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21-104105-TIR-10-06-2021-V1 TWIN TRAILS Plat Preliminary TIR & Downstream Analysis October 5, 2021 Prepared for Prospect Development Puyallup, WA 98372 (206) 276-7526 Submitted by ESM Consulting Engineers, LLC 33400 8th Avenue S, Suite 205 Federal Way, WA 98003 253.838.6113 tel 253.838.7104 fax www.esmcivil.com 10/05/2021 Table of Contents Section 1. Project Overview Section 2. Conditions and Requirements Summary Section 3. Off-Site Analysis Section 4. Flow Control & Water Quality Facility Analysis and Design Section 5. Conveyance System Analysis and Design Section 6. Special Reports and Studies Section 7. Other Permits Section 8. ESC Analysis and Design Section 9. Bond Quantities, Facility Summaries, & Declaration of Covenant Section 10. Operations and Maintenance Manual List of Figures 1.1 TIR Worksheet 1.2 Site Location 1.3A Existing Drainage Basin 1.3B Developed Drainage Basin 1.4 Soils Map 3.1 303d Polluted Waters 3.2 Downstream Study Area 3.3A Downstream Corridor 3.3B Downstream Corridor Appendix Appendix A Hydrology Model Output Appendix B Geotechnical Study \\esm8\engr\esm-jobs\1316\003\020\stormreport\rprt - stormreport.docx Plat of Twin Trails Preliminary TIR Page 2 October 5, 2021 ESM Consulting Engineers 1. Project Overview The purpose of this report is to encapsulate the documents and analysis required by the Drainage Review as defined by FWRC 16.25.020 in accordance with the 2016 King County Surface Water Design Manual (KCSWDM) and the 2016 Federal Way Addendum to the King County Washington Surface Water Design Manual (Federal Way Addendum), the King County Stormwater Prevention Manual (KCSPPM), and the latest edition of the LID Technical Guidance Manual for the Puget Sound (LID Manual), for the proposed 38 Lot single family plat. Existing Site: The proposed Twin Trails project site is located in Section 24, Township 21 N, Range 03 E, W.M., situated on two separate parcels, Parcel A of an ongoing BLA with the adjacent WSDOT park and ride and Parcel 2421039051. The site is south of 341st Place and north of SW 344th St and split down the center by 19th Ave SW, a public Right-of-Way. Both parcels are within the Residential Comprehensive Plan Designation and zoned Single Family Residential (RS-7.2). Parcel A (7978200085): Parcel A, [after ongoing BLA]: 295,435 sf (6.782 ac) is located at SW 344th St & 21st Ave SW (Twin Lakes Park & Ride). The future property will be rectangular in shape, approximately 667’ x 443’. Adjacent to Parcel A is S SW 344th Street to the south, the Twin Lakes WSDOT Park & Ride to the west, and undeveloped parcel to the east, and the Saghalie Firs plat to the north as well the second project parcel Parcel A is currently undeveloped, and access is provided off of S 344th St from the south. Across the property is a dense scattering of evergreen and deciduous trees with a vertical relief of approximately 20 feet over slopes average 3.5 percent (ranging from 1 to up 15 percent in isolated areas) from the southeast down to the northwest. A Lakehaven sewer main is located down through the center of the property running south- north from SW 341st down to the north through to Saghalie Firs. The sewer main alignment is set within the future 19th Ave SW right-of-way. Parcel (7978200086): The second parcel is 108,900 sf (2.50 ac) located at 1605 SW 341st Pl. The property is rectangular in shape, approximately 410’ x 265’. Adjacent to this parcel is 19th Ave SW/Saghalie Firs to the west, an undeveloped parcel to the east, and SW 341st Place to the north, and Parcel A to the south. This property has been previously developed with a single-family residence built in 1949 and a detached garage built in 1980 with gravel access off of SW 341st Place. Both structures will be demolished as a part of the plat improvements. A majority of the parcel has been cleared from past improvements and consists of landscaping and short grasses and a few scatterings of trees. Grades across the property range from 1 to 12 percent down from the southeast to the northwest corner and a vertical relief of approximately 11 feet. Refer to Figures 1.2 and 1.3A for a vicinity map and a visual representation of the existing site conditions. Proposed improvements: The plat infrastructure improvements include new public internal roadways, private alleys and tracts, existing public right-of-way improvements, associated utilities, and landscaping. The improvements slated to be built in a single phase. The new internal roadways will be constructed to a Type W standard section. The new 19th Ave SW extension to SW 344th Street will be construction to a Type ‘S’ standard section. Plat of Twin Trails Preliminary TIR Page 3 October 5, 2021 ESM Consulting Engineers Additionally, half street improvements will be provided along the adjacent existing rights-of- way. This includes the south half of SW 341st Pl to complete a Type ‘S’ road section, the east half of 19th Ave SW along Saghalie First to complete a Type ‘S’ road section, and the north half of SW 344th St to start a Type ‘K’ road section. Stormwater will be conveyed and collected by a series of pipes and catch basins. An onsite Best Management Practice (BMP), an above ground retention (infiltration) pond, is proposed to meet the flow control requirements conditioned with this project. The pond will retain most runoff generated from the project site with the exception of a small amount of bypass near the southwest area of the site. Additionally, the pond has been sized to manage the potential run- on from the wetlands (discussed below) east of the project site. A stormwater treatment unit is proposed to treat stormwater at a minimum to Basic treatment standard is also proposed to meet the City’s required level of treatment. See section 4 of this report for a further discussion of the existing and proposed hydrology and design details. Critical Areas Wetlands: A wetland and fish and wildlife habitat assessment report was completed by Soundview Consultants, dated September 21. In summary, using current wetland delineation methodology, no potentially regulated wetlands or fish and wildlife habitat conservation areas were identified on the subject property. However, three wetlands were identified offsite to the east that were previously delineated and assessed by Wetland Resources in 2020 associated with the Saghalie Heights projects. This includes two category III depressional wetland subject to an 80-foot buffer and a category IV depressional wetland with no buffer. None of the wetland buffers project on to the Twin Trails project site. Refer to Sound View Consultants assessment for further information included as a part of the plat application submittal package. Geotechnical: A geotechnical study has been completed by Earth Solutions NW, LLC (ESI) dated September 13, 2021. A total of eight exploration pits were dug across the project site including one at the location of the proposed infiltration pond (TP-1). See Figure 1.4 for the Soils Map provided by AESI for test pit locations. In summary, topsoil (where encountered) generally extended to depths ranging from about two to eight inches below ground surface. Underlying the topsoil, native soils were encountered consisting primarily of silty sand with gravel that extended to the maximum exploration depth of about 9 feet. At test pit locations TP-1 through TP-3, underlying about five to dix feet of silty sand, medium dense to dense outwash-type sand and gravel deposits that extended to the termination depth of the test pits was observed. The majority of the native soils encountered are consistent with ice contact deposits, and the sandy soils encountered below the glacially consolidated cap within test pit locations TP-1 through TP-3 are consistence with outwash deposits. The outwash soils may be suitable for infiltration pending in-situ infiltration testing. Based on textural analyses and using the DOE saturated hydraulic conductivity calculation, for feasibility considerations, a preliminary infiltration rate of 10 inches per hour is recommended. Plat of Twin Trails Preliminary TIR Page 4 October 5, 2021 ESM Consulting Engineers During explorations completed in May 2021, groundwater seepage was not encountered at the test pit location. However perched seepage should be expected within the weathered zone of soils on the site depending on the time of year grading occurs. Plat of Twin Trails Preliminary TIR Page 5 October 5, 2021 ESM Consulting Engineers Figure 1.1 – TIR Worksheet TIR Worksheet can be provided with the Final TIR as requested by the City. Plat of Twin Trails Preliminary TIR Page 6 October 5, 2021 ESM Consulting Engineers FIGURE 1.2 – VICINITY MAP Plat of Twin Trails Preliminary TIR Page 7 October 5, 2021 ESM Consulting Engineers L L CFederal Way, WA 9800333400 8th Ave S, Suite 205C O N S U L T I N G E N G I N E E R SEXISTING BASIN MAPTWIN TRAILSn SCALE: 1" = 0 125' 62.5 125 SW 344TH ST SW 341ST PL SW 342ND PL 19TH AVE SW18TH AVE SW15TH PL SWSW 344TH ST Plat of Twin Trails Preliminary TIR Page 8 October 5, 2021 ESM Consulting Engineers L L CFederal Way, WA 9800333400 8th Ave S, Suite 205C O N S U L T I N G E N G I N E E R SDEVELOPED BASIN MAPTWIN TRAILSn SCALE: 1" = 0 125' 62.5 125 SW 344TH ST SW 341ST PL SW 342ND PL 19TH AVE SW1 2 3 4 5678 9 10 11 12 13 14 15 16 17 18192021 2223 24 25 26 27 28 29 3031 32 33 34 35 36 3738 18TH AVE SW15TH PL SWSW 344TH ST Plat of Twin Trails Preliminary TIR Page 9 October 5, 2021 ESM Consulting Engineers MRS SSR 09/13/2021 7961 2TestPitLocationPlan TwinTrailsFederalWay,WashingtonNORTH NOT -TO -SCALE 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-7961,May 2021 Subject Site Existing Building Proposed Lot Number TP-1 19 S.W.341ST PLACE S.W.342ND PLACE S.W.344TH STREET 19THavenueS.W.road a road B 1 2 3 4 5678 9 10 11 12 13 14 15 16 17 18192021 2223 24 25 26 27 28 29 30 31 32 33 34 35 36 3738 Tract A Tract B Tract C Tract D Tract E Tract F340338336334Close-Up of Tract A with Pond TP-1 TP-2 TP-3 TP-4TP-5 TP-6 TP-7 TP-8 Plate Proj.No. Date Checked By Drwn.ByEarthSolutionsNWLLCGeotechnicalEngineering,ConstructionObservation/TestingandEnvironmentalServicesEarthSolutionsNWLLCEarthSolutionsNWLLCPlat of Twin Trails Preliminary TIR Page 10 October 5, 2021 ESM Consulting Engineers FIGURE 1.4 2. Conditions and Requirements Summary The proposed project is subject to Stormwater Review as a result of being subject to a City of Federal way development permit and results in more than 2,000 sf of new impervious surface according to FWRC 16.15.010. This review includes KCSWDM’s Small Project Drainage Requirements, Core Requirements 1-9, and Special Requirements 1-5. Review of the 9 Core Requirements and 5 Special Requirements This section describes how the project will meet the KCSWDM Core and Special Requirements. Core Requirement No. 1 Discharge at the Natural Location The project site, located on 2 parcels, is within one Threshold Discharge Area with two natural discharge locations. The site (both parcels) naturally drains down from the southeast to the northwest. Similarly, all runoff generated from the developed basin will continue to be directed the northwest to an infiltration/retention pond in the northwest corner of the site. The proposed infiltration pond will reduce the burden of the downstream stormwater network and reduce potential downstream flooding. However, if the infiltration pond were to fail, overflow will be directed toward the natural discharge location over overflow toward SW 341st Pl. Core Requirement No. 2 Off-site Analysis An off-site analysis has been documented in Section 3 of this report. Core Requirement No. 3 Flow Control The site is located within a Conservation Flow Control Area requiring at minimum Level 2 flow control standards per Section 1.2.3.1.B of the KCSWDM and the Federal Way Addendum Flow Control Application map. Additionally, the project has been conditioned to meet Level 3 flow control requirements as directed by the city due to documented capacity/flooding issues downstream. The flow control standard will be met through the use of an above ground infiltration pond. By retaining most all stormwater from the site’s target surfaces the developed (discharge) flow rates are greatly reduce where the flow control facility requirement can be waved for the threshold discharge area according to Section 1.2.3.1.C of the manual. Refer to Section 4.C of this report for further discussion of the flow control performance standards. Core Requirement No. 4 Conveyance System The new conveyance network will be analyzed and designed with sufficient capacity to convey and contain the 25-year peak flow. Events that exceed the 25-year event that may over top the system structure will be contained within the public right-of-way. Core Requirement No. 5 Erosion and Sediment Control Erosion and sediment controls to prevent the transport of sediment from the project site to downstream drainage facilities, water resources, and adjacent properties will be provided on the construction plans and discussed in the Erosion and Sediment Control Report. Core Requirement No. 6 Maintenance and Operations The Operations and Maintenance manual will be included with the final TIR as requested by the city. Core Requirement No. 7 Financial Guarantees and Liability All drainage facilities constructed or modified for projects will comply with the financial guarantee requirements as provided in the City of Federal Way Bond Quantities Worksheet. Bond Quantities will be provided in the final TIR as requested by the City. Plat of Twin Trails Preliminary TIR Page 11 October 5, 2021 ESM Consulting Engineers Core Requirement No. 8 Water Quality All projects with the city are initially subject to the Enhanced-Basic treatment standard unless otherwise exempt according to Section 1.2.8.1 of the SWDM upon which the treatment standard may be reduced to Basic. Basic treatment may be allowed for lower density developments resulting in less than 8 du/acre. The Twin Trails plat is zoned RS—7.2 resulting in approximately 6 dwelling units per acres. As a result, at a minimum, the site is subject to Basic Treatment. A stormwater treatment facility approved by the Washington State Department of Ecology for at least Basic treatment is proposed proceeding the retention pond. Refer to Section 4 of this report for further detail. Core Requirement No. 9 Flow Control BMP’s All applicable Flow Control BMP’s requirements are listed and discussed in Section 4.F of this report. Special Requirement No. 1 Other Adopted Area-Specific Requirements According to the city preapplication meeting notes, dated March 15, 2021, there is a known off-site, downstream stormwater conveyance restriction. It has been informed by the city that the applicant may choose to fix the conveyance restriction OR provide a Level 3 flow control pond on-site. To assist with addressing the downstream conveyance restriction, a stormwater retention pond is proposed store and infiltrate stormwater onsite. This is similar to the Lakehaven Estates project north of the proposed project. Other than the flow control area specific requirements, there are no known master drainage plans, basin plans, salmon conservation plans, stormwater compliance plans, flood hazard reduction plan updates, or shared facility drainage plans for this project. Special Requirement No. 2 Flood Hazard Area Delineation The site falls outside of the 500-year flood plain and located in an area of minimal flood hazard according to FEMA FIRM Map Number 53033C1250 G. Special Requirement No. 3 Flood Protection Facilities The developed project site is not protected by an existing flood protection facility. The proposed site improvements do not include the modification of an existing flood protection facility. Special Requirement No. 3 does not apply. Special Requirement No. 4 Source Control The site is a residential development and is not subject to this requirement. Special Requirement No. 4 does not apply. Special Requirement No. 5 Oil Control The project does not have a “high-use site” characteristic and is not a redevelopment of a high-use site. Special Requirement No. 5 does not apply. A “high-use site” is a commercial or industrial site that typically generates or is subject to runoff containing high concentrations of oil due to high traffic turnover, on-site vehicle or heavy or stationary equipment use, or the frequent transfer of liquid petroleum or coal derivative products. Plat of Twin Trails Preliminary TIR Page 12 October 5, 2021 ESM Consulting Engineers 3. Level 1 Off-Site Analysis This narrative is to provide a Level 1 Downstream Analysis for the proposed Twin Trails development per Core Requirement #2, Section 1.2.2 of the 2016 King County Surface Water Design Manual (SWDM). The analysis is to identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project. The primary component of this offsite analysis is the downstream corridor. The second component is to evaluate the upstream drainage system to verify any offsite run-on that may impact the project. The following Level 1 downstream analysis is a qualitative survey of the downstream system of the project site and is composed of the following four tasks: Task 1 – Define and map the study area Task 2 – Downstream Resource Review for 1-mile downstream Task 3 – Field Inspection Task 4 – Drainage System Description and Problem Descriptions Task 1: Study Area Definition and Maps The project site is located within the Hylebos Drainage basin, within the Central Puget Sound watershed which is a part of the Duwamish-Green water resource inventory area (WRIA #9). The presented drainage study area is approximately a one-mile-long path encompassing the site’s downstream corridor. Refer to Figure 3.2 for a map of the basic study area. The study area also includes a 1/4 mile downstream field investigation of stormwater released from the project site. See Figure 1.3A and Figure 3.3 for the Existing Site Conditions and a map of the downstream corridor. Task 2: Resource Review Flow Control Map According to the Federal Way Flow Control Applications Map, the project is located within the Conservation Flow Control Area and required to comply with Level 2 Flow Control Standards and has been additionally conditioned with meeting Level 3 Flow Control due to downstream conveyance restriction. Site Soils Refer to the project overview in Section 1 of this report. Federal Way Critical Areas Map Properties and streams along the 1-mile downstream study area and portions of the upstream basin were researched such that offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project can be identified and evaluated: The following items were investigated according to the available studies and the City’s 2016 Critical Areas Map. Refer to Figure 3.2 for further in this section for maps of environmental hazards (if any) within the 1-mile study area. • Drainage Studies ......................... None Mapped • Streams ....................................... None Mapped • Lakes ........................................... None Mapped • Potential Steep Slope Hazard ...... None Mapped Plat of Twin Trails Preliminary TIR Page 13 October 5, 2021 ESM Consulting Engineers • Erosion Hazard Area .................... None Mapped • Landslide Hazard Area................. None Mapped • FEMA Floodway or Floodplain ..... None Mapped 100 Year Floodplain The Federal emergency Management Agency prepared maps for all areas within the City of Federal Way. These maps can be found on the FEMA website. Panel #53033C1250G, effective 05/16/1995 depicts the areas, if any, subject to flooding in the vicinity of this project. By inspection of this map, the project is located in Zone X, which is designated as areas outside the 0.2 percent annual chance floodplain. The site falls outside of the 500-year flood plain and located in an area of minimal flood hazard according to FEMA FIRM Map Number 53033C1250 G. Wetlands A wetland and fish and wildlife habitat assessment report was completed by Sound View Consultants, dated September 21. In summary, using current wetland delineation methodology, no potentially regulated wetlands or fish and wildlife habitat conservation areas were identified on the subject property. However, three wetlands were identified offsite to the east that were previously delineated and assessed by Wetland Resources in 2020 associated with the Saghalie Heights projects. This includes two category III depressional wetland subject to an 80-foot buffer and a category IV depressional wetland with no buffer. None of the wetland buffers project on to the Twin Trails project site. Refer to Sound View Consultants assessment for further information included as a part of the plat application submittal package. Downstream Drainage Complaints Research of relevant drainage complaints within the downstream corridor is currently on going. When the data becomes available from the City, it will be added to this report as applicable. Water Quality Assessment The Department of Ecology Water Quality Assessment 303(d)/305(b) lists were reviewed to see if there are any known downstream water quality concerns. Waters whose beneficial uses are impaired by pollutants that require a water improvement project are placed in the polluted water category (Category 5) and put on the 303(d) list. The 305(b) lists all waters and all categories. Pollutants of concerns could be Bacteria, Dissolved oxygen, temperature, metals, phosphorus, turbidity, or high pH. From the site, water is tributary to Panther Lake (tributary to Hylebos Creek). According to the assessment, the downstream path to Panther Lake, approximately 1.5 miles downstream, is not comprised of any assessed impairments. Refer to Figure 3.1 at the end of this section of the DOE 303(d) water quality assessment map. Designated water quality problems There are no designated water quality problems known at this time. Plat of Twin Trails Preliminary TIR Page 14 October 5, 2021 ESM Consulting Engineers Task 3: Field Inspection (Level 1 Inspection) A site visit was conducted in July 2021 by ESM Consulting Engineers for the purpose of analyzing the project site and its upstream and downstream corridors. The weather conditions were sunny with no prior rainfall for several weeks. The ground surface was dry. A description of the drainage path is provided below. Refer to Figures 3.3A and 3.3B at the end of the downstream analysis for maps of the downstream reach and point locations. Upstream and onsite runoff Based on upstream topography, it is assumed that the adjacent offsite wetlands to the east are tributary to the project site. The upstream area is approximated to be 12.8 acres of both saturated forested (wetland), as unsaturated forest. It is estimated that there are two main areas along the project’s east boundary could direct overflow from the wetland onto the project site area. One location being at the midpoint of the project site, the other being further north at the east end of SW 341st Pl. Additional upstream stormwater converges at the southwest corner of the project site from upstream developments. This includes discharge from developments (Danville Station Div. No.01 and others) to the south and additional city conveyance networks to the southeast from SW 344th St and 15th Pl SW. It is this upstream basin that as resulted in the past installation of a bypass conveyance network through the current WSDOT Twin Trails Park and Ride and through Saghalie Firs. Reach 1A: The conveyance system currently in place through the park and ride is a densely vegetated channel that run near the west boundary of the project. This conveyance channel is proposed to remain and as a part of the Twin Trail project. The channel continues north adjacent to the Plat of Saghalie Firs until reaching SW 341st where runoff is then intercepted by a 12” culvert and conveyed to a 30” bypass line. Natural Discharge #1 Downstream Corridor This natural discharge location is at the Northwest corner of Parcel A. Runoff that may be generated form the densely vegetated parcel sheets flows north and west and is intercepted by the conveyance channel described in Reach 1A. Reach 2A: After runoff reaches the 30” bypass line, water is conveyed through the Plat of Saghalie First, bypassing the Saghalie Firs detention pond, to the intersection of SW 340th St and 19th Ave SW approximately 1,000 feet downstream, where stormwater joins up with the stormwater from Natural Discharge #2, described below. Natural Discharge #2 Downstream Corridor The following reaches describe the downstream corridor from Parcel 7978200086 starting the natural discharge location of the parcel. This is also the location of the proposed infiltration pond’s emergency overflow. Reach 1B: Upon leaving the site, stormwater is intercepted by the stormwater network installed with the Lakehaven Estates plat where water is then conveyed to the Lakehaven Estates infiltration pond. Reach 2B: The Lakehaven Estates infiltration pond was installed with an emergency overflow riser. Emergency overflow is conveyed to the 30” bypass line at the intersection of SW 340th St and 19th Ave SW where it joins up with Reach 2A from Plat of Twin Trails Preliminary TIR Page 15 October 5, 2021 ESM Consulting Engineers Natural Discharge Location #1 approximately 450 feet downstream of discharge location #2. Reach 3: Runoff at this point continues within 30-to-48-inch diameter pipes north in the 19th Ave SW right-of-way until reaching Campus Dr approximately 1,825 feet downstream of discharge location #2. Task 4: Drainage Description and Problem Descriptions The downstream drainage system is largely comprised of manmade conveyance pipes with the exception of the open conveyance channel along the west side of the project and the Plat of Saghalie Firs. This upstream and downstream system was study extensively in the Saghalie Firs Technical Information Report prepared by ESM Consulting Engineers in 2005 for the installation of the bypass system through Saghalie Firs. Additional study of the downstream system was prepared with the Lakehaven Estates plat prepared by Barghausen Consulting Engineers in 2006. Noted in Lakehaven Estates TIR from studies previously completed was a history of capacity concerns in the SW 19th Street conveyance system. In addition, the same capacity concerns were passed on in the conditions of approval for the Saghalie Firs project, which was required to provide additional detention due to the downstream system. The Twin trails project proposes to reduce the contributing stormwater flows to the bypass network and the SW 19th Street Conveyance system the use of an infiltration pond. Plat of Twin Trails Preliminary TIR Page 16 October 5, 2021 ESM Consulting Engineers Figure 3.1 - 303d Polluted Waters Sources: Esri, HERE, Garmin, Intermap, increment P Corp., GEBCO, USGS,FAO, NPS, NRCAN, GeoBase, IGN, Kadaster NL, Ordnance Survey, EsriJapan, METI, Esri China (Hong Kong), (c) OpenStreetMap contributors, and October 1, 2021 0 0.25 0.50.125Miles K Assessed Water/SedimentWater Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 Sediment Category 5 - 303d Category 4C Category 4B Category 4A Category 2 Category 1 Plat of Twin Trails Preliminary TIR Page 17 October 5, 2021 ESM Consulting Engineers DOWNSTREAM CORRIDOR PROJECT SITE Plat of Twin Trails Preliminary TIR Page 18 October 5, 2021 ESM Consulting Engineers FIGURE 3.2 - DOWNSTREAM STUDY AREA SITE DOWNSTREAM CORRIDOR UPSTREAM RUNON 1.5 MILES DOWNSTREAM NATURAL DISCHARGE LOCATION #2 NATURAL DISCHARGE LOCATION #1 King C ou n ty, Eag le Vie w Figure 3.3A - Dow nstream Cor rid or Dat e: 9 /3 0 /2 021 Not es: Th e inform a tion in clud e d o n th is ma p h a s b e en comp ile d by K in g Count y s ta ff fr o m a varie ty of so u rc es a nd is sub je ct tochange w ith o ut n o tic e. King C ou n ty mak es n o re p res e nt a tion s or w a rra ntie s, express or imp lie d , a s to ac cu ra cy,comp let e ne s s, time lin ess, o r rig h ts to t he u se o f su ch inf o rma tio n . This d o cu men t is no t int ende d f or u se a s a su rve yproduct. K in g Cou n ty sh a ll n o t be lia ble f or a ny g en eral, spe cial, ind ire ct , incid en ta l, or co n se qu e nt ial d am a ge s in clu d ing ,but n ot limit e d to, lo st re ve nu e s or lo st p rof its re sultin g fro m th e use or misu se of t he inf o rma tio n co ntain e d on th is ma p.An y sa le of th is ma p o r in formation o n t h is m ap is p ro h ibit ed e xce pt by wr it te n p e r missio n o f K ing Co u n ty.± Plat of Twin Trails Preliminary TIR Page 19 October 5, 2021 ESM Consulting Engineers NATURAL DISCHARGE LOCATION #1 EXISTING CONVEYANCE CHANNEL REACH 1A EXISTING CONVEYANCE CHANNEL REACH 1B 12" STORM REACH 2A EXISTING 30" BYPASS STORM SYSTEM NATURAL DISCHARGE LOCATION #2 REACH 2B EXISTING CONVEYANCE CHANNEL REACH 3 30" STORM INFILTRATION POND UPSTREAM RUNON UPSTREAM RUNON 640 LF DOWNSTREAM REFER TO FIGURE 3.3B FOR DOWNSTREAM CONTINUATION SITE #7978200085 (PARCEL A) PLAT OF SAGHALIE FIRS SITE #7978200086 TWIN LAKES PARK & RIDE PLAT OF LAKEHAVEN ESTATES FRED MEYER King C ou n ty, Eag le Vie w Figu re 3.3 B - Downstrea m Corr idor Dat e: 9 /3 0 /2 021 Not es: Th e inform a tion in clud e d o n th is ma p h a s b e en comp ile d by K in g Count y s ta ff fr o m a varie ty of so u rc es a nd is sub je ct tochange w ith o ut n o tic e. King C ou n ty mak es n o re p res e nt a tion s or w a rra ntie s, express or imp lie d , a s to ac cu ra cy,comp let e ne s s, time lin ess, o r rig h ts to t he u se o f su ch inf o rma tio n . This d o cu men t is no t int ende d f or u se a s a su rve yproduct. K in g Cou n ty sh a ll n o t be lia ble f or a ny g en eral, spe cial, ind ire ct , incid en ta l, or co n se qu e nt ial d am a ge s in clu d ing ,but n ot limit e d to, lo st re ve nu e s or lo st p rof its re sultin g fro m th e use or misu se of t he inf o rma tio n co ntain e d on th is ma p.An y sa le of th is ma p o r in formation o n t h is m ap is p ro h ibit ed e xce pt by wr it te n p e r missio n o f K ing Co u n ty.± Plat of Twin Trails Preliminary TIR Page 20 October 5, 2021 ESM Consulting Engineers 640 LF DOWNSTREAM REFER TO FIGURE 3.3A FOR UPSTREAM CONTINUATION REACH 3 30" to 48" STORM 1,825 LF DOWNSTREAM 4. Flow Control & Water Quality Facility Analysis and Design 4.1 Existing Site Hydrology Much of the existing project site is vegetated with either short grasses, or dense vegetation and ranges in grades between 1 and 15 percent. Stormwater that does not infiltrate into soils and turns into runoff generally sheet flows across the property through native vegetation toward the northwest. Based on upstream topography, it is assumed that the adjacent offsite wetlands to the east are tributary to the project site. The upstream area is approximated to be 12.8 acres of both saturated forested (wetland), as unsaturated forest. It is estimated that there are two main areas along the project’s east boundary could direct overflow from the wetland onto the project site area. One location being at the midpoint of the project site, the other being further north at the east end of SW 341st Pl. Additional upstream stormwater converges at the southwest corner of the project site from upstream developments. This includes discharge from developments (Danville Station Div. No.01 and others) to the south and additional city conveyance networks to the southeast from SW 344th St and 15th Pl SW that convey water across the south side of the project site within the future SW 344th St right-of-way improvements. 4.2 Developed Site Hydrology In summary, the proposed project proposes a series of catch basins and pipe leading to a new infiltration pond located in the northwest corner of the project, natural discharge location #2. A proprietary treatment facility will be provided upstream of the pond to provide at a minimum basic treatment for polluted runoff. This includes stormwater runoff from the plat infrastructure as well as runoff from future single-family rooftops and driveways. A small portion of runoff will bypass the pond due to topographic constraints will be directed toward the conveyance channel west of the project site around the areas of Natural Discharge #1. As discussed in Section 4.1, there is anticipated to be upstream run-on from the eastern wetlands. To accommodate potential run on, a couple interceptor pipes have been preliminarily proposed along the eastern boundary of the site. This will be further elaborated on further into the plat process. Accommodation of the upstream city storm system along the south side of the project within the SW 344th St right-of-way improvements, made up of open channels and 24” pipes, has also been provided. The existing 24” pipes coming from the intersection of 15th Pl SW and SW 344th St will be extended (eliminating the existing open channel) under the proposed sidewalk to the existing city system at the SW 16th Ave SW and SW 344th Pl where water then discharges to the open channel along the west side of the Twin Trails Project. The following tables represent the pre-developed and developed conditions for the project Plat of Twin Trails Preliminary TIR Page 21 October 5, 2021 ESM Consulting Engineers Table 4.1 – Historic Conditions (POC #1) Threshold Discharge Area #1 Area Forest, C, Flat sf (ac) Forest SAT, Flat (ac) Impervious (ac) Total** (ac) #2421039008 (Parcel A) 295,435 6.782 0 0 295,435 6.782 #2421039051 108,900 2.500 0 0 108,900 2.500 SW 344th St, 19th Ave SW, & SW 341st Pl Improvements* 40,091 (0.920) 0 0 40,091 (0.920) Total 444,426 (10.202) - - 444,426 (10.202) Upstream Run-on (Predeveloped and Developed to Pond) Eastern Wetlands #1921049019, 1921049018, 1921049024, 1921049026, 640370TRCT 456,908 (10.489) 103,307 (2.372) 560,215 (12.861) *Area is equal to the limits of construction under developed conditions within the ROW for surfaces considered ‘New’ or ‘Replaced’. Areas of asphalt overlay are not included. Table 4.2 – Developed Conditions POC #1 Area Stormwater BMP Impervious Infiltration Pond sf (ac) Amended Soil & Infiltration Pond Impervious Bypass sf (ac) Amended Soil & Bypass Total sf (ac) Lot 1-38* (except 8 & 12) 170,376 (3.911) 113,584 (2.608) 0 0 283,960 (6.519) Lots 8 & 12* 0 0 10,128 (0.233) 6,752 (0.155) 16,880 (0.388) SW 344th St Bypass+ 0 0 20,286 (0.466) 0 20,286 (0.466) ROW/Tracts+ (except SW 344th bypass) 100,929 (2.317) 0 0 0 100,929 (2.317) Pond (Tract A)** 11,186 (0.257) 11,186 (0.257) 0 0 22,371 (0.514) Total 282,491 (0.213) 124,770 (0.071) 30,414 (0.698) 6,752 (0.155) 444,426 (10.202) Credits Applied Credit for Flow Control Exception Analysis Infiltration (Subtract surface) Infiltration (Subtract surface) Impervious (no credit) Lawn (no credit) - Applied credit for infiltration pond modeling Impervious (no credit) Lawn (no credit) NA NA - *60% max impervious surface per lot per FWRC 19.200.010 +Assumed to be 100% impervious. **Assumed 50% impervious, 50% lawn Plat of Twin Trails Preliminary TIR Page 22 October 5, 2021 ESM Consulting Engineers To determine the increase between predeveloped and developed conditions, WWHMv2012 was used to analyze and compare the pre-developed and developed flow rates after considering amended soils and utilizing flow credits for Full Infiltration as found in Table 1.2.9.A of the SWDM. The WWHM model input is shown in Table 4.3 below. Table 4.3 – WWHM Input POC #1 (After Applied Credits) Area Forest, C, Flat (ac) Lawn, C, Flat (ac) Pasture, C Flat (ac) Roads/Flat (ac) Total (ac) 100-year flow (cfs) Predeveloped 10.202 0 0 0 10.202 0.7885 Developed 0 0.155 0 0.698 0.853 0.5783 NET DIFFERENCE - - - - - (0.2102) 4.3 Performance Standards Flow Control (Sec 1.2.3.2 SWDM) Stormwater systems onsite are to be designed to mitigate runoff generated from the project per the requirements of the KCSWDM. The Federal Way flow Control Applications Map provides the area-specific flow control facility standard. The project site falls within the Conservation Flow Control Area. Conservation Flow Control areas are to apply the Level 2 flow control standard. The Level 2 flow control standard is to match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow. Also match developed peak discharge rates to predeveloped peak discharge rates for the 2- and 10- year return periods. This also assumes historic site conditions as the predeveloped condition. Due to the limited increase in developed stormwater flow rates a flow control exception can be applied to this project. The required flow control facility can be waived for any threshold discharge area in which there is no more than a 0.15-cfs difference in the sum of the developed 100-year peak flows for those target surfaces and the sum of the historic site conditions. A stormwater analysis of the respective target surfaces for this project indicate there will be less than 0.15-cfs difference. Bypass (Sec 1.2.3.2.E) On some sites, topography can make it difficult or costly to collect all target surface runoff for conveyance to the onsite flow control facility. Section 1.2.3.2.E of the SWDM allows for bypass of the flow control facility. Compensatory mitigation by the flow control facility must be provided so that the net effect at the point of convergence downstream is the same with or without the bypass. This mitigation may be waived if the existing site conditions 100-year peak discharge from the area of the bypassed target surfaces is increased by no more than 0.15 cfs and flow control BMPs are applied to the impervious surfaces. Water Quality (Sec 1.2.8.1 SWDM & 1.2.8.2.D) Stormwater treatment is required for this project as a result of exceeding the 5,000 sf PGIS site area threshold. to provide a water quality benefit to stormwater coming in contact with pollution generating impervious surfaces (PGIS) prior to discharging from the project site. All projects with the city are initially subject to the Enhanced-Basic treatment standard unless otherwise exempt according to Section 1.2.8.1 of the SWDM upon which the treatment Plat of Twin Trails Preliminary TIR Page 23 October 5, 2021 ESM Consulting Engineers standard may be reduced to Basic. The goal for this level of treatment is 80% removal of total suspended solids (TSS) for flows or volumes up to and including the Water Quality design flow or volume for a typical rainfall year. Basic treatment may be allowed for lower density developments resulting in less than 8 du/acre. The Twin Trails plat is zoned RS—7.2 resulting in approximately 6 dwelling units per acres. As a result, the site, at a minimum, the site is subject to Basic Treatment. A stormwater treatment facility approved by the Washington State Department of Ecology for at least Basic treatment is proposed proceeding the retention pond. Preceding retention/detention, the design water quality flow rate is the flow rate form the contributing drainage basin at or below which 91% of the total runoff volume will be treated. per Section 6.2.1 of the SWDM. These flows are determined using WWHM. Additional design information will be provided with the engineering application further along in the plat process. 4.4 Flow Control System The flow control system proposed is the infiltration pond to be located at the low point of the project. The pond is designed to mitigate runoff generated from the project per the requirements of the Manual as discussed in Section 1 of this report, Minimum Requirement #3. The infiltration pond was proposed following the consideration procedures found in Section 5.2 of the SWDM and the Design Criteria in Section 5.2.2. A discussion of infiltration pond consideration and design is provided below. Infiltration Pond Consideration Procedures Step 1 – Site Reconnaissance: Review of the site topography helped determine the best location of the infiltration pond being the northwest corner of the project site. Step 2 – Minimum requirement for infiltration facilities Minimum requirements were evaluated to confirm the infiltration facility would be feasible in the selected location. a. The base of the infiltration basin shall be a minimum of 3 feet above seasonal high groundwater levels, bedrock, or other low permeability layers. Infiltration basins may not be constructed within a floodplain area. After review of the geotechnical study for the infiltration pond, 3 feet of separation between a restrictive layer and the bottom of the infiltration pond is achievable. Also, available maps indicate that the site is not within a floodplain area. b. The location of the infiltration pond will be downgradient from the rest of the site. c. Infiltration facilities shall not be placed closer than 100 feet from drinking water wells and Septic systems and 200 feet from springs used for drinking water supplies. To the best of our knowledge, there are no known water wells, springs, or septic fields within 200 feet of the proposed pond. d. The pond shall be setback 20 feet from property lines, tracts, or easements but may be reduced to 5 feet with support from the Plat of Twin Trails Preliminary TIR Page 24 October 5, 2021 ESM Consulting Engineers geotechnical engineer. The pond surface water is setback approximately 10 to 20 feet from the tract property line. e. Building setback lines for adjacent internal lots shall be 20 feet. This may be reduced to the minimum allowed by zoning with support from the geotechnical engineer. f. If the depth of the infiltration facility being considered is greater than the largest surface dimensions, it is considered an injection well. This proposed infiltration basin is not considered an injection well due to the shallow depth when compared to the length and width. g. The proposed depth of the infiltration facility is approximately 8 feet below existing grade, below the 20-foot maximum. Step 3 – Method of Analysis The size of the pond is determined using the hydrologic analysis and routing methods approved by the SWDM and Department of Ecology. The storage volume in the pond is used to retain runoff prior to infiltration. To determine compliance with the flow control requirements, WWHMv2012 was utilized. The pond facility was represented by using the stage-storage data from the proposed pond and entering the predetermined infiltration rate. The pond was then sized to achieve 100 percent infiltration. Step 4 – Geotechnical Analysis Earth Solutions NW, LLC has prepared an infiltration evaluation of the site in their report dated August 23, 2021, and documents their support for an infiltration pond. The evaluation includes soil testing results, discussion on groundwater, and a feasibility design infiltration rate. One of the site soil tests (TP-1) was conducted in the proposed pond area to observe the in-situ conditions of the soils in the area of the pond. Outwash soils consisting of gray well-graded gravel with silt and sand soils were encountered approximately 5 feet below grade. Based on a textural analysis and using the DOE saturated hydraulic conductivity calculation, a preliminary design rate of 10 inches per hour was recommended for infiltration facilities targeting glacial outwash deposits. The test pit terminated at 9 feet deep, groundwater was not observed during the exploration. Design Criteria The size of the infiltration basin was determined by routing the influent runoff file generated by the continuous runoff model through the proposed pond element. The infiltration facility has two discharge methods. The primary mode of discharge is into the ground. The second mode of discharge is any overflow that may occur if the infiltration capacity was reached. Plat of Twin Trails Preliminary TIR Page 25 October 5, 2021 ESM Consulting Engineers Table 4.4 below summarizes the input values to evaluate the proposed pond. Table 4.4 Infiltration Pond Parameters Parameter WWHM input Proposed Length Stage-Storage - Width Stage-Storage - Bottom Square footage 7,163 7,163 Storage Depth 5 ft 5 ft Effective Depth 6 ft - Side Slopes 3:1 3:1 Total Live Storage 51,174 sf 1.174 ac-ft 51,174 sf 1.174 ac-ft Design Infiltration Rate 10 in/hr - Percent Infiltrated 100% - The following table provides a summary of the proposed pond Basin. Refer to Table 4.1 and Table 4.2 in this report for further explanation of the assumed and calculated basin areas. Table 4.5 Infiltration Basin Land Use Area Impervious sf (ac) C, Lawn, Flat sf (ac) Forest, C Flat sf (ac) Forest, SAT Flat sf (ac) Lot 1-38 (Except 8 & 12) 170,376 (3.911) 113,584 (2.608) 0 0 ROW/Tracts (Except SW 344th Bypass) 100,929 (2.317) 0 0 0 Pond Tract (Tract A) 11,186 (0.257) 11,186 (0.257) 0 0 Offsite wetland overflow 0 0 456,908 (10.489) 103,307 (2.372) Total 282,491 sf (6.485) 124,770 sf (2.864) 456,908 (10.489) 103,307 (2.372) Plat of Twin Trails Preliminary TIR Page 26 October 5, 2021 ESM Consulting Engineers The following table provides a summary of the proposed pond stage/storage/discharge information representing the retention facility. Table 4.6 PROPOSED Stage-Storage-Discharge Table Retention Stage Elevation Stage Depth (feet) Recurrence Interval Surface Area sf (ac) Cumulative Storage cf (ac-ft) Discharge (cfs) 330.0 0.0 - 7,163 0.164 0 1.925 INFILTRATION (10 IN/HR) 331.0 1.0 - 8,316 (0.191) 7,740 (0.178) 1.925 332.0 2.0 - 9,532 (0.219) 16,663 (0.382) 1.925 333.0 3.0 - 10,811 (0.248) 26,835 (0.616) 1.925 334.0 4.0 12,153 (0.279) 38,317 (0.879) 1.925 335.0 5.0 *100% Infiltration 13,559 (0.311) 51,174 (1.174) 1.925 330.0-330.5 5.0-5.5 100-year** Overflow Spillway 6.457 330.5-331.0 5.5-6.0 Freeboard *Proposed depth to achieve 100% infiltration. Minimum 1 foot below top of pond. **See 100-year overflow depth analysis below. 100-year Overflow Conveyance An overflow route will be provided for stormwater flow that overtop the infiltration basin when infiltration capacity is exceeded, or the facility becomes plugged and fails. The proposed overflow system is to be an emergency spillway located on the north side of the pond to direct water into the SW 341st Pl right-of-way where water would eventually enter into the Lakehaven Estates infiltration/downstream bypass conveyance system. The spillway will be analyzed to pass the 100-year, design storm event for developed conditions provided in Table 4.6 above. The 100-year peak flow analysis can be found in the WWHM output located in Appendix A of this report under POC #3 of the infiltration pond analysis. Pretreatment A facility to remove a portion of the influent suspended solids is proposed preceding the infiltration basin. A basic treatment facility is proposed upstream of the infiltration facility. Further discussion and analysis of the treatment facilities are provided in the Water Quality analysis in this section. 4.5 Water Quality System: As discussed in Section 4.3, Basic water quality treatment standards apply to this project. The Department of Ecology provides a list of treatment devises that have received a General Use Level Designation (GULD). Prior to discharge from the project site, stormwater from the project parcels will be treated by a GULD facility. Further detail will be provided during review of the construction permit application. 4.6 Flow Control BMPs (Core Requirement #9): On-Site BMPs were evaluated for the project site as outlined in the KCSWDM under Section 1.2.9.1.D.3 – subdivision projects on sites less than 5 acres in size OR within the UGA. Target surfaces for application of Core Requirement #9 include new impervious surfaces, new pervious surface, and replaced impervious surfaces. Plat of Twin Trails Preliminary TIR Page 27 October 5, 2021 ESM Consulting Engineers Implementation of flow control BMPs will be deferred until a permit is obtained for construction on each lot. The BMP’s required for the plat infrastructure per KCSWDM Section 1.2.9.4 (e.g. road and sidewalk etc.) are described in order of precedence below with feasibility determined. Requirement #1 Full Dispersion (Section C.2.1) has been evaluated for the project site. There is insufficient onsite native vegetated flow path to which target impervious surfaces may be dispersed. Therefore, minimum design requirement #1 (specified in Section C.2.1.1) cannot be met; hence, this BMP is infeasible for the project site. Requirement #2 Full Infiltration (Section C.2.2 or Section 5.2, whichever is applicable) has been evaluated. A geotechnical study has been conducted by Earth Solutions NW, LLC, dated August 23, 2021. Underlying the weathered till in the north portion of the site are outwash soils suitable for infiltration. Infiltration has been proposed as the main method of stormwater management for the project with the exception of select areas that are difficult to route to the pond due to topographic limitations. This includes runoff from lots 8 and 12. Pending an evaluation of feasible onsite BMPs for these lots, it is proposed to direct discharge runoff from these lots at Natural Discharge Area #1. Requirement #3 Further BMP requirements are not applicable for the plat infrastructure. Refer to Requirement #2. Requirement #6 All new pervious surfaces will be amended in accordance with KCC 16.82.100(F) and (G) to satisfy the requirements specified therein (notes will be included on the final landscape plans). Plat of Twin Trails Preliminary TIR Page 28 October 5, 2021 ESM Consulting Engineers 5. Conveyance System Analysis and Design Onsite Conveyance System: Runoff from the developed project site will be collected from the developed lot lawns, roofs, drive aisles, and parking area by the proposed the conveyance system located throughout the project. The proposed stormwater drainage system is composed of catch basin structures with 6-inch and 12-inch diameter pipes. A conveyance and backwater analysis will be completed to verify the capacity of the critical pipes in the system. The proposed stormwater drainage system will be designed to convey the 25-year peak flow rate generated by the developed tributary basin as required in the SWDM. Conveyance calculations will be provided with the future engineering application. Plat of Twin Trails Preliminary TIR Page 29 October 5, 2021 ESM Consulting Engineers 6. Special Reports and Studies Geotechnical Study Preliminary Geotechnical Engineering Study, Twin Trails Residential Subdivision, August 23, 2021, updated September 13, 2021; prepared by Earth Solutions NW, LLC. This study has been provided in Appendix B of this report. Environmental Study Wetland and Fish and Wildlife Habitat Assessment Report, Twin Trails, September 2021; prepared by Soundview Consultants. This study has been included as a part of the project submittal package. Plat of Twin Trails Preliminary TIR Page 30 October 5, 2021 ESM Consulting Engineers 7. Other Permits Building permits will be required for this project, together with permits for utility connections. Plat of Twin Trails Preliminary TIR Page 31 October 5, 2021 ESM Consulting Engineers 8. ESC Analysis and Design The Erosion and Sedimentation Control will be provided with the final TIR. Plat of Twin Trails Preliminary TIR Page 32 October 5, 2021 ESM Consulting Engineers 9. Bond Quantities, Facility Summaries, and Declaration of Covenant The Bond Quantities worksheet will be provided with the final TIR. Plat of Twin Trails Preliminary TIR Page 33 October 5, 2021 ESM Consulting Engineers 10. Operations and Maintenance The Operations and Maintenance manual will be provided with the final TIR. Plat of Twin Trails Preliminary TIR Page 34 October 5, 2021 ESM Consulting Engineers Appendix A – Hydrology Model Output WWHM2012 PROJECT REPORT TWIN TRAILS PRELIMINARY FLOW AND POND ANALYSIS Pond 10/4/2021 4:47:03 PM Page 2 General Model Information Project Name:Pond Site Name: Site Address: City: Report Date:10/4/2021 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 Low Flow Threshold for POC2:50 Percent of the 2 Year High Flow Threshold for POC2:50 Year Low Flow Threshold for POC3:50 Percent of the 2 Year High Flow Threshold for POC3:50 Year Pond 10/4/2021 4:47:03 PM Page 3 Landuse Basin Data Predeveloped Land Use Historic Basin Bypass:No GroundWater:No Pervious Land Use acre C, Forest, Flat 10.202 Pervious Total 10.202 Impervious Land Use acre Impervious Total 0 Basin Total 10.202 Element Flows To: Surface Interflow Groundwater Pond 10/4/2021 4:47:03 PM Page 6 Mitigated Land Use Pond Basin Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 2.864 C, Forest, Flat 10.489 SAT, Forest, Flat 2.372 Pervious Total 15.725 Impervious Land Use acre ROADS FLAT 6.485 Impervious Total 6.485 Basin Total 22.21 Element Flows To: Surface Interflow Groundwater Infiltration Pond SSD Infiltration Pond SSD Pond 10/4/2021 4:47:03 PM Page 7 Developed Basin Bypass:No GroundWater:No Pervious Land Use acre C, Lawn, Flat 0.155 Pervious Total 0.155 Impervious Land Use acre ROADS FLAT 0.698 Impervious Total 0.698 Basin Total 0.853 Element Flows To: Surface Interflow Groundwater Pond 10/4/2021 4:47:03 PM Page 8 Routing Elements Predeveloped Routing Pond 10/4/2021 4:47:03 PM Page 9 Mitigated Routing Infiltration Pond SSD Depth:6 ft. Discharge Structure: 1 Riser Height:5 ft. Riser Diameter:12 in. Element Flows To: Outlet 1 Outlet 2 SSD Table Hydraulic Table Stage Area Volume Outlet Infilt/ (feet) (ac.) (ac-ft.) Struct Recharge NotUsed NotUsed NotUsed 0.000 0.164 0.000 0.000 1.925 0.000 0.000 0.000 1.000 0.191 0.178 0.000 1.925 0.000 0.000 0.000 2.000 0.219 0.382 0.000 1.925 0.000 0.000 0.000 3.000 0.248 0.616 0.000 1.925 0.000 0.000 0.000 4.000 0.279 0.879 0.000 1.925 0.000 0.000 0.000 5.000 0.311 1.174 0.000 1.925 0.000 0.000 0.000 6.000 0.345 1.502 3.150 1.925 0.000 0.000 0.000 Pond 10/4/2021 4:47:03 PM Page 10 Analysis Results POC 1 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #1 Total Pervious Area:10.202 Total Impervious Area:0 Mitigated Landuse Totals for POC #1 Total Pervious Area:0.155 Total Impervious Area:0.698 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.299947 5 year 0.471079 10 year 0.568062 25 year 0.670807 50 year 0.734386 100 year 0.788463 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.276022 5 year 0.352175 10 year 0.404317 25 year 0.472366 50 year 0.524728 100 year 0.578586 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.295 0.367 1950 0.367 0.376 1951 0.661 0.229 1952 0.208 0.193 1953 0.168 0.209 1954 0.259 0.225 1955 0.412 0.252 1956 0.328 0.249 1957 0.265 0.289 1958 0.298 0.227 DEVELOPED BASIN ANALYSIS Pond 10/4/2021 4:48:42 PM Page 18 POC 2 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #2 Total Pervious Area:0 Total Impervious Area:1 Mitigated Landuse Totals for POC #2 Total Pervious Area:15.725 Total Impervious Area:6.485 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #2 Return Period Flow(cfs) 2 year 0.381265 5 year 0.481582 10 year 0.54974 25 year 0.638151 50 year 0.705819 100 year 0.77513 Flow Frequency Return Periods for Mitigated. POC #2 Return Period Flow(cfs) 2 year 0 5 year 0 10 year 0 25 year 0 50 year 0 100 year 0 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #2 Year Predeveloped Mitigated 1949 0.494 0.000 1950 0.534 0.000 1951 0.308 0.000 1952 0.274 0.000 1953 0.296 0.000 1954 0.310 0.000 1955 0.352 0.000 1956 0.346 0.000 1957 0.392 0.000 1958 0.317 0.000 1959 0.323 0.000 POND OUTFLOW Pond 10/4/2021 4:51:01 PM Page 26 POC 3 + Predeveloped x Mitigated Predeveloped Landuse Totals for POC #3 Total Pervious Area:0 Total Impervious Area:1 Mitigated Landuse Totals for POC #3 Total Pervious Area:15.725 Total Impervious Area:6.485 Flow Frequency Method:Log Pearson Type III 17B Flow Frequency Return Periods for Predeveloped. POC #3 Return Period Flow(cfs) 2 year 0.381265 5 year 0.481582 10 year 0.54974 25 year 0.638151 50 year 0.705819 100 year 0.77513 Flow Frequency Return Periods for Mitigated. POC #3 Return Period Flow(cfs) 2 year 2.87155 5 year 3.749484 10 year 4.361403 25 year 5.171229 50 year 5.802054 100 year 6.457173 Annual Peaks Annual Peaks for Predeveloped and Mitigated. POC #3 Year Predeveloped Mitigated 1949 0.494 3.950 1950 0.534 3.775 1951 0.308 2.944 1952 0.274 1.918 1953 0.296 1.962 1954 0.310 2.412 1955 0.352 2.551 1956 0.346 2.494 1957 0.392 3.134 1958 0.317 2.247 1959 0.323 2.099 INFLOW TO POND Pond 10/4/2021 4:51:33 PM Page 35 Appendix Predeveloped Schematic Pond 10/4/2021 4:51:33 PM Page 36 Mitigated Schematic Pond 10/4/2021 4:51:34 PM Page 48 Disclaimer Legal Notice This program and accompanying documentation are provided 'as-is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. 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Clear Creek Solutions, Inc. 6200 Capitol Blvd. Ste F Olympia, WA. 98501 Toll Free 1(866)943-0304 Local (360)943-0304 www.clearcreeksolutions.com Appendix B – Geotechnical Report EarthSolutionsNWLLC EarthSolutions NW LLC 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 PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341ST PLACE FEDERAL WAY, WASHINGTON ES-7961 PREPARED FOR PROSPECT DEVELOPMENT, LLC August 23, 2021 Updated September 13, 2021 __________________________ Scott S. Riegel, L.G., L.E.G. Senior Project Manager __________________________ Kyle R. Campbell, P.E. Principal Engineer PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341ST PLACE FEDERAL WAY, WASHINGTON ES-7961 Earth Solutions NW, LLC 15365 Northeast 90th Street, Suite 100 Redmond, Washington 98052 Phone: 425-449-4704 | Fax: 425-449-4711 www.earthsolutionsnw.com 09/13/2021 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 Report Geotechnical-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 Times Geotechnical 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 Guidance Some 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 August 23, 2021 Updated September 13, 2021 ES-7961 Prospect Development, LLC 2913 – 5th Avenue Northeast, Suite 201 Puyallup, Washington 98372 Attention: Mr. Justin Holland Dear Mr. Holland: Earth Solutions NW, LLC (ESNW) is pleased to present this preliminary report that supports the current project. Based on the results of our investigation, construction of the proposed residential subdivision is feasible from a geotechnical standpoint. Our study indicates the site is underlain primarily by glacially consolidated soils with areas of outwash type deposits. This update includes the current site plan. In general, typical residences up to three stories in height may be supported on conventional continuous and spread footing foundations bearing on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general, competent native soil, suitable for support of the new foundations, will likely be encountered beginning at depths of about two to four feet below the existing ground surface. Where loose or unsuitable soil conditions are exposed at foundation subgrade elevations, compaction of soils to the specifications of structural fill, or overexcavation and replacement with suitable structural fill, will be necessary. Because no design details were available at the time of this report, ESNW should review the project details to confirm the recommendations in this report are applicable. Infiltration is feasible in the northeastern portion of the site (TP-1 through TP-3) where outwash type soils were encountered at depth. Infiltration facilities planned for this site must expose the clean outwash sand and gravel soils across the entirety of the system. We appreciate the opportunity to be of service to you on this project. If you have questions regarding the content of this geotechnical engineering study, please contact us. Sincerely, EARTH SOLUTIONS NW, LLC Scott S. Riegel, L.G., L.E.G. Senior Project Manager 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-7961 PAGE INTRODUCTION ................................................................................. 1 General..................................................................................... 1 Project Description ................................................................. 1 SITE CONDITIONS ............................................................................. 2 Surface ..................................................................................... 2 Subsurface .............................................................................. 2 Topsoil and Fill ............................................................. 2 Native Soil ..................................................................... 3 Geologic Setting ........................................................... 3 Groundwater ................................................................. 3 GEOLOGICALLY HAZARDOUS AREAS ........................................... 3 DISCUSSION AND RECOMMENDATIONS ....................................... 3 General..................................................................................... 3 Site Preparation and Earthwork ............................................. 4 Temporary Erosion Control ......................................... 4 Stripping ....................................................................... 5 Excavations and Slopes .............................................. 5 In-situ and Imported Soils ........................................... 5 Wet-Season Grading .................................................... 6 Structural Fill ................................................................ 6 Foundations ............................................................................ 6 Seismic Design ....................................................................... 7 Slab-on-Grade Floors ............................................................. 8 Retaining Walls ....................................................................... 8 Landscape Retaining Walls ......................................... 9 Drainage................................................................................... 9 Infiltration Evaluation ................................................... 9 Utility Support and Trench Backfill ....................................... 10 Preliminary Pavement Sections ............................................. 10 LIMITATIONS ...................................................................................... 11 Additional Services ................................................................. 11 Earth Solutions NW, LLC Table of Contents Cont’d ES-7961 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 PRELIMINARY GEOTECHNICAL ENGINEERING STUDY TWIN TRAILS RESIDENTIAL SUBDIVISION 1605 SOUTHWEST 341ST PLACE FEDERAL WAY, WASHINGTON ES-7961 INTRODUCTION General This preliminary geotechnical engineering study (study) was prepared for the proposed residential development to be constructed off the south side of Southwest 341st Place near 16th Avenue Southwest in Federal Way, Washington. The purpose of this study was to develop geotechnical recommendations for the proposed project. The scope of services for completing this study included the following:  Subsurface exploration consisting of test pit excavations;  Laboratory testing of soil samples collected at the test pit locations;  Engineering analyses and recommendations for the proposed development, and;  Preparation of this report. The following documents and maps were reviewed as part of preparing this study:  Preliminary Site Plan, Sheet PP-03 and PP-04, prepared by ESM Consulting Engineers dated September 3, 2021;  Geologic Map of the Poverty Bay 7.5’ Quadrangle, King County, Washington, prepared by Tabor, et al, 2014;  Federal Way Municipal Code (FWMC) section19.145 Environmentally Critical Areas, and;  Web Soil Survey (WSS), provided by the United States Department of Agriculture (USDA), Natural Resources Conservation Service. Project Description Based on review of the referenced site plan, the subject site will be developed with 38 detached residential lots, access roadways, a stormwater detention tract and utility improvements. The stormwater tract will be located in the northern portion of the site. Prospect Development, LLC ES-7961 August 23, 2021 Page 2 Updated September 13, 2021 Earth Solutions NW, LLC Based on existing grades, we anticipate mass grading activities will include minor cuts and fills of up to about seven feet. Perimeter footing loads will likely be 1 to 2 kips per lineal foot. Slab- on-grade loading is anticipated to be approximately 150 pounds per square foot (psf). The referenced plan indicates a stormwater infiltration pond will be constructed in the northern portion of the site where outwash soils were encountered during our fieldwork. The pond base will be excavated at least seven feet below existing grade to expose outwash soils and will use a combination of cut and fill berms. If the above design assumptions are incorrect or change, ESNW should be contacted to review the recommendations provided in this report. ESNW should review final designs to confirm that our geotechnical recommendations have been incorporated into the plans. SITE CONDITIONS Surface The subject site is located off the south side Southwest 341st Place in Federal Way, Washington. The approximate location of the property is illustrated on Plate 1 (Vicinity Map). The site consists of two adjoining tax parcels (King County Parcel Numbers 2421039051 and the east half of 2421039008). The property at 1605 Southwest 341 st Place is occupied by a residence, outbuildings and landscaping in the northeastern portion of the site, but is otherwise forested. The site topography generally descends gently to the west and north from the southeastern portion of the property with about 30 feet of total elevation change. Subsurface A representative of ESNW observed, logged, and sampled eight test pits excavated across the overall project area, on May 28, 2021 using a trackhoe and operator provided by the client. The test pits were completed for purposes of assessing soil conditions, classifying site soils, and characterizing near-surface groundwater conditions within the overall development area. 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 (where encountered) generally extended to depths of ranging from about two to eight inches below the existing ground surface (bgs). The topsoil was characterized by the observed dark brown color, the presence of fine organics, and root intrusions extending into the shallow, weathered soils. Fill was not encountered during our exploration; however, fill is likely present near the existing development areas of the site. Prospect Development, LLC ES-7961 August 23, 2021 Page 3 Updated September 13, 2021 Earth Solutions NW, LLC Native Soil Underlying topsoil, native soils encountered on the subject site were consisting primarily of silty sand with gravel (USCS: SM) that extended to the maximum exploration depth of about nine and one-half feet except test pit locations TP-1 through TP-3. At test pit locations TP-1 through TP- 3, underlying about five to six feet of silty sand, we encountered medium dense to dense, outwash-type sand and gravel deposits (USCS: SP and GW-GM) that extended to the termination depths of the test pits. Geologic Setting The referenced geologic map resource identifies ice contact (Qvi) deposits as the primary geologic unit underlying the site and surrounding areas. The referenced WSS map resource identifies Alderwood gravelly sandy loam (Map Unit Symbol: AgC) across the property. The Alderwood series soils formed in glacial till plains. Based on our field observations, the majority of the native soils encountered during our fieldwork are consistent with ice contact deposits, and the sandy soils encountered below the glacially consolidated cap within test pit locations TP-1 through TP-3 are consistent with outwash deposits. Groundwater During our subsurface exploration completed on May 2021, groundwater seepage was not encountered at the test pit locations. However, perched seepage should be expected within the weathered zone of soils on this site depending on the time of year grading occurs. In general, groundwater flow rates and elevations are higher during the winter, spring, and early summer months. GEOLOGICALLY HAZARDOUS AREAS Based on our review of the referenced Federal Way municipal code section and site conditions encountered during our fieldwork, there are no geologic hazard areas (erosion, landslide, seismic, or mine hazards) on or within 300 feet of the subject site. Standard development BMPs may be used for this site development plans. DISCUSSION AND RECOMMENDATIONS General Based on the results of our investigation, construction of typical single-family residences on this site is feasible from a geotechnical standpoint. The primary geotechnical considerations associated with the proposed development include site grading, infiltration facility construction, foundation support, slab-on-grade subgrade support, and the suitability of using on-site soils as structural fill. Prospect Development, LLC ES-7961 August 23, 2021 Page 4 Updated September 13, 2021 Earth Solutions NW, LLC Typical single-family residences may be supported on conventional continuous and spread footing foundations bearing on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general, competent native soil, suitable for support of the new foundations, will likely be encountered beginning at depths of about two to four feet bgs. Where loose or unsuitable soil conditions are exposed at foundation subgrade elevations, compaction of soils to the specifications of structural fill, or overexcavation and replacement with suitable structural fill, will be necessary. ESNW should review the proposed plans to confirm the recommendations in this report remain applicable. The relatively clean sand and gravel deposits encountered at test pit locations TP-1 through TP- 3 are suitable for infiltration but in-situ testing will be required to determine the infiltration rate for formal designs. Due to the low infiltration capacity of the glacially consolidated soils on this site, infiltration is not recommended for the southern portion of the development area. This study has been prepared for the exclusive use of Prospect Development, LLC, and their representatives. A warranty is neither expressed nor implied. This study has been prepared in a manner consistent with the level of care and skill ordinarily exercised by other members of the profession currently practicing under similar conditions in this area. Site Preparation and Earthwork Initial site preparation activities will consist of installing temporary erosion control measures, establishing grading limits, removing structural improvements, and clearing and stripping the site. Subsequent earthwork activities will involve site grading and related infrastructure improvements. Temporary Erosion Control The following temporary erosion control measures are offered:  Temporary construction entrances and drive lanes, consisting of at least six inches of quarry spalls, should be considered to both minimize off-site soil tracking and provide a stable access entrance surface. Placing geotextile fabric underneath the quarry spalls will provide greater stability, if needed.  Silt fencing should be placed around the site perimeter.  When not actively graded, soil stockpiles should be covered or otherwise protected.  Temporary measures for controlling surface water runoff, such as interceptor trenches, sumps, or swales, should be installed prior to beginning earthwork activities.  Dry soils disturbed during construction should be wetted to minimize dust and airborne soil erosion. Additional Best Management Practices (BMPs), as specified by the project civil engineer and indicated on the plans, should be incorporated into construction activities. Temporary erosion control measures should be actively managed and may be modified during construction as site conditions require, to ensure proper performance. Prospect Development, LLC ES-7961 August 23, 2021 Page 5 Updated September 13, 2021 Earth Solutions NW, LLC Stripping Topsoil was generally encountered within the upper approximately four to eight inches at the test pit locations. The organic-rich topsoil should be stripped and segregated into a stockpile for later use on site or to haul off site. The material remaining immediately below the topsoil may have some root zones and will likely be variable in composition, density, and/or moisture content. The material exposed after initial topsoil stripping will likely not be suitable for direct structural support as is and will likely need to be compacted in place or stripped and stockpiled for reuse as fill; depending on the time of year stripping occurs, the soil exposed below the topsoil may be too wet to compact and may need to be aerated or treated. ESNW should observe initial stripping activities to provide recommendations regarding stripping depths and material suitability. Excavations and Slopes Based on the soil conditions observed at the subsurface exploration locations, the maximum allowable temporary slope inclinations provided below may be used. 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. 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. In-situ and Imported Soils The majority of the near-surface soils encountered during our subsurface exploration have a high sensitivity to moisture and were generally in a damp to moist condition at the time of the exploration (May 2021). Exposed soils will degrade rapidly if exposed to wet weather and/or construction traffic. In general, soils encountered during site excavations that are excessively over the optimum moisture content will 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. A representative of ESNW should determine the suitability of in-situ soils for use as structural fill at the time of construction. Prospect Development, LLC ES-7961 August 23, 2021 Page 6 Updated September 13, 2021 Earth Solutions NW, LLC Imported soil intended for use as structural fill should consist of a well-graded, granular soil with a moisture content that is at (or slightly above) the optimum level. 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). Wet-Season Grading Because the site soils are highly sensitive to moisture, grading during the rainy season will be very difficult. If grading takes place during the winter, spring, or early summer months, a contingency in the project budget should be included to allow for export of native soil and import of structural fill. Structural Fill Structural fill is defined as compacted soil placed in foundation, slab-on-grade, roadway, permanent slope, retaining wall, utility trench, and vault backfill areas. Soils placed in structural areas should consist of a granular material devoid of deleterious debris and organics, placed in loose lifts of 12 inches or less and compacted to a relative compaction of 95 percent, based on the laboratory maximum dry density as determined by the Modified Proctor Method (ASTM D- 1557). Foundations Typical two to three story residential structures may be supported on conventional spread and continuous footings bearing on competent native soil, recompacted native soil, or new structural fill placed directly on competent native soil. In general, competent native soil suitable for the support of foundations will likely be encountered at depths of about two to four feet bgs. ESNW should evaluate the design subgrade conditions to confirm suitable conditions are exposed and to provide additional preparation recommendations, where necessary. Where loose, organic, or otherwise unsuitable soil conditions are observed at foundation subgrade elevations, compaction of the soils to the specifications of structural fill, or overexcavation and replacement with granular structural fill, will likely be necessary. Provided residential structures will be supported as described above, the following parameters can be used for design of the new foundations:  Allowable soil bearing capacity 2,500 psf  Passive earth pressure 300 pcf (equivalent fluid)  Coefficient of friction 0.40 The passive earth pressure and coefficient of friction values include a safety factor of 1.5. A one- third increase in the allowable soil bearing capacity may be assumed for short-term wind and seismic loading conditions. With structural loading as expected, total settlement in the range of 1 inch is anticipated, with differential settlement of about 0.5 inch. The majority of settlement should occur during construction, as dead loads are applied. Prospect Development, LLC ES-7961 August 23, 2021 Page 7 Updated September 13, 2021 Earth Solutions NW, LLC Seismic Design The 2018 International Building Code (2018 IBC) recognizes the most recent edition of the Minimum Design Loads for Buildings and Other Structures manual (ASCE 7-16) for seismic design, specifically with respect to earthquake loads. Based on the soil conditions encountered at the boring locations, the parameters and values provided below are recommended for seismic design per the 2018 IBC. Parameter Value Site Class D* Mapped short period spectral response acceleration, S S (g) 1.336 Mapped 1-second period spectral response acceleration, S 1 (g) 0.459 Short period site coefficient, Fa 1 Long period site coefficient, Fv 1.86 Adjusted short period spectral response acceleration, S MS (g) 1.336 Adjusted 1-second period spectral response acceleration, S M1 (g) 0.854 Design short period spectral response acceleration, S DS (g) 0.891 Design 1-second period spectral response acceleration, S D1 (g) 0.569 * Assumes medium dense soil conditions, encountered to a maximum depth of nine and one-half feet bgs during the May 2021 field exploration, remain medium dense or better to at least 100 feet bgs. Based on our experience with the project geologic setting (glacial till) across the Puget Sound region, soil conditions are likely consistent with this assumption. Further discussion between the project structural engineer, the project owner (or their representative), and ESNW may be prudent to determine the possible impacts to the structural design due to increased earthquake load requirements under the 2018 IBC. ESNW can provide additional consulting services to aid with design efforts, including supplementary geotechnical and geophysical investigation, upon request. Liquefaction is a phenomenon where saturated or loose soil suddenly loses internal strength and behaves as a fluid. This behavior is in response to increased pore water pressures resulting from an earthquake or another intense ground shaking. In our opinion, site susceptibility to liquefaction may be considered negligible. The absence of a shallow groundwater table and the relative density of the native soil were the primary bases for this opinion. Prospect Development, LLC ES-7961 August 23, 2021 Page 8 Updated September 13, 2021 Earth Solutions NW, LLC Slab-on-Grade Floors Slab-on-grade floors should be supported on a firm and unyielding subgrade consisting of competent native soil or new structural fill. Unstable or yielding areas of the subgrade should be recompacted or overexcavated and replaced with suitable structural fill prior to construction of the slab. 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 design:  Active earth pressure (unrestrained condition) 35 pcf (equivalent fluid)  At-rest earth pressure (restrained condition) 55 pcf  Traffic surcharge (passenger vehicles) 70 psf (rectangular distribution) *  Passive earth pressure 300 pcf (equivalent fluid)  Coefficient of friction 0.40  Seismic surcharge 8H psf** * Where applicable ** Where H equals the retained height (in feet) The above design parameters are based on a level backfill condition and level grade at the wall toe. Revised design values will be necessary if sloping grades are to be used above or below retaining walls. Additional surcharge loading from adjacent foundations, sloped backfill, or other relevant loads should be included in the retaining wall design, where applicable. A safety factor of 1.5 is included in the passive earth pressure and coefficient of friction values. Retaining walls should be backfilled with free-draining material or suitable sheet drainage 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 can 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. Prospect Development, LLC ES-7961 August 23, 2021 Page 9 Updated September 13, 2021 Earth Solutions NW, LLC Landscape Retaining Walls Based on the existing site grades, retaining walls may be used along the portions of the lots to raise grades for new building pads. Final wall heights, alignments and facing materials have not been determined at the time of this report. Walls over four feet in total height, including toe embedment will require building permits supported by an engineered design. ESNW can prepare and engineered retaining wall design, upon request. ESNW should review the final grading plans to confirm the recommendations are incorporated and to provide additional recommendations where appropriate. Drainage Groundwater seepage was not encountered during our exploration; however, groundwater seepage will likely be encountered within site excavations, particularly utility trenches and deeper excavations such as detention vault/pond areas. Temporary measures to control surface water runoff and groundwater during construction would likely involve passive elements, such as interceptor trenches and sumps. ESNW should be consulted during preliminary grading to identify areas of groundwater and to provide recommendations to reduce the potential for instability related to groundwater effects. Depending on the flow volumes encountered during grading, an interceptor trench drain system may be warranted along the up-slope perimeter of the work area to help mitigate or otherwise control shallow perched groundwater flows. Finish grades must be designed to direct surface water away from the new structures and/or slopes for a distance of at least 10 feet or as setbacks allow. Water must not be allowed to pond adjacent to the new structures and/or slopes. A typical foundation drain detail is provided on Plate 4. Infiltration Evaluation Outwash type soils were encountered at depth within test pits TP-1 through TP-3 with corresponding fines content of about 5 percent. The outwash soils may be suitable for infiltration pending in-situ infiltration testing. Based on textural analyses and using the DOE saturated hydraulic conductivity calculation, for feasibility considerations, a preliminary infiltration rate of 10 inches per hour was recommended to the project civil engineer for the relatively clean outwash sand and gravel deposits such as those encountered at test pit locations TP-1 through TP-3. The facility illustrated on the referenced preliminary plans were developed based on information provided by ESNW during the early phase of design. ESNW should conduct targeted infiltration testing to better characterize the infiltration potential for the outwash type soils. Based on the conditions observed during our fieldwork and anecdotal information provided by the project civil engineer, groundwater separation will likely be feasible. We understand projects nearby to the north also use infiltration systems and these are functioning as intended. The native soils in the southern portion of the site consist predominately of silty sand with gravel, glacially consolidated deposits that exhibit fines contents ranging from about 33 to 49 percent (passing the U.S. No. 200 sieve), and are not suitable for infiltration. Prospect Development, LLC ES-7961 August 23, 2021 Page 10 Updated September 13, 2021 Earth Solutions NW, LLC Utility Support and Trench Backfill The native soils observed at the test pit locations are generally suitable for support of utilities; however, the native soils may not be suitable for use as structural backfill in the utility trench excavations unless the soil is at or near the optimum moisture content at the time of placement and compaction. Moisture conditioning or cement treatment of the soils may be necessary at some locations prior to use as structural fill. If utility backfill occurs during wet weather, cement treatment of native soils or import of a suitable material will be necessary. Utility trench backfill should be placed and compacted to the specifications of structural fill provided in this report, or to the applicable requirements of presiding jurisdiction. Preliminary Pavement Sections The performance of site pavements is largely related to the condition of the underlying subgrade. To ensure adequate pavement performance, the subgrade should be in a firm and unyielding condition when subjected to proofrolling with a loaded dump truck. Structural fill in pavement areas should be compacted to the specifications detailed in the Site Preparation and Earthwork section of this report. It is possible that soft, wet, or otherwise unsuitable subgrade areas may still exist after base grading activities. Areas of unsuitable or yielding subgrade conditions may require remedial measures such as overexcavation and replacement with structural fill or thicker crushed rock sections prior to pavement. For lightly loaded pavement areas subjected primarily to passenger vehicles such as driveways, the following preliminary pavement sections may be considered:  A minimum of two inches of hot mix asphalt (HMA) placed over four inches of crushed rock base (CRB), or;  A minimum of two inches of HMA placed over three inches of asphalt treated base (ATB). Heavier traffic areas generally require thicker pavement sections depending on site usage, pavement life expectancy, and site traffic. For preliminary design purposes, the following pavement sections for occasional truck traffic areas may be considered:  Three inches of HMA placed over six inches of crushed rock base (CRB), or;  Three inches of HMA placed over four-and-one-half inches of ATB. The HMA, CRB and ATB materials should conform to WSDOT specifications. If pavement areas will include a reverse crown, additional drainage should be used to effectively convey water that may enter the subgrade toward the storm drainage system. ESNW can provide recommendations for enhanced drainage upon request. Prospect Development, LLC ES-7961 August 23, 2021 Page 11 Updated September 13, 2021 Earth Solutions NW, LLC LIMITATIONS The recommendations and conclusions provided in this 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 neither expressed nor 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 study if variations are encountered. Additional Services ESNW should have an opportunity to review final project plans with respect to the geotechnical recommendations provided in this report. ESNW should also be retained to provide testing and consultation services during construction. Drwn.MRS Checked SSR Date Aug.2021 Date 08/13/2021 Proj.No.7961 Plate 1 Earth Solutions NWLLC Geotechnical Engineering,Construction EarthSolutionsNWLLC EarthSolutions NW LLC Observation/Testing and Environmental Services Vicinity Map Twin Trails Federal Way,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 MRS SSR 09/13/2021 7961 2TestPitLocationPlan TwinTrailsFederalWay,WashingtonNORTH NOT -TO -SCALE 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-7961,May 2021 Subject Site Existing Building Proposed Lot Number TP-1 19 S.W.341ST PLACE S.W.342ND PLACE S.W.344TH STREET 19THavenueS.W.road a road B 1 2 3 4 5678 9 10 11 12 13 14 15 16 17 18192021 2223 24 25 26 27 28 29 30 31 32 33 34 35 36 3738 Tract A Tract B Tract C Tract D Tract E Tract F340338336334Close-Up of Tract A with Pond TP-1 TP-2 TP-3 TP-4TP-5 TP-6 TP-7 TP-8 Plate Proj.No. Date Checked By Drwn.ByEarthSolutionsNWLLCGeotechnicalEngineering,ConstructionObservation/TestingandEnvironmentalServicesEarthSolutionsNWLLCEarthSolutionsNWLLC Drwn.MRS Checked SSR Date Aug.2021 Date 08/13/2021 Proj.No.7961 Plate 3 Earth Solutions NWLLCEarthSolutionsNWLLC EarthSolutions NW LLC Geotechnical Engineer ing,C onstr uction Observation/Testing and Environmental Services Retaining Wall Drainage Detail Twin Trails Federal Way,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 SSR Date Aug.2021 Date 08/13/2021 Proj.No.7961 Plate 4 Earth Solutions NWLLC Geotechnical Engineering,Construction Observation/Testing and Environmental Services EarthSolutionsNWLLC EarthSolutions NW LLC Footing Drain Detail Twin Trails Federal Way,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 DRAW ING Earth Solutions NW, LLC Appendix A Subsurface Exploration Test Pit Logs ES-7961 Subsurface conditions at the subject site were explored on May 28, 2021 by excavating eight test pits using a mini-trackhoe and operator provided by the client. The approximate locations test pits are illustrated on Plate 2 of this study. The test pit logs are provided in this Appendix. The maximum exploration depth was approximately nine and one-half feet bgs and were terminated in firm native soils. 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 GRAVELLY SOILS CLAYEY GRAVELS, GRAVEL - SAND - CLAY MIXTURES WELL-GRADED SANDS, GRAVELLY SANDS, LITTLE OR NO FINES POORLY-GRADED SANDS, GRAVELLY SAND, LITTLE OR NO FINES SILTY SANDS, SAND - SILT MIXTURES CLAYEY SANDS, SAND - CLAY MIXTURES INORGANIC SILTS AND VERY FINE SANDS, ROCK FLOUR, SILTY OR CLAYEY FINE SANDS OR CLAYEY SILTS WITH SLIGHT PLASTICITY INORGANIC CLAYS OF LOW TO MEDIUM PLASTICITY, GRAVELLY CLAYS, SANDY CLAYS, SILTY CLAYS, LEAN CLAYS ORGANIC SILTS AND ORGANIC SILTY CLAYS OF LOW PLASTICITY INORGANIC SILTS, MICACEOUS OR DIATOMACEOUS FINE SAND OR SILTY SOILS INORGANIC CLAYS OF HIGH PLASTICITY SILTS AND CLAYS MORE THAN 50% OF MATERIAL IS LARGER THAN NO. 200 SIEVE SIZE MORE THAN 50% OF MATERIAL IS SMALLER THAN NO. 200 SIEVE SIZE MORE THAN 50% OF COARSE FRACTION PASSING ON NO. 4 SIEVE MORE THAN 50% OF COARSE FRACTION RETAINED ON NO. 4 SIEVE SOIL CLASSIFICATION CHART (APPRECIABLE AMOUNT OF FINES) (APPRECIABLE AMOUNT OF FINES) (LITTLE OR NO FINES) FINE GRAINED SOILS SAND AND SANDY SOILS SILTS AND CLAYS ORGANIC CLAYS OF MEDIUM TO HIGH PLASTICITY, ORGANIC SILTS PEAT, HUMUS, SWAMP SOILS WITH HIGH ORGANIC CONTENTS LETTERGRAPH SYMBOLSMAJOR DIVISIONS COARSE GRAINED SOILS TYPICAL DESCRIPTIONS WELL-GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES POORLY-GRADED GRAVELS, GRAVEL - SAND MIXTURES, LITTLE OR NO FINES SILTY GRAVELS, GRAVEL - SAND - SILT MIXTURES CLEAN GRAVELS GRAVELS WITH FINES CLEAN SANDS (LITTLE OR NO FINES) SANDS WITH FINES LIQUID LIMIT LESS THAN 50 LIQUID LIMIT GREATER 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 MC = 13.3% MC = 4.5% Fines = 5.3% MC = 4.8% TPSL SM GW- GM Dark brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, moist (Weathered Till) -becomes gray, dense Gray well-graded GRAVEL with silt and sand, dense, damp [USDA Classification: extremely gravelly loamy coarse SAND] Test pit terminated at 9.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 0.5 5.0 9.0 NOTES Depth of Topsoil & Sod 6": grass LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35659 LATITUDE 47.29579 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-1 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 10.1% MC = 5.3% TPSL SM GW- GM Dark brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, damp (Weathered Till) -becomes gray, dense Gray well-graded GRAVEL with silt and sand, dense, damp (Unweathered Till) Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 0.5 5.5 8.0 NOTES Depth of Topsoil & Sod 4": grass LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35649 LATITUDE 47.29516 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-2 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 7.3% MC = 6.6% TPSL SM SP Dark brown TOPSOIL, minimal root intrusions Brown silty SAND with gravel, medium dense, damp -large cobbles 1.5' diameter Brown poorly graded SAND with gravel, medium dense, damp Test pit terminated at 8.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. 0.5 6.0 8.5 NOTES Depth of Topsoil & Sod 8": grass LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35591 LATITUDE 47.29502 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-3 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 15.7% Fines = 49.1% MC = 12.0% SM Gray silty SAND with gravel, medium dense, moist [USDA Classification: gravelly LOAM] -becomes dense, damp -weakly cemented Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 8.0 NOTES Depth of Topsoil & Sod 2": duff LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35622 LATITUDE 47.29358 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-4 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 10.5% MC = 13.2% Fines = 33.5% SM Gray silty SAND, medium dense, damp -increasing sand content [USDA Classification: slightly gravelly sandy LOAM] Test pit terminated at 9.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. 9.5 NOTES Depth of Topsoil & Sod 3": duff LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35755 LATITUDE 47.29382 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-5 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 14.7% MC = 13.2% Fines = 33.3% SM Brown silty SAND, medium dense, moist -becomes gray -becomes dense -iron oxide staining [USDA Classification: gravelly sandy LOAM] Test pit terminated at 8.0 feet below existing grade. No groundwater encountered during excavation. No caving observed. 8.0 NOTES Depth of Topsoil & Sod 3": duff LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35822 LATITUDE 47.2944 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-6 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 14.4% MC = 14.1% TPSL SM Dark brown TOPSOIL, shallow root intrusions Brown silty SAND with gravel, medium dense, moist -becomes gray, dense -weakly cemented -becomes gray -iron oxide staining Test pit terminated at 8.5 feet below existing grade. No groundwater encountered during excavation. No caving observed. 0.5 8.5 NOTES Depth of Topsoil & Sod 6": duff LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.3571 LATITUDE 47.29438 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-7 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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 MC = 10.8% Fines = 39.2% MC = 10.4% SM Gray silty SAND with gravel, medium dense, damp -becomes dense -weakly cemented [USDA Classification: slightly gravelly sandy LOAM] -becomes very dense Test pit terminated at 6.5 feet below existing grade due to refusal on very dense soils. No groundwater encountered during excavation. No caving observed. 6.5 NOTES Depth of Topsoil & Sod 3": duff LOGGED BY SES EXCAVATION METHOD EXCAVATION CONTRACTOR Client Provided CHECKED BY SSR DATE STARTED 5/28/21 COMPLETED 5/28/21 GROUND WATER LEVEL: GROUND ELEVATION LONGITUDE -122.35624 LATITUDE 47.29431 AT TIME OF EXCAVATION SAMPLE TYPENUMBERDEPTH(ft)0 5 PAGE 1 OF 1 TEST PIT NUMBER TP-8 PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GENERAL BH / TP / WELL - 7961.GPJ - GRAPHICS TEMPLATE.GDT - 9/13/21Earth 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-7961 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 HYDROMETER 304 5.3 1.7 49.1 33.5 33.3 101/2 COBBLES Specimen Identification 4 coarse 20 401.5 8 14 USDA: Brown Extremely Gravelly Loamy Coarse Sand. USCS: GW-GM with Sand. USDA: Brown Very Gravelly Coarse Sand. USCS: SP with Gravel. USDA: Gray Gravelly Loam. USCS: SM. USDA: Gray Slightly Gravelly Sandy Loam. USCS: SM. USDA: Gray Gravelly Sandy Loam. USCS: SM. 6 60 PERCENT FINER BY WEIGHTD10 2.869 0.663 15.214 2.435 0.233 0.256 0.324 GRAIN SIZE DISTRIBUTION 100 50.84 7.83 LL TP-01 TP-03 TP-04 TP-05 TP-06 0.299 0.311 3/4 U.S. SIEVE OPENING IN INCHES U.S. SIEVE NUMBERS GRAVEL SAND 37.5 19 19 19 37.5 %Silt 1.81 0.58 TP-01 TP-03 TP-04 TP-05 TP-06 2 2003 Cc CuClassification %Clay 16 PID60 D30 coarse SILT OR CLAYfinemedium GRAIN SIZE IN MILLIMETERS 3/8 50 6.0ft. 8.5ft. 2.5ft. 9.5ft. 8.0ft. 6.00ft. 8.50ft. 2.50ft. 9.50ft. 8.00ft. PL PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GRAIN SIZE USDA ES-7961 TWIN TRAILS.GPJ GINT US LAB.GDT 6/4/21Earth 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 HYDROMETER 304 39.2 101/2 COBBLES Specimen Identification 4 coarse 20 401.5 8 14 USDA: Gray Slightly Gravelly Sandy Loam. USCS: SM. 6 60 PERCENT FINER BY WEIGHTD10 0.224 GRAIN SIZE DISTRIBUTION 100 LL TP-08 3/4 U.S. SIEVE OPENING IN INCHES U.S. SIEVE NUMBERS GRAVEL SAND 19 %Silt TP-08 2 2003 Cc CuClassification %Clay 16 PID60 D30 coarse SILT OR CLAYfinemedium GRAIN SIZE IN MILLIMETERS 3/8 50 3.0ft. 3.00ft. PL PROJECT NUMBER ES-7961 PROJECT NAME Twin Trails GRAIN SIZE USDA ES-7961 TWIN TRAILS.GPJ GINT US LAB.GDT 6/4/21Earth 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-7961 EMAIL ONLY Prospect Development, LLC 2913 – 5th Avenue Northeast, Suite 201 Puyallup, Washington 98372 Attention: Mr. Justin Holland EMAIL ONLY Mr. Clay Loomis