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Home My WebLink AboutRS_Drainage_Technical_Information_Report_191015_v1.pdfcoterra 321 3rd Avenue South, Suite 406 Seattle, Washington 98104 206.596.7115 TECHNICAL INFORMATION REPORT Project: Habitat For Humanity La Fortuna Townhomes Renton, WA 98058 Prepared For: Tonkin Architecture 2701 1st Ave #520 Seattle, WA 98121 Prepared By: Max Berde, PE Reviewed By: Peter Apostol, PE Date: October 15, 2019 ENGINEERING PLLC RECEIVED 11/04/2019 amorganroth PLANNING DIVISION TABLE OF CONTENTS Section No. Subject Page No. SECTION I PROJECT OVERVIEW.....................................................................1 SECTION II CONDITIONS AND REQUIREMENTS SUMMARY...................12 SECTION III OFFSITE ANALYSIS......................................................................15 SECTION IV FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN..........................................17 SECTION V CONVEYANCE SYSTEM ANALYSIS AND DESIGN................17 SECTION VI SPECIAL REPORTS AND STUDIES.............................................19 SECTION VII OTHER PERMITS ...........................................................................19 SECTION VIII EROSION AND SEDIMENT CONTROL ......................................19 SECTION IX BOND QUANTITIES AND DECLARATION OF COVENANTS...............................................20 SECTION X OPERATIONS AND MAINTENANCE MANUAL.......................20 LIST OF FIGURES FIGURE 1 VICINITY MAP.................................................................................3 FIGURE 2 TIR WORKSHEET ............................................................................4 FIGURE 3 EXISTING SITE SOILS...................................................................11 LIST OF APPENDICIES APPENDIX A CIVIL PLANS APPENDIX B FLOW CONTROL/DETENTION CALCULATIONS APPENDIX C WATER QUALITY CALCULATIONS APPENDIX D GEOTECHNICAL REPORT APPENDIX E DRAINAGE REVIEW FLOWCHART La Fortuna Townhomes Page 1 Project No. 19006 Renton, WA Coterra Engineering PLLC SECTION I – PROJECT OVERVIEW General Description: The current phase of the proposed La Fortuna Townhomes project includes the construction of three multi-story townhome buildings, totally twelve additional units, on a partially developed 4.51 acre site. The proposed project includes associated parking areas and development of several open space areas. Included in the project are landscape and street lighting improvements, as well as associated storm drainage and utility improvements for the proposed residential buildings. Existing improvements including the private access drive and water and sewer mains were installed previously with the first phase of the La Fortuna Townhomes project. The project site is located on 127th Ave SE – a private road, south of SE 172nd St and north of SE Petrovisky Rd in the City of Renton, WA. See Figure 1 for Vicinity Map. 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: Per the project geotechnical report, prepared by South Sound Geotechnical Consulting, and dated July 31, 2019, fill was discovered at all geotechnical borings to an approximate depth of 3 ft. Below the fill layer, native soils consist of silty sand with gravel and occasional cobbles. Per the geotechnical report, it is “medium dense condition below the surface soils and graded dense at shallow depth. This soil is interpreted to be glacial till and continued to the termination depth of the test pits.” For more information, see the geotechnical report included in Appendix D. Predeveloped Conditions: The overall project site is partially developed from a previous phase of the La Fortuna Townhomes project. The existing site conditions of the current phase – totaling 1.08 acres – consists of mostly stripped, cleared, and graded land. For the purposes of storm drainage modelling, the existing site condition is forested. The project site consists of 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.” The existing land coverage of the project site is presented in the table below. The downstream path of stormwater runoff is described in the offsite analysis in Section III. Table 1 – Existing Site Land Coverage for Storm Drainage Modeling (Acres) Impervious (Acres) Pervious Till Forest (Acres) Total (Acres) Project Site 0.00 1.081 1.081 La Fortuna Townhomes Page 2 Project No. 19006 Renton, WA Coterra Engineering PLLC Developed Conditions: The proposed developed condition includes three townhome buildings; paved parking areas; pedestrian walkways; and associated storm drainage, utility, and landscape improvements. The following table quantifies the areas of the proposed land coverage. Table 2 – Developed Site Land Coverage (Acres) Rooftop (Acres) Hardscape (Acres) Grass/ Landscape (Acres) Total (Acres) Project Site 0.228 0.298 0.555 1.081 The existing site topography will not be dramatically altered in the proposed condition. In general, the site will continue to drain from north to the southwest to the existing wetland. The proposed improvements within the project site 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 under the western parking lot and which will discharge to the existing wetland west of the project site. Permeable pavement and bioretention cells will attenuate flows from stormwater runoff. See Section IV for a detailed description of the design of the flow control and water quality facilities. La Fortuna Townhomes Page 3 Project No. 19006 Renton, WA Coterra Engineering PLLC FIGURE 1 – VICINITY MAP PROJECT SITE La Fortuna Townhomes Page 4 Project No. 19006 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 __________________ __________________ __________________ Habitat for Humanity (425)453-2950 560 Naches Ave SW Suite 110 Seattle,WA 98057 PETER APOSTOL,PE COTERRA ENGINEERING,PLLC (206)596-7115 HH Lafortuna 23N 5E 28 12710-12748 SE 173rd Street, 17210-17324 127th Street SE RENTON,WA 980058 X X NPDES CONSTRUCTION STORMWATER X X X X 10/15/2019 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 ______________________  _____________________________________ TBD -RENTON SUB BASIN:TBD -Petrovitsky/Wetland MIN REQUIREMENTS #1 -#9;ENHANCED BASIN WQ TREATMENT RENTON X 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: _________________________ AmC -ARDENTS,ALDERWOOD MATERIALS 6 -15%LOW SITE CORE REQUIREMENTS #1 -#9 1 PEAK RATE FLOW CONTROL STANDARD REQUIRED MEF PER CORE REQUIREMENT #9 TBD TBD Sk -Seattle muck 0 -5%LOW AgC -Alderwood gravelly sandy loam 8 -1 5%LOW 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: MULTI-FAMILY DETENTION FACILITY -DETENTION VAULT OR TANK -Additional study and verification of potential additional capacity of existing stormwater facility needs to be determined at Civil Construction permit stage of project. 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 _______________________________ X X X X XX X X X X X X X DETENTION VAULT/ TANK X X BIORETENTION CELLS X AS REQUIRED AT CIVIL CONSTRUCTION PERMIT 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 10/10/2019 La Fortuna Townhomes Page 11 Project No. 19006 Renton, WA Coterra Engineering PLLC FIGURE 3 – EXISTING SITE SOIL CONDITIONS La Fortuna Townhomes Page 12 Project No. 19006 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. See Appendix F for drainage review type flow chart. 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 Flow Control Duration Standard (Forested Condition). The proposed improvements 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 Flow Control Duration 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. Req. #5 Erosion and Sediment Control: Construction erosion and sediment control systems will be designed and provided for review at civil construction permit submittal. La Fortuna Townhomes Page 13 Project No. 19006 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 Renton Housing Authority. 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 for targeted PGIS. Bioretention cells are proposed to provide water quality treatment of pollution- generating impervious surfaces. Additionally, pervious concrete sidewalk pavement is proposed for pedestrian paths. 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. Following the City of Renton SWDM required list method for Individual Large Lots implementing on-site BMPs to comply with Core Requirement #9, all onsite BMPs were considered per the list approach in the SWDM. For further details see Section IV. La Fortuna Townhomes Page 14 Project No. 19006 Renton, WA Coterra Engineering PLLC Special Requirements: Req. #1 Area Specific Requirements: No known area specific requirements. 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 facilities which will introduce stormwater to groundwater given the project soil conditions. La Fortuna Townhomes Page 15 Project No. 19006 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 City of Renton Basin Name: John’s Creek Basin 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 None found 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 medium dense sand with silt that was underlain by impermeable glacial till soil. Wetlands Inventory Maps o There is a mappe wetland to the west of the project site which project stormwater discharges to. La Fortuna Townhomes Page 16 Project No. 19006 Renton, WA Coterra Engineering PLLC Task 3 – Field Inspection A full Level 1 downstream analysis will be performed for civil construction permit submittal. No known 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 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 There is no significant upslope unmitigated drainage areas flowing onto the proposed project site. La Fortuna Townhomes Page 17 Project No. 19006 Renton, WA Coterra Engineering PLLC SECTION IV – FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Flow Control The project site is located in the Flow Control Duration Standard (Forested Conditions) area per the 2017 City of Renton Surface Water Design Manual. 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 durations standard and implement on-site flow control BMPs. MGS Flood stormwater modeling program – utilizing a continuous hydrologic model – was implemented to size a detention facility which matched the existing site condition (forested) flow duration from ½ the 2 year flow through the 50 year duration flow standard.. At this preliminary stage of design, a detention vault was sized in MGS Flood to meet the flow duration requirement. As the project stormwater design is refined, the type of detention structure on the project may be revised. The existing onsite detention vault will be analyzed to determine excess storage capacity and possibly utilized this excess capacity for the proposed project. Based on the preliminary MGS Flood analysis, an approximately 16,000 CF detention vault is required for the proposed project. The MGS Flood calculations are included in Appendix B of this report. The proposed detention vault was designed per the City of Renton SWDM requirements including 0.5’ sediment storage (dead storage). Sizing details are shown on the Utility Plan and the future civil construction permit submittal. On-Site Flow Control BMPs The proposed site is classified as a Large Lot BMP Site as the proposed site is over 22,000 SF. 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 1.2.9.2.2 for Large Lot BMP Projects in the 2017 City of Renton Surface Water Design Manual. 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 borings per geotechnical report prepared South Sound Geotechnical Consulting dated 7/31/2019, are classified as “silty sand with gravel and occasional cobbles.”This soil was interpreted by the geotechnical engineer as glacial till. These soils were found at a depth of typical on-site infiltration facilities or infiltration BMPs. La Fortuna Townhomes Page 18 Project No. 19006 Renton, WA Coterra Engineering PLLC Bioretention: Feasible. Bioretention is proposed to mitigate runoff from PGIS parking areas in the propose project. Permeable Pavement: Feasible. Permeable concrete pavement is proposed for pedestrian pathways on the project site. 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 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. The proposed project will provide mitigation of on-site PGIS using bioretention cells to treat targeted surfaced. All proposed PGIS in the current phase of development is treated via bioretention. La Fortuna Townhomes Page 19 Project No. 19006 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. Conveyance capacity calculations will be included with the civil construction permit submittal. 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 Site Plan Review, 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 will be provided on all drainage inlets on, adjacent to, and downstream of the project site. 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. La Fortuna Townhomes Page 20 Project No. 19006 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 the civil construction permit submittal. 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. Appendix A Civil Plans coterra321 3rd Ave South, Suite 406Seattle, Washington 98104ph 206.596.7115coterraengineering.comENGINEERING PLLCNOT FORCONSTRUCTIONCIVIL COVERSHEET ANDNOTES coterra321 3rd Ave South, Suite 406Seattle, Washington 98104ph 206.596.7115coterraengineering.comENGINEERING PLLCNOT FORCONSTRUCTIONUTILITY PLAN coterra321 3rd Ave South, Suite 406Seattle, Washington 98104ph 206.596.7115coterraengineering.comENGINEERING PLLCNOT FORCONSTRUCTIONDRAINAGECONTROL PLAN coterra321 3rd Ave South, Suite 406Seattle, Washington 98104ph 206.596.7115coterraengineering.comENGINEERING PLLCNOT FORCONSTRUCTIONGRADING ANDPAVING PLAN Appendix B Flow Control/Detention Calculations ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.38 Program License Number: 201510001 Project Simulation Performed on: 10/15/2019 11:47 AM Report Generation Date: 10/15/2019 11:48 AM ————————————————————————————————— Input File Name: LA FORTUNA - PRELIM SIZING - NEW PORTION2.fld Project Name: Renton Habitat - La Fortuna Analysis Title: Match Duration - Forested Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Climatic Region Number: 14 Full Period of Record Available used for Routing Precipitation Station : 96004405 Puget East 44 in_5min 10/01/1939-10/01/2097 Evaporation Station : 961044 Puget East 44 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) 1.081 1.081 Area of Links that Include Precip/Evap (acres) 0.000 0.000 Total (acres) 1.081 1.081 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Existing ---------- -------Area(Acres) -------- Till Forest 1.081 Till Pasture 0.000 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 1.081 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Proposed ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 Till Grass 0.555 Outwash Forest 0.000 Outwash Pasture 0.000 Outwash Grass 0.000 Wetland 0.000 Green Roof 0.000 User 2 0.000 Impervious 0.526 ---------------------------------------------- Subbasin Total 1.081 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: Detention Vault - New Portion Link Type: Structure Downstream Link: None Prismatic Pond Option Used Pond Floor Elevation (ft) : 100.50 Riser Crest Elevation (ft) : 105.00 Max Pond Elevation (ft) : 105.50 Storage Depth (ft) : 4.50 Pond Bottom Length (ft) : 132.5 Pond Bottom Width (ft) : 26.5 Pond Side Slopes (ft/ft) : L1= 0.00 L2= 0.00 W1= 0.00 W2= 0.00 Bottom Area (sq-ft) : 3511. Area at Riser Crest El (sq-ft) : 3,511. (acres) : 0.081 Volume at Riser Crest (cu-ft): 15,801. (ac-ft) : 0.363 Area at Max Elevation (sq-ft) : 3511. (acres) : 0.081 Vol at Max Elevation (cu-ft): 17,907. (ac-ft) : 0.411 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) : 0.010 Riser Crest Elevation : 105.00 ft Hydraulic Structure Geometry Number of Devices: 2 ---Device Number 1 --- Device Type : Circular Orifice Control Elevation (ft) : 100.50 Diameter (in) : 0.56 Orientation : Horizontal Elbow : No ---Device Number 2 --- Device Type : Circular Orifice Control Elevation (ft) : 103.50 Diameter (in) : 1.00 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: Detention Vault - New Portion ********** Link WSEL Stats WSEL Frequency Data(ft) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) WSEL Peak (ft) ====================================== 1.05-Year 101.837 1.11-Year 101.949 1.25-Year 102.169 2.00-Year 102.829 3.33-Year 103.314 5-Year 103.674 10-Year 104.142 25-Year 104.561 50-Year 104.693 100-Year 104.966 ***********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 205.649 _____________________________________ Total: 205.649 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin A - Proposed 71.482 Link: Detention Vault - Ne 0.000 _____________________________________ Total: 71.482 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 1.302 ac-ft/year, Post Developed: 0.452 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ********** Link: Detention Vault - New Portion ********** Basic Wet Pond Volume (91% Exceedance): 3265. cu-ft Computed Large Wet Pond Volume, 1.5*Basic Volume: 4898. cu-ft Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 400.33 Inflow Volume Including PPT-Evap (ac-ft): 400.33 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): 400.27 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: Detention Vault - New Portion *** 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 2.780E-02 2-Year 1.300E-02 5-Year 4.456E-02 5-Year 2.572E-02 10-Year 5.811E-02 10-Year 3.651E-02 25-Year 7.987E-02 25-Year 4.322E-02 50-Year 0.100 50-Year 4.512E-02 100-Year 0.107 100-Year 4.870E-02 200-Year 0.170 200-Year 6.148E-02 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals Appendix C Water Quality Calculation ————————————————————————————————— MGS FLOOD PROJECT REPORT Program Version: MGSFlood 4.38 Program License Number: 201510001 Project Simulation Performed on: 10/15/2019 11:49 AM Report Generation Date: 10/15/2019 11:49 AM ————————————————————————————————— Input File Name: LA FORTUNA - PRELIM SIZING - WQ.fld Project Name: Renton Habitat - La Fortuna Analysis Title: WQ - Bioretention Comments: ———————————————— PRECIPITATION INPUT ———————————————— Computational Time Step (Minutes): 15 Extended Precipitation Time Series Selected Climatic Region Number: 14 Full Period of Record Available used for Routing Precipitation Station : 96004405 Puget East 44 in_5min 10/01/1939-10/01/2097 Evaporation Station : 961044 Puget East 44 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) 1.081 0.103 Area of Links that Include Precip/Evap (acres) 0.000 0.002 Total (acres) 1.081 0.105 ----------------------SCENARIO: PREDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Existing ---------- -------Area(Acres) -------- Till Forest 1.081 Till Pasture 0.000 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 1.081 ----------------------SCENARIO: POSTDEVELOPED Number of Subbasins: 1 ---------- Subbasin : Basin A - Proposed ---------- -------Area(Acres) -------- Till Forest 0.000 Till Pasture 0.000 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 0.103 ---------------------------------------------- Subbasin Total 0.103 ************************* LINK DATA ******************************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ************************* LINK DATA ******************************* ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ------------------------------------------ Link Name: New Bio Lnk1 Link Type: Bioretention Facility Downstream Link: None Base Elevation (ft) : 100.00 Riser Crest Elevation (ft) : 101.00 Storage Depth (ft) : 1.00 Bottom Length (ft) : 10.0 Bottom Width (ft) : 8.0 Side Slopes (ft/ft) : L1= 3.00 L2= 3.00 W1= 3.00 W2= 3.00 Bottom Area (sq-ft) : 80. Area at Riser Crest El (sq-ft) : 224. (acres) : 0.005 Volume at Riser Crest (cu-ft): 170. (ac-ft) : 0.004 Infiltration on Bottom only Selected Soil Properties Biosoil Thickness (ft) : 1.50 Biosoil Saturated Hydraulic Conductivity (in/hr) : 2.00 Biosoil Porosity (Percent) : 20.00 Maximum Elevation of Bioretention Soil : 101.00 Native Soil Hydraulic Conductivity (in/hr) : 0.00 Underdrain Present Orifice NOT Present in Under Drain Riser Geometry Riser Structure Type : Circular Riser Diameter (in) : 6.00 Common Length (ft) : 0.000 Riser Crest Elevation : 101.00 ft Hydraulic Structure Geometry Number of Devices: 0 **********************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: New Bio Lnk1 ********** Link WSEL Stats WSEL Frequency Data(ft) (Recurrence Interval Computed Using Gringorten Plotting Position) Tr (yrs) WSEL Peak (ft) ====================================== 1.05-Year 101.017 1.11-Year 101.020 1.25-Year 101.023 2.00-Year 101.031 3.33-Year 101.037 5-Year 101.039 10-Year 101.044 25-Year 101.048 50-Year 101.055 100-Year 101.057 ***********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 205.649 _____________________________________ Total: 205.649 Total Post Developed Recharge During Simulation Model Element Recharge Amount (ac-ft) ----------------------------------------------------------------------------------------------- Subbasin: Basin A - Proposed 0.000 Link: New Bio Lnk1 0.000 _____________________________________ Total: 0.000 Total Predevelopment Recharge is Greater than Post Developed Average Recharge Per Year, (Number of Years= 158) Predeveloped: 1.302 ac-ft/year, Post Developed: 0.000 ac-ft/year ***********Water Quality Facility Data ************* ----------------------SCENARIO: PREDEVELOPED Number of Links: 0 ----------------------SCENARIO: POSTDEVELOPED Number of Links: 1 ********** Link: New Bio Lnk1 ********** Infiltration/Filtration Statistics-------------------- Inflow Volume (ac-ft): 51.70 Inflow Volume Including PPT-Evap (ac-ft): 53.02 Total Runoff Infiltrated (ac-ft): 0.00, 0.00% Total Runoff Filtered (ac-ft): 48.76, 91.97% Primary Outflow To Downstream System (ac-ft): 53.03 Secondary Outflow To Downstream System (ac-ft): 0.00 Percent Treated (Infiltrated+Filtered)/Total Volume: 91.97% ***********Compliance Point Results ************* Scenario Predeveloped Compliance Subbasin: Basin A - Existing Scenario Postdeveloped Compliance Link: New Bio Lnk1 *** 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 2.780E-02 2-Year 3.331E-02 5-Year 4.456E-02 5-Year 4.556E-02 10-Year 5.811E-02 10-Year 5.290E-02 25-Year 7.987E-02 25-Year 6.018E-02 50-Year 0.100 50-Year 7.202E-02 100-Year 0.107 100-Year 7.472E-02 200-Year 0.170 200-Year 8.442E-02 ** Record too Short to Compute Peak Discharge for These Recurrence Intervals Appendix D Geotechnical Report South Sound Geotechnical Consulting July 31, 2019 Habitat for Humanity – Seattle/King County 560 Naches Avenue SW, Suite 110 Renton, WA 98057 Attention: Mr. Brett VanSlyke Subject: Geotechnical Engineering Report LaFortuna Development 127th Avenue E. Renton, Washington SSGC Project No. 19061 Mr. VanSlyke, South Sound Geotechnical Consulting (SSGC) has completed a geotechnical assessment for the above referenced project. Our services have been completed in general conformance with our proposal P19056 (dated June 13, 2019) and authorized per signature of our services agreement. Our evaluation included completion of four test pits on the property, engineering analyses, and preparation of this report. PROJECT INFORMATION Three town-house buildings are planned on the property, located near the looped portion of 127th Street SE, south of SE 172nd Street. Conventional spread footing foundations will be used for support of the structures, with concrete slab-on-grade floors. SITE CONDITIONS The property is characterized with west-facing sloping ground in the eastern portion, with relatively level ground in the western portion. Overall elevation change across the site is on the order of 24 (+/-) feet. The upper eastern site is covered with grass. The western portions are used for equipment storage and construction offices. It appears that cuts on the east side of 127th Street SE were completed for the road and have resulted in steeper cut slopes in this area. SUBSURFACE CONDITIONS Subsurface conditions were characterized by completing four test pits on the site on July 9, 2019. Test pits were advanced to final depths between about 3.5 and 5 feet below existing ground surface. Approximate locations of the test pits are shown on Figure 1, Exploration Plan. Logs of the test pits are provided in Appendix A. A summary description of observed subgrade conditions is provided below. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 2 Soil Conditions Fill was observed below the surface in two of the test pits (TP-1 and TP-2). Fill at the TP-1 location was crushed gravel for a working surface and on the order of 6 inches thick. Fill at the TP-2 location consisted of mixed silt, sand, and gravel in a loose condition. This fill extended to about 3 feet. An approximate 6 inch topsoil layer was at the surface of the remaining test pits. Native soil below the fill (or topsoil) was silty sand with gravel and occasional cobbles. It was generally in a medium dense condition below the surface soils and graded dense at shallow depth. This soil is interpreted to be glacial till and continued to the termination depth of the test pits. Groundwater Conditions Groundwater was not observed in the test pits at the time of excavation. Dense glacial till is considered impermeable to vertical groundwater flow and can cause perched groundwater conditions, particularly during the wetter seasons of the year. Wetlands are west of the site on lower elevated ground. Groundwater levels should be anticipated to fluctuate due to seasonal precipitation variations and on- and off-site drainage patterns. Geologic Setting Soils within the project area have been classified by the NRCS Soil Survey. Site soils are mapped as Alderwood gravelly sandy loam. Alderwood soils reportedly formed in glacia l till. Native soils in the excavations appear to conform to the mapped soil type. GEOTECHNICAL DESIGN CONSIDERATIONS Planned development of this site is considered feasible based on observed soil conditions in the test pits. Properly prepared native soils can be used for support of conventional spread footing foundations and pavements. Daylight basement structures are anticipated for buildings on the east side of 127 th Avenue SE where grades rise to the east from street elevation. Infiltration to assist in stormwater control is not considered feasible at this site. The dense glacial till at shallow depth will create a barrier to vertical groundwater flow. Recommendations presented in the following sections should be considered general and may require modifications when earthwork and grading occur. They are based upon the subsurface conditions observed in the test pits and the assumption that finish site grades will be similar to existing grades. It should be noted subsurface conditions across the site may vary from those depicted on the exploration logs and can change with time, especially on sites with previous development. Therefore, proper site preparation will depend upon the weather and soil conditions encountered at the time of construction. We recommend SSGC review final plans and further assess subgrade conditions at the time of construction, as warranted. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 3 General Site Preparation Site grading and earthwork should include procedures to control surface water runoff. Grading the site without adequate drainage control measures may negatively impact site soils, resulting in increased export of impacted soil and import of fill materials, potentially increasing the cost of the earthwork and subgrade preparation phases of the project. Site grading should include removal (stripping) of topsoil and fill or very loose or soft soils encountered in building and pavement areas. Subgrades should consist of firm, undisturbed native till following stripping. Stripping depths in the area of test pit TP-2 may approach 4 feet (or more), but can only be determined at the time of construction. Shallower stripping depths are expected over most of the remaining site. General Subgrade Preparation Subgrades in building footprints and pavement areas should consist of firm, undisturbed native soil. We recommend exposed subgrades in building and conventional pavement areas are proofrolled using a large roller, loaded dump truck, or other mechanical equipment to assess subgrade conditions following stripping. Proofrolling efforts should result in the upper 1 foot of subgrade soils in building and conventional pavement areas achieving a compaction level of at least 95 percent of the maximum dry density (MDD) per the ASTM D1557 test method. Wet, loose, or soft subgrades that cannot achieve this compaction level should be removed (over-excavated) and replaced with structural fill. The depth of over-excavation should be based on soil conditions at the time of construction. A representative of SSGC should be present to assess subgrade conditions during proofrolling. Grading and Drainage Positive drainage should be provided during construction and maintained throughout the life of the development. Allowing surface water into cut or fill areas, utility trenches and building footprints should be prevented. Temporary and permanent drainage systems should prevent stormwater from flowing onto the steeper south-facing slope. Structural Fill Materials The suitability of soil for use as structural fill will depend on the gradation and moisture content of the soil when it is placed. Soils with higher fines content (soil fraction passing the U.S. No. 200 sieve) will become sensitive with higher moisture content. It is often difficult to achieve adequate compaction if soil moisture is outside of optimum ranges for soils that contain more than about 5 percent fines. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 4 Site Soils: Topsoil and observed fill are not considered suitable as structural fill. They could be used in non-structural areas, such as lawns. Native glacial till could be suitable for use as structural fill provided it can be moisture conditioned to within optimal ranges. Till can have considerable fine (silt) content and therefore will be moisture sensitive and difficult to use as structural fill if wet. Optimum moisture is considered within about +/- 2 percent of the moisture content required to achieve the maximum dry density (MDD) per the ASTM D-1557 test method. If moisture content is higher or lower than optimum, soils would need to be dried or wetted prior to placement as structural fill. Import Fill Materials: We recommend import structural fill placed during dry weather consist of material which meets the specifications for Gravel Borrow as described in Section 9-03.14(1) of the 2018 Washington State Department of Transportation (WSDOT) Specifications for Road, Bridge, and Municipal Construction (Publication M 41-10). Gravel Borrow should be protected from disturbance if exposed to wet conditions after placement. During wet weather, or for backfill on wet subgrades, import soil suitable for compaction in wetter conditions should be provided. Imported fill for use in wet conditions should conform to specifications for Select Borrow as described in Section 9-03.14(2), or Crushed Surfacing per Section 9-03.9(3) of the 2018 WSDOT M-41 manual, with the modification that a maximum of 5 percent by weight shall pass the U.S. No. 200 sieve for these soil types. Structural fill placement and compaction is weather-dependent. Delays due to inclement weather are common, even when using select granular fill. We recommend site grading and earthwork be scheduled for the drier months of the year. Structural fill should not consist of frozen material. Structural Fill Placement We recommend structural fill is placed in lifts not exceeding about 10 inches in loose measure. It may be necessary to adjust lift thickness based on site and fill conditions during placement and compaction. Finer grained soil used as structural fill and/or lighter weight compaction equipment may require significantly thinner lifts to attain required compaction levels. Granular soil with lower fines contents could potentially be placed in thicker lifts if they can be adequately compacted. Structural fill should be compacted to attain the recommended levels presented in Table 1, Compaction Criteria. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 5 Table 1. Compaction Criteria Fill Application Compaction Criteria* Footing areas 95 % Upper 2 feet in pavement areas, slabs and sidewalks, and utility trenches 95 % Below 2 feet in pavement areas, slabs and sidewalks, and utility trenches 92 % Utility trenches or general fill outside of paved or building areas 90 % *Per the ASTM D 1557 test method. Trench backfill within about 2 feet of utility lines should not be over-compacted to reduce the risk of damage to the line. In some instances the top of the utility line may be within 2 feet of the surface. Backfill in these circumstances should be compacted to a firm and unyielding condition. We recommend fill procedures include maintaining grades that promote drainage and do not allow ponding of water within the fill area. The contractor should protect compacted fill subgrades from disturbance during wet weather. In the event of rain during structural fill placement, the exposed fill surface should be allowed to dry prior to placement of additional fill. Alternatively, the we t soil can be removed. We recommend consideration is given to protecting haul routes and other high traffic areas with free-draining granular fill material (i.e. sand and gravel containing less than 5 percent fines) or quarry spalls to reduce the potential for disturbance to the subgrade during inclement weather. Structural or embankment fill placed on slopes should be benched into firm (dense) native glacial till. Benches should be excavated level (or with a slight incline into the hillside). Benches should be a maximum of 2 feet high and wide enough to accommodate a conventional vibratory smooth-drum roller capable of compacting fill to at least 95 percent of the MDD per the ASTM D 1557 test method. Earthwork Procedures Conventional earthmoving equipment should be suitable for earthwork at this site. Earthwork may be difficult during periods of wet weather or if elevated soil moisture is present. Excavated site soils may not be suitable as structural fill depending on the soil moisture content and weather conditions at the time of earthwork. If soils are stockpiled and wet weather is anticipated, the stockpile should be protected with securely anchored plastic sheeting. If stockpiled soils become wet and unusable, it will become necessary to import clean, granular soils to complete wet weather site work. Wet or disturbed subgrade soils should be over-excavated to expose firm, non-yielding, non-organic soils and backfilled with compacted structural fill. We recommend the earthwork portion of this project be completed during extended periods of dry weather. If earthwork is completed during the wet season (typically October through May) it may be necessary to take extra measures to protect subgrade soils. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 6 If earthwork takes place during freezing conditions, we recommend the exposed subgrade is allowed to thaw and re-compacted prior to placing subsequent lifts of structural fill. Alternatively, the frozen soil can be removed to unfrozen soil and replaced with structural fill. The contractor is responsible for designing and constructing stable, temporary excavations (including utility trenches) as required to maintain stability of excavation sides and bottoms. Excavations should be sloped or shored in the interest of safety following local and federal regulations, including current OSHA excavation and trench safety standards. Temporary excavation cuts should be sloped at inclinations of 1H:1V (Horizontal:Vertical) or flatter, unless the contractor can demonstrate the safety of steeper cut slopes. Permanent cut and fill slopes should be inclined at 2H:1V, or flatter. Erosion control measures should be implemented on all temporary and permanent cut or fill slopes immediately after grading. A qualified geotechnical engineer and materials testing firm should be retained during the construction phase of the project to observe earthwork operations and to perform necessary tests and observations during subgrade preparation, placement and compaction of structural fill, and backfilling of excavations. Foundations Foundations can be placed on native glacial till or on a zone of structural fill above prepared native subgrades as described in this report. The following recommendations are for conventional spread footing foundations: Bearing Capacity (net allowable): 3,000 pounds per square foot (psf) for footings supported on firm native till subgrades or structural fill prepared as described in this report. Footing Width (Minimum): 16 inches (Strip) 24 inches (Column) Embedment Depth (Minimum): 18 inches (Exterior) 12 inches (Interior) Settlement: Total: < 1 inch Differential: < 1/2 inch (over 30 feet) Allowable Lateral Passive Resistance: 325 psf/ft* (below 12 inches) Allowable Coefficient of Friction: 0.40* *These values include a factor of safety of approximately 1.5. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 7 The net allowable bearing pressures presented above may be increased by one -third to resist transient, dynamic loads such as wind or seismic forces. Lateral resistance to footings should be ignored in the upper 12-inches from exterior finish grade. Foundation Construction Considerations All foundation subgrades should be free of water and loose soil prior to placing concrete , and should be prepared as recommended in this report. Concrete should be placed soon after excavating and compaction to reduce disturbance to bearing soils. Should soils at foundation level become excessively dry, disturbed, saturated, or frozen, the affected soil should be removed prior to placing concrete. We recommend SSGC observe all foundation subgrades prior to placement of concrete. Foundation Drainage Ground surface adjacent foundations should be sloped away to facilitate drainage. We recommend footing drains are installed around perimeter footings. Footing drains should include a minimum 4- inch diameter perforated rigid plastic or metal drain line installed along the exterior base of the footing. The perforated drain lines should be connected to a tight line pipe that discharges to an approved storm drain receptor. The drain line should be surrounded by a zone of clean, free-draining granular material having less than 5 percent passing the No. 200 sieve or meeting the requirements of section 9-03.12(2) “Gravel Backfill for Walls” in the 2018 WSDOT (M41-10) manual. The free- draining aggregate zone should be at least 12 inches wide and wrapped in filter fabric. The granular fill should extend to within 6 inches of final grade where it should be capped with compacted fill containing sufficient fines to reduce infiltration of surface water into the footing drains. Alternately, the ground surface can be paved with asphalt or concrete. Cleanouts are recommended for maintenance of the drain system. On-Grade Floor Slabs On-grade floor slabs should be placed on native soils or structural fill prepared as described in this report. We recommend a modulus subgrade reaction of 200 pounds per square inch per inch (psi/in) for upper native soil or compacted granular structural fill over properly prepared native soil. An increased subgrade reaction of 250 (psi/in) can be used for slabs placed on dense glacial till. We recommend a capillary break is provided between the prepared subgrade and bottom of slab. Capillary break material should be a minimum of 4 inches thick and consist of compacted clean, free- draining, well graded course sand and gravel. The capillary break material should contain less than 5 percent fines, based on that soil fraction passing the U.S. No. 4 sieve. Alternatively, a clean angular gravel such as No. 7 aggregate per Section 9-03.1(4) C of the 2018 WSDOT (M41-10) manual could be used for this purpose. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 8 We recommend positive separations and/or isolation joints are provided between slabs and foundations, and columns or utility lines to allow independent movement where needed. Backfill in interior trenches beneath slabs should be compacted in accordance with recommendations presented in this report. A vapor retarder should be considered beneath concrete slabs that will be covered with moisture sensitive or impervious coverings (such as tile, wood, etc.), or when the slab will support equipment or stored materials sensitive to moisture. We recommend the slab designer refer to ACI 302 and/or ACI 360 for procedures and limitations regarding the use and placement of vapor retarders. Lateral Earth Pressures Below grade and retaining walls will be subject to lateral earth pressures. Subgrade walls are typi cally designed for “active” or “at-rest” earth pressure conditions. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes lateral movement at the top of the wall of around 0.002H to 0.004H, where H is the height of the wall. The at-rest condition assumes no wall movement. The following recommended earth pressures (Table 2) should be applied as a triangular distribution starting at the top of the wall (for active and at-rest) and bottom of wall (for passive) and assume:  Backfill behind walls is level and no surcharge loads will be applied;  Drainage is provided behind the wall to prevent the development of hydrostatic pressures. Table 2. Lateral Earth Pressures Soil Type Earth Pressure Coefficient* Equivalent Fluid Pressure (pcf)* Native Glacial Till Active: 0.28 At-rest: 0.44 Passive: 3.50 Active: 30 At-rest: 45 Passive: 350 * A factor of safety of about 1.5 should be applied to these values. Additional lateral pressure should be added to these values to model surcharges such as sloped backfill, traffic, construction, or seismic loads. We recommend an active seismic pressure of 4H psf (where H is the height of the subgrade wall) and an at-rest seismic pressure of 7H. The effects of other surcharge loads should be accounted for as appropriate. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 9 Wall Backfill Backfill behind the drainage zone should consist of granular material that satisfies the criteria of Section 9-03.12(2) “Gravel Backfill for Walls” per the 2018 WSDOT (M 41-10) manual, or as approved by the engineer. Backfill should be placed in lifts not exceeding 8 inches and compacted with hand-operated compaction equipment. Compaction of wall backfill should be between 90 to 92 percent of the maximum dry density (MDD) per the ASTM D1557 test method within 3 feet of the back of the wall to limit additional lateral pressures. At a distance greater than 3 feet behind the back of the wall, backfill can be compacted using conventional rollers, with backfill compacted to at least 92 percent of the MDD. Seismic Considerations Seismic parameters and values in Table 3 are based on the 2015 International Building Code (IBC). Table 3. Seismic Parameters PARAMETER VALUE 2015 International Building Code (IBC) Site Classification1 C Ss Spectral Acceleration for a Short Period 1.381 S1 Spectral Acceleration for a 1-Second Period 0.515g Fa Site Coefficient for a Short Period 1.00 Fv Site Coefficient for a 1-Second Period 1.3 1 Note: In general accordance with 2015 International Building Code, Section 1613.3.1 for risk categories I,II,III. IBC Site Class is based on estimated characteristics of the upper 100 feet of the subsurface profile. Ss, S1, Fa, and Fv values based on the OSHPD Seismic Design Maps website. Liquefaction Soil liquefaction is a condition where loose, typically granular soils located below the groundwater surface lose strength during ground shaking, and is often associated with earthquakes. The King County “Liquefaction Susceptibility” map (Map 11-5) shows the property in an area with low susceptibility to liquefaction. Native soils at fairly shallow depth consists of dense to very dense glacial till. The risk of liquefaction at this site is considered low for the design level earthquake. Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 10 Infiltration Characteristics Infiltration to control stormwater is not considered feasible at this site due to the presence of dense glacial till at shallow depths. Assessment of infiltration rates using small-scale Pilot Infiltration Test (PIT) procedures in similar soils in the area have shown negligible infiltration during the soak period of the test. Other detention/retention facilities will be required to control stormwater runoff at this site. Critical Areas Site slopes with heights of 10 feet or more have average inclinations between about 15 to 20 percent based on topographic information on the “Boundary and Topographic Survey” of the site completed by 4Site Surveying and Consulting, dated April 3, 2019. The site is not identified on the City of Renton GIS Landslide Hazard Severity map. Test pits on the site and geologic/soil maps indicate underlying native soils that comprise site slopes consist of glacial till. Slopes comprised of dense glacial till with inclinations of less than 20 percent are generally considered stable and not subject to significant movement. Evidence of recent or historic landslides was not observed on site slopes. We are unaware of any known deeper seated landslides within 300 feet of the site. The proposed development will not adversely impact site or neighboring slopes, in our opinion. REPORT CONDITIONS This report has been prepared for the exclusive use of Habitat for Humanity for specific application to the project discussed, and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or i mplied, are intended or made. The analysis and recommendations presented in this report are based on observed soil conditions and test results at the indicated locations, and from other geologic information discussed. This report does not reflect variations that may occur across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplementa l recommendations can be provided. This report was prepared for the planned type of development of the site as discussed herein. It is not valid for third party entities or alternate types of development on the site without the express written consent of SSGC. If development plans change we should be notified to review those changes and modify our recommendations as necessary. N South Sound Geotechnical Consulting P.O. Box 39500 Lakewood, WA 98496 (253) 973-0515 Figure 1 – Exploration Plan LaFortuna Renton, WA SSGC Project #19061 Approximate Test Pit Location PIT - 1 TP - 1 TP-1 TP-1 Scale: NTS Base map from “Boundary and Topographic Survey of 12710 – 173rd Pl”, by 4Site Surveying and Consulting, dated 4-3-19. Legend TP-2 TP-1 TP-3 TP-1 TP-4 TP-1 Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 A-1 Appendix A Field Exploration Procedures and Test Pit Logs Geotechnical Engineering Report SSGC LaFortuna Development 127th Avenue SE Renton, Washington SSGC Project No. 19061 July 31, 2019 Field Exploration Procedures Our field exploration for this project included four test pits completed on July 9, 2019. The approximate locations of the explorations are shown on Figure 1, Exploration Plan. The exploration locations were determined by pacing from site features. Ground surface elevations referenced on the logs were inferred from Google Earth satellite imagery. Exploration locations and elevations should be considered accurate only to the degree implied by the means and methods used. A client provided excavator dug the test pits. Soil samples were collected and stored in moisture tight containers for further assessment and laboratory testing. Explorations were backfilled with excavated soils and tamped when completed. Please note that backfill in the explorations will likely settle with time. Backfill material located in building areas should be re-excavated and recompacted, or replaced with structural fill. The following logs indicate the observed lithology of soils and other materials observed in the explorations at the time of excavation. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also indicate the approximate depth to groundwater (where observed at the time of excavation), along with sample numbers and approximate sample depths. Soil descriptions on the logs are based on the Unified Soil Classification System. Project: LaFortuna SSGC Job # 19061 TEST PIT LOGS PAGE 1 OF 2 Location: Renton, WA TEST PIT LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 TO TP-4 Logged by: THR Test Pit TP-1 Depth (feet) Material Description 0 – 0.5 0.5 – 4 Fill: Crushed Gravel Silty SAND with gravel and occasional cobble: Medium dense to dense, moist, brownish gray. (Glacial Till) Test pit completed at approximately 4 feet on 7/9/19. Groundwater not observed at time of excavation. Approximate surface elevation: 388 feet Test Pit TP-2 Depth (feet) Material Description 0 – 3 3 – 5 Fill: Silt, sand, gravel: Loose, damp, brown. Silty SAND with gravel and occasional cobble: Medium dense to dense, moist, brownish gray. (Glacial Till) Test pit completed at approximately 5 feet on 7/9/19. Groundwater not observed at time of excavation. Approximate surface elevation: 400 feet Test Pit TP-3 Depth (feet) Material Description 0 – 0.5 0.5 – 3.5 Topsoil Silty SAND with gravel and occasional cobble: Medium dense to dense, moist, light brown. (Glacial Till) Test pit completed at approximately 3.5 feet on 7/9/19. Groundwater not observed at time of excavation. Approximate surface elevation: 403 feet UNIFIED SOIL CLASSIFICATION SYSTEM Criteria for Assigning Group Symbols and Group Names Using Laboratory TestsA Soil Classification Group Symbol Group NameB Coarse Grained Soils More than 50% retained on No. 200 sieve Gravels More than 50% of coarse fraction retained on No. 4 sieve Clean Gravels Less than 5% finesC Cu  4 and 1  Cc  3E GW Well-graded gravelF Cu  4 and/or 1  Cc  3E GP Poorly graded gravelF Gravels with Fines More than 12% finesC Fines classify as ML or MH GM Silty gravelF,G, H Fines classify as CL or CH GC Clayey gravelF,G,H Sands 50% or more of coarse fraction passes No. 4 sieve Clean Sands Less than 5% finesD Cu  6 and 1  Cc  3E SW Well-graded sandI Cu  6 and/or 1  Cc  3E SP Poorly graded sandI Sands with Fines More than 12% finesD Fines classify as ML or MH SM Silty sandG,H,I Fines Classify as CL or CH SC Clayey sandG,H,I Fine-Grained Soils 50% or more passes the No. 200 sieve Silts and Clays Liquid limit less than 50 inorganic PI  7 and plots on or above “A” lineJ CL Lean clayK,L,M PI  4 or plots below “A” lineJ ML SiltK,L,M organic Liquid limit - oven dried  0.75 OL Organic clayK,L,M,N Liquid limit - not dried Organic siltK,L,M,O Silts and Clays Liquid limit 50 or more inorganic PI plots on or above “A” line CH Fat clayK,L,M PI plots below “A” line MH Elastic SiltK,L,M organic Liquid limit - oven dried  0.75 OH Organic clayK,L,M,P Liquid limit - not dried Organic siltK,L,M,Q Highly organic soils Primarily organic matter, dark in color, and organic odor PT Peat A Based on the material passing the 3-in. (75-mm) sieve B If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name. C Gravels with 5 to 12% fines require dual symbols: GW -GM well-graded gravel with silt, GW -GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay. D Sands with 5 to 12% fines require dual symbols: SW -SM well-graded sand with silt, SW -SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP-SC poorly graded sand with clay E Cu = D60/D10 Cc = 6010 2 30 DxD )(D F If soil contains  15% sand, add “with sand” to group name. G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM. HIf fines are organic, add “with organic fines” to group name. I If soil contains  15% gravel, add “with gravel” to group name. J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay. K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel,” whichever is predominant. L If soil contains  30% plus No. 200 predominantly sand, add “sandy” to group name. M If soil contains  30% plus No. 200, predominantly gravel, add “gravelly” to group name. N PI  4 and plots on or above “A” line. O PI  4 or plots below “A” line. P PI plots on or above “A” line. Q PI plots below “A” line. Project: LaFortuna SSGC Job # 19061 TEST PIT LOGS PAGE 2 OF 2 Location: Renton, WA TEST PIT LOGS FIGURE A-1 South Sound Geotechnical Consulting TP-1 TO TP-4 Logged by: THR Test Pit TP-4 Depth (feet) Material Description 0 – 0.5 0.5 – 4 Topsoil Silty SAND with gravel and occasional cobble: Medium dense to dense, moist, light brown. (Glacial Till) Test pit completed at approximately 4 feet on 7/9/19. Groundwater not observed at time of excavation. Approximate surface elevation: 410 feet Appendix E Drainage Review Flow Chart SECTION 1.1 DRAINAGE REVIEW 12/12/2016 2017 City of Renton Surface Water Design Manual 1-14 FIGURE 1.1.2.A FLOW CHART FOR DETERMINING TYPE OF DRAINAGE REVIEW REQUIRED