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ENG SI REPORT 4.11.2024NELSON GEOTECHNICAL ASSOCIATES. INC. MEMORANDUM DATE: April 11, 2024 TO: Mr. Garth Hitchens C/O: Robin Kearsley — Kearsley Homes FROM: Khaled M. Shawish, PE Sarah L. Dunn, GIT RE: Geotechnical Engineering Memorandum Hitchens Residence Development 17901 Vista Del Mar Drive Edmonds, Washington NGA File No. 1065318 17311-1351" Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com t024 This memorandum presents our comments and recommendations regarding the drywell and permanent erosion control at the Hitchens Residence Development project located at 17901 Vista Del Mar Drive in Edmonds, Washington. INTRODUCTION A two-story, single-family residence with a daylight basement was recently constructed within the upper east -central portion of the property. The western portion of the structure consists of cantilevered building space, as well as main floor cantilevered deck areas. The upper eastern and central portions of the site are generally relatively level to gently sloping with a steep northwest -facing bluff slope descending from the central portion of the site down to the Burlington Northern Santa Fe (BNSF) railroad and the shores of Puget Sound. We previously prepared a geotechnical engineering evaluation for the residential development dated December 20, 2018, a memorandum regarding bluff setbacks and vegetation management dated February 1, 2019, a plan review letter dated April 22, 2020, a dry season grading extension letter dated April 2, 2021, and a memorandum dated June 17, 2021 regarding fencing near the top of the bluff. We also provided geotechnical construction monitoring for the residence development between May 13, 2021 and July 16, 2021 and recorded our observation in Field Reports 1 through 8. We understand that a drywell has been installed beneath the driveway on the south side of the site and that the City of Edmonds is requiring a geotechnical review of the design and installation. We also understand that final erosion control has been implemented. Geotechnical Engineering Memorandum Hitchens Residence Development Edmonds, Washington NGA File No. 1065318 April 11, 2024 Page 2 For our use in preparing this memorandum we were provided with the following documents: • An engineering memorandum titled "Installation of structural slab over infiltration gallery, dated August 14, 2023, and prepared by Greg Guillen with CG Engineering. • Drywell plans titled "Downspout Infiltration Drywell," dated July 5, 2023, and prepared by Kearsley Homes, Inc. • A site plan titled "Hitchens Residence," dated July 27, 2023, and prepared by Kearsley Homes, Inc. • A landscaping plan titled "Landscape Plan," dated August 15, 2020, and prepared by Kearsley Homes, Inc. • A site plan titled "Site Plan," dated March 28, 2024, and prepared by Kearsley Homes, Inc. REVIEW Drywell We understand that a drywell has been constructed underlying the driveway on the south side of the site to manage runoff from an additional 745 square feet of impervious surfaces. The plans indicate that the system is 10 feet wide by 10 feet long, lined with filter fabric, and terminates 5.1 feet below finished grade. Plans also show the bottom 2.5 feet of the system filled with 1%-inch clean crushed rock, topped with 1 foot of 5/8 inch clean crushed rock, a 6-inch-thick layer of rebar reinforced concrete, and 6 inches of compacted 5/8 inch minus crushed rock. We previously performed a geotechnical boring within the vicinity of the new drywell, which extended 25.3 feet below the surface and encountered native outwash soils at a depth of 4.0 feet, consisting of fine to coarse sand with gravel and silt. We have reviewed photographs taken during installation and the base of the system appears to be within the native recessional outwash soils. We understand the system was sized and designed using the rtment of Ecoloev's 2019 Stormwater Manual for Western Washington (2019 SWMMWW standards for fine or loamy sand. In our opinion, this is the appropriate classification for the native soils within this site. No groundwater was encountered within the site during construction or within any of our previous geotechnical borings. It is our opinion that the newly installed drywell has been designed and installed in accordance with the 2019 SWMMWW and should be adequate to manage runoff generated by the additional impervious areas without negatively affecting the subject or neighboring structures. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Memorandum Hitchens Residence Development Edmonds, Washington NGA File No. 1065318 April 11, 2024 Page 3 Permanent Erosion Control We visited the site on April 3, 2024 to evaluate the site conditions and landscaping. We observed that turf has been placed within the yard area west of the residence. All other areas disturbed during construction that were not covered by impervious materials were surfaced with mulch and planted with native deep-rooted vegetation including: Weeping Canadian Hemlock, Mikawa yatsubusa Japanese Maple, Salal, Oregon grape, and Winter green. We did not observe any exposed soils within the site during our visit. Plans call for installation of matting secured with wire staples between the edge of the grass and top of the slope. This was not installed at the time of our visit. However, in our opinion, with the erosion control already in place, the matting can be omitted without negatively impacting site or slope stability. It is also our opinion that the implemented planting and ground coverage provides adequate permanent erosion control for this site. Fencing While onsite, we observed that the chain link fence previously situated at the top of the steep slope on the west side of the site had been removed and replaced with a metal bar fence. We were not present for the installation of the fence, nor did we provide any consultation during its design. However, the homeowner informed us that the fence post extended a minimum of 2.0 feet below the ground surface. We did not observe any leaning within the fence or signs of instability within the steep slope. In our opinion, the newly installed fence has not negatively impacted site slopes and should be stable in its current configuration. We trust that this memorandum should satisfy your needs at this time. Please do not hesitate to contact us should you require anything further. NELSON GEOTECHNICAL ASSOCIATES, INC.