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Home My WebLink AboutEX_11_Updated TIR_March 2022 Western Washington Division Eastern Washington Division 165 NE Juniper St., Ste 201, Issaquah, WA 98027 407 Swiftwater Blvd, Cle Elum, WA 98922 Phone: (425) 392-0250 Fax: (425) 391-3055 Phone: (509) 674-7433 Fax: (509) 674-7419 www.EncompassES.net PRELIMINARY TECHNICAL INFORMATION REPORT For Meadow Ave Short Plat 3804 Meadow Avenue N Renton, WA 98056 January 29, 2021 Revised March 7, 2022 Prepared by: Ian Dahl Encompass Engineering Job No. 20591 Prepared For: Huy Nguyen 15400 SE 155th Place, Unit 99 Renton, WA 98058 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL 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 _________________________ DPER Permit # ________________________ Location Township ______________ Range ________________ Section ________________ Site Address __________________________ _____________________________________ Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS  Landuse (e.g.,Subdivision / Short Subd. / UPD)  Building (e.g.,M/F / Commercial / SFR)  Clearing and 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 ____________________________________ __________________ Part 6 SWDM ADJUSTMENT APPROVALS Type (circle one): Standard / Experimental / Blanket Description: (include conditions in TIR Section 2) ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ Approved Adjustment No. ______________________ Date of Approval: ______________________ 2016 Surface Water Design Manual 4/24/2016 1 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: _______________________ Completion Date: _______________________ Describe: _________________________________ _________________________________________ _________________________________________ Re: KCSWDM 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 ______________________  _____________________________________ Part 10 SOILS Soil Type _________________ _________________ _________________ _________________ Slopes _________________ _________________ _________________ _________________ Erosion Potential _________________ _________________ _________________ _________________  High Groundwater Table (within 5 feet)  Other ________________________________  Sole Source Aquifer  Seeps/Springs  Additional Sheets Attached 2016 Surface Water Design Manual 4/24/2016 2 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 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) Level: 1 / 2 / 3 or Exemption Number ____________ Flow Control 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: _________________________ 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. ______________________ Landscape Management Plan: Yes / No Special Requirements (as applicable): Area Specific Drainage Requirements Type: CDA / SDO / MDP / BP / LMP / 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: 2016 Surface Water Design Manual 4/24/2016 3 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) 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: 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  Protection of Flow Control BMP Facilities (existing and proposed)  Maintain 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 Facilities, restore operation of Flow Control BMP Facilities as necessary  Flag limits of SAO 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  Flow Control BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________  Vegetated Flowpath  Wetpool  Filtration  Oil Control  Spill Control  Flow Control BMPs  Other ________________ ________________ ________________ ________________ ________________ ________________ ________________ 2016 Surface Water Design Manual 4/24/2016 4 KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 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 ______________________________ 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 2016 Surface Water Design Manual 4/24/2016 5 Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 1 I. PROJECT OVERVIEW Project: Meadow Ave Short Plat Site Address: 3804 Meadow Avenue N, Renton, WA 98056 (See Vicinity Map) Tax Parcel #: 334270-0570 Zoning District: R-6, Single Family Residential Site Area: 25,536 SF (0.59 Acres) Site Location: The site is in the City of Renton within the NW quarter of Section 32, Township 24 North, Range 5 East, W.M, King County, Washington. The site is located on the northeast corner of the intersection of Meadow Ave N and N 38th St. Figure 2: Vicinity Map Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 2 Pre-developed Site Conditions The project site is located in the City of Renton on a 25,536 SF (0.59 Acres) lot that is zoned R-6 (single- family residential). The site is accessed from Meadow Ave N, north of the intersection of N 38th St and Meadow Ave N. The site is bordered to the north and south by single family residences, to the east by I- 405, and to the west by Meadow Ave N right-of way (ROW). The site is currently developed with a single-family residence accessed from Meadow Ave N via two gravel driveways on the western side of the site. The site slopes gently to the northwest at approximately 2%. The property is located within the May Creek drainage basin. Stormwater runoff currently leaves the site at a single natural discharge location at the northwest corner, creating one Threshold Discharge Area for the site. The majority of the site gently slopes at approximately 2%, allowing stormwater to sheet flow northwest into Meadow Ave. A cleared area on the adjacent parcel 334270-0630 is tributary to the site. This off-site area sheet flows to the north over approximately 10-15% slopes onto the existing gravel driveway located in the southern portion of the site. See full downstream analysis in Section III of this Technical Information Report (TIR). An Existing Conditions Map is included as Figure 4 at the end of this Section. Critical Areas According to the City of Renton online mapping system, there is a High Erosion Hazard Area in the northeast corner of the project site. There are no required buffers or building setbacks associated with the erosion hazard area. No other critical areas have been identified on the project site. Soils Per the US Department of Agriculture (USDA), Natural Resources Conservation Service (NRCS) Web Soil Survey (WSS) information, the majority of the project site is underlain with Indianola loamy sand, 5 to 15 percent slopes with a small portion of the site in the northeast corner underlain with Norma sandy loam (See Figure 3 on the following page). Soil explorations detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 (Appendix A) encountered primarily medium dense silty sand and poorly graded sand. These findings are consistent with geologic and soil mapping resources. An in- situ infiltration test was performed and a design infiltration rate of 5 inches per hour was determined by the Geotech, which is suitable for stormwater infiltration. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 3 Figure 3: Soil Map and Legend Developed Site Conditions The project proposes the development of three (3) single-family lots within the 25,536 SF (0.59 Acres) parcel, as well as a 7.5-foot right-of-way dedication with frontage improvements. Lot 1 is 8,230 SF (0.19 AC) and is located in the north portion of the site. Lot 2 is 8,043 SF (0.18 Acres) and is located in the central portion of the site. Lot 3 is 7,868 SF (0.18 AC) and is located in the southern portion of the site. All three (3) lots will have driveway access off of Meadow Ave N. Frontage improvements will include the addition of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 4 The total disturbed area for this project, including the construction of frontage improvements, is 27,320 SF (0.627 AC). The parcel is zoned R-6, which allows for a maximum building coverage of 40% and a maximum impervious surface coverage of 55%. The maximum allowable building/impervious surface for each lot is discussed below: · Lot 1: The maximum allowable building coverage for Lot 1 is 8,230 SF*0.4 = 3,292 SF, and the maximum impervious surface coverage is 8,230 SF*0.55 = 4,527 SF. The final site layout of Lot 1 has not been determined at this stage; therefore, this report assumes that the future impervious surfaces required to construct the residence on Lot 1 will use the maximum coverages stated above. · Lot 2: The maximum allowable building coverage for Lot 2 is 8,043 SF*0.4 = 3,217 SF, and the maximum impervious surface coverage is 8,043 SF*0.55 = 4,424 SF. The final site layout of Lot 2 has not been determined at this stage; therefore, this report assumes that the future impervious surfaces required to construct the residence on Lot 2 will use the maximum coverages stated above. · Lot 3: The maximum allowable building coverage for Lot 2 is 7,868 SF*0.4 = 3,147 SF, and the maximum impervious surface coverage is 7,868 SF*0.55 = 4,327 SF. The final site layout of Lot 3 has not been determined at this stage; therefore, this report assumes that the future impervious surfaces required to construct the residence on Lot 3 will use the maximum coverages stated above. Stormwater runoff from the proposed development of Lots 1-3 will be managed as follows: · Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 1 to the west of the proposed structure to mitigate stormwater from impervious areas. An emergency overflow to the public storm system from the infiltration trench will be provided. This report assumes that the maximum impervious surface per zoning (4,527 SF) will be conveyed to this infiltration trench. · Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 2 to the west of the proposed structure to mitigate stormwater from impervious areas. An emergency overflow to the public storm system from the infiltration trench will be provided. This report assumes that the maximum impervious surface per zoning (4,424 SF) will be conveyed to this infiltration trench. · Lot 3: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 3 Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 5 to the west of the proposed structure to mitigate stormwater from impervious areas. An emergency overflow to the public storm system from the infiltration trench will be provided. This report assumes that the maximum impervious surface per zoning (4,327 SF) will be conveyed to this infiltration trench. Please refer to Core Requirement # 9 in Section II and Section IV of this TIR for additional discussion on stormwater BMPs. A Developed Conditions Map is provided as Figure 5 at this end of this Section. N 38TH STREET MEADOW AVENUE NEastern Washington Division407 Swiftwater Blvd. ▪ Cle Elum, WA 98922 ▪ Phone: (509) 674-7433 Western Washington Division165 NE Juniper Street, Suite 201 ▪ Issaquah, WA 98027 ▪ Phone: (425) 392-0250 ncompass ENGINEERING & SURVEYING E Know what'sCallbelow. before you dig. R NORTHEXISTING CONDITIONS AREAS PREDEVELOPED MODELED AREA FIGURE 4 - EXISTING CONDITIONS MAP Eastern Washington Division407 Swiftwater Blvd. ▪ Cle Elum, WA 98922 ▪ Phone: (509) 674-7433 Western Washington Division165 NE Juniper Street, Suite 201 ▪ Issaquah, WA 98027 ▪ Phone: (425) 392-0250 ncompass ENGINEERING & SURVEYING E Know what'sCallbelow. before you dig. R NORTHDEVELOPED CONDITIONS AREAS MITIGATED MODELED AREAS FIGURE 5 - DEVELOPED CONDITIONS MAP Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 8 II. CONDITIONS AND REQUIREMENTS SUMMARY The 2017 City of Renton Surface Water Design Manual (RSWDM) was utilized to determine and address all core and special requirements. Based on the criteria specified in Figure 1.1.2.A of the RSWDM, the project falls under Full Drainage Review. Per Section 1.1.2.4 of the RSWDM, the project must meet all nine (9) core and all six (6) special requirements. See Figure 6 below for more information on how the type of drainage review was determined. Figure 6: Drainage Review Flow Chart Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 9 Core Requirements Core Requirement #1: Discharge at the Natural Location The proposed development runoff will follow existing drainage patterns toward the Meadow Ave N storm system or will be fully infiltrated on-site. Refer to the Level 1 Downstream Analysis in Section III of this TIR for a complete description of the existing drainage paths. Core Requirement #2: Downstream Analysis A Level 1 Downstream analysis has been completed for the site and no existing or potential drainage problems have been identified. This analysis is included in Section III of this TIR. Core Requirement #3: Flow Control Facilities Based on the City of Renton’s flow control application map, the project site is located within the Peak Rate Flow Control Standard (Existing Site Conditions). Flow control facilities are required to match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10- and 100-year return periods. The proposed stormwater BMPs result in a 0.2289 CFS decrease in the 100-year flow using 15- minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum increase per Section 1.2.3.1.B of the RSWDM; therefore, the project meets flow control exemption 2. No formal flow control facilities are proposed at this time; however, flow control BMPs will be implemented as described in Core Requirement #9. Please refer to Section IV of this TIR for additional discussion. Core Requirement #4: Conveyance System Conveyance in compliance with the requirements detailed in Section 1.2.4.1 of the City of Renton 2017 SWDM will be provided with final engineering. Core Requirement #5: Construction Stormwater Pollution Prevention The limits of construction for the project are less than 1 acre. A temporary erosion and sediment control (TESC) plan providing details on best management practices (BMPs) to be implemented during construction is included in the engineering plan set. A Construction Stormwater Pollution Prevention Plan (CSWPPP) will be provided with final engineering. Please refer to Section VIII of this TIR for additional discussion. Core Requirement #6: Maintenance and Operations An Operation and Maintenance Manual will be provided with final engineering. Core Requirement #7: Financial Guarantees and Liability The owner will arrange for any financial guarantees and liabilities required by the permit. Core Requirement #8: Water Quality Facilities In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not required for this project as new plus replaced pollution generating impervious surfaces (PGIS) does not exceed 5,000 SF. The development results in 768 SF of concrete driveway within the ROW dedication area, 485 of pavement replacement within the existing ROW, and 3,700 SF of on- site driveway is estimated for the future build-out of Lots 1-3. These quantities total to 4,953 SF Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 10 of PGIS that will be created or replaced as a result of this project; therefore, the project meets surface area exemption 1. Core Requirement #9: Flow Control BMPs Per section 1.2.9.3.1 of the RSWDM, this project is classified as a small subdivision; therefore, it is subject to the Small Road Improvement and Urban Road Improvement Project BMP Requirements detailed in Section 1.2.9.3.2 in the RSWDM. Although implementation of individual lot BMPs is not required until building permit application, BMPs have been considered for the future improvements on Lots 1-3 based on Section 1.2.9.2 of the RSWDM. See Section IV of this TIR for further discussion and flow control analysis. Impervious Surface BMPs Full Dispersion: Infeasible for Lots 1-3. · Lots 1-3: The space required for a 100-foot native vegetated flowpath segment is not available on these sites. Full Infiltration: Feasible for Lots 1-3. · Lots 1-3: Soil explorations detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 (Appendix A) encountered primarily medium dense silty sand and poorly graded sand. An in-situ infiltration test was performed and a design infiltration rate of 5 inches per hour was determined which is suitable for stormwater infiltration. Per section C.2.2.3 of the RSWDM, 30 liner feet of 2’ wide trench per 1,000 SF of tributary impervious surface can be used to design full infiltration trenches in coarse sands. A 30’x2’ trench has an infiltration area of 60 SF; therefore, 60 SF of infiltration area is required per 1,000 SF of tributary impervious surface. For Lots 1-3, three infiltration trenches of 30’x9’ are proposed which provides an infiltration area of 270 SF each. A 30’x9’ trench has the ability to mitigate up to (270 SF/60 SF)x1,000 SF= 4500 SF of tributary impervious surface. This allows each trench to mitigate the entire maximum impervious surface coverage for each lot. As an additional safety measure, an emergency overflow to the public storm system from the infiltration trenches will be provided. As all target impervious surfaces on the project site have been mitigated with flow control BMPs, no further BMPs have been considered. Pervious Surface BMPs Soil Amendment: Feasible for Lots 1-3 · Lots 1-3: All disturbed, pervious areas of the project will meet soil amendment requirements as detailed in Section C.2.13 of the RSWDM. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 11 Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 12 Special Requirements Special Requirement #1: Other Adopted Area-Specific Requirements Critical Drainage Area – N/A Master Drainage Plan – N/A Basin Plan – N/A Lake management Plan – N/A Shared Facility Drainage Plan – N/A Special Requirement #2: Flood Hazard Area Delineation The limits of this project do not lie within a delineated FEMA 100-year floodplain. Special Requirement #3: Flood Protection Facilities This project does not rely on or propose to modify/construct a new flood protection facility. Special Requirement #4: Source controls The project is a single-family residential development; therefore, this requirement is not applicable. Special Requirement #5: Oil Control This project is not considered high-use in need of oil control. Special Requirement #6: Aquifer Protection Area The site is not located within an Aquifer Protection Area Zone per the City of Renton Groundwater Protection Area Map Reference 15-B. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 13 III. DOWNSTREAM ANALYSIS A Level 1 Downstream analysis has been conducted per the requirements in Section 1.2.2.1 of the RSWDM. Please see Tasks 1 through 4 below for a summary of the results. Task 1: Define and Map the Study Area The area of analysis extends from the site discharge points along Meadow Ave N and the eastern limit of the site along the I-405 barrier wall to approximately a quarter-mile downstream where the two discharge areas converge. A Downstream Map is provided in Figure 7 below. Figure 7: Downstream Map Task 2: Review All Available Information on the Study Area Per King county resources, there have been no significant drainage complaints within a quarter-mile downstream of the site. Task 3: Field Inspect the Study Area A field inspection was performed by Encompass Engineering & Surveying on January 26, 2021. Please refer to Task 4 for a detailed description of the downstream drainage system and analysis. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 14 Task 4: Describe the Drainage System Stormwater runoff currently leaves the site at a single natural discharge location at the northwest corner creating one Threshold Discharge Area for the site. The majority of the site gently slopes at approximately 2%, allowing stormwater to sheet flow northwest into Meadow Ave. A cleared area on the adjacent parcel 334270-0630 is tributary to the site. This off-site area and sheet flows to the north over approximately 10-15% slopes onto the existing gravel driveway in the southern portion of the site. Runoff in the southeast portion of the site sheet flows on gravel driveway and grass towards a low point along the I-405 wall on the eastern side of the site (A). From here, stormwater is assumed to infiltrate directly into the soil as no outlet exists. This observation is consistent with type A/B outwash soils found and detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 (Appendix A). Runoff from the southwestern portion of the site sheets flows towards Meadow Ave N where it enters the City of Renton (COR) storm system in a Type 1 CB (B) on the edge of asphalt. Runoff from the northern portion of the site sheet flows to the north and west, eventually entering Meadow Ave N and entering a Type 1 CB (C) just northwest of the site at the edge of asphalt. The southern of these two Type 1 CBs (B) outlets to the NW through a 10” DI storm pipe into a Type 2 CB approximately 12 feet away (D). From here, stormwater flows to the north through an 18” Poly storm pipe (E) into another Type 2 CB (F). This Type 2 CB collects stormwater from the Type 1 CB (C) just northwest of the site via an 8” DI storm pipe. Stormwater continues north on Meadow Ave N through a series of Type 2 CBs and 18” Poly storm pipe. It turns to the west when it reaches the intersection of Meadow Ave N and N 40th St, and runs westward on N 40th St downhill towards lake Washington. Stormwater continues through a series of Type 2 CBs and 18” poly pipe, through the intersection of N 40th St and Park Ave N, to the intersection of N 40th St and Wells Ave N. The Type 2 CB in this location (G) is where the analysis was completed, just over ¼ mile downstream of the site. Please refer to Figure 7 on the previous page, for the approximate location of identified drainage features. If conveyance system nuisance, severe erosion, severe flooding, or wetland hydrology problems are identified downstream of the site under Core Requirement #2, additional impact analysis and/or mitigation may be required. As discussed in Task 2, there have been no recent significant drainage complaints within a quarter-mile downstream of the site. As no existing drainage or conveyance issues have been recently identified, further impact analysis and mitigation of the downstream stormwater system is not required. Please refer to the conveyance analysis in Section V of this TIR for additional discussion and evaluation of the on-site conveyance system capacity. Photographs from the site visit are included on the pages that follow. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 15 Element A- Low point on eastern side of site by I-405 Wall Element B- Type 1 CB on Meadow Ave N collecting runoff from southern site frontage and yard Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 16 Element C- Type 1 CB on Meadow Ave N collecting water from the northern site frontage and yard Element G- Type 2 CB on N 40th St Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 17 IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN Part A: Existing Site Hydrology The 25,536 SF (0.59 AC) site is currently developed with a single-family residence accessed from Meadow Ave N via two gravel driveways on the west side of the site. The property is located within the May Creek drainage basin. Stormwater runoff currently leaves the site at a single natural discharge location at the northwest corner creating one Threshold Discharge Area for the site. The majority of the site gently slopes at approximately 2%, allowing stormwater to sheet flow northwest into Meadow Ave. A cleared area on the adjacent parcel 334270-0630 is tributary to the site. This off-site area and sheet flows to the north over approximately 10-15% slopes onto the existing gravel driveway in the southern portion of the site. See full downstream analysis in Section III of this Technical Information Report (TIR). Soil explorations detailed in the Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 (Appendix A) encountered primarily medium dense silty sand and poorly graded sand. An in-situ infiltration test was performed and a design infiltration rate of 5 inches per hour was determined by the Geotech, which is suitable for stormwater infiltration. WWHM 2012 was used to model the existing condition within the 27,320 SF (0.627 AC) limits of disturbance using existing surfaces found on site including rooftop, driveway, lawn and walkways. See the summary of existing and developed analyses on the following pages, as well as the existing conditions map provided as Figure 4 for more details. Part B: Developed Site Hydrology The project proposes the development of three (3) single-family lots within the 25,536 SF (0.59 Acres) parcel, as well as a 7.5-foot right-of-way dedication with frontage improvements. Lot 1 is 8,230 SF (0.19 AC) and is located in the north portion of the site. Lot 2 is 8,043SF (0.18 Acres) and is located in the central portion of the site. Lot 3 is 7,868 SF (0.18 AC) and is located in the southern portion of the site. All three (3) lots will have driveway access off of Meadow Ave N. Frontage improvements will include the addition of a 5-foot sidewalk, 8-foot planter strip, as well as curb and gutter along the edge of asphalt roadway. Stormwater runoff from the proposed development of Lots 1-3 will be managed as follows: · Lot 1: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 1 to the west of the proposed structure to mitigate stormwater from impervious areas. This report assumes that the maximum impervious surface per zoning (4,527 SF) will be conveyed to this infiltration trench. Additionally, an emergency overflow to the public storm system from the infiltration trench will be provided. · Lot 2: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 2 to the west of the proposed structure to mitigate stormwater from impervious areas. This report assumes that the maximum impervious surface per zoning (4,424 SF) will be conveyed to this infiltration trench. Additionally, an emergency overflow to the public storm system from the infiltration trench will be provided. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 18 · Lot 3: Stormwater BMP’s satisfying Core Requirement #9 will be used to the maximum extent feasible to mitigate new runoff created by the proposed single-family residence and other impervious surfaces on-site. A 30’x9’ gravel filled full infiltration trench is proposed on Lot 3 to the west of the proposed structure to mitigate stormwater from impervious areas. This report assumes that the maximum impervious surface per zoning (4,327 SF) will be conveyed to this infiltration trench. Additionally, an emergency overflow to the public storm system from the infiltration trench will be provided. WWHM 2012 was used to model the proposed condition as follows: · Lot 1: The modeled limits of disturbance for Lot 1 is 8,230 SF. The maximum impervious surface per R-6 zoning of 4,527 SF was assumed, with the entirety of this area being conveyed to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration can be subtracted from the model. The remaining 3,703 SF of pervious surface on Lot 1 will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with flat slopes in type A/B soils. · Lot 2: The modeled limits of disturbance for Lot 2 is 8,043 SF. The maximum impervious surface per R-6 zoning of 4,424 SF was assumed, with the entirety of this area being conveyed to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration can be subtracted from the model. The remaining 3,619 SF of pervious surface on Lot 1 will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with flat slopes in type A/B soils. · Lot 3: The modeled limits of disturbance for Lot 3 is 7,868 SF. The maximum impervious surface per R-6 zoning of 4,327 SF was assumed, with the entirety of this area being conveyed to the full infiltration trench. Per Table 1.2.9.A in the RSWDM, the area going to full infiltration can be subtracted from the model. The remaining 3,541 SF of pervious surface on Lot 1 will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with flat slopes in type A/B soils. · ROW Dedication & Road Widening: The modeled limits of disturbance for the ROW dedication area and road widening is 2,995 SF. This will include 838 SF of sidewalk, 768 SF of driveway entrance as well as 485 SF of asphalt road repair. The remaining 904 SF of pervious surface will meet soil amendment requirement as detailed in Section C.2.13 of the RSWDM; therefore, it has been modeled as 100% pasture with moderate slopes in type C soils. Please refer to Core Requirement #9 in Section II of the TIR for additional discussion. A Developed Conditions Map is provided as Figure 5. A summary of the existing and developed analyses is provided in the table on the following page. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 19 On-site + Off-Site Existing Proposed Condition Measured Modeled Measured Modeled Lawn, Mod: 14,050 SF (0.322 AC) 14,050 SF (0.322 AC) 11,850 (0.272 AC) Pasture, Mod: 11,850 (0.272 AC) Roof, Flat: 1,995 SF (0.046 AC) 1,995 SF (0.046 AC) 9,729 SF (0.223 AC) Driveway, Flat: 10,122 SF (0.232 AC) 10,122 SF (0.232 AC) 4,418 SF (0.101 AC) 768 SF (0.018 AC) Sidewalk, Flat: 719 (0.017 AC) 719 (0.017 AC) 838 SF (0.019 AC) 838 SF (0.019 AC) Road, Flat: 434 SF (0.010 AC) 434 SF (0.010 AC) 485 SF (0.011 AC) 485 SF (0.011 AC) Total Area: 27,320 SF (0.627 AC) 27,320 SF (0.627 AC) 27,320 SF (0.627 AC) 13,941 SF (0.320 AC) Part C: Performance Standards Based on the City of Renton’s flow control map, the project site is located within the Peak Rate Flow Control Standard (Existing Site Conditions). Flow control facilities are required to match the developed peak discharge rates to existing site conditions peak discharge rates for 2-, 10- and 100-year return periods. However, as determined in Part D on the following page, the site meets flow control exemption 2 per Section 1.2.3.1.B of the RSWDM, and flow control facilities are not required. Per section 1.2.9.3.1 of the RSWDM, this project is classified as a small subdivision; therefore, it is subject to the Small Road Improvement and Urban Road Improvement Project BMP Requirements detailed in Section 1.2.9.3.2 in the RSWDM. The site falls within a Basic Water Quality treatment area in accordance with Section 1.2.8.1.A of the RSWDM, however new plus replaced impervious pollution generating areas within the project site is under 5,000 SF. Therefore, water quality treatment is not required. Part D: Flow Control System As shown in the table below, the proposed stormwater BMPs result in a 0.2289 CFS decrease in the 100- year flow using 15-minutes time steps. This is below the allowable threshold of a 0.15 CFS maximum increase per Section 1.2.3.1.B of the RSWDM; therefore, the project meets flow control exemption 2. No formal flow control facilities are proposed at this time; however, flow control BMPs will be implemented as described in Core Requirement #9 in Section II of this TIR. Please refer to Appendix B for a copy of the full WWHM data output. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 20 Part E: Water Quality System In accordance with Section 1.2.8.1.A of the RSWDM, Basic Water Quality Treatment is not required for this project as new plus replaced pollution generating impervious surfaces (PGIS) does not exceed 5,000 SF. The development results in 768 SF of concrete driveway within the ROW dedication area, 485 SF of pavement replacement within the existing ROW, and 3,700 SF of on-site driveway is estimated for the future build-out of Lots 1-3. These quantities total to 4,953 SF of PGIS that will be created or replaced as a result of this project; therefore, the project meets surface area exemption 1. Meadow Ave Short Plat Preliminary Technical Information Report 3/7/2022 P a g e | 21 V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN A conveyance system analysis will be provided with final engineering VI. SPECIAL REPORTS AND STUDIES · Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 · Arborist Report/Tree Plan by Layton Tree Consulting, LLC dated February 12, 2020 VII. OTHER PERMITS · Civil Construction Permit · Final Short Plat · Building Permits · Right-of-Way Use Permit VIII. CSWPP ANALYSIS AND DESIGN A CSWPPP will be provided with final engineering. IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION of COVENANT Bond Quantities, Facility Summary and Declaration of Covenant to be provided with Final Engineering. X. OPERATION AND MAINTENANCE MANUAL An Operation and Maintenance Manual to be provided with Final Engineering. Appendix A Geotechnical Engineering Study by Earth Solutions NW dated March 12, 2020 EarthSolutionsNWLLC EarthSolutionsNWLLC 15365 N.E. 90th Street, Suite 100 Redmond, WA98052 (425) 449-4704 Fax (425) 449-4711 www.earthsolutionsnw.com Geotechnical Engineering Construction Observation/Testing Environmental Services GEOTECHNICAL ENGINEERING STUDY PROPOSED SHORT PLAT 3804 MEADOW AVENUE NORTH RENTON, WASHINGTON ES-7126 PREPARED FOR MR. HUY NGUYEN March 12, 2020 _________________________ Brian C. Snow, G.I.T. Staff Geologist _________________________ Keven D. Hoffmann, P.E. Senior Project Manager GEOTECHNICAL ENGINEERING STUDY PROPOSED SHORT PLAT 3804 MEADOW AVENUE NORTH RENTON, WASHINGTON ES-7126 Earth Solutions NW, LLC 15365 Northeast 90th Street, Suite 100 Redmond, Washington 98052 Phone: 425-449-4704 | Fax: 425-449-4711 www.earthsolutionsnw.com 03/16/2020 Geotechnical-Engineering Report Important Information about This Subsurface problems are a principal cause of construction delays, cost overruns, claims, and disputes. While you cannot eliminate all such risks, you can manage them. The following information is provided to help. The Geoprofessional Business Association (GBA) has prepared this advisory to help you – assumedly a client representative – interpret and apply this geotechnical-engineering report as effectively as possible. In that way, you can benefit from a lowered exposure to problems associated with subsurface conditions at project sites and development of them that, for decades, have been a principal cause of construction delays, cost overruns, claims, and disputes. If you have questions or want more information about any of the issues discussed herein, contact your GBA-member geotechnical engineer. Active engagement in GBA exposes geotechnical engineers to a wide array of risk-confrontation techniques that can be of genuine benefit for everyone involved with a construction project. Understand the Geotechnical-Engineering Services Provided for this ReportGeotechnical-engineering services typically include the planning, collection, interpretation, and analysis of exploratory data from widely spaced borings and/or test pits. Field data are combined with results from laboratory tests of soil and rock samples obtained from field exploration (if applicable), observations made during site reconnaissance, and historical information to form one or more models of the expected subsurface conditions beneath the site. Local geology and alterations of the site surface and subsurface by previous and proposed construction are also important considerations. Geotechnical engineers apply their engineering training, experience, and judgment to adapt the requirements of the prospective project to the subsurface model(s). Estimates are made of the subsurface conditions that will likely be exposed during construction as well as the expected performance of foundations and other structures being planned and/or affected by construction activities. The culmination of these geotechnical-engineering services is typically a geotechnical-engineering report providing the data obtained, a discussion of the subsurface model(s), the engineering and geologic engineering assessments and analyses made, and the recommendations developed to satisfy the given requirements of the project. These reports may be titled investigations, explorations, studies, assessments, or evaluations. Regardless of the title used, the geotechnical-engineering report is an engineering interpretation of the subsurface conditions within the context of the project and does not represent a close examination, systematic inquiry, or thorough investigation of all site and subsurface conditions. Geotechnical-Engineering Services are Performed for Specific Purposes, Persons, and Projects, and At Specific TimesGeotechnical engineers structure their services to meet the specific needs, goals, and risk management preferences of their clients. A geotechnical-engineering study conducted for a given civil engineer will not likely meet the needs of a civil-works constructor or even a different civil engineer. Because each geotechnical-engineering study is unique, each geotechnical-engineering report is unique, prepared solely for the client. Likewise, geotechnical-engineering services are performed for a specific project and purpose. For example, it is unlikely that a geotechnical- engineering study for a refrigerated warehouse will be the same as one prepared for a parking garage; and a few borings drilled during a preliminary study to evaluate site feasibility will not be adequate to develop geotechnical design recommendations for the project. Do not rely on this report if your geotechnical engineer prepared it: • for a different client; • for a different project or purpose; • for a different site (that may or may not include all or a portion of the original site); or • before important events occurred at the site or adjacent to it; e.g., man-made events like construction or environmental remediation, or natural events like floods, droughts, earthquakes, or groundwater fluctuations. Note, too, the reliability of a geotechnical-engineering report can be affected by the passage of time, because of factors like changed subsurface conditions; new or modified codes, standards, or regulations; or new techniques or tools. If you are the least bit uncertain about the continued reliability of this report, contact your geotechnical engineer before applying the recommendations in it. A minor amount of additional testing or analysis after the passage of time – if any is required at all – could prevent major problems. Read this Report in Full Costly problems have occurred because those relying on a geotechnical- engineering report did not read the report in its entirety. Do not rely on an executive summary. Do not read selective elements only. Read and refer to the report in full. You Need to Inform Your Geotechnical Engineer About Change Your geotechnical engineer considered unique, project-specific factors when developing the scope of study behind this report and developing the confirmation-dependent recommendations the report conveys. Typical changes that could erode the reliability of this report include those that affect: • the site’s size or shape; • the elevation, configuration, location, orientation, function or weight of the proposed structure and the desired performance criteria; • the composition of the design team; or • project ownership. As a general rule, always inform your geotechnical engineer of project or site changes – even minor ones – and request an assessment of their impact. The geotechnical engineer who prepared this report cannot accept responsibility or liability for problems that arise because the geotechnical engineer was not informed about developments the engineer otherwise would have considered. Most of the “Findings” Related in This Report Are Professional Opinions Before construction begins, geotechnical engineers explore a site’s subsurface using various sampling and testing procedures. Geotechnical engineers can observe actual subsurface conditions only at those specific locations where sampling and testing is performed. The data derived from that sampling and testing were reviewed by your geotechnical engineer, who then applied professional judgement to form opinions about subsurface conditions throughout the site. Actual sitewide-subsurface conditions may differ – maybe significantly – from those indicated in this report. Confront that risk by retaining your geotechnical engineer to serve on the design team through project completion to obtain informed guidance quickly, whenever needed. This Report’s Recommendations Are Confirmation-Dependent The recommendations included in this report – including any options or alternatives – are confirmation-dependent. In other words, they are not final, because the geotechnical engineer who developed them relied heavily on judgement and opinion to do so. Your geotechnical engineer can finalize the recommendations only after observing actual subsurface conditions exposed during construction. If through observation your geotechnical engineer confirms that the conditions assumed to exist actually do exist, the recommendations can be relied upon, assuming no other changes have occurred. The geotechnical engineer who prepared this report cannot assume responsibility or liability for confirmation-dependent recommendations if you fail to retain that engineer to perform construction observation. This Report Could Be Misinterpreted Other design professionals’ misinterpretation of geotechnical- engineering reports has resulted in costly problems. Confront that risk by having your geotechnical engineer serve as a continuing member of the design team, to: • confer with other design-team members; • help develop specifications; • review pertinent elements of other design professionals’ plans and specifications; and • be available whenever geotechnical-engineering guidance is needed. You should also confront the risk of constructors misinterpreting this report. Do so by retaining your geotechnical engineer to participate in prebid and preconstruction conferences and to perform construction-phase observations. Give Constructors a Complete Report and GuidanceSome owners and design professionals mistakenly believe they can shift unanticipated-subsurface-conditions liability to constructors by limiting the information they provide for bid preparation. To help prevent the costly, contentious problems this practice has caused, include the complete geotechnical-engineering report, along with any attachments or appendices, with your contract documents, but be certain to note conspicuously that you’ve included the material for information purposes only. To avoid misunderstanding, you may also want to note that “informational purposes” means constructors have no right to rely on the interpretations, opinions, conclusions, or recommendations in the report. Be certain that constructors know they may learn about specific project requirements, including options selected from the report, only from the design drawings and specifications. Remind constructors that they may perform their own studies if they want to, and be sure to allow enough time to permit them to do so. Only then might you be in a position to give constructors the information available to you, while requiring them to at least share some of the financial responsibilities stemming from unanticipated conditions. Conducting prebid and preconstruction conferences can also be valuable in this respect. Read Responsibility Provisions Closely Some client representatives, design professionals, and constructors do not realize that geotechnical engineering is far less exact than other engineering disciplines. This happens in part because soil and rock on project sites are typically heterogeneous and not manufactured materials with well-defined engineering properties like steel and concrete. That lack of understanding has nurtured unrealistic expectations that have resulted in disappointments, delays, cost overruns, claims, and disputes. To confront that risk, geotechnical engineers commonly include explanatory provisions in their reports. Sometimes labeled “limitations,” many of these provisions indicate where geotechnical engineers’ responsibilities begin and end, to help others recognize their own responsibilities and risks. Read these provisions closely. Ask questions. Your geotechnical engineer should respond fully and frankly. Geoenvironmental Concerns Are Not Covered The personnel, equipment, and techniques used to perform an environmental study – e.g., a “phase-one” or “phase-two” environmental site assessment – differ significantly from those used to perform a geotechnical-engineering study. For that reason, a geotechnical-engineering report does not usually provide environmental findings, conclusions, or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Unanticipated subsurface environmental problems have led to project failures. If you have not obtained your own environmental information about the project site, ask your geotechnical consultant for a recommendation on how to find environmental risk-management guidance. Obtain Professional Assistance to Deal with Moisture Infiltration and Mold While your geotechnical engineer may have addressed groundwater, water infiltration, or similar issues in this report, the engineer’s services were not designed, conducted, or intended to prevent migration of moisture – including water vapor – from the soil through building slabs and walls and into the building interior, where it can cause mold growth and material-performance deficiencies. Accordingly, proper implementation of the geotechnical engineer’s recommendations will not of itself be sufficient to prevent moisture infiltration. Confront the risk of moisture infiltration by including building-envelope or mold specialists on the design team. Geotechnical engineers are not building-envelope or mold specialists. Copyright 2019 by Geoprofessional Business Association (GBA). Duplication, reproduction, or copying of this document, in whole or in part, by any means whatsoever, is strictly prohibited, except with GBA’s specific written permission. Excerpting, quoting, or otherwise extracting wording from this document is permitted only with the express written permission of GBA, and only for purposes of scholarly research or book review. Only members of GBA may use this document or its wording as a complement to or as an element of a report of any kind. Any other firm, individual, or other entity that so uses this document without being a GBA member could be committing negligent or intentional (fraudulent) misrepresentation. Telephone: 301/565-2733 e-mail: info@geoprofessional.org www.geoprofessional.org March 12, 2020 ES-7126 Mr. Huy Nguyen 15400 Southeast 155th Place, Unit 99 Renton, Washington 98058 Dear Mr. Nguyen: Earth Solutions NW, LLC (ESNW), is pleased to present this geotechnical report for the subject site. While specific site plans were not available at the time of this report, we presume the site will be developed into a short plat, comprised of several two- to three-story residential structures and related infrastructure improvements. Based on the results of our study, construction of a short plat is feasible from a geotechnical standpoint. Based on the conditions observed during the fieldwork, the subject site is underlain primarily by medium dense recessional outwash deposits. The proposed structures can be supported on conventional spread and continuous foundations bearing on undisturbed competent native soil, recompacted native soil, or new structural fill. We anticipate competent native soil suitable for support of foundations will be encountered beginning at depths of about two to four feet below existing grades across the site. This report provides geotechnical analyses and recommendations for the proposed short plat. The opportunity to be of service to you is appreciated. If you have any questions regarding the content of this geotechnical engineering study, please call. Sincerely, EARTH SOLUTIONS NW, LLC Brian C. Snow, G.I.T. Staff Geologist 15365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 •(425) 449-4704 • FAX (425) 449-4711 Earth Solutions NW LLC Geotechnical Engineering, Construction Observation/Testing and Environmental Services Earth Solutions NW, LLC Table of Contents ES-7126 PAGE INTRODUCTION ................................................................................. 1 General..................................................................................... 1 Project Description ................................................................. 2 SITE CONDITIONS ............................................................................. 2 Surface ..................................................................................... 2 Subsurface .............................................................................. 2 Topsoil and Fill ............................................................. 2 Native Soil ..................................................................... 3 Geologic Setting ........................................................... 3 Groundwater ................................................................. 3 GEOLOGICALLY HAZARDOUS AREAS EVALUATION .................. 3 Erosion Hazard ........................................................................ 4 Landslide Hazard .................................................................... 4 Regulated Slopes .................................................................... 5 Seismic Hazard ....................................................................... 5 Analysis of Proposal .............................................................. 6 DISCUSSION AND RECOMMENDATIONS ....................................... 6 General..................................................................................... 6 Site Preparation and Earthwork ............................................. 6 Temporary Erosion Control ......................................... 6 In-Situ and Imported Soil ............................................. 6 Structural Fill ................................................................ 7 Subgrade Preparation .................................................. 7 Excavations and Slopes .............................................. 7 Foundations ............................................................................ 8 Seismic Design ....................................................................... 9 Slab-on-Grade Floors ............................................................. 9 Retaining Walls ....................................................................... 9 Drainage................................................................................... 10 Infiltration Evaluation ................................................... 10 Utility Support and Trench Backfill ....................................... 11 LIMITATIONS ...................................................................................... 11 Additional Services ................................................................. 11 Earth Solutions NW, LLC Table of Contents Continued ES-7126 GRAPHICS Plate 1 Vicinity Map Plate 2 Test Pit Location Plan Plate 3 Retaining Wall Drainage Detail Plate 4 Footing Drain Detail APPENDICES Appendix A Subsurface Exploration Test Pit Logs Appendix B Laboratory Test Results Earth Solutions NW, LLC GEOTECHNICAL ENGINEERING STUDY PROPOSED SHORT PLAT 3804 MEADOW AVENUE NORTH RENTON, WASHINGTON ES-7126 INTRODUCTION General This geotechnical engineering study (study) was prepared for the proposed short plat to be constructed on the east side of Meadow Avenue North, nearest the intersection with North 38th Street, in the Kennydale neighborhood of Renton, Washington. To complete the scope of services, we performed the following:  Subsurface exploration to characterize the soil and groundwater conditions;  In-situ infiltration testing;  Laboratory testing of representative soil samples collected on site;  Review of on-site geologically hazardous areas;  Engineering analyses and recommendations for the proposed short plat, and;  Preparation of this report. The following documents and resources were reviewed as part of our report preparation:  Preliminary Geologic Map of Seattle and Vicinity, Washington, compiled by H.H. Waldron et al., dated 1961;  Web Soil Survey (WSS) online resource, maintained by the Natural Resources Conservation Service (NRCS) under the United States Department of Agriculture (USDA);  Soil Survey of Snoqualmie Pass Area, Parts of King and Pierce Counties, Washington, prepared by the United States Department of Agriculture Soil Conservation Service, dated 1990;  Soil Survey of King County Area, Washington, prepared by the United States Department of Agriculture Soil Conservation Service, dated 1973;  Liquefaction Susceptibility Map 11-5, prepared by the King County Flood Control District, dated May 2010;  Renton Municipal Code (RMC);  City of Renton Critical Areas Map: Online “COR Maps”, and;  City of Renton Surface Water Design Manual. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 2 Earth Solutions NW, LLC Project Description We understand the proposed project will likely consist of constructing a three-lot short plat and associated improvements. At the time this report was prepared, neither site plans nor building load values were available for review. However, we anticipate the proposed residential structures will be two to three stories and constructed using relatively lightly loaded wood framing supported atop conventional foundation systems. Based on our experience with similar developments, we estimate wall loads of about 1 to 2 kips per linear foot and slab-on-grade loading of 150 pounds per square foot (psf) will be incorporated into final designs. Grade cuts and fills to establish individual-lot subgrade and finish grade elevations are expected to be less than five feet. Stormwater management will presumably use infiltration to the extent feasible. If the above design assumptions are incorrect or change, ESNW should be contacted to review the recommendations provided in this report. ESNW should review the final designs to verify the geotechnical recommendations provided in this report have been incorporated into the plans. SITE CONDITIONS Surface The subject site is located northeast of the intersection between North 38th Street and Meadow Avenue North, in Renton, Washington. The approximate location of the property is illustrated on Plate 1 (Vicinity Map). The site is comprised of one tax parcel (King County Parcel No. 334270- 0570), totaling about 0.59 acres. The site is currently developed with a single-family residence and a detached shed structure. The existing topography is relatively level, with less than two feet of elevation change across the site. To the east of the sound barrier wall, grades descend sharply to the Interstate 405 corridor. Subsurface An ESNW representative observed, logged, and sampled five test pits on February 17, 2020. The test pits were excavated at accessible locations within the property, using a mini-trackhoe and operator retained by our firm. The approximate locations of the test pits are depicted on Plate 2 (Test Pit Location Plan). Please refer to the test pit logs provided in Appendix A for a more detailed description of subsurface conditions. Representative soil samples collected at the test pit locations were analyzed in general accordance with Unified Soil Classification System (USCS) and USDA methods and procedures. Topsoil and Fill Topsoil was generally encountered within the upper 6 to 42 inches of existing grades where test pits were excavated. In some instances, relic topsoil horizons were observed beneath fill soil. The topsoil was characterized by a dark brown color, the presence of fine organic material, and small root intrusions. Root zones extended between about four to five feet below the existing ground surface (bgs). Mr. Huy Nguyen ES-7126 March 12, 2020 Page 3 Earth Solutions NW, LLC Fill was observed at test pit locations TP-1, TP-2, TP-3, and TP-4, extending to a maximum depth of about five feet bgs. The fill soil generally consisted of tan silty sand with minor interbedded topsoil. In TP-4, rebar, plastic bags, and wood debris was observed. Native Soil Underlying the topsoil and fill, the native soil consisted primarily of medium dense silty sand and poorly graded sand with variable silt and gravel content (USCS: SM, SP, and SP-SM). The native deposits were primarily observed in a wet condition at the time of our exploration, and weak iron oxide staining was noted at several locations. The maximum exploration depth was roughly nine feet bgs. Geologic Setting Geologic mapping of the area indicates the site is underlain by younger gravel (Qyg) and younger sand (Qys) as part of the Vashon recessional outwash series. Recessional outwash was deposited by glacial meltwater and is readily comprised of silt, clay, sand, and gravel. The referenced WSS resource identifies Indianola loamy sand (Map Unit Symbol: InC) across the majority of the site, with Norma sandy loam (Map Unit Symbol: No) mapped along the eastern edge. The Indianola loamy sands were formed in eskers, kames, and terraces, and the Norma sandy loams were formed in flood plains. Based on our field observations, the native soil on site is generally consistent with the geologic and soil mapping resources outlined in this section. Groundwater During the subsurface exploration, perched groundwater seepage was encountered at test pit locations TP-3 and TP-4 at eight-and-one-half and five-and-one-half feet bgs, respectively. Groundwater seepage may be encountered within site excavations depending on the time of year and extent of grading activities. Seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. In general, groundwater flow rates are higher during the winter, spring, and early summer months. GEOLOGICALLY HAZARDOUS AREAS EVALUATION To evaluate geologically hazardous areas, we reviewed Chapter 4-3 of the City of Renton (City) Municipal Code (RMC). The RMC provides designation and definition criteria for identifying specific geologically hazardous areas and developing appropriate site development plans which will not adversely impact the site or surrounding properties. According to the referenced City Critical Areas Map, erosion hazards, landslide hazards, and regulated slopes are all mapped within 50 feet of the subject site. The mapped hazards are largely associated with the hillslope east of the subject site (outside of the property boundaries) and bordering Interstate 405. Additionally, a seismic hazard area is mapped within 500 feet of the subject site. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 4 Earth Solutions NW, LLC Erosion Hazard According to RMC 4-3-050G5c, erosion hazard areas can be defined as: i. Low Erosion Hazard (EL): Areas with soils characterized by the NRCS as having slight or moderate erosion potential and a slope less than 15 percent. ii. High Erosion Hazard (EH): Areas with soils characterized by the NRCS as having severe or very severe erosion potential and a slope greater than 15 percent. The referenced City Critical Areas Map designates a high erosion hazard on the eastern slope of the site (bordering Interstate 405). The site soils are predominantly mapped as Indianola loamy sand (Map Unit Symbol: InC), with Norma sandy loam (Map Unit Symbol: No) along the eastern edge. The USDA SCS 1973 King County Soils Survey classifies the erosion hazard of Indianola soils as slight to moderate. The USDA SCS 1990 Snoqualmie Pass Area Soil Survey states there is no hazard of erosion associated with Norma soils. In our opinion, given that the site topography is relatively level, the site soils within the proposed development areas generally exhibit low erosivity potential in a typical construction setting. Best Management Practices (BMPs), in general accordance with City surface water and stormwater regulations, should be used for site design and development. At a minimum, silt fencing should be placed along the entire development envelope, and soil stockpiles should be covered when not in use. If construction occurs during periods of wet weather, methods to control surface water runoff will likely be necessary. Construction entrances should be surfaced with quarry spalls to minimize off-site tracking of silt and soil generated during site construction. Landslide Hazard Per RMC 4-3-050G5b, landslide hazard areas can be defined as: i. Low Landslide Hazard (LL): Areas with slopes less than 15 percent. ii. Medium Landslide Hazard (LM): Areas with slopes between 15 percent and 40 percent, underlain by soils that consist largely of sand, gravel, or glacial till. iii. High Landslide Hazards (LH): Areas with slopes greater than 40 percent and areas with slopes between 15 percent and 40 percent underlain by soils consisting largely of silt and clay. iv. Very High Landslide Hazards (LV): Areas of known mapped or identified landslide deposits. The referenced City Critical Areas Map designates a landslide hazard on the eastern slope bordering Interstate 405. In our opinion, given that the slope is largely vegetated with blackberries, trees, and other shrubs; has a slope of about 31 percent; and is outside of the proposed development area, the landslide hazard may be characterized as low to medium. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 5 Earth Solutions NW, LLC Regulated Slopes According to RMC 4-3-050G5a, steep slopes may be categorized into two types: i. Sensitive Slopes: A hillside, or portion thereof (excluding engineering retaining walls), characterized by: a. An average slope of 25 percent to less than 40 percent, as identified in the City Steep Slope Atlas or in a method approved by the City, or; b. An average slope of 40 percent or greater, with a vertical rise of less than 15 feet, as identified in the City Steep Slope Atlas or in a method approved by the City; c. Abutting an average slope of 25 percent to 40 percent, as identified in the City Steep Slope Atlas or in a method approved by the City. ii. Protected Slopes: A hillside, or portion thereof, characterized by an average slope of 40 percent or greater grade and having a minimum vertical rise of 15 feet, as identified in the City Steep Slope Atlas or in a method approved by the City. The referenced City Critical Areas Map designates portions of the eastern slope both as sensitive and protected slopes. Because the eastern slope is located outside of and well away from the anticipated grading and construction activities, it is our opinion that the designations of sensitive and/or protected slopes on site should not impact the proposed short plat from a geotechnical standpoint. Seismic Hazard Per RMC 4-3-050G5d, seismic hazard areas can be defined as: i. Low Seismic Hazard (SL): Areas underlain by dense soils or bedrock. These soils generally have site classifications of A through D, as defined in the 2012 International Building Code (2012 IBC). ii. High Seismic Hazard (SH): Areas underlain by soft or loose, saturated soils. These soils generally have site classifications E or F, as defined in the 2012 IBC. The referenced City Critical Areas Map designates a seismic hazard area approximately 260 feet to the east of the subject site. Based on the conditions observed during our subsurface exploration and the lack of an established, shallow groundwater table, it is our opinion that the seismic hazard on site is low. The referenced Liquefaction Susceptibility Map identifies low potential for liquefaction at the subject site. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 6 Earth Solutions NW, LLC Analysis of Proposal ESNW should be contacted to review both the preliminary and final project plans to further evaluate the proposed construction and any potential impacts to geologically hazardous areas. If more significant grading activities or larger-than-anticipated residential structures are proposed, ESNW should reevaluate the potential impacts to the adjacent hazard areas. DISCUSSION AND RECOMMENDATIONS General Based on the results of our investigation, construction of the proposed short plat is feasible from a geotechnical standpoint. The primary geotechnical considerations associated with the proposed development include site preparation and earthwork, suitability of on-site soil as structural fill, subgrade preparation, temporary excavations, building foundations, retaining walls, drainage, and infiltration feasibility. Site Preparation and Earthwork Site preparation activities should consist of installing temporary erosion control measures and performing site stripping within the designated clearing limits. Subsequent earthwork activities may involve infrastructure and utility installations. Temporary Erosion Control Erosion control measures should conform to the standards and requirements of the Washington State Department of Ecology, King County, and City, where applicable. Please refer to the Erosion Hazard section of this report for a more detailed discussion on recommended temporary erosion and sediment control measures during construction. In-Situ and Imported Soil The majority of the soils encountered during our subsurface exploration have a moderate to high sensitivity to moisture and were generally in a wet condition at the time of exploration. The soils anticipated to be exposed at this site will degrade if exposed to wet weather and construction traffic. Compaction of the soils to the levels necessary for use as structural fill may be difficult or impossible during wet weather conditions. Soils encountered during site excavations that are excessively over the optimum moisture content will likely require aeration or treatment prior to placement and compaction. Conversely, soils that are substantially below the optimum moisture content will require moisture conditioning through the addition of water prior to use as structural fill. An ESNW representative should determine the suitability of in-situ soils for use as structural fill at the time of construction. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 7 Earth Solutions NW, LLC Imported soil intended for use as structural fill should be evaluated by ESNW during construction. The imported soil must be workable to the optimum moisture content, as determined by the Modified Proctor Method (ASTM D1557), at the time of placement and compaction. During wet weather conditions, imported soil intended for use as structural fill should consist of a well-graded, granular soil with a fines content of 5 percent or less (where the fines content is defined as the percent passing the Number 200 sieve, based on the minus three-quarter-inch fraction). Structural Fill Structural fill is defined as compacted soil placed in foundation, slab-on-grade, roadway, permanent slope, retaining wall, and utility trench backfill areas. Structural fill placed and compacted during site grading activities should meet the following specifications and guidelines:  Structural fill material Granular soil*  Moisture content At or slightly above optimum**  Relative compaction (minimum) 95 percent (Modified Proctor)  Loose lift thickness (maximum) 12 inches * The existing soil may not be suitable for use as structural fill unless the soil is at (or slightly above) the optimum moisture content at the time of placement and compaction ** Soil shall not be placed dry of optimum and should be evaluated by ESNW during construction With respect to underground utility installations and backfill, local jurisdictions may dictate the soil type(s) and compaction requirements. Areas of otherwise unsuitable material and debris should be removed from structural areas and replaced with structural fill. Subgrade Preparation Following site stripping, ESNW should observe the subgrade to confirm soil conditions are as anticipated and to provide supplementary recommendations for subgrade preparation as necessary. In general, foundation subgrade surfaces should be compacted in situ to a minimum depth of one foot below the design subgrade elevation. Uniform compaction of the foundation and slab subgrade areas will establish a relatively consistent subgrade condition below the foundation and slab elements. Supplementary recommendations for subgrade improvement may be provided at the time of construction and would likely include further mechanical compaction or overexcavation and replacement with suitable structural fill. Excavations and Slopes Excavation activities on site are likely to expose loose to medium dense native soil beginning at depths of approximately two to four feet bgs. Based on the soil conditions observed at the subsurface exploration locations, the following maximum allowable temporary slope inclinations may be used. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 8 Earth Solutions NW, LLC The applicable Federal Occupation Safety and Health Administration and Washington Industrial Safety and Health Act soil classifications are also provided:  Areas exposing groundwater seepage 1.5H:1V (Type C)  Loose soil; fill 1.5H:1V (Type C)  Medium dense to dense native soil 1H:1V (Type B) Permanent slopes should be planted with vegetation to both enhance stability and minimize erosion and should maintain a gradient of 2H:1V or flatter. The presence of perched groundwater may cause localized sloughing of temporary slopes. An ESNW representative should observe temporary and permanent slopes to confirm the slope inclinations are suitable for the exposed soil conditions and to provide additional excavation and slope recommendations as necessary. If the recommended temporary slope inclinations cannot be achieved, temporary shoring may be necessary to support excavations. Foundations The proposed residential structures can be supported on conventional spread and continuous footings bearing on undisturbed competent native soil, compacted native soil, or new structural fill. We anticipate competent native soil suitable for support of foundations will be encountered at a depth of about two to four feet below existing grades. Where loose or unsuitable soil conditions are encountered at foundation subgrade elevations, compaction of the soils to the specifications of structural fill or overexcavation and replacement with suitable structural fill will likely be necessary. Provided the structure will be supported as described above, the following parameters may be used for design of the new foundations:  Allowable soil bearing capacity 2,500 psf  Passive earth pressure 300 pcf  Coefficient of friction 0.40 A one-third increase in the allowable soil bearing capacity can be assumed for short-term wind and seismic loading conditions. The passive earth pressure and coefficient of friction values include a safety factor of 1.5. With structural loading as expected, total settlement in the range of one inch is anticipated, with differential settlement of about one-half inch. The majority of the settlement should occur during construction when dead loads are applied. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 9 Earth Solutions NW, LLC Seismic Design The 2015 IBC recognizes the American Society of Civil Engineers (ASCE) for seismic site class definitions. In accordance with Table 20.3-1 of the ASCE Minimum Design Loads for Buildings and Other Structures Manual, Site Class D should be used for design. Please refer to the Seismic Hazard section of this report for an opinion of the site-specific seismic hazard. Slab-on-Grade Floors Slab-on-grade floors should be supported on a firm and unyielding subgrade consisting of competent native soil or at least 12 inches of new structural fill. Unstable or yielding areas of the subgrade should be recompacted or overexcavated and replaced with suitable structural fill prior to slab construction. A capillary break consisting of a minimum of four inches of free-draining crushed rock or gravel should be placed below the slab. The free-draining material should have a fines content of 5 percent or less defined as the percent passing the number 200 sieve, based on the minus three- quarters-inch fraction. In areas where slab moisture is undesirable, installation of a vapor barrier below the slab should be considered. If used, the vapor barrier should consist of a material specifically designed to function as a vapor barrier and should be installed in accordance with the manufacturer’s specifications. Retaining Walls Retaining walls must be designed to resist earth pressures and applicable surcharge loads. The following parameters may be used for retaining wall design:  Active earth pressure (unrestrained condition) 35 pcf  At-rest earth pressure (restrained condition) 55 pcf  Traffic surcharge (passenger vehicles) 70 psf (rectangular distribution)  Passive earth pressure 300 pcf  Coefficient of friction 0.40  Seismic surcharge 6H psf* * Where H equals the retained height (in feet) The passive earth pressure and coefficient of friction values include a safety factor of 1.5. Additional surcharge loading from adjacent foundations, sloped backfill, or other loads should be included in the retaining wall design. Drainage should be provided behind retaining walls such that hydrostatic pressures do not develop. If drainage is not provided, hydrostatic pressures should be included in the wall design. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 10 Earth Solutions NW, LLC Retaining walls should be backfilled with free-draining material that extends along the height of the wall and a distance of at least 18 inches behind the wall. The upper 12 inches of the wall backfill may consist of a less permeable soil, if desired. A perforated drainpipe should be placed along the base of the wall and connected to an approved discharge location. A typical retaining wall drainage detail is provided on Plate 3. If drainage is not provided, hydrostatic pressures should be included in the wall design. Drainage Discrete zones of perched groundwater seepage should be anticipated in site excavations depending on the time of year grading operations take place. Temporary measures to control surface water runoff and groundwater during construction would likely involve interceptor trenches, interceptor swales, and sumps. ESNW should be consulted during preliminary grading to both identify areas of seepage and provide recommendations to reduce the potential for seepage-related instability. Finish grades must be designed to direct surface drain water away from structures and slopes. Water must not be allowed to pond adjacent to structures or slopes. In our opinion, foundation drains should be installed along building perimeter footings. A typical foundation drain detail is provided on Plate 4. Infiltration Evaluation In accordance with the 2017 City of Renton Surface Water Design Manual (2017 RSWDM), a small-scale Pilot Infiltration Test (PIT) was completed at test pit location TP-2. The PIT was completed by excavating a roughly three-foot by four-foot (infiltration surface) test pit to a depth of about three and one-half feet bgs and following the prescribed PIT procedure as outlined in the 2017 RSWDM. Based on the results of the PIT, the following design parameters are recommended:  Measured infiltration rate 10 inches per hour (iph)  Total correction factor 0.5  Design infiltration rate 5 iph The correction factor of 0.5 was selected based on the guidelines provided in the 2017 RSWDM. In our opinion, a correction factor of 0.5 is suitable for the observed conditions at the testing location. At this time, the design infiltration rate of 5 iph is applicable only at the location of TP-2 and only at the infiltration test depth. ESNW should be contacted to review stormwater management plans if infiltration is used for design. Supplementary recommendations and/or testing may be necessary depending on the size, depth, and siting of infiltration facilities. Mr. Huy Nguyen ES-7126 March 12, 2020 Page 11 Earth Solutions NW, LLC Utility Support and Trench Backfill The soils observed at the subsurface exploration locations are generally suitable for support of utilities. Use of the native soil as structural backfill in the utility trench excavations will depend on the in-situ moisture content at the time of placement and compaction. If native soil is placed below the optimum moisture content, settlement will likely occur once wet weather impacts the trenches. Native soil will be difficult or impossible to use as utility trench backfill during wet weather conditions. Moisture conditioning or treatment of the soils may be necessary at some locations prior to use as structural fill. Utility trench backfill should be placed and compacted to the specifications of structural fill provided in this report or to the applicable requirements of the presiding jurisdiction. LIMITATIONS This study has been prepared for the exclusive use of Mr. Huy Nguyen and his representatives. The recommendations and conclusions provided in the geotechnical engineering study are professional opinions consistent with the level of care and skill that is typical of other members in the profession currently practicing under similar conditions in this area. A warranty is not expressed or implied. Variations in the soil and groundwater conditions observed at the test locations may exist and may not become evident until construction. ESNW should reevaluate the conclusions provided in this geotechnical engineering study if variations are encountered. Additional Services ESNW should have an opportunity to review the final design with respect to the geotechnical recommendations provided in this report. ESNW should also be retained to provided testing and consultation services during construction. Drwn. MRS Checked BCS Date Mar. 2020 Date 03/13/2020 Proj. No. 7126 Plate 1 Earth Solutions NWLLC Geotechnical Engineering,Construction EarthSolutionsNWLLC EarthSolutionsNWLLC Observation/Testing and Environmental Services Vicinity Map Meadow Avenue Short Plat Renton, Washington Reference: King County, Washington OpenStreetMap.org NORTH NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. SITE Newcastle Renton Drwn. MRS Checked BCS Date Mar. 2020 Date 03/13/2020 Proj. No. 7126 Plate 2 Earth Solutions NWLLC Geotechnical Engineering,Construction EarthSolutionsNWLLC EarthSolutionsNWLLC Observation/Testing and Environmental Services Test Pit Location Plan Meadow Avenue Short Plat Renton, Washington NORTH 0 30 60 120 Scale in Feet 1"=60' NOTE: This plate may contain areas of color. ESNW cannot be responsible for any subsequent misinterpretation of the information resulting from black & white reproductions of this plate. NOTE: The graphics shown on this plate are not intended for design purposes or precise scale measurements, but only to illustrate the approximate test locations relative to the approximate locations of existing and / or proposed site features. The information illustrated is largely based on data provided by the client at the time of our study. ESNW cannot be responsible for subsequent design changes or interpretation of the data by others. LEGEND Approximate Location of ESNW Test Pit, Proj. No. ES-7126, Feb. 2020 Subject Site Existing Building Proposed Lot Letter A B C Sound Barrier Wall TP-1 TP-2 TP-3 TP-4 TP-5 TP-1 130 130MEADOW AVENUE N.SR-405A N. 38TH STREET Drwn. MRS Checked BCS Date Mar. 2020 Date 03/13/2020 Proj. No. 7126 Plate 3 Earth Solutions NWLLCEarthSolutionsNWLLC EarthSolutionsNWLLC Geotechnical Engineering,Construction Observation/Testing and Environmental Services Retaining Wall Drainage Detail Meadow Avenue Short Plat Renton, Washington NOTES: Free-draining Backfill should consist of soil having less than 5 percent fines. Percent passing No. 4 sieve should be 25 to 75 percent. Sheet Drain may be feasible in lieu of Free-draining Backfill, per ESNW recommendations. Drain Pipe should consist of perforated, rigid PVC Pipe surrounded with 1-inch Drain Rock. LEGEND: Free-draining Structural Backfill 1-inch Drain Rock 18" Min. Structural Fill Perforated Rigid Drain Pipe (Surround in Drain Rock) SCHEMATIC ONLY - NOT TO SCALE NOT A CONSTRUCTION DRAWING Drwn. MRS Checked BCS Date Mar. 2020 Date 03/13/2020 Proj. No. 7126 Plate 4 Earth Solutions NWLLC Geotechnical Engineering,Construction Observation/Testing and Environmental Services EarthSolutionsNWLLC EarthSolutionsNWLLC Footing Drain Detail Meadow Avenue Short Plat Renton, Washington Slope Perforated Rigid Drain Pipe (Surround in Drain Rock) 18" Min. NOTES: Do NOT tie roof downspouts to Footing Drain. Surface Seal to consist of 12" of less permeable, suitable soil. Slope away from building. LEGEND: Surface Seal: native soil or other low-permeability material. 1-inch Drain Rock SCHEMATIC ONLY - NOT TO SCALE NOT A CONSTRUCTION DRAWING Earth Solutions NW, LLC Appendix A Subsurface Exploration Test Pit Logs ES-7126 Subsurface conditions on site were explored on February 17, 2020, by excavating five test pits using a mini-trackhoe and operator retained by our firm. The approximate locations of the test pits are illustrated on Plate 2 of this study. The test pit logs are provided in this Appendix. The test pits were advanced to a maximum depth of about nine feet bgs. The final logs represent the interpretations of the field logs and the results of laboratory analyses. The stratification lines on the logs represent the approximate boundaries between soil types. In actuality, the transitions may be more gradual. GRAVEL AND GRAVELLYSOILS CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES WELL-GRADED SANDS, GRAVELLYSANDS, LITTLE OR NO FINES POORLY-GRADED SANDS,GRAVELLY SAND, LITTLE OR NO FINES SILTY SANDS, SAND - SILTMIXTURES CLAYEY SANDS, SAND - CLAYMIXTURES INORGANIC SILTS AND VERY FINESANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEYSILTS WITH SLIGHT PLASTICITY INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLYCLAYS, SANDY CLAYS, SILTY CLAYS,LEAN CLAYS ORGANIC SILTS AND ORGANICSILTY CLAYS OF LOW PLASTICITY INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND ORSILTY SOILS INORGANIC CLAYS OF HIGHPLASTICITY SILTSANDCLAYS MORE THAN 50% OF MATERIAL ISLARGER THANNO. 200 SIEVE SIZE MORE THAN 50%OF MATERIAL IS SMALLER THANNO. 200 SIEVESIZE MORE THAN 50%OF COARSEFRACTION PASSING ON NO.4 SIEVE MORE THAN 50%OF COARSEFRACTION RETAINED ON NO.4 SIEVE SOIL CLASSIFICATION CHART (APPRECIABLEAMOUNT OF FINES) (APPRECIABLE AMOUNT OF FINES) (LITTLE OR NO FINES) FINEGRAINEDSOILS SAND AND SANDY SOILS SILTS AND CLAYS ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY, ORGANIC SILTS PEAT, HUMUS, SWAMP SOILS WITHHIGH ORGANIC CONTENTS LETTERGRAPH SYMBOLSMAJOR DIVISIONS COARSE GRAINEDSOILS TYPICAL DESCRIPTIONS WELL-GRADED GRAVELS, GRAVEL -SAND MIXTURES, LITTLE OR NO FINES POORLY-GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLEOR NO FINES SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES CLEANGRAVELS GRAVELS WITH FINES CLEAN SANDS (LITTLE OR NO FINES) SANDS WITH FINES LIQUID LIMITLESS THAN 50 LIQUID LIMITGREATER THAN 50 HIGHLY ORGANIC SOILS DUAL SYMBOLS are used to indicate borderline soil classifications. The discussion in the text of this report is necessary for a proper understanding of the nature of the material presented in the attached logs. GW GP GM GC SW SP SM SC ML CL OL MH CH OH PT Earth Solutions NW LLC 127.5 126.5 123.0 122.0 MC = 16.20% MC = 18.40% MC = 2.90% Fines = 1.40% MC = 21.90% SM TPSL SP SM Brown silty SAND, medium dense, moist (Fill) Relic TOPSOIL Horizon Tan poorly graded SAND with gravel, medium dense, moist [USDA Classification: very gravelly coarse SAND] Tan silty SAND, medium dense, moist Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 2.5 3.5 7.0 8.0 NOTES Surface Conditions: gravel GROUND ELEVATION 130 ft LOGGED BY BCS EXCAVATION METHOD TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: CHECKED BY KDH DATE STARTED 2/17/20 COMPLETED 2/17/20 AT TIME OF EXCAVATION --- AT END OF EXCAVATION --- AFTER EXCAVATION ---SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-1 PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GENERAL BH / TP / WELL - 7126.GPJ - GINT STD US.GDT - 3/16/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TESTS U.S.C.S.MATERIAL DESCRIPTION GRAPHICLOG 128.5 127.5 127.0 125.0 120.5 MC = 11.00% MC = 20.50%Fines = 33.20% MC = 27.70% Fines = 8.20% MC = 19.20% MC = 14.70%Fines = 5.10% TPSL SM TPSL SM SP-SM Dark brown TOPSOIL, root intrusions to 5' (Fill) Tan silty SAND, medium dense, moist (Fill) Relic TOPSOIL Horizon Tan silty SAND, medium dense, moist -slight caving to BOH -infiltration test, [USDA Classification: slightly gravelly very fine sandy LOAM] Gray poorly graded SAND with silt, medium dense, wet [USDA Classification: slightly gravelly SAND] -weak iron oxide staining to 8' -becomes poorly graded sand with gravel [USDA Classification: gravelly coarse SAND] Test pit terminated at 8.5 feet below existing grade. No groundwater encountered duringexcavation. Caving observed from 3.0 feet to BOH. 0.5 1.5 2.0 4.0 8.5 NOTES Depth of Topsoil & Sod 6": grass GROUND ELEVATION 129 ft LOGGED BY BCS EXCAVATION METHOD TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: CHECKED BY KDH DATE STARTED 2/17/20 COMPLETED 2/17/20 AT TIME OF EXCAVATION --- AT END OF EXCAVATION --- AFTER EXCAVATION ---SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-2 PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GENERAL BH / TP / WELL - 7126.GPJ - GINT STD US.GDT - 3/16/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TESTS U.S.C.S.MATERIAL DESCRIPTION GRAPHICLOG 129.5 127.0 126.0 121.5 MC = 15.20% MC = 11.40% Fines = 15.20% MC = 22.90% MC = 23.10% TPSL SM TPSL SM Dark brown TOPSOIL (Fill) Brown silty SAND, medium dense, moist (Fill) Relic TOPSOIL Horizon -charcoal fragments Tan silty SAND, medium dense, moist -weak iron oxide staining -at 4' [USDA Classification: slightly gravelly loamy SAND] -becomes gray, light groundwater seepage at 8.5' Test pit terminated at 8.5 feet below existing grade. Groundwater seepage encountered at8.5 feet during excavation. No caving observed. 0.5 3.0 4.0 8.5 NOTES Depth of Topsoil & Sod 6": grass GROUND ELEVATION 130 ft LOGGED BY BCS EXCAVATION METHOD TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: CHECKED BY KDH DATE STARTED 2/17/20 COMPLETED 2/17/20 AT TIME OF EXCAVATION --- AT END OF EXCAVATION --- AFTER EXCAVATION ---SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-3 PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GENERAL BH / TP / WELL - 7126.GPJ - GINT STD US.GDT - 3/16/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TESTS U.S.C.S.MATERIAL DESCRIPTION GRAPHICLOG 125.5 121.0 MC = 22.60% MC = 24.30% MC = 18.40% SM SM Tan silty SAND, medium dense, moist (Fill) -becomes brown -interbedded dark brown TOPSOIL in fill -rebar, plastic bags, and wood debris Gray silty SAND, medium dense, moist -weak iron oxide staining -light groundwater seepage Test pit terminated at 9.0 below existing grade. Groundwater seepage encountered at 5.5feet during excavation. Caving observed from 1.5 to 5.0 feet. 4.5 9.0 NOTES Surface Conditions: gravel GROUND ELEVATION 130 ft LOGGED BY BCS EXCAVATION METHOD TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: CHECKED BY KDH DATE STARTED 2/17/20 COMPLETED 2/17/20 AT TIME OF EXCAVATION --- AT END OF EXCAVATION --- AFTER EXCAVATION ---SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-4 PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GENERAL BH / TP / WELL - 7126.GPJ - GINT STD US.GDT - 3/16/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TESTS U.S.C.S.MATERIAL DESCRIPTION GRAPHICLOG 125.5 124.5 120.0 MC = 10.90% MC = 6.60% Fines = 7.80% MC = 45.40% TPSL SM SP- SM Dark brown TOPSOIL -root intrusions to 4' Tan silty SAND, medium dense, moist Gray poorly graded SAND with silt, medium dense, moist [USDA Classification: gravelly SAND] -8" tan silty sand lens -becomes wet Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 2.5 3.5 8.0 NOTES Depth of Topsoil & Sod 30": grass GROUND ELEVATION 128 ft LOGGED BY BCS EXCAVATION METHOD TEST PIT SIZE EXCAVATION CONTRACTOR NW Excavating GROUND WATER LEVELS: CHECKED BY KDH DATE STARTED 2/17/20 COMPLETED 2/17/20 AT TIME OF EXCAVATION --- AT END OF EXCAVATION --- AFTER EXCAVATION ---SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-5 PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GENERAL BH / TP / WELL - 7126.GPJ - GINT STD US.GDT - 3/16/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 TESTS U.S.C.S.MATERIAL DESCRIPTION GRAPHICLOG Earth Solutions NW, LLC Appendix B Laboratory Test Results ES-7126 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 0.0010.010.1110100 3 D100 140 Specimen Identification 1 fine 6 HYDROMETER304 1.4 33.2 8.2 5.1 15.2 101/2 COBBLES Specimen Identification 4 coarse 20 401.5 8 14 USDA: Tan Very Gravelly Coarse Sand. USCS: SP with Gravel. USDA: Tan Slightly Gravelly Very Fine Sandy Loam. USCS: SM. USDA: Gray Slightly Gravelly Sand. USCS: SP-SM. USDA: Gray Gravelly Coarse Sand. USCS: SP-SM with Gravel. USDA: Tan Slightly Gravelly Loamy Sand. USCS: SM. 6 60 PERCENT FINER BY WEIGHTD10 0.526 0.174 0.399 0.111 7.406 0.137 0.259 1.314 0.253 GRAIN SIZE DISTRIBUTION 100 36.98 3.06 8.55 LL TP-01 TP-02 TP-02 TP-02 TP-03 0.2 0.085 0.154 3/4U.S. SIEVE OPENING IN INCHES U.S. SIEVE NUMBERS GRAVEL SAND 37.5 9.5 9.5 19 9.5 %Silt 0.19 1.38 0.79 TP-01 TP-02 TP-02 TP-02 TP-03 2 2003 Cc CuClassification %Clay 16 PID60 D30 coarse SILT OR CLAYfinemedium GRAIN SIZE IN MILLIMETERS 3/8 50 5.0ft. 3.5ft. 5.0ft. 8.5ft. 4.0ft. 5.00ft. 3.50ft. 5.00ft. 8.50ft. 4.00ft. PL PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GRAIN SIZE USDA ES-7126 MEADOW AVENUE SHORT PLAT.GPJ GINT US LAB.GDT 3/10/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 0.0010.010.1110100 3 D100 140 Specimen Identification 1 fine 6 HYDROMETER304 7.8 101/2 COBBLES Specimen Identification 4 coarse 20 401.5 8 14 USDA: Gray Gravelly Sand. USCS: SP-SM. 6 60 PERCENT FINER BY WEIGHTD10 0.2650.664 GRAIN SIZE DISTRIBUTION 100 7.71 LL TP-05 0.086 3/4U.S. SIEVE OPENING IN INCHES U.S. SIEVE NUMBERS GRAVEL SAND 19 %Silt 1.23 TP-05 2 2003 Cc CuClassification %Clay 16 PID60 D30 coarse SILT OR CLAYfinemedium GRAIN SIZE IN MILLIMETERS 3/8 50 4.0ft. 4.00ft. PL PROJECT NUMBER ES-7126 PROJECT NAME Meadow Avenue Short Plat GRAIN SIZE USDA ES-7126 MEADOW AVENUE SHORT PLAT.GPJ GINT US LAB.GDT 3/10/20Earth Solutions NW, LLC 15365 N.E. 90th Street, Suite 100 Redmond, Washington 98052 Telephone: 425-449-4704 Fax: 425-449-4711 Earth Solutions NW, LLC Report Distribution ES-7126 EMAIL ONLY Mr. Huy Nguyen 15400 Southeast 155th Place, Unit 99 Renton, Washington 98058 Appendix B WWHM Output WWHM2012 PROJECT REPORT 20591 WWHM 1/27/2021 6:46:41 PM Page 2 General Model Information Project Name:20591 WWHM Site Name: Site Address: City: Report Date:1/27/2021 Gage:Seatac Data Start:1948/10/01 Data End:2009/09/30 Timestep:15 Minute Precip Scale:1.000 Version Date:2019/09/13 Version:4.2.17 POC Thresholds Low Flow Threshold for POC1:50 Percent of the 2 Year High Flow Threshold for POC1:50 Year 20591 WWHM 1/27/2021 6:46:41 PM Page 3 Landuse Basin Data Predeveloped Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre A B, Lawn, Flat 0.322 Pervious Total 0.322 Impervious Land Use acre ROADS FLAT 0.01 ROOF TOPS FLAT 0.046 DRIVEWAYS FLAT 0.232 SIDEWALKS FLAT 0.017 Impervious Total 0.305 Basin Total 0.627 Element Flows To: Surface Interflow Groundwater 20591 WWHM 1/27/2021 6:46:41 PM Page 4 Mitigated Land Use Basin 1 Bypass:No GroundWater:No Pervious Land Use acre A B, Pasture, Flat 0.272 Pervious Total 0.272 Impervious Land Use acre ROADS FLAT 0.011 DRIVEWAYS FLAT 0.018 SIDEWALKS FLAT 0.019 Impervious Total 0.048 Basin Total 0.32 Element Flows To: Surface Interflow Groundwater 20591 WWHM 1/27/2021 6:46:41 PM Page 5 Routing Elements Predeveloped Routing 20591 WWHM 1/27/2021 6:46:41 PM Page 6 Mitigated Routing 20591 WWHM 1/27/2021 6:46:41 PM Page 7 Analysis Results POC 1 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.322 Total Impervious Area:0.305 Mitigated Landuse Totals for POC #1 Total Pervious Area:0.272 Total Impervious Area:0.048 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.117712 5 year 0.152554 10 year 0.177699 25 year 0.211909 50 year 0.239222 100 year 0.268153 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.018418 5 year 0.023514 10 year 0.027077 25 year 0.031804 50 year 0.035494 100 year 0.039335 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.152 0.024 1950 0.163 0.026 1951 0.104 0.016 1952 0.084 0.013 1953 0.090 0.014 1954 0.098 0.015 1955 0.107 0.017 1956 0.106 0.017 1957 0.120 0.019 1958 0.097 0.015 20591 WWHM 1/27/2021 6:47:13 PM Page 8 1959 0.099 0.016 1960 0.097 0.015 1961 0.102 0.016 1962 0.089 0.014 1963 0.101 0.016 1964 0.097 0.015 1965 0.126 0.019 1966 0.083 0.013 1967 0.149 0.022 1968 0.162 0.025 1969 0.112 0.018 1970 0.108 0.017 1971 0.129 0.020 1972 0.154 0.021 1973 0.081 0.013 1974 0.118 0.019 1975 0.136 0.021 1976 0.092 0.014 1977 0.099 0.016 1978 0.121 0.019 1979 0.166 0.026 1980 0.149 0.024 1981 0.122 0.019 1982 0.171 0.027 1983 0.139 0.022 1984 0.088 0.014 1985 0.121 0.019 1986 0.105 0.017 1987 0.162 0.026 1988 0.099 0.015 1989 0.123 0.019 1990 0.272 0.033 1991 0.183 0.026 1992 0.087 0.014 1993 0.075 0.012 1994 0.082 0.013 1995 0.108 0.017 1996 0.131 0.022 1997 0.119 0.018 1998 0.113 0.018 1999 0.231 0.036 2000 0.115 0.018 2001 0.126 0.020 2002 0.147 0.023 2003 0.116 0.018 2004 0.216 0.034 2005 0.099 0.016 2006 0.094 0.014 2007 0.261 0.041 2008 0.176 0.026 2009 0.150 0.024 Ranked Annual Peaks Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.2721 0.0411 2 0.2607 0.0363 3 0.2310 0.0340 20591 WWHM 1/27/2021 6:47:14 PM Page 9 4 0.2161 0.0329 5 0.1825 0.0270 6 0.1764 0.0263 7 0.1714 0.0261 8 0.1656 0.0256 9 0.1628 0.0256 10 0.1621 0.0256 11 0.1616 0.0254 12 0.1538 0.0237 13 0.1521 0.0237 14 0.1503 0.0235 15 0.1495 0.0232 16 0.1486 0.0224 17 0.1473 0.0220 18 0.1395 0.0219 19 0.1358 0.0214 20 0.1312 0.0212 21 0.1292 0.0203 22 0.1263 0.0201 23 0.1258 0.0195 24 0.1230 0.0194 25 0.1215 0.0193 26 0.1212 0.0191 27 0.1210 0.0190 28 0.1198 0.0188 29 0.1191 0.0186 30 0.1179 0.0181 31 0.1164 0.0180 32 0.1154 0.0178 33 0.1129 0.0177 34 0.1123 0.0177 35 0.1084 0.0171 36 0.1078 0.0170 37 0.1072 0.0169 38 0.1056 0.0167 39 0.1051 0.0166 40 0.1036 0.0163 41 0.1023 0.0157 42 0.1008 0.0157 43 0.0990 0.0157 44 0.0989 0.0156 45 0.0986 0.0155 46 0.0985 0.0155 47 0.0981 0.0154 48 0.0972 0.0152 49 0.0971 0.0152 50 0.0966 0.0149 51 0.0945 0.0144 52 0.0920 0.0143 53 0.0904 0.0142 54 0.0891 0.0138 55 0.0882 0.0138 56 0.0871 0.0137 57 0.0839 0.0132 58 0.0827 0.0130 59 0.0823 0.0129 60 0.0808 0.0129 61 0.0755 0.0120 Appendix C Arborist Report/Tree Plan by Layton Tree Consulting, LLC dated February 12, 2020