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20-103186-Geotechnical (Soils) Report-08-20-2020-V1 DE-Civil, PLLC P.O. Box 446 Tacoma, WA 98401 Ph (253) 228-0513 www.de-civil.com March 16, 2020 Bill and Jenny Horner 35507 – 6th Avenue Southwest Federal Way, Washington 98023 Geotechnical Report Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington PN: 3021049112 Job: 18-217 Horner.GR INTRODUCTION This geotechnical report summarizes our site observations and provides geotechnical recommendations and design criteria for the proposed Short Plat to be located at 35507 – 6th Avenue Southwest in the City of Federal Way, Washington. The site is currently developed and has been cleared of native vegetation and is well vegetated grass and ornamental trees. The general location of the site is shown on the attached Site Vicinity Map, Figure 1. Our understanding of the project is based on our discussions with you, and our experience in the area. We understand that the proposed development will include subdividing the property into a total of three lots with the future construction a single-family residence on each lot. It is anticipated that the buildings will consists of typical wood frame construction supported on conventional concrete spread footings. The existing site configuration is illustrated on the attached Site Plan, Figure 2. SCOPE The purpose of our services is to evaluate the surface and subsurface conditions at the site as a basis for developing and providing geotechnical recommendations and design criteria for the proposed drainage system. Specifically, our scope of services for the project included the following: 1. Conducting a geologic reconnaissance of the site area. 2. Exploring the subsurface conditions by monitoring the excavation of four track-hoe excavated test pits at select locations across the site. 3. Addressing the appropriate geotechnical regulatory requirements for the proposed site development. 4. Determining the local groundwater depth, for dewatering and drainage design purposes. 5. Provide our opinion with regard to the feasibility of on -site stormwater infiltration/dispersal and provide an infiltration rate, if appropriate. SITE CONDITIONS Surface Conditions The parcel is mostly rectangular in shape and measures approximately 150 feet in width, east to west, by 290 feet in length, north to south, and encompasses 1.08 acres. The site is currently developed with a single-family residence onsite. The site slopes down from south to north at grades of approximately 5 percent. Overall topographic relief across the site is approximately 12 feet. Off-site to the north is a regional drainage facility that includes a 2:1 slope down to the facility with an overall elevation change of approximately 30 feet. A site plan showing the existing contours is attached as Figure 2. 18-217 Horner.GR March 16, 2020 Page 2 No deep-seated slopes instability was observed at the site or the adjacent areas. No erosion or other soil movement was observed at the time of our site visit. Site Soils The USDA Natural Resource Conservation Service Web Soil Survey for King County maps the soils as Everett gravelly sandy loam (Evb & Evd). The Eveerett soils consist of moderately deep, well drained soils formed in glacial drift and outwash on slopes of 0 to 8 percent and 15 to 30 percent. Theses soils are known to have a high rate of water transmission. An excerpt of the USDA Soil map is included as Figure 3. Site Geology According to the 7.5 Minute Quadrangle for Poverty Bay, by Derek B. Booth, Howard H. Waldron and Kathy Goetz Troost, 2003, shows the site as underlain by Vashon Ice Contact (Qvi). These glacial soils were deposited during the Vashon Stade of the Fraser Glaciation, approximately 12,000 to 15,000 years ago. The ice contact deposits consists of well to poorly sorted mixture of sand and gravel that was partially overridden by glacial ice. Since they were partially overridden by the ice mass and can be considered normally consolidated, are in a dense condition, and exhibit moderate strength and compressibility characteristics where undisturbed. Based on our test pits the site soils are consistent with the available mapping for the area. An excerpt of the Geological map is included as Figure 4. Subsurface Explorations On October 30, 2018 we explored subsurface conditions at the site by monitoring the excavation of two test pits to depths varying between 4.5 - 5.5 feet. The test pits were approximately located in the field by our representative by pacing from existing features such as roadways and lathe left by the previous survey of the parcel. The approximate location of the explorations are indicated on the attached Site Plan as Figure 2. An engineer from our office maintained logs of the subsurface conditi ons encountered in each test pit, obtained representative soil samples, and observed pertinent site features. Representative soil samples obtained from the test pits were placed in sealed containers and taken to a laboratory for possible further examination and testing. Subsurface Conditions Our explorations encountered consistent subsurface conditions. The test pits showed a silty sand and gravel to a depth of approximately 4.5 feet. Soil logs are included as Figure 4. Groundwater Conditions No groundwater was encountered in either Test Pit. Based on our test pits, we do not anticipate that local groundwater will occur at shallow elevations. However, groundwater levels will occur in response to precipitation patterns, off-site construction activities, and site utilization. Perched groundwater typically develops when the vertical infiltration of precipitation through a more permeable soil is slowed at depth by a deeper, less permeable soil type. Geologically Hazardous Areas – per City of Federal Way Municipal Code, Chapter 13.6.4.7 Geologically hazardous areas means areas which because of their susceptibility to erosion, landsliding, seismic or other geological events require specific studies to determine appropriate buffers or property use. Geologically hazardous areas include the following areas: erosion hazard areas, landslide hazard areas, and seismic hazard areas. 18-217 Horner.GR March 16, 2020 Page 3 No critical areas are present on or near this site. CONCLUSIONS Based on the results of our data review, site reconnaissance, subsurface explorations and our experience in the area, it is our opinion that the site is suitable for development. Conventional footings may be utilized in areas of medium dense to dense native soil. The infiltration of stormwater runoff is feasible for the project. Pertinent conclusions and geotechnical recommendations regarding the design and construction of the proposed development are presented below. Site Preparation Areas to be graded should be cleared of deleterious matter including any existing structures, foundations, abandoned utility lines, debris and vegetation. The portions of the site covered with vegetation should be stripped of any duff and organic-laden soils. These materials can be stockpiled and later used for erosion control. Material that cannot be utilized on site should be removed from the site. Where placement of fill material is required, the stripped/exposed subgrade areas should be compacted to a firm and unyielding surface prior to placement of any fill. Excavations for debris removal should be backfilled with structural fill compacted to the densities described in the “Structural Fill” section of this report. We recommend that a member of our staff evaluate the exposed subgrade conditions after removal of vegetation and topsoil/fill stripping is completed and prior to placement of structural fill. The exposed subgrade soil should be proof-rolled with heavy rubber-tired equipment during dry weather or probed with a 1/2 inch-diameter steel rod during wet weather conditions. Any soft, loose, or otherwise unsuitable areas delineated during proof-rolling or probing should be recompacted, if practical, or over-excavated and replaced with structural fill, based on the recommendations of our site representative. The areas of old fill material should be evaluated during grading operations to determine if they need mitigation; recompaction or removal. Structural Fill All material placed as fill associ ated with mass grading, as utility trench backfill, under building areas, or under roadways should be placed as structural fill. The structural fill should be placed in horizontal lifts of appropriate thickness to allow adequate and uniform compaction of each lift. Fill should be compacted to at least 95 percent of MDD (maximum dry density as determined in a ccordance with ASTM D 1557). The appropriate lift thickness will depend on the fill characteristics and compaction equipment used. We recommend that the appropriate lift thickness be evaluated by our field representative during construction. We recommend that our representative be present during site grading activities to observe the work and perform field density tests. The suitability of material for use as structural fill will depend on the gradation and moisture content of the soil. As the amount of fines (material passing US No. 200 sieve) increases, soil becomes increasingly sensitive to small changes in moisture content and adequate compaction becomes more difficult to achieve. During wet weather, we recommend use of well -graded sand and gravel with less than 5 percent (by weight) passing the US No. 200 sieve based on that fraction passing the 3/4-inch sieve, such as Gravel Backfill for Walls (9-03.12(2)). If prolonged dry weather prevails during the earthwork and 18-217 Horner.GR March 16, 2020 Page 4 foundation installation phase of construction, higher fines content (up to 10 to 12 percent) will be acceptable. Material placed for structural fill should be free of debris, organi c matter, trash and cobbles greater than 6-inches in diameter. The moisture content of the fill material should be adjusted as necessary for proper compaction. Suitability of On-Site Materials as Fill During dry weather construction, any non-organic on-site soil may be considered for use as structural fill; provided it meets the criteria described above in the structural fill section and can be compacted as recommended. If the soil material is over-optimum in moisture content when excavated, it will be necessary to aerate or dry the soil prior to placement as structural fill. We generally did not observe the site soils to be excessively moist at the time of our su bsurface exploration program. The areas of native sandy gravel material are comparable to sandy “pit run” and may be used as structural fill. We recommend that completed graded-areas be restricted from traffic or protected prior to wet weather conditions. The graded areas may be protected by paving, placing asphalt-treated base, a layer of free-draining material such as pit run sand and gravel or clean crushed rock material containing less than 5 percent fines, or some combination of the above. Site Drainage All ground surfaces, pavements and sidewalks at the site should be sloped away from structures. The lot should also be carefully graded to ensure positive drainage away from all structures and property lines. Surface water runoff from the roof area, driveways, perimeter footing drains, and wall drains, should be collected, tightlined, and conveyed to an appropriate discharge point. We recommend that footing drains are installed for the townhome(s). The roof drains should not be connected to the footing drain. Pilot Infiltration Test On December 1, 2018 we returned to the site to perform a full-scale infiltration test, also known as a Pilot Infiltration Test (PIT). During the test we excavated a pit to the depth of approximately 3 feet exposing the existing sand and gravel soils. The test was located in the north east area of the site (see Figure 2 Site Plan) and was performed in general conformance with the City drainage manual requirements and determined an average field measured rate of 24.1 inches per hour. The correction factors should be 0.5 for testing, 0.65 for geometry and 0.9 for plugging, resulting in a long -term design infiltration rate of 7 inches per hour. After the test we excavated the bottom of the pit an additional foot and found no evidence of groundwater or mounding a fter the test was performed. LID Feasibility The following table presents our recommendations for various Best Management Practices (BMP’s) for various Low Impact Development (LID) BMP’s. 18-217 Horner.GR March 16, 2020 Page 5 BMP Viable Limitations or Infeasibility Criteria Lawn and Landscape Areas T5.13: Post Construction Soil Quality and Depth Yes None Roofs T5.30: Full Dispersion No There is insufficient native vegetation. T5.10A: Downspout full infiltration systems. Yes Full infiltration can be accomplished if there is sufficient room for the individual infiltration trenches on each lot. Bio Retention Yes This BMP is feasible for the same general reasons as for full infiltration. T5.10B Downspout Dispersion systems No For similar reasons given for full dispersion. T5.10C Perforate stub connections No Other Hard Surfaces T5.30 Full Dispersion No There is insufficient native vegetation. T5.15 Permeable Surfacing Yes The site infiltration rates are sufficient to allow for permeable surfacing. Bio Retention Yes This BMP is feasibile for the same general reasons as for full infiltration. T5.12: Sheet Flow Dispersion T5.11 Concentrated flow dispersion No For similar reasons given for full dispersion. Site Buildable Area Based on our test pits and correlation of site vegetation with an ticipated limits of the peat we have delineated the approximate limits of the peat on the Site plan Figure 2. We recommend that any proposed development occur outside of the peat areas. Should development be proposed within the peat area special considerations will need to be employed which are beyond the scope of this report. LIMITATIONS We have prepared this report for Gian Singh and other members of the design team for use in evaluating a portion of this project. Su bsurface conditions described herein are based on our observations of exposed soils on the parcel. This report may be made available to regulatory agencies or others, but this report and conclusions should not be construed as a warranty of subsurface conditions. Subsurface conditions can vary over short distances and can change with time. Within the limitations of scope, schedule and budget, our services have been executed in accordance with generally accepted practices in this area at the time this report was prepared. No warranty, express or implied, should be understood. 18-217 Horner.GR March 16, 2020 Page 6 We trust this is sufficient for your current needs. Should you have any questions, or require additional information, please contact us at your earliest convenience. Respectfully submitted, Development Engineering, PLLC Glen Coad, PE Owner WGC Doc ID: 18-217 Horner.GR Attachments: Figure 1: Vicinity Map Figure 2: Site Plan Figure 3: USDA Web Soil Survey Figure 4: USGS Geological Map Figure 5: Soil Classification Chart Figure 6: Test Pit Logs DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com VICINITY MAP Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 1 Not to Scale Approximate site location Not to Scale Approximate Test Pit location DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com Site Plan Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 2 TP-X TP-1 TP-2 DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com USDA WEB SOIL SURVEY Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 3a Not to Scale Approximate site location DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com USDA WEB SOIL SURVEY LEGEND Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 3b DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com USGS GEOLOGICAL MAP – POVERTY BAY Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 4 Not to Scale Approximate site location DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com SOIL CLASSIFICATION CHART Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 5 Test Pit TP-1 Location: (See Figure 2) Depth (ft) Soil Type Description 0.0 – 1.0 Topsoil 1.0 – 2.5 SM Brown silty SAND and gravel (loose, moist) 2.5 – 4.5 SM Tan silty SAND and gravel (med dense, moist) 4.5 – 5.5 SM Grey silty SAND w/gravel (dense, moist) Terminated at 5.5 feet below ground surface. No caving observed. No Seepage observed. Slightly cemented material present at depth. Test Pit TP-2 Location: (See Figure 2) Depth (ft) Soil Type Description 0.0 – 1.0 Topsoil 1.0 – 3.0 SM Brown silty SAND and gravel (loose, moist) 3.0 – 4.5 SM Tan silty SAND and gravel (loose, moist) Terminated at 4.5 feet below ground surface. No caving observed. No Groundwater encountered. No cemented material present. Logged by WGC, 10/30/2018 DEVELOPMENT ENGINEERING, PLLC P.O. Box 446 Tacoma, Washington 98401 (253) 228-0513 www.de-civil.com SOIL LOGS Proposed Short Plat 35507 – 6th Avenue Southwest Federal Way, Washington Job No: 18-217 December 2018 Figure 6