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LETTER 1065318 Hitchens Residence Plan ReviewNELSON GEOTECHNICAL ASSOCIATES. INC. April 22, 2020 Mr. Garth Hitchens 3380 — 126t" Avenue NE Bellevue, Washington 98005 C/O Mr. Robin Kearsely— Kearsely Homes Geotechnical Plan Review Letter Hitchens Residence Development 17901 Vista Del Mar Drive Edmonds, Washington NGA File No. 1065318 Dear Mr. Hitchens: 17311-135tf' Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com This letter presents the results of our geotechnical engineering review of the plans for your residence development project located at 17901— Vista Del Mar Drive in Edmonds, Washington. INTRODUCTION Project plans consist of constructing a new two-story, single-family residence with a daylight basement within the upper east -central portion of the property. The western portion of the structure will consist 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 have previously prepared a geotechnical engineering evaluation for the project titled "Geotechnical Engineering Evaluation - REVISED — Hitchens Residence Development — 17901 Vista Del Mar Drive — Edmonds, Washington," dated December 20, 2018. We also prepared a memorandum titled "Bluff Setback Vegetation Management — Hitchens Residence Development," dated February 1, 2019. For our use in preparing this letter, we were provided with an architectural plan set titled "Hitchens Residence," prepared by Kearsley Homes, Inc., dated October 6, 2018 and a preliminary structural plan set titled, "Hitchens Residence," prepared by CG Engineering. Geotechnical Plan Review Letter NGA Project No. 1065318 Hitchens Residence Development April 22, 2020 Edmonds, Washington Page. 2 The site falls within severe erosion hazard and landslide hazard area as indicated on the City of Edmonds GIS web map. This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, and the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. PLAN REVIEW AND CONCLUSIONS We have reviewed the geotechnical aspects of the most recent plans and found the plans to be in general compliance with our recommendations as presented in our previous geotechnical report and memorandum. The structural plans indicate that the western portion of the proposed residence as well as the western 10 feet of the northern and southern foundation elements are to be supported on 15, 18- inch diameter auger cast piles as recommended in our report. Foundations for the residence will consist of conventional shallow spread footings and a grade beam along the western edge of the residence to support cantilevered portions of the building. A minor discrepancy was found regarding the pile diameter on sheet S1.1 (structural notes) and S2.1 (foundation plan); referencing 18-inch dimeter piles within the structural notes and 20-inch diameter piles on the foundation plan. The plans should be corrected to indicate consistent pile diameter specifications. The plans specify augercast piles are to be embedded per geotech report and ultimately determined in the field, however we anticipate the embedment depth will be approximately 30-feet below the existing ground surface. Structure setbacks are not indicated on the provided plans, however we recommend the proposed residence foundations maintain a minimum of 35 feet from the top of the steep west- to northwest - facing bluff slope while the exterior western patio and cantilevered portions of the residence maintain a minimum of 30-foot setback. The setback area should not be disturbed during construction and any surface water developed during the construction process should be directed away from the steep slopes. Stormwater plans were also not indicated in the provided plans, however we understand runoff associated with this property is to be directed into a series of catch basins and tightlined to flow into the existing stormwater drainage system located immediately to the east of the property. We understand that this existing system ultimately flows out and discharges near the railroad tracks to the northeast of the property. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter NGA Project No. 1065318 Hitchens Residence Development April 22, 2020 Edmonds, Washington Page. 3 We also understand that temporary erosion control measures such as silt fences and a stabilized construction entrance are incorporated into the plans. Permanent erosion control measures such as compost amended soils and permanent seeding and planting will also be incorporated into the plans. Per our supplemental vegetation management memorandum we recommend areas of exposed soils below and to the west of the proposed residence are covered with erosion control matting and vegetated with deep-rooted plants as needed. We should be retained to review more detailed permanent erosion control plans once they are developed. The site soils are considered extremely moisture sensitive and will disturb easily when wet. We recommend that earthwork construction take place during periods of extended dry weather, and suspended during periods of precipitation if possible. If work is to take place during periods of wet weather, care should be taken during site preparation not to disturb the site soils. This can be accomplished by utilizing large excavators equipped with smooth buckets and wide tracks to complete earthwork, and diverting surface and groundwater flow away from the prepared subgrades. Also, construction traffic should not be allowed on the exposed subgrade. A blanket of rock spalls should be used in construction access areas if wet conditions are prevalent. The thickness of this rock spall layer should be based on subgrade performance at the time of construction. For planning purposes, we recommend a minimum one -foot thick layer of rock spalls. We also recommend that all of our recommendations provided in our previous report and memo, and this letter be strictly followed during construction. CLOSURE 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 during the work differ from those anticipated, and to evaluate whether or not earthwork activities comply with contract plans and specifications. ... NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Hitchens Residence Development Edmonds, Washington NGA Project No. 1065318 April 22, 2020 Page. 4 We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this letter or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Alex B. Rinaldi, GIT Project Geologist O M. 4 of wA��-Y >1 z �i 35215 %w �' GISTB�r'��jti NAL Khaled M. Shawish, PE Principal ABR:KMS:sw NELSON GEOTECHNICAL ASSOCIATES, INC.