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BLD2020-0564+Updated GEO_Other+9.14.2020_8.48.53_AMNELSON GEOTECHNICAL ASSOCIATES. INC. July 10, 2020 Carl and Sharon Larsen 9832 Marine View Drive Mukilteo, Washington 98275 Geotechnical Plan Review Letter Larsen Residence Garage Addition 7329 — 158th Street SW Edmonds, Washington NGA File No. 999017 Dear Mr. and Mrs. Larsen: 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com This letter presents the results of our updated geotechnical engineering review of the plans for your residence garage addition project located at 7329 — 158th Street SW in Edmonds, Washington. INTRODUCTION We previously prepared a report for this property that included recommendations for supporting the proposed garage addition and an overall report for the property dated February 28, 2018. Project plans consist of constructing a new garage addition along the eastern side of the existing residence near the toe of steep west -facing slope above. The proposed garage addition will be supported on 4-inch driven pin piles. Lateral loads on the eastern portion of the structure are to be restrained by helical anchors. The site falls within "Zones B through D" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds. 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, the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. We have addressed all of these requirements in our previous report. For our use in preparing this letter, we were provided with an overall plan set titled "Larsen Residence," dated July 1, 2020 and prepared by 1­12D Architecture + Design. Geotechnical Plan Review Letter NGA File No. 999017 Larsen Residence Garage Addition July 10, 2020 Edmonds, Washington Page 2 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. We understand that vertical loads for the proposed addition will be supported on 4-inch driven pin piles with a design capacity of 8 tons. Helical anchors with three helices (8, 10, and 12 inch diameter) are also planned to restrain lateral loads of the addition along the upper eastern side of the addition. We recommend that NGA be retained to observe the installation of the pin piles and helical anchors on a full-time basis during construction. The proposed garage addition will be located in close proximity to the steep west -facing slope above and to the east of the proposed garage addition. As a result, we understand that the uphill eastern side of the garage addition foundation was designed as a concrete debris catchment retaining wall extending four feet above the finished ground surface as recommended in our previous report. The debris wall has been designed to resist an active earth pressure of 100 PCF as recommended in our previous report. We understand that all stormwater runoff associated with this property is to be directed and tightlined to flow into the existing stormwater drainage system located within the southern portion of the property. We understand that this existing system ultimately flows out and discharges to the west of the property. Temporary erosion control measures such as silt fences along the downhill side of the proposed development area are incorporated into the plans. In our opinion, the proposed temporary erosion control measures appear to be feasible. We do not anticipate significant excavations associated with the proposed development as foundations consist of deep foundations. Any disturbed soils remaining after the proposed development should be revegetated in accordance with our previous report. Permanent erosion control measures such as compost amended soils, ground cover, and permanent seeding and planting have also been incorporated into the plans. The proposed garage addition development should not adversely impact or decrease the existing stability of the steep slope areas, neighboring properties, or critical areas; and is in compliance with the City of Edmonds Codes 23.80.060 and 23.80.070, provided our recommendations are incorporated into the proposed plans and followed during construction. We recommend that all of our recommendations provided in our previous report and this letter be strictly followed during construction. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Larsen Residence Garage Addition Edmonds, Washington NGA File No. 999017 July 10, 2020 Page 3 MINIMUM RISK STATEMENT Provided that the recommendations in this letter, and the geotechnical report dated February 28, 2018 are followed during construction, the areas disturbed by construction should remain stable. Therefore, the risk of damage to the proposed development or to adjacent properties form soil instability should be minimal, and the proposed grading and development should not increase the potential for soil movement. 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 Larsen Residence Garage Addition Edmonds, Washington NGA File No. 999017 July 10, 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. of washI� CO 2883 7�ed Gea�o LEE S. BELLAH Lee S. Bellah, LG Project Geologist Khaled M. Shawish, PE Principal LSB:KMS:sw NELSON GEOTECHNICAL ASSOCIATES, INC.