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REVIEWED STM_RESUB1 BLD2021-0657+INFILTRATION+12.16.2021_4.48.36_PM+2578440NELSON GEOTECHNICAL ASSOCIATES. INC. September 3, 2021 Mr. Kevin Kopak 346 Sunset Avenue North Edmonds, Washington VIA Email: jkopak@eastsidestaple.com Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration 346 Sunset Avenue North Edmonds, Washington NGA File No. 1287921 17311-135" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com Dear Mr. Kopak: This letter documents our explorations and provides our opinions and recommendations for the feasibility of stormwater infiltration for the proposed detached garage addition located at 346 Sunset Avenue North in Edmonds, Washington as shown in the Vicinity Map in Figure 1. The parcel number for the subject property is 27032300100400. INTRODUCTION The site is currently occupied by a single-family residence and a detached shed within the western and northeastern portions of the property, respectively. The ground surface within the site is generally level. We understand the proposed development will include constructing a new detached garage structure within the north -central portion of the property. The existing site layout is shown on the Schematic Site Plan in Figure 2. For use in preparing this letter, we have been provided with a plan set titled "Kopak Residence," dated May 10, 2021 and prepared by Strobl Design, LLC, 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 2 We have been requested to evaluate the infiltration capacity of the site soils within the property. The City of Edmonds utilizes the 2014 Department of Ecology (DOE) Stormwater Management in Western Washington Manual (2014 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). SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions and provide general recommendations for site development. Specifically, our scope of services included: 1. Reviewing available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site with trackhoe excavated test pits. The trackhoe was provided by NGA. 3. Performing grain -size sieve analysis on selected soil samples, as needed. 4. Providing long-term design infiltration rates based on one on -site small-scale Pilot Infiltration Test (PIT) per the 2014 SWMMWW. Location and depth of the test was determined by civil engineer. Water for the test was secured by client. 5. Providing recommendations for infiltration system installation. 6. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical letter. SITE CONDITIONS Surface Conditions The site consists of a rectangular -shaped parcel covering approximately 0.24 acres. The site is occupied by a single-family residence with a detached shed within the western and northeastern portions of the property, respectively. Topography within the site is generally level. Vegetation within the site consists of grass -covered yard areas bordered by shrubs. The site is bordered by Sunset Avenue North to the northwest and by residential properties to all other side. We did not observe surface water within the site during our site reconnaissance on August 12, 2021. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation NGA File No. 1287921 Kopak Detached Garage Addition Infiltration September 3, 2021 Edmonds, Washington Page 3 Subsurface Conditions Geology: The geologic units for this area are shown on Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by Minard, J.P., (USGS, 1983). The site is mapped as Whidbey Formation (Qw) with Vashon glacial till (Qvt) mapped nearby. Whidbey formation is generally described as bedded, compact, commonly oxidized, medium- to coarse -grained sand. Glacial till is described as a non -sorted, compact diamict of silt, sand, and sub -rounded to well-rounded clasts. Our explorations generally encountered undocumented fill soils underlain by gray to gray -brown, silty fine to medium sand with varying amounts of gravel and iron -oxide staining, consistent with the description of glacial till deposits at depth. Explorations: The subsurface conditions within the site were explored on August 12, 2021 with one test pit to an approximate depth of 4.0 feet below the existing ground surface using a mini-trackhoe. We also performed one hand auger to a depth of approximately 4.0 feet below the existing ground surface. The approximate locations of our explorations are shown on the Schematic 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 test pits. 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 logs of the explorations should be reviewed. At the surface of Infiltration Pit One and Hand Auger One, we generally encountered approximately 2.0 - feet of dark brown, silty sand with varying amounts of roots, gravel, and organics which we interpreted as topsoil and/or undocumented fill soils. Underlying the topsoil and/or undocumented fill soils in each of the exploration, we generally encountered gray -brown, fine to medium silty sand with varying amounts of iron -oxide staining, gravel, and trace boulders which we interpreted as native glacial till soils. Infiltration Pit One and Hand Auger One were both terminated within the native glacial till soils at a depth of 4.0-feet below the existing ground surface. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 4 Hydrogeologic Conditions We did not encounter ground water seepage in any of our explorations. If groundwater is encountered within the site, we would interpret this to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered soils and undocumented fill. The low permeability soil consists of relatively silty native deposits. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. INFILTRATION TESTING We conducted a Small PIT in accordance with the 2014 Stormwater Management Manual for Western Washington (2014 SWMMWW) within Infiltration Test Pit One located as shown on the attached Site Plan in Figure 2. Infiltration Pit 1 measured 4.0 feet long by 3.0 feet wide by 4.0 feet deep. The subsurface soils generally consisted of silty fine to medium sand with gravel, which we interpreted as native glacial till deposits. The pit was filled with 12-inches of water and this level was maintained for six hours for the pre-soak period. At the end of the pre-soak the water level within the pit had remained at 12-inches with no additional water being added to the pit. As a result, the infiltration testing was concluded. CONCLUSIONS Based on the results of the small -PIT performed within the site, and the relatively silty compact nature of the native glacial till deposits that underlie the site, it is our opinion that the on -site native glacial soils encountered at depth are not conducive for traditional stormwater infiltration systems. Stormwater generated from proposed structures and other hard surfaces within the site should be directed to on - site detention systems and ultimately into an approved point of discharge likely found within the adjacent roadways. NELSON GEOTECHNICAL ASSOCIATES, INC. Stormwater Infiltration Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 5 USE OF THIS LETTER NGA has prepared this letter for Mr. Kevin Kopak and their agents, for use in the planning and design of 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 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 differ from those anticipated, and to evaluate whether or not earthwork and foundation installation 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 report 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 Evaluation Kopak Detached Garage Addition Infiltration Edmonds, Washington NGA File No. 1287921 September 3, 2021 Page 6 It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Sarah L. Dunn Staff Geologist Lee S. Bellah, LG Project Geologist 09.03.2021 Khaled M. Shawish, PE Principal SLD:LSB:KMS:dy Four Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale �Q Portofino o�� \ Restaurant & Bar \ Maplewood P,90 Dr _Puget Ot & Gerr MdodrLn Edmonds E T z Elementary School < z e N Mlndev Ln I V4w1a06 Way � n.. Project Z J Edmonds Food Bank Site 0 - theta PI THE BOWL Brackett s OF EDMONDS a Landing Shoreline z r Sanctuary... Pf' 104 Brackett s -• "y SI I A Daley St Landing North F Sprague St >' Hummingbird z Hill Park !nEdmonds Ferry Terminal sell St D Bill St > dsa z z Edmonds ant Main St ds W oayton s Dayton St gMapla Way Port of Edmonds Maple St ' > Maple St w J Yost Pool Q Alder St Alder St a All Beck Ln y Walnut Street CoffeeQ Walnut St Yost Park Edmonds Shelleberger „�„yn, o° Marsh Creek �`��`�' �dO�n waY N SDlucest-_ < r Spruce St Edmonds D CityPark = q hemlock Way N w _ 214th PI SW Edwards Q a n miniums Edmonds, WA Project Number IIELson GEOTECHIIICAL No. Date Revision By cK Z Kopak Residence Detachedlimb1 1287921 Garage Addition Infiltration ASSOCIATES, incR 8/25/21 Original DPN ABR N Figure 1 Vicinity Map W135th -NE,e Wenatohee° t. 17311-1351h Ave. NE, A-500 105 Palouse St. Wood-ille, WA 98072 Wenatchee, WA 98801 0 OU i vrvuw.nelsongeolech.com (425)486-1669/Fax. 481-2510 (509)665-7696/Fax : 665-7692 5 N Schematic Site Plan w _ 4 z fD CONJ C CD Cl)v a Cj- � (0 3 CD m 3 a� CD ; ? m Existing Residence CD CD v n 0 3 � CD Q 3 M - 6Na a Cn cn O D , ° cn = a m r m —1 cn m - � :0 n.- All ,Z LEGEND v — — Property line INF-1 —�— Number and approximate ° I location of infiltration test pit 3 HA-1 W _ I Number and approximate Z ` I location of hand auger D 0 M X Reference: Site plan based on field measurements, observations, and aerial Mill\company\2021 NGA Project Folders\12879-21 Kopak Residence Detached Garage Addition Infiltration Edmonds\Drafting\SP.dwg map review. Existing Shed 0 30 60 Approximate Scale: 1 inch = 30 feet _40 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 nELSOn GEOTEnninl_ No. Date Revision By cK Kopak Residence Detached � 1287921 Garage Addition Infiltration ASSOCIATES, inc1 8/25/21 Original DPN ABR Figure 3 Soil Classification Chart ""135thA°e°""e Wa"a`°hee.S"e 17311-1351h Ave. NE, A-500 105 Palouse St. Woodinville, WA 98072 Wenatchee, WA 98801 www.nelsongeoiech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION INFILTRATION TEST PIT ONE 0.0-2.0 SOD UNDERLAIN BY DARK BROWN, ORGANIC -RICH, SILTY SAND WITH ROOTS AND GRAVEL (LOOSE, DRY TO MOIST) (UNDOCUMENTED FILL) 2.0-4.0 SM ORANGE -BROWN TO GRAY -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON -OXIDE STAINING, AND TRACE BOULDERS (MEDIUM DENSE TO DENSE, MOIST) (TILL) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 4.0 FEET ON 08/12/21 HAND AUGER ONE 0.0 - 2.0 SOD UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ORGANICS, AND ROOTS (LOOSE, DRY) (UNDOCUMENTED FILL) 2.0-4.0 SM LIGHT BROWN TO GRAY -BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND IRON -OXIDE WEATHERING (MEDIUM DENSE, MOIST) (TILL) SAMPLE WAS COLLECTED AT 4.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED HAND AUGER CAVING WAS NOT ENCOUNTERED HAND AUGER WAS COMPLETED AT 4.0 FEET ON 08/12/21 SLD:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1287921 FIGURE 4