dense to very dense undisturbed glacial till or on structural fill placed over these soils. Footings bearing on dense to very dense undisturbed glacial till may be designed using an allowable soil bearing value of 6,000 pounds per square foot (psf). Footings bearing on structural fill placed over undisturbed dense to very dense glacial till may be designed using an allowable bearing value of 3,000 psf. All existing fill, highly weathered glacial till or otherwise unsuitable soils should be removed from below foundations prior to constructing foundations or placing structural fill. The allowable bearing value may be increased by one-third for short duration loads such as wind or seismic events. ■ Lateral foundation loads may be resisted by passive resistance on the sides of the footings and by friction on the base of the footings. For footings supported and surrounded by either dense native soils or compacted structural fill, a coefficient of friction of 0.35 and a passive resistance of 350 pounds per cubic foot (pcf) may be used. ■ A subgrade modulus of 100 pounds per cubic inch (pci) may be used for design of the slabs-on-grade for the townhomes. Concrete slabs-on-grade should be supported on a 4-inch-thick capillary break layer overlain by a vapor retarder (in conditioned spaces or enclosed rooms, such as mechanical or storage space). ■ The pavement section extending to the west and north from the southeast corner of the project site may be supported on existing fill soils provided that the upper 2 feet of the fill is placed and compacted as structural fill. A proof roll test can also be conducted to reduce excavation costs if native soils are encountered within the upper 2 feet and to check that the soils can perform adequately under planned loads. If any soft spots are observed, the upper 2 feet of subgrade soils should be removed and replaced with at least 2 feet of structural fill compacted to at least 95 percent of the maximum dry density (MDD) per ASTM International (ASTM) D 1557. Suitable on-site soils may be used as structural November 15, 2018 | Page 4 File No. 23656-001-00 fill under the planned road and associated hardscape provided that earthwork is accomplished during dry weather conditions in the summer months. Earthquake Engineering We evaluated the site for seismic hazards including liquefaction, lateral spreading, fault rupture and earthquake-induced landsliding. Our evaluation indicates that the site does not have liquefiable soils present and, therefore, also has no risk of liquefaction-induced lateral spreading. In addition, the site has a low risk of fault rupture and earthquake-induced landsliding. 2015 IBC Seismic Design Information For the planned townhomes, we recommend the IBC 2015 parameters for average field standard penetration resistance, site class, short period spectral response acceleration (SS), 1-second period spectral response acceleration (S1), and seismic coefficients FA and FV presented in Table 1. TABLE 1. 2015 IBC SEISMIC PARAMETERS 2015 IBC Parameter Recommended Value Average Field Standard Penetration Resistance >50 Site Class C Short Period Spectral Response Acceleration, SS (percent g) 143.2 1-Second Period Spectral Response Acceleration, S1 (percent g) 53.8 Seismic Coefficient, FA 1.000 Seismic Coefficient, FV 1.300 Liquefaction Potential Liquefaction is a phenomenon where soils experience a rapid loss of internal strength as a consequence of strong ground shaking. Ground settlement, lateral spreading and/or sand boils may result from soil liquefaction. Structures supported on liquefied soils could suffer foundation settlement or lateral movement that could be severely damaging to the structures. Conditions favorable to liquefaction occur in loose to medium dense, clean to moderately silty sand, which is below the groundwater level. Based on our evaluation of the subsurface conditions observed in the explorations, it is our opinion that potentially liquefiable soils are not present at the project site. Ground Rupture Ground rupture from lateral spreading is associated with liquefaction. Lateral spreading involves lateral displacements of large volumes of liquefied soil, and can occur on near-level ground as blocks of surface soils displace relative to adjacent blocks. In our opinion, ground rupture resulting from lateral spreading at the site is unlikely because potentially liquefiable soils are not present at the site as discussed above. Because of the thickness of the Quaternary sediments below the site, which are commonly more than 1,000 feet thick, the potential for surface fault rupture is considered low. November 15, 2018 | Page 5 File No. 23656-001-00 Landslides Because site topography is relatively flat and dense to very dense glacial till deposits occur at shallow depths, it is our opinion that landsliding as a result of strong ground shaking is unlikely at the site. Foundations We recommend that the buildings be supported on shallow spread footings founded on the dense to very dense native glacial till soil encountered in the explorations, or on properly compacted structural fill extending down to medium dense to dense glacial till. The following recommendations for the buildings are based on the subsurface conditions observed in the explorations and the site survey. Foundation Design For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed townhomes. The design frost depth in the Puget Sound area is 12 inches, therefore, we recommend that the footings be founded at least 18 inches below lowest adjacent finished grade. Unsuitable soils consisting of fill, topsoil, and/or highly weathered glacial soils will vary across the site and must be removed from below planned footings. Based on our explorations, up to 3½ feet of fill and/or looser weathered native soils exist under the proposed north building unit (GEI-TP-1 and GEI-TP-2), approximately 3 feet under the central building unit (GEI-TP-3 and GEI-TP-4) and 2½ feet under the south building unit (GEI-TP-5 and GEI-TP-6). Therefore, depending on the foundation locations and depths, overexcavation under the footings may be necessary. For foundations supported on medium dense native glacial till or structural fill extending down to medium dense to dense native glacial till, we recommend footings be designed using a maximum allowable bearing pressure of 3,000 psf. A maximum allowable bearing pressure of 6,000 psf may be used in design where foundations are bearing on dense to very dense relatively unweathered glacial till or on controlled density fill (CDF) extending down to the dense to very dense native till. All existing fill and looser native soils should be removed from below planned footings. These allowable bearing pressures apply to the total dead and long-term live loads and may be increased up to one-third for short-term live loads such as wind or seismic forces. The overexcavated areas should be backfilled with: (1) CDF having a design strength of at least 200 pounds per square inch (psi) where 6,000 psf bearing pressures are used or, (2) imported gravel borrow where 3,000 psf is used. Where structural fill is placed below footings, the fill should extend beyond the edges of the foundations by the depth of the overexcavation, while the CDF should extend beyond the edges of the foundations by half the depth of the excavation. Foundation Settlement We estimate that the post-construction settlement of footings founded on the very dense glacial till or structural fill extending to the medium dense to very dense till, as recommended above, will be between ½ and 1 inch. Differential settlement between comparably loaded column footings or along a 25-foot section of continuous wall footing should be less than ½ inch. We expect most of the footing settlements will occur as loads are applied. Loose or disturbed soils not removed from footing excavations prior to placing concrete will result in additional settlement. November 15, 2018 | Page 6 File No. 23656-001-00 Lateral Resistance Lateral loads can be resisted by passive resistance on the sides of the footings and by friction on the base of the footings. Passive resistance should be evaluated using an equivalent fluid density of 350 pcf where footings are poured neat against native soil or are surrounded by structural fill compacted to at least 95 percent of MDD, as recommended. Resistance to passive pressure should be calculated from the bottom of adjacent floor slabs and paving or below a depth of 1 foot where the adjacent area is unpaved, as appropriate. Frictional resistance can be evaluated using 0.35 for the coefficient of base friction against footings. The above values incorporate a factor of safety of about 1.5. If soils adjacent to footings are disturbed during construction, the disturbed soils must be recompacted, otherwise the lateral passive resistance value must be reduced. Construction Considerations Immediately prior to placing concrete, all debris and loose soils that accumulated in the footing excavations during forming and steel placement must be removed. Debris or loose soils not removed from the footing excavations will result in increased settlement. If wet weather construction is planned, we recommend that all footing subgrades be protected using a lean concrete mud mat or 3 inches of compacted crushed base course. The mud mat or base course should be placed the same day that the footing subgrade is excavated and approved for foundation support. We recommend that all completed footing excavations be observed by a representative of our firm prior to placing mud mat, reinforcing steel, and structural concrete. Our representative will confirm that the bearing surface has been prepared in a manner consistent with our recommendations and that the subsurface conditions are as expected. Footing Drains We recommend that perimeter footing drains be installed around each building. The perimeter drains should be installed at the base of the exterior footings. The perimeter drains should be provided with cleanouts and should consist of at least 4-inch-diameter perforated pipe placed on a 3-inch bed of drainage material, and surrounded by 6 inches of drainage material enclosed in a non-woven geotextile fabric such as TenCate Mirafi 140N (or approved equivalent) to prevent fine soil from migrating into the drain material. We recommend against using flexible tubing for footing drainpipes. The perimeter drains should be sloped to drain by gravity, if practicable, to a suitable discharge point, preferably a storm drain. We recommend that the cleanouts be covered, and be placed in flush-mounted utility boxes. Water collected in roof downspout lines must not be routed to the footing drain lines. Slab-on-Grade Floors We expect that the lower level concrete slab-on-grade can be supported on the medium dense to very dense native soil encountered in our explorations or on properly compacted structural fill. A subgrade modulus of 100 pci may be used for design of the slabs-on-grade at the site. We recommend that an appropriate capillary break and vapor retarder be installed below concrete slabs to reduce the risk of moisture migration through the floor slab. This is especially important since zones of groundwater seepage may be present at the planned floor slab level in more permeable layers above the dense native glacial till or in looser soils on top of the dense glacial till. November 15, 2018 | Page 7 File No. 23656-001-00 Prior to placing the gravel layer, the subgrade should be proofrolled as described previously in the “Earthwork” section of this report. If necessary, the building slab subgrades should be recompacted to a firm and unyielding condition. We recommend that concrete slabs-on-grade be constructed on a gravel layer to provide uniform support and drainage and to act as a capillary break. The gravel layer below slabs-on-grade should consist of at least 4 inches of clean crushed gravel with a maximum particle size of 1 inch and negligible sand or silt in accordance with Washington State Department of Transportation (WSDOT) Standard Specification 9-03.1(4)C American Association of State Highway and Transportation Officials (AASHTO) Grading No. 67. If prevention of moisture migration through the slab is essential, a vapor retarder such as heavy plastic sheeting should be installed between the slab and the gravel layer. It may also be prudent to apply a sealer to the slab to further retard the migration of moisture through the floor. We recommend that the plastic sheet be placed over the capillary break layer. Pavement Recommendations Recommendations for typical pavements (asphalt and concrete) are provided in the following sections. The City of Renton may have standard pavement sections that could apply to the site, therefore the project civil engineer should review the City’s standards, if applicable. Subgrade Preparation We recommend the subgrade soils in new pavement areas be prepared and evaluated as described in the “Earthwork” section of this report. All new pavement and hardscape areas should be supported on subgrade soils that have been proof rolled or probed as described in the “Clearing and Site Preparation” section of this report. If the exposed subgrade soils are loose or soft, it may be necessary to excavate localized areas and replace them with structural fill or gravel base course. Pavement subgrade conditions should be observed during construction and prior to placing the subbase materials in order to evaluate the presence of zones of unsuitable subgrade soils and the need for overexcavation and replacement of these zones. New Hot Mix Asphalt Pavement In light-duty pavement areas (e.g., automobile parking), we recommend a pavement section consisting of at least a 2½ -inch thickness of ½-inch hot-mix asphalt (HMA) per WSDOT Sections 5-04 and 9-03, over a 4-inch thickness of densely compacted crushed surfacing base course (CSBC) per WSDOT Section 9-03.9(3). In heavy-duty pavement areas (such as the driveway), we recommend a pavement section consisting of at least a 3-inch thickness of ½-inch HMA over a 6-inch thickness of densely compacted CSBC. The base course should be compacted to at least 95 percent of the MDD obtained using ASTM D 1557. We recommend that proof rolling of the subgrade and compacted base course be observed by a representative from our firm prior to paving. Soft or yielding zones observed during proof rolling may require overexcavation and replacement with compacted structural fill. The pavement sections recommended above are based on our experience. Thicker asphalt sections may be needed in accordance with the City of Renton or based on the actual traffic data, truck loads, and intended use. All paved and landscaped areas should be graded so that surface drainage is directed to appropriate catch basins. November 15, 2018 | Page 8 File No. 23656-001-00 Portland Cement Concrete Pavement Portland cement concrete (PCC) sections may be considered for areas where concentrated heavy loads may occur. We recommend that these pavements consist of at least 6 inches of PCC over 6 inches of CSBC. A thicker concrete section may be needed based on the actual load data for use of the area. If the concrete pavement will have doweled joints, we recommend that the concrete thickness be increased by an amount equal to the diameter of the dowels. The base course should be compacted to at least 95 percent of the MDD. We recommend PCC pavements incorporate construction joints and/or crack control joints spaced at maximum distances of 12 feet apart, center-to-center, in both the longitudinal and transverse directions. Crack control joints may be created by placing an insert or groove into the fresh concrete surface during finishing, or by saw cutting the concrete after it has initially set up. We recommend the depth of the crack control joints be approximately one fourth the thickness of the concrete; or about 1½ inches deep for the recommended concrete thickness of 6 inches. We also recommend the crack control joints be sealed with an appropriate sealant to help restrict water infiltration into the joints. Asphalt-Treated Base If pavements are constructed during the wet seasons, consideration may be given to covering the areas to be paved with asphalt-treated base (ATB) for protection. Light-duty pavement areas should be surfaced with 3 inches of ATB, and heavy-duty pavement areas should be surfaced with 6 inches of ATB. Thicker ATB sections may be needed based on construction equipment loads. Prior to placement of the final pavement sections, we recommend the ATB surface be evaluated and areas of ATB pavement failure be removed and the subgrade repaired. If ATB is used and is serviceable when final pavements are constructed, the CSBC can be eliminated, and the design PCC or asphalt concrete pavement thickness can be placed directly over the ATB. Earthwork Based on the subsurface soil conditions encountered in the explorations, we expect that the soils at the site may be excavated using conventional heavy-duty construction equipment. Very dense glacial till was encountered at relatively shallow depths at the planned building locations; therefore, glacial till soils within deeper portions of excavations may require a large excavator to accomplish the excavations. Cobbles were observed in most of the test pits and glacial till deposits in the area commonly contain boulders that may be encountered during excavation. Accordingly, the contractor should be prepared to deal with boulders, if encountered. The glacial till contains sufficient fines (material passing the U.S. standard No. 200 sieve) to be highly moisture-sensitive and susceptible to disturbance, especially when wet. Ideally, earthwork should be undertaken during extended periods of dry weather when the surficial soils will be less susceptible to disturbance and provide better support for construction equipment. Dry weather construction will help reduce earthwork costs and increase the potential for using the native soils as structural fill. Trafficability on the site is not expected to be difficult during dry weather conditions. However, the fill and native soils will be susceptible to disturbance from construction equipment during wet weather conditions and pumping and rutting of the exposed soils under equipment loads may occur. November 15, 2018 | Page 9 File No. 23656-001-00 Clearing and Site Preparation Areas to be developed or graded should be cleared of surface and subsurface deleterious matter including any debris, shrubs, trees and associated stumps and roots. Graded areas should be stripped of organic soils. The organic soils can be stockpiled and used later for landscaping purposes or may be spread over disturbed areas following completion of grading. If spread out, the organic strippings should be in a layer less than 1-foot thick, should not be placed on slopes greater than 3H:1V (horizontal to vertical) and should be track-rolled to a uniformly compacted condition. Materials that cannot be used for landscaping or protection of disturbed areas should be removed from the project site. Undocumented fill may be present in various areas of the site and will be required to be removed under building foundations and within the upper two feet of pavement, hardscape and slab subgrade levels. Where existing fill and looser native soils are removed, they may be reused and recompacted as structural fill, if conditions allow. If medium dense to dense native soils are encountered below slab subgrade, additional excavation is not required. If old fill is encountered below slab subgrade, the fill should be evaluated and possibly removed up to 2 feet below slab subgrade or until medium dense to dense native soils are encountered (less than 2 feet below slab subgrade). Excavations for slab subgrade preparation likely do not need to extend more than 2 feet below slab subgrade. The upper two feet below pavement subgrade should also be removed and replaced as structural fill; however, if existing fill soils are suitable and adequately compacted based on evaluations after the pavement is removed, the contractor can perform a proof roll on the exposed surface at or below slab subgrade level, and if approved by the geotechnical engineer, the fill may be left in place. Subgrade Preparation Prior to placing new fills, pavement base course materials or gravel below on-grade floor slabs, subgrade areas should be proof rolled to locate any soft or pumping soils. Prior to proof rolling, all unsuitable soils should be removed from below the building footprints. Proof rolling can be completed using a piece of heavy tire-mounted equipment such as a loaded dump truck. During wet weather, the exposed subgrade areas should be probed to determine the extent of soft soils. If soft or pumping soils are observed, they should be removed and replaced with compacted structural fill. If deep pockets of soft or pumping soils are encountered outside the building areas, it may be possible to limit the depth of overexcavation by placing a non-woven geotextile fabric such as TenCate Mirafi 500X (or equivalent) on the overexcavated subgrade prior to placing structural fill. The geotextile will provide additional support by bridging over the soft material and will help reduce fines contamination into the structural fill. After completing the proof rolling, the subgrade areas should be recompacted to a firm and unyielding condition, if possible. The degree of compaction that can be achieved will depend on when the construction is performed. If the work is performed during dry weather conditions, we recommend that all subgrade areas be recompacted to at least 95 percent of the MDD in accordance with the ASTM D 1557 test procedure (modified Proctor). If the work is performed during wet weather conditions, it may not be possible to recompact the subgrade to 95 percent of the MDD. In this case, we recommend that the subgrade be compacted to the extent possible without causing undue heaving or pumping of the subgrade soils. November 15, 2018 | Page 10 File No. 23656-001-00 Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. If the subgrade deteriorates during proof rolling or compaction, it may become necessary to modify the proof rolling, compaction criteria, or methods. Structural Fill All fill, whether existing on-site glacial till soil or imported soil, that will support building foundations and floor slabs, pavement and hardscape areas, or be placed in utility trenches should generally meet the criteria for structural fill presented below. The suitability of soil for use as structural fill depends on its gradation and moisture content. Materials Materials used on the project site, under buildings, pavement, hardscape areas, and to backfill utility trenches are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use as described below: 1. Structural fill placed below all building elements (except footing designed for greater than 3,000 psf bearing pressure) and during wet weather conditions should consist of imported Gravel Borrow, as described in Section 9-03.14(1) of the 2018 WSDOT Standard Specifications, with the additional restriction that the fines content be limited to no more than 5 percent. On-site soils may be used as structural fil provided it is placed during the summer months, is properly moisture conditioned to within 2 percent of the optimum moisture content, and can be compacted to at least 95 percent of the MDD. 2. CDF having a design strength of at least 200 psi should be used under all foundations designed for greater than 3,000 psf bearing pressure. 3. Structural fill placed to construct embankment and parking areas and to backfill utility trenches may consist of on-site fill and glacial till provided that the soils are moisture conditioned for the required compaction. On-site till soils may be suitable for use as structural fill during dry weather conditions in areas needing 95 percent compaction. If structural fill is placed during wet weather, the structural fill should consist of imported gravel borrow. 4. Structural fill placed as CSBC below pavements should conform to Section 9-03.9(3) of the 2018 WSDOT Standard Specifications. 5. Structural fill placed as capillary break below slabs should consist of 1-inch minus clean crushed gravel with negligible sand or silt in conformance with Section 9-03.1(4)C, grading No. 67 of the 2018 WSDOT Standard Specifications. Reuse of On-site Native Soils The existing fill and till soils contain a high percentage of fines and will be sensitive to changes in moisture content and difficult to handle and compact during wet weather. The existing fill (free of organic debris) and till deposits are expected to be suitable for structural fill in areas requiring compaction to at least 95 percent of MDD (per ASTM D 1557), provided the work is accomplished during the normally dry season (June through September) and that the soil can be properly moisture conditioned to within 2 percent of the optimum moisture content. Imported structural fill consisting of sand and gravel (WSDOT gravel borrow) should be planned under all building foundation elements, especially if construction occurs during wet weather. November 15, 2018 | Page 11 File No. 23656-001-00 The use of existing on-site fill and till soils as structural fill during wet weather should be planned only for areas requiring compaction to 90 percent of MDD, as long as the soils are properly protected from wet weather, not placed during periods of precipitation, and that they can be dried if needed to achieve proper compaction. The contractor should plan to cover and maintain all fill stockpiles with plastic sheeting if it will be used as structural fill. The reuse of on-site soils is highly dependent on the skill of the contractor and schedule, and we will work with the design team and contractor to maximize the reuse of on-site till soils during the wet and dry seasons. Fill Placement and Compaction Criteria Structural fill should be mechanically compacted to a firm, non-yielding condition. Structural fill should be placed in loose lifts not exceeding 12 inches in thickness when using heavy compaction equipment and not more than 6 inches when using hand-operated compaction equipment. The actual thickness will be dependent on the structural fill material used and the type and size of compaction equipment. Each lift should be moisture conditioned to within about 2 percent of the optimum moisture content to achieve proper compaction to the specified density before placing subsequent lifts. Compaction of all structural fill at the site should be in accordance with the ASTM D 1557 (modified Proctor) test method. Structural fill should be compacted to the following criteria: 1. Structural fill placed below floor slabs and foundations should be compacted to 95 percent of the MDD. 2. Structural fill in new pavement and hardscape areas, including utility trench backfill, should be compacted to at least 90 percent of the MDD, except that the upper 2 feet of fill below final subgrade should be compacted to at least 95 percent of the MDD, see Figure 3, Compaction Criteria for Trench Backfill. 3. Structural fill placed as CSBC below pavements should be compacted to 95 percent of the MDD. 4. Non-structural fill, such as fill placed in landscape areas, should be compacted to at least 90 percent of the MDD. Weather Considerations Disturbance of near-surface soils should be expected if earthwork is completed during periods of wet weather. During dry weather, the soils will: (1) be less susceptible to disturbance, (2) provide better support for construction equipment, and (3) be more likely to meet the required compaction criteria. The wet weather season generally begins in October and continues through May in western Washington; however, periods of wet weather may occur during any month of the year. For earthwork activities during wet weather, we recommend that the following steps be taken: ■ The ground surface in and around the work area should be sloped so that surface water is directed away from the work area. The ground surface should be graded so that areas of ponded water do not develop. Measures should be taken by the contractor to prevent surface water from collecting in excavations and trenches. Measures should be implemented to remove surface water from the work area. ■ Earthwork activities should not take place during periods of moderate to heavy precipitation. ■ Slopes with exposed soils should be covered with plastic sheeting. November 15, 2018 | Page 12 File No. 23656-001-00 ■ The contractor should take necessary measures to prevent on-site soils and soils to be used as fill from becoming wet or unstable. These measures may include the use of plastic sheeting, sumps with pumps, and grading. The site soils should not be left uncompacted and exposed to moisture. Sealing the surficial soils by rolling with a smooth-drum roller prior to periods of precipitation will help reduce the extent that these soils become wet or unstable. ■ The contractor should cover all soil stockpiles that will be used as structural fill with plastic sheeting. ■ Construction traffic should be restricted to specific areas of the site, preferably areas that are surfaced with the existing asphalt or working pad materials not susceptible to wet weather disturbance. ■ Construction activities should be scheduled so that the length of time that soils are left exposed to moisture is reduced to the extent practical. Routing of equipment on the existing fill and native till subgrade soils during the wet weather months will be difficult and the subgrade will likely become highly disturbed and rutted. In addition, a significant amount of mud can be produced by routing equipment directly on the glacial soils in wet weather. Therefore, to protect the subgrade soils and to provide an adequate wet weather working surface for the contractor’s equipment and labor, we recommend that the contractor protect exposed subgrade soils with sand and gravel, crushed gravel, or ATB. Permanent Cut and Fill Slopes We recommend that permanent cut or fill slopes be constructed at inclinations of 2H:1V or flatter, and be blended into existing slopes with smooth transitions. To achieve uniform compaction, we recommend that fill slopes be overbuilt slightly and subsequently cut back to expose well compacted fill. It is our experience that permanent cut slopes made in dense to very dense glacial till are difficult to establish vegetation on and difficult to place and maintain topsoil on. Therefore, 3H:1V or flatter permanent cut slopes should be considered for landscape purposes if site conditions allow for their use. To reduce erosion, newly constructed slopes should be planted or hydroseeded shortly after completion of grading. Until the vegetation is established, some sloughing and raveling of the slopes should be expected. This may necessitate localized repairs and reseeding. Temporary covering, such as clear heavy plastic sheeting, jute fabric, or erosion control blankets (such as American Excelsior Curlex 1 or North American Green SC150) could be used to protect the slopes during periods of rainfall. Utility Trenches Trench excavation, pipe bedding, and trench backfilling should be completed using the general procedures described in the 2018 WSDOT Standard Specifications or other suitable procedures specified by the project civil engineer. The native glacial deposits and fill soils encountered at the site are generally of low corrosivity based on our experience in the Puget Sound area. Utility trench backfill should consist of structural fill and should be placed in loose lifts not exceeding 12 inches in thickness when using heavy compaction equipment and not more than 6 inches when using hand-operated compaction equipment such that adequate compaction can be achieved throughout the lift. Each lift must be compacted prior to placing the subsequent lift. The backfill should be compacted in accordance with the criteria discussed above. Figure 3 illustrates recommended trench compaction criteria under pavement and non-structural areas. November 15, 2018 | Page 13 File No. 23656-001-00 Sedimentation and Erosion Control In our opinion, the erosion potential of the on-site soils is low to moderate. Construction activities including stripping and grading will expose soils to the erosional effects of wind and water. The amount and potential impacts of erosion are partly related to the time of year that construction actually occurs. Wet weather construction will increase the amount and extent of erosion and potential sedimentation. Erosion and sedimentation control measures may be implemented by using a combination of interceptor swales, straw bale barriers, silt fences and straw mulch for temporary erosion protection of exposed soils. All disturbed areas should be finish graded and seeded as soon as practicable to reduce the risk of erosion. Erosion and sedimentation control measures should be installed and maintained in accordance with the requirements of the City of Renton. Excavations We anticipate that excavations are limited and will be primarily associated with footing excavations and underground utilities. These cuts can likely be made as temporary open cut slopes depending on site constraints. The stability of open cut slopes is a function of soil type, groundwater seepage, slope inclination, slope height and nearby surface loads. The use of inadequately designed open cuts could impact the stability of adjacent work areas, existing utilities, and endanger personnel. The contractor performing the work has the primary responsibility for protection of workmen and adjacent improvements. In our opinion, the contractor will be in the best position to observe subsurface conditions continuously throughout the construction process and to respond to variable soil and groundwater conditions. Therefore, the contractor should have the primary responsibility for deciding whether or not to use open cut slopes for much of the excavations rather than some form of temporary excavation support, and for establishing the safe inclination of the cut slope. Acceptable slope inclinations for utilities and ancillary excavations should be determined during construction. Because of the diversity of construction techniques and available shoring systems, the design of temporary shoring is most appropriately left up to the contractor proposing to complete the installation. Temporary cut slopes and shoring must comply with the provisions of Title 296 WAC, Part N, “Excavation, Trenching and Shoring.” The excavations for the buildings and utilities will be completed primarily in loose to medium dense fill and dense to very dense glacial till deposits. The following sections summarize the general excavation recommendations. Temporary Cut Slopes For planning purposes, temporary unsupported cut slopes more than 4 feet high may be inclined at 1H:1V maximum steepness within the dense to very dense glacial till (below a depth of about 3 feet) and 1½H:IV maximum steepness in the overlying fill (upper 3 feet). If significant seepage is present on the cut face then the cut slopes may have to be flattened. However, temporary cuts should be discussed with the geotechnical engineer during final design development to evaluate suitable cut slope inclinations for the various portions of the excavation. The contractor should scale slopes cut at 1H:1V to remove loose materials and cobbles. The above guidelines assume that surface loads such as traffic, construction equipment, stockpiles or building supplies will be kept away from the top of the cut slopes a sufficient distance so that the stability November 15, 2018 | Page 14 File No. 23656-001-00 of the excavation is not affected. We recommend that this distance be at least 5 feet from the top of the cut for temporary cuts made at 1H:1V or flatter, and no closer than a distance equal to one-half the height of the slope for cuts made steeper than 1H:IV. Water that enters the excavation must be collected and routed away from prepared subgrade areas. We expect that this may be accomplished by installing a system of drainage ditches and sumps along the toe of the cut slopes. Some sloughing and raveling of the cut slopes should be expected. Temporary covering, such as heavy plastic sheeting with appropriate ballast, should be used to protect these slopes during periods of wet weather. Surface water runoff from above cut slopes should be prevented from flowing over the slope face by using berms, drainage ditches, swales or other appropriate methods. If temporary cut slopes experience excessive sloughing or raveling during construction, it may become necessary to modify the cut slopes to maintain safe working conditions. Slopes experiencing problems can be flattened, regraded to add intermediate slope benches, or additional dewatering can be provided if the poor slope performance is related to groundwater seepage. Drainage Considerations We anticipate shallow groundwater seepage may enter excavations for utilities depending on the time of year construction takes place, especially in the winter months. However, we expect that this seepage water can be handled by digging interceptor trenches in the excavations and pumping from sumps. The seepage water, if not intercepted and removed from the excavations, will make it difficult to place and compact structural fill and may destabilize cut slopes. All paved and landscaped areas should be graded so that surface drainage is directed away from the buildings to appropriate catch basins. Water collected in roof downspout lines must not be routed to the footing drain lines. Collected downspout water should be routed to appropriate discharge points in separate pipe systems. Infiltration Considerations Sieve analyses were performed on selected soil samples collected from the test pits that were completed as part of this study. The soil samples typically consisted of native weathered or relatively unweathered glacial till. The design infiltration value described below is based on the results of the grain size analyses, the United States Department of Agriculture (USDA) Textural Triangle, and the Washington State Department of Ecology Storm Water Management Manual (2005). The grain size analyses are presented in Appendix B. Based on our analysis, it is our opinion that the on-site native glacial till soils have a very low infiltration capacity. The majority of the soils across the site contain significant fines, which limits the infiltration capacity. The results of the sieve analyses indicated that the fines content (material passing the U.S. No. 200 sieve) typically ranges from 30 to 35 percent. Due to the density, high fines content, and relative impermeability of the glacial till, infiltration should be assumed to be very low when designing infiltration systems. We recommend a preliminary infiltration rate of not more than 0.2 inches per hour be used for design of the infiltration facilities. Depending on the depth of proposed infiltration facilities, the infiltration rate will vary; however, we recommend site specific pilot infiltration testing be performed to determine the design infiltration rate if specific infiltration facilities are being considered. November 15, 2018 | Page 15 File No. 23656-001-00 RECOMMENDED ADDITIONAL GEOTECHNICAL SERVICES Throughout this report, recommendations are provided where we consider additional geotechnical services to be appropriate. These additional services are summarized below: ■ GeoEngineers should be retained to review the project plans and specifications when complete to confirm that our design recommendations have been implemented as intended. ■ During construction, GeoEngineers should observe and evaluate the suitability of the foundation subgrades, evaluate the suitability of floor slab and pavement subgrades, observe installation of subsurface drainage measures including footing drains, observe and test structural backfill, and provide a summary letter of our construction observation services. The purposes of GeoEngineers construction phase services are to confirm that the subsurface conditions are consistent with those observed in the explorations and other reasons described in Appendix D, Report Limitations and Guidelines for Use. LIMITATIONS We have prepared this report for use by Homestead Community Land Trust and members of the project team for use in design of this project. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in the field of geotechnical engineering in this area at the time this report was prepared. No warranty or other conditions, express or implied, should be understood. Please refer to Appendix D for additional information pertaining to use of this report. REFERENCES International Code Council, 2015, “International Building Code.” Mullineaux, D. R., 1965, “Geologic Map of the Renton Quadrangle, King County, Washington.” Soil & Environmental Engineers, Inc., 2011, “Report of Geotechnical Investigation, Proposed Residential Development, Edmonds and Glennwood Ave NE, Renton, Washington.” United States Geological Survey – Earthquake Hazards Program Software, “U.S. Seismic Design Maps,” 2008 data, 2012/2015 IBC, https://earthquake.usgs.gov/designmaps/us/application.php. Washington State Department of Transportation, 2018, “Standard Specifications for Road, Bridge and Municipal Construction.” FIGURES µ SITE Vicinity Map Figure 1 Willow Crest Townhomes Renton, Washington 2,000 2,0000 Feet Data Source: Mapbox Open Street Map, 2018 Notes:1. The locations of all features shown are approximate.2. This drawing is for information purposes. It is intended to assist inshowing features discussed in an attached document. GeoEngineers, Inc.cannot guarantee the accuracy and content of electronic files. The masterfile is stored by GeoEngineers, Inc. and will serve as the official record ofthis communication. Projection: NAD 1983 UTM Zone 10N P:\23\23656001\GIS\2365600100_F01_VicinityMap.mxd Date Exported: 10/17/18 by cchelf 310315320325 330335340345308309311312313314316317318319321322323324326 3 2 7 328329331332333 3343363373383393413423 4 3 3 4 4 3463053103153203253303353403453453 0 1 3 0 2 3 03 3043063073083093113123133143163173183193213223233243263 2 7 3 2 8 3 2 9 3313323333 3 4 336337338339341342343344345344346343 3443 2 0 325 33033534031 8 31 9 32132 2 32 3 324 3263273283293313323333343343343363373373373 3 8 3393413 4 2 342342 343 295 300 3 0 5 3 1 0 292 293 294296297 298 299 301 302 30 3 30 4 3 0 63073 0 8 3 0 9 31 1 312 3103153203253303353 4 0 3 4 5 3093113123133143163173183193213223233243263 27 328329331332333334336337338 3 3 9 34134234334 4 3 4 6 347 340341342343345343344345346345342343344346 3 4 7 34 7347 3473473463473473 4 7 Edmonds Ave NEGlenwood Ave NEGEI-TP-1 GEI-TP-2 GEI-TP-3 GEI-TP-4 GEI-TP-5 GEI-TP-6 B-1 B-2 TP-1 TP-2 TP-3 TP-4 TP-5 TP-6 Figure 2 Site Plan - Proposed Development Willow Crest Townhomes Renton, Washington W E N S P:\23\23656001\CAD\00\GeoTech\2365600100_F02_Site Plan.dwg TAB:F02 Date Exported: 10/31/18 - 10:19 by hmaraLegendNotes: 1. The locations of all features shown are approximate. 2. This drawing is for information purposes. It is intended to assist in showing features discussed in an attached document. GeoEngineers, Inc. cannot guarantee the accuracy and content of electronic files. The master file is stored by GeoEngineers, Inc. and will serve as the official record of this communication. Data Source: Base survey by Lank Tree Land Surveying dated 10/17/18. Proposed site features from Third Place Design Co-Operative dated 10/7/18. Projection: WA State Plane, North Zone, NAD83, US Foot Feet 040 40 Test Pit by GeoEngineers, 2018 (Current Study)GEI-TP-1 Boring by Soil & Environmental Engineers, 2011B-1 Test Pit by Soil & Environmental Engineers, 2011TP-1 Site Boundary 4 Unit Town Homes 4 Unit Town Homes 4 Unit Town Homes Figure 3 P:\23\23656001\CAD\00\GeoTech\2365600100_F03_Compaction Criteria.dwg TAB:F03 Date Exported: 10/19/18 - 13:02 by hmara95 90 90 95 90 Pipe Varies Varies (See Note 1) 2 Feet Varies (Modified Proctor) Pipe Bedding Trench Backfill Base Course Concrete or Asphalt Pavement Maximum Dry Density, by Test Method ASTM D1557 Recommended Compaction as a Percentage of Legend 95 Notes: 1. All backfill under building areas should be compacted to at least 95 percent per ASTM D1557. Non-structural Areas Hardscape Or Pavement Areas Ground Surface Not To Scale Compaction Criteria for Trench Backfill Willow Crest Townhomes Renton, Washington APPENDICES APPENDIX A Field Explorations November 15, 2018 | Page A-1 File No. 23656-001-00 APPENDIX A FIELD EXPLORATIONS Subsurface soil and groundwater conditions were evaluated by completing six test pits (GEI-TP-1 through GEI-TP-6) at the approximate locations shown on Figure 2, Site Plan. Locations of the test pits were determined in the field by pacing and tape measuring distances from the test pit locations to existing site features. Ground surface elevations were interpolated from a site topographic map. Test pits (GEI-TP-1 through GEI-TP-6) were completed on October 22, 2018 at the approximate locations shown on Figure 2. The test pits were excavated to depths of about 8 feet below the existing ground surface (bgs). The test pits were completed using a mini rubber track mounted Takeuchi TB138BFR excavator owned and operated by Kelly’s Excavating. The test pits were continuously monitored by a geotechnical engineer from our firm who evaluated and classified the soils encountered, obtained representative soil samples, and observed groundwater conditions (if present). Our representative maintained a detailed log of each test pit. Disturbed samples of representative soil types were obtained from the excavator bucket at representative depths. Soils encountered in the test pits were classified in the field in general accordance with ASTM D 2488, the Standard Practice for Classification of Soils, Visual-Manual Procedure, which is summarized in Figure A-1. Logs of the test pits are provided in Figures A-2 through A-7. Measured groundwater level in exploration, well, or piezometer Measured free product in well or piezometer Distinct contact between soil strata Approximate contact between soil strata Contact between geologic units SYMBOLS TYPICAL DESCRIPTIONS GW GP SW SP SM FINE GRAINED SOILS SILTS AND CLAYS NOTE: Multiple symbols are used to indicate borderline or dual soil classifications MORE THAN 50% RETAINED ON NO. 200 SIEVE MORE THAN 50% PASSING NO. 200 SIEVE GRAVEL AND GRAVELLY SOILS SC LIQUID LIMIT LESS THAN 50 (APPRECIABLE AMOUNT OF FINES) (APPRECIABLE AMOUNT OF FINES) COARSE GRAINED SOILS MAJOR DIVISIONS GRAPH LETTER GM GC ML CL OL SILTS AND CLAYS SANDS WITH FINES SAND AND SANDY SOILS MH CH OH PT (LITTLE OR NO FINES) CLEAN SANDS GRAVELS WITH FINES CLEAN GRAVELS (LITTLE OR NO FINES) WELL-GRADED GRAVELS, GRAVEL -SAND MIXTURES CLAYEY GRAVELS, GRAVEL - SAND -CLAY MIXTURES WELL-GRADED SANDS, GRAVELLYSANDS POORLY-GRADED SANDS, GRAVELLYSAND SILTY SANDS, SAND - SILT MIXTURES CLAYEY SANDS, SAND - CLAYMIXTURES INORGANIC SILTS, ROCK FLOUR,CLAYEY SILTS WITH SLIGHTPLASTICITY INORGANIC CLAYS OF LOW TOMEDIUM PLASTICITY, GRAVELLYCLAYS, SANDY CLAYS, SILTY CLAYS,LEAN CLAYS ORGANIC SILTS AND ORGANIC SILTYCLAYS OF LOW PLASTICITY INORGANIC SILTS, MICACEOUS ORDIATOMACEOUS SILTY SOILS INORGANIC CLAYS OF HIGHPLASTICITY ORGANIC CLAYS AND SILTS OFMEDIUM TO HIGH PLASTICITY PEAT, HUMUS, SWAMP SOILS WITHHIGH ORGANIC CONTENTSHIGHLY ORGANIC SOILS SOIL CLASSIFICATION CHART MORE THAN 50% OF COARSE FRACTION RETAINED ON NO. 4 SIEVE MORE THAN 50% OF COARSE FRACTION PASSING ON NO. 4 SIEVE SILTY GRAVELS, GRAVEL - SAND -SILT MIXTURES POORLY-GRADED GRAVELS,GRAVEL - SAND MIXTURES LIQUID LIMIT GREATER THAN 50 Continuous Coring Bulk or grab Direct-Push Piston Shelby tube Standard Penetration Test (SPT) 2.4-inch I.D. split barrel Contact between soil of the same geologic unit Material Description Contact Graphic Log Contact NOTE: The reader must refer to the discussion in the report text and the logs of explorations for a proper understanding of subsurface conditions. Descriptions on the logs apply only at the specific exploration locations and at the time the explorations were made; they are not warranted to be representative of subsurface conditions at other locations or times. Groundwater Contact Blow count is recorded for driven samplers as the number of blows required to advance sampler 12 inches (or distance noted). See exploration log for hammer weight and drop. "P" indicates sampler pushed using the weight of the drill rig. "WOH" indicates sampler pushed using the weight of the hammer. Key to Exploration Logs Figure A-1 Sampler Symbol Descriptions ADDITIONAL MATERIAL SYMBOLS NS SS MS HS No Visible Sheen Slight Sheen Moderate Sheen Heavy Sheen Sheen Classification SYMBOLS Asphalt Concrete Cement Concrete Crushed Rock/ Quarry Spalls Topsoil GRAPH LETTER AC CC SOD Sod/Forest Duff CR DESCRIPTIONS TYPICAL TS Laboratory / Field Tests %F %G AL CA CP CS DD DS HA MC MD Mohs OC PM PI PP SA TX UC VS Percent fines Percent gravel Atterberg limits Chemical analysis Laboratory compaction test Consolidation test Dry density Direct shear Hydrometer analysis Moisture content Moisture content and dry density Mohs hardness scale Organic content Permeability or hydraulic conductivity Plasticity index Pocket penetrometer Sieve analysis Triaxial compression Unconfined compression Vane shear Blackberry brambles Dark brown fine to medium sand with silt, occasional gravel, roots (loose, moist) (fill) Light brown silty fine to medium sand with gravel, occasional cobbles, trace roots (medium dense, moist) (weathered glacial till) Gray silty fine to medium sand with gravel (very dense, moist) (glacial till) WD SP-SM SM SM 1 SA 2 %F 3 4 4 3 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 6 inches 0.5 kg/cm2 / 4 inches >4.5 kg/cm2 / ½ inch >4.5 kg/cm2 / <½ inch 21 30 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit GEI-TP-1 Willow Crest Townhomes Figure A-2Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306462 186084 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. Blackberry brambles Dark brown fine to medium sand with silt, roots (loose, moist) (fill) Brown silty coarse gravel with sand and occasional cobbles, roots (medium dense to dense, moist) (weathered glacial till) No roots below 2 feet Gray silty fine sand with occasional gravel and cobbles (very dense, moist) (glacial till) WD SP-SM GM SM 1 SA 2 3 4 3 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 8 inches 1.0 kg/cm2 / 3 inches >4.5 kg/cm2 / 1 inch >4.5 kg/cm2 / ¼ inch 17 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit TEI-TP-2 Willow Crest Townhomes Figure A-3Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306453 186046 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. Blackberry brambles Dark brown fine to medium sand with silt and occasional gravel, roots (loose, moist) (fill) Light brown silty fine sand with gravel and occasional cobbles, roots (loose to medium dense, moist) (weathered glacial till) No roots below 2 feet Light gray silty fine to medium sand with occasional gravel and cobbles (very dense, moist) (glacial till) Gray fine to medium sand with silt, gravel and occasional cobbles (very dense, moist) WD SP-SM SM SM SP-SM 1 SA 2 3 4 5 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 12 inches 2.0 kg/cm2 / 3 inches >4.5 kg/cm2 / ½ inch >4.5 kg/cm2 / ¼ inch 32 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit GEI-TP-3 Willow Crest Townhomes Figure A-4Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306422 185979 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. Blackberry brambles Dark brown fine to medium sand with silt and occasional gravel, roots (loose, moist) (fill) Light brown silty fine sand with gravel and occasional cobbles (loose to medium dense, moist) (weathered glacial till) Light gray silty fine to medium sand with gravel and occasional cobbles; partially cemented (very dense, moist) (glacial till) Boulder encountered at 3½ feet WD SP-SM SM SM 1 2 SA 3 4 6 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 12 inches 2.25 kg/cm2 / 4 inches >4.5 kg/cm2 / 1 inch >4.5 kg/cm2 / ¼ inch 35 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit GEI-TP-4 Willow Crest Townhomes Figure A-5Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306466 185974 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. Blackberry brambles Brown fine to medium sand with silt and occasional gravel (loose, moist) (fill) Light brown silty fine sand with gravel and cobbles, roots (loose to medium dense, moist) (weathered glacial till) Gray silty fine sand with gravel and occasional cobbles; partially cemented (dense to very dense, moist) (glacial till) WD SP-SM SM SM 1 %F 2 3 4 8 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 12 inches 1.5 kg/cm2 / 5 inches 2.5 kg/cm2 / 2 inches >4.5 kg/cm2 / ¼ inch 26 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit GEI-TP-5 Willow Crest Townhomes Figure A-6Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306422 185911 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. Blackberry brambles Brown fine to medium sand with silt and occasional gravel (loose, moist) (fill) Light brown silty fine to medium sand with gravel and cobbles, roots (loose to medium dense, moist) (weathered till) Gray silty fine to medium sand with gravel and occasional cobbles (dense to very dense, moist) (glacial till) WD SP-SM SM SM 1 2 SA 3 4 5 Pocket Pen / Probe Penetration 0.0 kg/cm2 / 12 inches 1.5 kg/cm2 / 5 inches 4.0 kg/cm2 / 1½ inches >4.5 kg/cm2 / ¼ inch 33 Notes: See Figure A-1 for explanation of symbols. The depths on the test pit logs are based on an average of measurements across the test pit and should be considered accurate to ½ foot. Coordinates Data Source: Horizontal approximated based on Aerial Imagery. Vertical approximated based on Google Earth.Date:11/5/18 Path:W:\PROJECTS\23\23656001\GINT\2365600100.GPJ DBLibrary/Library:GEOENGINEERS_DF_STD_US_JUNE_2017.GLB/GEI8_TESTPIT_1P_GEOTEC_%FSheet 1 of 1Project Number: Project Location: Project: Renton, Washington 23656-001-00 Log of Test Pit GEI-TP-6 Willow Crest Townhomes Figure A-7Elevation (feet)339338337336335334333332Depth (feet)1 2 3 4 5 6 7 8 Testing SampleGraphic LogSAMPLE MATERIAL DESCRIPTION GroupClassificationSample NameTestingMoistureContent (%)REMARKS FinesContent (%)Date Excavated Surface Elevation (ft) Vertical Datum Coordinate System Horizontal Datum Easting (X) Northing (Y) Total Depth (ft)10/22/2018 8 340 NAVD88 1306474 185878 WA State Plane North NAD83 (feet) AJF Checked By MAG Groundwater not observed Caving not observedEquipment Takeuchi TB138FR Logged By Excavator Kelly's Excavating, Inc. APPENDIX B Laboratory Testing November 15, 2018 | Page B-1 File No. 23656-001-00 APPENDIX B LABORATORY TESTING Soil samples obtained from the test pits were transported to our laboratory and examined to confirm or modify field classifications, as well as to evaluate engineering properties of the soil. Representative samples were selected for laboratory testing consisting of moisture content, percent fines (material passing the U.S. No. 200 sieve), grain size determinations (sieve analysis), and modified Proctor testing. The tests were performed in general accordance with test methods of the ASTM International (ASTM) or other applicable procedures. Soil Classifications Soil samples obtained from the test pits were visually classified in the field and/or in our laboratory using a system based on the Unified Soil Classification System (USCS) and ASTM classification methods. ASTM test method D 2488 was used to visually classify the soil samples, while ASTM D 2487 was used to classify the soils based on laboratory tests results. These classification procedures are incorporated in the test pit logs shown in Figures A-2 through A-7 in Appendix A. Moisture Content Determinations Moisture contents tests were completed in general accordance with ASTM D 2216 for representative samples obtained from the test pits. The test results are presented on the test pit logs in Appendix A at the respective sample depth. Percent Fines Tests Tests to evaluate the percent fines (particles passing the No. 200 sieve) were completed using ASTM D 1140. The wet sieve method was used to determine the percentage of soil particles larger than the U.S. No. 200 sieve opening. The results of the percent fines tests are presented on the test pit logs at the depths at which the samples were obtained. Sieve Analysis Sieve analyses were performed on selected samples in general accordance with ASTM D 422. The wet sieve analysis method was used to estimate the percentage of soil greater than the U.S. No. 200 mesh sieve. The results of the sieve analyses were plotted, classified in general accordance with the USCS, and presented on Figures B-1 and B-2. Modified Proctor The modified Proctor tests were completed in general accordance with ASTM D1557. The results of the modified Proctor testing are presented on Figure B-3. 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.11101001000PERCENT PASSING BY WEIGHT GRAIN SIZE IN MILLIMETERS U.S. STANDARD SIEVE SIZE SAND SILT OR CLAYCOBBLES GRAVEL COARSE MEDIUM FINECOARSEFINE Boring Number Depth (feet)Soil Description TP-1 TP-2 TP-3 TP-4 2 2 1.5 4 Silty fine to medium sand with gravel (SM) Silty coarse gravel with sand (GM) Silty fine sand with gravel (SM) Silty fine to medium sand with gravel (SM) Symbol Moisture (%) 4 3 5 6 3/8”3”1.5”#4 #10 #20 #40 #60 #1003/4”Figure B-1Sieve Analysis ResultsWillow Crest Townhomes Renton, Washington23656-001-00 Date Exported: 10/31/18 Note:This report may not be reproduced,except in full,without written approval of GeoEngineers,Inc.Test results are applicable only to the specific sample on which they were performed,and should not be interpreted as representative of any other samples obtained at other times,depths or locations,or generated by separate operations or processes. The grain size analysis results were obtained in general accordance with ASTM D 6913.GeoEngineers 17425 NE Union Hill Road Ste 250,Redmond,WA 98052 #200 0 10 20 30 40 50 60 70 80 90 100 0.0010.010.11101001000PERCENT PASSING BY WEIGHT GRAIN SIZE IN MILLIMETERS U.S. STANDARD SIEVE SIZE SAND SILT OR CLAYCOBBLES GRAVEL COARSE MEDIUM FINECOARSEFINE Boring Number Depth (feet)Soil Description TP-6 4 Silty fine to medium sand with gravel (SM) Symbol Moisture (%) 5 3/8”3”1.5”#4 #10 #20 #40 #60 #1003/4”Figure B-2Sieve Analysis ResultsWillow Crest Townhomes Renton, Washington23656-001-00 Date Exported: 10/31/18 Note:This report may not be reproduced,except in full,without written approval of GeoEngineers,Inc.Test results are applicable only to the specific sample on which they were performed,and should not be interpreted as representative of any other samples obtained at other times,depths or locations,or generated by separate operations or processes. The grain size analysis results were obtained in general accordance with ASTM D 6913.GeoEngineers 17425 NE Union Hill Road Ste 250,Redmond,WA 98052 #200 Figure B-3 Compaction Test Results (TP-3) Willow Crest Townhomes Renton, Washington 23656-001-00 Date Exported: 10/30/18Note:This report may not be reproduced,except in full,without written approval of GeoEngineers,Inc.Test results are applicable only to the specific sample on which they were performed,and should not be interpreted as representative of any other samples obtained at other times,depths or locations,or generated by separate operations or processes. The Proctor results were obtained in general accordance with ASTM D 1557. Boring Number Soil Description Optimum Moisture (%) Maximum Dry Density (pcf) Uncorrected TP-3 Silty fine sand with gravel (SM)11.8 120.0 Symbol 100 105 110 115 120 125 130 135 140 0 5 10 15 20 MOISTURE CONTENT (%)DRY DENSITY (pcf)100% Saturation (Gs=2.65) Depth (feet) 1.5 APPENDIX C Boring and Test Pit Logs from Previous Studies November 15, 2018 | Page C-1 File No. 23656-001-00 APPENDIX C BORING AND TEST PIT LOGS FROM PREVIOUS STUDIES Included in Appendix C are the boring and test pit logs from available previous studies completed in the immediate vicinity of the project site. ■ The logs of two borings (B-1 and B-2) and six test pits (TP-1 through TP-6) completed by Soil and Environmental Engineers, Inc., in 2010 for the Proposed Development at Edmonds Avenue NE and Glennwood Avenue NE project. APPENDIX D Report Limitations and Guidelines for Use November 15, 2018 | Page D-1 File No. 23656-001-00 APPENDIX D REPORT LIMITATIONS AND GUIDELINES FOR USE 1 This appendix provides information to help you manage your risks with respect to the use of this report. Geotechnical Services Are Performed for Specific Purposes, Persons and Projects This report has been prepared for the exclusive use of Homestead Community Land Trust and other project team members for the Willow Crest Townhomes project. This report is not intended for use by others, and the information contained herein is not applicable to other sites. GeoEngineers structures our services to meet the specific needs of our clients. For example, a geotechnical or geologic study conducted for a civil engineer or architect may not fulfill the needs of a construction contractor or even another civil engineer or architect that are involved in the same project. Because each geotechnical or geologic study is unique, each geotechnical engineering or geologic report is unique, prepared solely for the specific client and project site. Our report is prepared for the exclusive use of our Client. No other party may rely on the product of our services unless we agree in advance to such reliance in writing. This is to provide our firm with reasonable protection against open-ended liability claims by third parties with whom there would otherwise be no contractual limits to their actions. Within the limitations of scope, schedule and budget, our services have been executed in accordance with our Agreement with the Client and generally accepted geotechnical practices in this area at the time this report was prepared. This report should not be applied for any purpose or project except the one originally contemplated. A Geotechnical Engineering or Geologic Report Is Based on a Unique Set of Project-specific Factors This report has been prepared for the Willow Crest Townhomes project in Renton, Washington. GeoEngineers considered a number of unique, project-specific factors when establishing the scope of services for this project and report. Unless GeoEngineers specifically indicates otherwise, do not rely on this report if it was: ■not prepared for you, ■not prepared for your project, ■not prepared for the specific site explored, or ■completed before important project changes were made. For example, changes that can affect the applicability of this report include those that affect: ■the function of the proposed structure; ■elevation, configuration, location, orientation or weight of the proposed structure; 1 Developed based on material provided by ASFE, Professional Firms Practicing in the Geosciences; www.asfe.org . November 15, 2018 | Page D-2 File No. 23656-001-00 ■ composition of the design team; or ■ project ownership. If important changes are made after the date of this report, GeoEngineers should be given the opportunity to review our interpretations and recommendations and provide written modifications or confirmation, as appropriate. Subsurface Conditions Can Change This geotechnical or geologic report is based on conditions that existed at the time the study was performed. The findings and conclusions of this report may be affected by the passage of time, by manmade events such as construction on or adjacent to the site, or by natural events such as floods, earthquakes, slope instability or groundwater fluctuations. Always contact GeoEngineers before applying a report to determine if it remains applicable. Most Geotechnical and Geologic Findings Are Professional Opinions Our interpretations of subsurface conditions are based on field observations from widely spaced sampling locations at the site. Site exploration identifies subsurface conditions only at those points where subsurface tests are conducted or samples are taken. GeoEngineers reviewed field and laboratory data and then applied our professional judgment to render an opinion about subsurface conditions throughout the site. Actual subsurface conditions may differ, sometimes significantly, from those indicated in this report. Our report, conclusions and interpretations should not be construed as a warranty of the subsurface conditions. Geotechnical Engineering Report Recommendations Are Not Final Do not over-rely on the preliminary construction recommendations included in this report. These recommendations are not final, because they were developed principally from GeoEngineers’ professional judgment and opinion. GeoEngineers’ recommendations can be finalized only by observing actual subsurface conditions revealed during construction. GeoEngineers cannot assume responsibility or liability for this report's recommendations if we do not perform construction observation. Sufficient monitoring, testing and consultation by GeoEngineers should be provided during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork activities are completed in accordance with our recommendations. Retaining GeoEngineers for construction observation for this project is the most effective method of managing the risks associated with unanticipated conditions. A Geotechnical Engineering or Geologic Report Could Be Subject to Misinterpretation Misinterpretation of this report by other design team members can result in costly problems. You could lower that risk by having GeoEngineers confer with appropriate members of the design team after submitting the report. Also retain GeoEngineers to review pertinent elements of the design team's plans and specifications. Contractors can also misinterpret a geotechnical engineering or geologic report. Reduce that risk by having GeoEngineers participate in pre-bid and preconstruction conferences, and by providing construction observation. November 15, 2018 | Page D-3 File No. 23656-001-00 Do Not Redraw the Exploration Logs Geotechnical engineers and geologists prepare final boring and testing logs based upon their interpretation of field logs and laboratory data. To prevent errors or omissions, the logs included in a geotechnical engineering or geologic report should never be redrawn for inclusion in architectural or other design drawings. Only photographic or electronic reproduction is acceptable, but recognize that separating logs from the report can elevate risk. Give Contractors a Complete Report and Guidance Some owners and design professionals believe they can make contractors liable for unanticipated subsurface conditions by limiting what they provide for bid preparation. To help prevent costly problems, give contractors the complete geotechnical engineering or geologic report, but preface it with a clearly written letter of transmittal. In that letter, advise contractors that the report was not prepared for purposes of bid development and that the report's accuracy is limited; encourage them to confer with GeoEngineers and/or to conduct additional study to obtain the specific types of information they need or prefer. A pre-bid conference can also be valuable. Be sure contractors have sufficient time to perform additional study. Only then might an owner be in a position to give contractors the best information available, while requiring them to at least share the financial responsibilities stemming from unanticipated conditions. Further, a contingency for unanticipated conditions should be included in your project budget and schedule. Contractors Are Responsible for Site Safety on Their Own Construction Projects Our geotechnical recommendations are not intended to direct the contractor’s procedures, methods, schedule or management of the work site. The contractor is solely responsible for job site safety and for managing construction operations to minimize risks to on-site personnel and to adjacent properties. Read These Provisions Closely Some clients, design professionals and contractors may not recognize that the geoscience practices (geotechnical engineering or geology) are far less exact than other engineering and natural science disciplines. This lack of understanding can create unrealistic expectations that could lead to disappointments, claims and disputes. GeoEngineers includes these explanatory “limitations” provisions in our reports to help reduce such risks. Please confer with GeoEngineers if you are unclear how these “Report Limitations and Guidelines for Use” apply to your project or site. Geotechnical, Geologic and Environmental Reports Should Not Be Interchanged The equipment, techniques and personnel used to perform an environmental study differ significantly from those used to perform a geotechnical or geologic study and vice versa. For that reason, a geotechnical engineering or geologic report does not usually relate any environmental findings, conclusions or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Similarly, environmental reports are not used to address geotechnical or geologic concerns regarding a specific project. November 15, 2018 | Page D-4 File No. 23656-001-00 Biological Pollutants GeoEngineers’ Scope of Work specifically excludes the investigation, detection, prevention or assessment of the presence of Biological Pollutants. Accordingly, this report does not include any interpretations, recommendations, findings, or conclusions regarding the detecting, assessing, preventing or abating of Biological Pollutants and no conclusions or inferences should be drawn regarding Biological Pollutants, as they may relate to this project. The term “Biological Pollutants” includes, but is not limited to, molds, fungi, spores, bacteria, and viruses, and/or any of their byproducts. If Client desires these specialized services, they should be obtained from a consultant who offers services in this specialized field.