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RESUB1 BLD2020-1386+Geotechnical_Letter+5.11.2021_9.55.05_AM+2189924NELSON GEOTECHNICAL ASSOCIATES. INC. May 10, 2021 Mr. Ryan Rotmark 17512 — 71s' Avenue West Edmonds, WA 98026 Geotechnical Plan Review Letter Rotmark Property Slope Stabilization and Retaining Wall 17512 — 715t Avenue West Edmonds, Washington NGA File No. 1184120 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com Dear Mr. Rotmark: This letter presents the results of our geotechnical engineering review of plans for the Rotmark Property Slope Stabilization and Retaining Wall project located at 17512 — 71" Avenue West in Edmonds, Washington. INTRODUCTION We previously prepared a geotechnical engineering evaluation for the project site September 16, 2020. The site is currently occupied by an existing single-family residence within the gently sloping upper central portion of the site. A steep west -facing slope is located below and to the west of the existing residence and yard area and extends down from the gently sloping upper bench area to the western property line and the neighboring property to the west. The slope is heavily vegetated with underbrush and trees. Some settlement has been observed within the top of slope area throughout the years. We understand that you proposed to stabilize the steep slope area utilizing a light retaining wall structure along the top of the steep slope as recommended in our previous report. We understand that the City of Edmonds has requested our review of the most recent set of plans in an email dated January 6, 2021. For our use in preparing this letter, we were provided with the structural plan set titled "Rotmark Residence Retaining Wall," dated December 18, 2020 and prepared by CG Engineering. Geotechnical Plan Review Letter Rotmark Property Slope Stabilization and Retaining Wall Edmonds, Washington NGA File No. 1184120 May 10, 2021 Page 2 PLAN REVIEW AND CONCLUSIONS We have reviewed the geotechnical aspects of the provided retaining wall plans and found the plans to be in general compliance with our recommendations as presented in our previous geotechnical report. It is our opinion that the proposed retaining wall and slope stabilization measures should generally be localized to the retaining wall area along the top of the steep slope and should not adversely impact existing slope stability conditions within the subject site and neighboring properties. Project plans indicate that 16, 3-inch square hollow steel pipe piles will be driven vertically along the top of slope area and spaced at a maximum 4-foot on center. The new walls are to be restrained laterally with 9, 3-inch diameter grouted anchor tiebacks extending back a minimum of 20 feet and spanned with a horizontal waler. The grouted anchors were designed with a capacity of 11 kips. The square piles will be spanned with new 4x12 pressure treated lagging. Spacing should be maintained between the lagging boards to facilitate drainage through the wall. Voids behind the new lagging are to be filled with clean crushed rock. The retaining wall stabilization measures appear to have utilized design values of 45 PCF for active pressure and a seismic surcharge of 8H. The overall exposed height of the retaining wall will be approximately four to six feet. We recommend that grading and the retaining wall construction be performed in a manner that minimizes disturbance to the areas outside of the wall area. We also recommend that all disturbed areas be revegetated and vegetation maintained until it is established. All other recommendations provided in our previous report should be strictly followed. MINIMUM RISK STATEMENT Based on our understanding of the proposed plans, and provided that the recommendations in our previous report and this letter are strictly followed during construction and the retaining wall and slope stabilization measures are constructed under the supervision of NGA, the areas disturbed by construction should remain stable. Therefore, the risk of damage to the proposed development or to adjacent properties from 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. o-o-o NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Rotmark Property Slope Stabilization and Retaining Wall Edmonds, Washington NGA File No. 1184120 May 10, 2021 Page 3 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. Lee S. Bellah, LG Project Geologist Khaled M. Shawish, PE Principal LSB:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.