Loading...
Geotech Letter 2NELSON GEOTECHNICAL ASSOCIATES. INC. October 14, 2021 Mr. Joe Tarqueno Via Email: jtargl2@yahoo.com Geotechnical Plan Review Letter Tarqueno Slope Stabilization 841 Elm Way Edmonds, Washington NGA File No. 119471320 Dear Joe: 17311-135'h 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 plans for the Elm Way Retaining Wall project located at 841 Elm Way in Edmonds, Washington. INTRODUCTION We previously prepared a geotechnical engineering evaluation for the project site dated October 1, 2020. The project consists of constructing a retaining wall below and immediately to the southwest of the existing residence to replace an existing failing timber retaining wall. The retaining wall construction is constrained by the existing residence, the southwestern property line, as well as a newly constructed CMU block fence along the adjoining southern neighbor's northern property line. We understand that the City of Edmonds has requested our review of the most recent set of plans. For our use in preparing this letter, we were provided with the structural plan set titled "Tarqueno Slope Stabilization," dated September 17, 2021 and prepared by CG Engineering. 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. After supplemental consultation with the homeowner and design team, we understand the proposed retaining system deviates from the soil nail wall recommended in our previous report and a cast -in -place wall was the agreed upon alternative. In our opinion, the proposed cast -in -place retaining wall is an adequate alternative and should maintain or improve existing stability conditions with respect to the timber -faced slope and existing residence. Geotechnical Plan Review Letter NGA File No. 119471320 Tarqueno Slope Stabilization October 14, 2021 Edmonds, Washington Page 2 The soil design parameters that were discussed with the design team and reflected on the reviewed plans were as follows. An active and passive lateral earth pressure of 40 and 200 pcf respectively, should be utilized assuming conditions behind the wall include a relatively level ground surface above the wall and granular structural fill or geofoam backfill. A shear key along the base of the wall was recommended to achieve higher passive pressure values, as needed, as well as a coefficient of friction along the base of the footing of 0.35. We also recommended a soil bearing capacity of 3,000 psf for footings bearing directly on competent native glacial soils interpreted to underly the site. All of this has been incorporated into the plans. We recommend that NGA be retained to verify subgrade conditions. The plans indicate the proposed concrete wall will range in overall height from approximately 4- to 6- feet and span the entire southwestern portion of the house along the southwestern property line, beginning at an existing block wall to the southwest of the house and terminating within the lower northwestern yard area. We understand the retained soil height will be 1- to 2-feet lower than top of wall elevation. A wall drain will be placed behind each proposed retaining wall as well as geofoam block backfill covered by filter fabric and surficial topsoil. Wall footing drains will be directed to an approximately 3-foot by 5-foot quarry spall splash pad within the very northwestern corner of the site. Given the generally level to gently sloping terrain in the proposed outfall area and likely minimal discharge volume, we consider this an appropriate system for managing drainage and should not adversely impact stability conditions within the site or on adjacent properties. Temporary excavations will consist of approximately 5- to 7-foot tall 1H:1V temporary cuts, roughly extending from the existing house foundations down to bottom of proposed wall elevation. We understand the existing house is supported on pipe piles; therefore, risk of house undermining is minimal. Nonetheless, extreme care should be taken during temporary grading activities; slopes should be covered with plastic sheeting when not in use and left exposed for as little time as possible. We recommend that all WISHA/OSHA requirements for safe worker access be strictly followed. Job site safety is the sole responsibility of the contractor. We recommend that NGA be retained to monitor temporary excavation grading and provide additional consultation as needed. Planter boxes are proposed above the cast -in -place wall and will consist of approximately 1- to 2-foot block wall steps along with a segment of approximately 2-foot-tall retaining wall spanning between the new retaining wall along the property line and the existing residence. In our opinion, this is appropriate. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Tarqueno Slope Stabilization Edmonds, Washington NGA File No. 11947B20 October 14, 2021 Page 3 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Tarqueno Slope Stabilization Edmonds, Washington NGA File No. 11947620 October 14, 2021 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, LG Project Geologist M. S� of wn. y 0 35215 �w ��ISTV�' C$ ss10 - ti� 10.14.2021 Khaled M. Shawish, PE Principal ABR:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.