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REVIEWEDRESUB1 BLD2023-0692+Geotechnical_Report+11.8.2023_8.07.23_AM+3887944October 16, 2023 NELSON GEOTECHNICAL ASSOCIATES. INC. RESUB Feb 20 2024 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Mr. BK Chen VIA Email:.bkchen@gmail.com 17311-135" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com BLD2023-0692 Stormwater Infiltration Letter (2"d Revision) — UPDATED Chen 210th Street SW Residential Development Infiltration 7103 — 210th Street SW Edmonds, Washington NGA File No. 1209720 ................................................. REVIEWED BY CITY OF EDMONDS BUILDING DEPARTMENT Dear Mr. Chen: This letter documents our explorations and provides our opinions and recommendations for the feasibility of stormwater infiltration for the proposed townhome residential development project located at 7103 — 210th Street SW in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. 1>; 1 1:Z611111110111i[9LJ The site is currently occupied by a single-family residence within the central portion of the property surrounded by grass -covered yard areas and few scattered young to mature trees. Topography within the site is generally level to gently sloping from north to south. We understand the proposed development will consist of removing the existing site structure and constructing several townhome residence structures within the site. For use in preparing this letter, we have been provided with an untitled and undated preliminary site plan showing the existing and proposed site layout. We understand that stormwater generated from the proposed development may be directed to onsite infiltration systems, if feasible. We have been requested to evaluate the infiltration capacity of the site soils within the property. The City of Edmonds utilizes the 2019 Department of Ecology (DOE) Stormwater Management in Western Washington Manual (2019 SWMMWW) to determine the design of infiltration or detention facilities. According to this manual, long-term design infiltration rates for this site are to be determined by performing on -site infiltration testing consisting of the Small Pilot Infiltration Test (PIT). Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 2101h Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 2 SCOPE The purpose of this study is to explore and characterize the subsurface conditions within the site and to provide opinions and recommendations for stormwater infiltration. Specifically, our scope of services included the following: 1. Review available soil and geologic maps of the area. 2. Explore the subsurface soil and groundwater conditions within the site with trackhoe excavated test pits. Trackhoe was subcontracted by NGA. 3. Perform grain -size sieve analysis on selected soil samples, as needed. 4. Provide long-term design infiltration rates based on one on -site small-scale Pilot Infiltration Tests (PIT) per the 2019 SWMMWW. Location and depth of tests to be determined by civil engineer. Water for the test to be secured by client. 5. Provide recommendations for infiltration system installation. 6. Document the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The property consists of a roughly rectangular -shaped parcel covering approximately 0.26 acres. The site is currently occupied by a single-family residence within the central portion of the property surrounded by grass -covered yard areas and few scattered young to mature trees. Topography within the site is generally level to gently sloping from north to south. We did not observe any surface water within the site on our visit on October 9, 2020. Subsurface Conditions: The geologic units for this area are shown on the Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by J. P. Minard (U.S.G.S., 1983). The site is mapped as Qva (Advance Outwash deposits), with Qvt (glacial till) nearby. Texture and structure vary from place to place but Advance Outwash generally is composed of moderate- to well -sorted, stratified, light gray to tan medium to coarse sand and pebbly sand containing minor amounts of fine sandy silt. Vashon till typically consists of a concrete -like mixture of relatively equal parts of silt, sand, and gravel. In our explorations, we encountered soils primarily consisting of fine to medium sand with varying amounts of gravel and silt, which we interpreted to be consistent with the description of the outwash deposits mapped in this area. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 2101h Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 3 Explorations: The subsurface conditions within the site were explored on October 9, 2020 by excavating three test pit explorations with a mini -excavator as shown in Plate 1 below. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the explorations. Plate 1: Excavator equipment utilized to dig explorations. Shown digging Test Pit 1 located to the west of the existing structure and looking north. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are attached to this letter and are presented in Figure 4. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions, the exploration logs should be reviewed. At the surface of Test Pits 1 and 2, and Infiltration Pit 1, we generally encountered approximately 1.0 feet of surficial topsoil. Underlying the surficial topsoil in Test Pit 1, we encountered approximately 2.0 feet of dark brown silty fine to medium sand with gravel and organics along with a buried topsoil layer that we interpreted as undocumented fill soil. Below the undocumented fill soils in Test Pit 1 and the surficial topsoil in Test Pit 2 and Infiltration Pit 1, we encountered medium dense to dense, brown gray fine to medium sand with varying amounts of sand, gravel and roots that we interpreted as native advance outwash deposits. All of our test pit explorations were terminated within the native advance outwash deposits at depths in the range of 7.0 to 9.5 feet below the existing ground surface. Soil conditions in Test Pit 1 are shown in Plate 2 below. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED Chen 2101h Street SW Townhome Residential Development Infiltration Edmonds, Washington NGA File No. 1209720 October 16, 2023 Page 4 Plate 2: Test Pit 1 soil conditions. Surficial undocumented fill soils underlain by native advance outwash deposits extending to the depths explored. Hydrogeologic Conditions We did not encounter groundwater seepage or observe any indications of groundwater seepage in any of our explorations completed within the site. If groundwater seepage is encountered during construction, we would interpret this water to be perched groundwater. It is our opinion that the potential for perched groundwater within the upper advance outwash soils is low due to the relatively granular nature of the native advance outwash soils underlying the site. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. Perched water tends to vary spatially and is dependent upon the amount of precipitation. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. Perched water does not represent a regional groundwater "table" within the upper soil horizons. We anticipate that the seasonal high groundwater table within the area is located at a greater depth below the site and should not adversely impact the proposed infiltration systems within the site. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 2101h Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 5 Stormwater Infiltration The subsurface soils generally consisted of topsoil and/or fill underlain by fine- to medium- grained sand with varying amounts of silt and gravel to the depths explored within the proposed infiltration areas. We conducted on -site infiltration testing in accordance with the 2019 SWMMWW, to determine the long term design infiltration rate of the site soils. On -site testing consisted of a Small Pilot Infiltration Test (Small PIT) to determine the long-term design infiltration rates. We conducted a Small PIT within Infiltration Pit 1, located as shown on the attached Site Plan in Figure 2 and in Plate 3 below. The test was conducted within a pit that measured 6.0-feet long by 4.5-feet wide by 5.0-feet deep. Due to the granular nature of the site soils and the maximum capacity of the on -site water source, we were only able to maintain approximately 8.5-inches of standing water within the hole instead of the recommended 12-inches. This level was maintained within the PIT for approximately 6 hours for the pre soak period of the test. At this time, the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 075 water flow meter. Plate 3: Infiltration PIT 1 location looking southeast. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 2101h Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 6 After the 6-hour soaking period was completed, the water level was maintained at approximately 8.5- inches for one hour for the steady-state period. The flow rate for Infiltration Pit 1 stabilized at 2.50 gallons per minute (150 gallons per hour), which equates to an approximate infiltration rate of 8.95 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes. After 60 minutes, the water level within the pit dropped approximately 8.0 inches, resulting in an infiltration rate of 8.0 inches per hour. At the conclusion of the testing, Infiltration PIT 1 was excavated an additional two feet to a total depth of 7.0 feet below the existing ground surface and no indications of groundwater seepage and/or impermeable zones were observed. CONCLUSIONS It is our opinion that the subsurface soils within the site are suitable for traditional stormwater infiltration. In accordance with the Table 3.5 of the 2019 SWMMWW, correction factors of 0.8, 0.5, and 0.9 for site variability and number of locations tested (CFv), testing method (CFt), and degree of influent control to prevent siltation and bio-buildup (CFm), respectively were applied to the field measured infiltration rate of 8.0 inches per hour, selected from the falling -head portion of the testing. A total correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head portion of the test to determine the long-term design infiltration rate. Using the above correction factor, we calculated a long-term design infiltration rate of approximately 2.88 inches per hour. The base of the infiltration systems should terminate within the native glacial outwash soils encountered at depth. The stormwater infiltration systems should be designed and maintained in accordance with 2019 SWMMWW. The stormwater manual recommends a minimum five-foot separation between the base of an infiltration system and any underlying bedrock, impermeable horizon, or groundwater. We did not observe any indications of or encounter groundwater and/or impermeable layers within our explorations to a depth of up to 9.5 feet below the existing ground surface within the site. We anticipate that the seasonal high groundwater table within the area is located at a greater depth below the site and should not adversely impact the proposed infiltration systems within the site. We recommend that any infiltration systems be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing property lines, structures, and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located within proposed fill areas within the site associated with site grading or retaining wall backfill as such condition could lead to failures of the placed fills and/or retaining structures. We should be retained to evaluate the infiltration system design and installation during construction, if necessary. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 2101h Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 7 USE OF THIS LETTER This letter was prepared for Mr. BK Chen and his agents, for use in planning and budgeting the above - referenced project only. Our services included an evaluation of the infiltration capability of the site soils at specific locations and should not be considered as an in-depth geotechnical study of the site or an evaluation of the overall site stability. This letter may be used for bidding and estimating purposes, but our letter, conclusions, and interpretations should not be construed as a warranty of the subsurface conditions. The subsurface conditions between explorations may vary. A contingency for varying conditions should be incorporated into the project plans. We recommend that NGA be retained to review the design and provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre -construction meetings if requested. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this letter was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. • • • NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Letter (2"d Revision) — UPDATED NGA File No. 1209720 Chen 210th Street SW Townhome Residential Development Infiltration October 16, 2023 Edmonds, Washington Page 8 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. LEE S. BELLAH Lee S. Bellah, LG Project Geologist .2023 Khaled M. Shawish, PE Principal LSB:KMS:dy Four Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale 204th St SW zoau SW Smart Foodservice Q 2041h St SW Warehouse Stores s � _ , s ' College Place Doug Hyundai 206thStSW Middle School _ D © El Antojo I Lynnwood a s � Project Walgreens Drug store 208th St SW r 208th St sw Site A T T L E HEIGHTS 208u, St sw P Pinewood Square 0 a Apartments > f � < W Snohomish County PUD Q > Lynnwood Office ` Burger King © se O sw Edmonds-Woodway © 212th St SW 212th St SW 212th St SW 212th St SW 2129 High School 01 a p Park 212 Q a > CarMax ` ® <' Q 30 Used car dealer g Southwest F'- 6 & Transf McDonald's 15 Swedish Edmonds Campus g E 216th S! SW / 2161h St SW Z Pr6mera Blue Cross Q : 0 QUpstaging Seattle 75 �4 0 Q � t e1i s. sv / � 21E1r s, "AI t ® WinCo Foods N Edmonds, WA N U N Project Number Chen 210th St SW NELSON GEOTECHNICAL No. Date Revision By I CK N 1209720 Residential Development NGA ASSOCIATES, INC. 1 10/23/20 Original DPN LSB N Infiltration GEOTECHNICAL ENGINEERS & GEOLOGISTS 6 E Figure 1 g Vicinity Map Woodinvillel�ce Wenatchee Office Woodi5ihe,WA9,A-500 105Pal,W St Wootllnville, WA 980]2 Wenatchee, WA 96801 9 (425) 486-16691 Fax:481-2510 www. ne,ongeotech—, (509) 665-76961 Fax: 665-7692 5 Existing Residence FF=413' --1ST LEVEL: 38.75'X21't (814SF) 2ND LEVEL: 39.75'X21'± (835S Ii -3RD LEVEL: 40.75'X21't (856SF) l� FF=411.5' i r 1ST LEVEL: 38.75'X21't (814SF) I 2ND LEVEL: 39.75'X21't (835SF) i� 3RD LEVEL: 40.75'X21't (856SF) 410 FF=409.5' 1ST LEVEL: 38-75'X21't (814SF) r 2ND LEVEL: 39.75'X21't (835SF) TP-1 3RD LEVEL: 40.75'X21't (856SF) I l i 408 1ST LEVEL: 38.75'X21't (814SF) J Proposed Townhome 2ND LEVEL: 39.75'X21't (835SF) m i 3RD EVEL: 40.75'X21't (typ) il- m F 406 _ FF=407.0' 111't (35SF)12' 0 7— N SB-2 3RD LEVEL: 40.75'X21't (856SF)TLV1S �Tll LEGEND I FF=406.5' 1ST LEVEL: 38.75'X21't (814SF) 2ND LEVEL: 39.75'X21't (835SF) 3RD LEVEL: 40.75'X21't (856SF) 25'-0" INF-1 TP-2 DRIVEWAY PAVEMENT: 41 L 74'-6" `a Property line la INF-1 v Number and approximate 0 20 40 r location of infiltration test pit TP-1 Number and approximate Scale: 1 inch = 20 feet Q location of test pit Reference: Site Plan based on an undated, untitled site plan. z Project Number Chen 210th St SW NELSON GEOTECHNICAL No. Date Revision By LL CK a 1209720 Residential Development Infiltration N G A ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 10/23/20 Original l LSB c z Figure 2 g Site Plan WoodZWk ore�a 17311.135th Ave. NE, A•500 Wood —me , WA98072 Wonatch oOMIw 105 P,,,--Sl wenamnae. WA 98801 cC. 14251486-1889+Fa, 481-2510 neiSIXgeon.' hmll 1509)665-7696l Fa,'. 665-7692 2 UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests is based on ASTM D 2488-93. Moist - Damp, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number Chen 210th St SW NELSON GEOTECHNICAL No. Date Revision By CK 1209720 Residential Development Infiltration IVGA ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS We dinville Office Wenatchee Office 1 10/23/20 Original DPN LSB Figure 3 Soil Classification Chart na,1-,65thA"e ..A-5°° Wenatchee, WA W1-135tlle WA980-2 Wenatchee,WA t. (425) 48fi-1669 / Fax:481-2510 www. nelsongeotech.com (509) 665-769fi /Fax: 665-7fi92 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION TEST PIT 1 0.0 - 1.0 GRASS UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOSE, MOIST) (TOPSOIL) 1.0 - 2.0 DARK BROWN TO BLACK SILTY FINE TO MEDIUM SAND WITH GRAVEL, ORGANICS AND DEBRIS (LOOSE, MOIST) (UNDOCUMENTED FILL) 2.0 - 3.0 BURIED TOPSOIL 3.0 - 9.5 SP-SM BROWN -GRAY, FINE TO MEDIUM SAND WITH SILT AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 5.0, 7.0, 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT COMPLETED AT 9.5 FEET ON 10/9/20 TEST PIT 2 0.0 - 1.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOSE, MOIST) (TOPSOIL) 1.0 - 9.5 SP-SM BROWN -GRAY, FINE TO MEDIUM SAND WITH SILT AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 4.0, 7.0, AND 8.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT COMPLETED AT 9.5 FEET ON 10/9/20 INFILTRATION PIT 101 0.0 - 1.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOSE, MOIST) (TOPSOIL) 1.0 - 7.0 SP-SM BROWN -GRAY, FINE TO MEDIUM SAND WITH SILT AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT COMPLETED AT 7.0 FEET ON 10/9/20 LSB:KMS NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1209720 FIGURE 4