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REVIEWED BLD RESUB2 GEO ENG BLD2023-0799+GO REPSONSE+6.13.2024_2.16.49_PM+4321173NELSON GEOTECHNICAL ASSOCIATES. INC. May 2, 2024 Ryan and Sydney Cochran Via Email: ryanrcochran@gmail.com BLD2023-0799 RES U B 17311-13511 Ave. N.E. Suite A-500 Jun 14 2024 Woodinville, WA 98072 CITY OF EDMONDS (425) 486-1669 DEVELOPMENT SERVICES www.nelsongeotech.com DEPARTMENT g •...........................................ti REVIEWED BY CITY OF EDMONDS BUILDING DEPARTMENT : ..............................................: Geotechnical Comment Response Letter Cochran Residence Development Infiltration 8475 Frederick Place Edmonds, Washington NGA File No. 102181323 Dear Mr. and Mrs. Cochran: This letter presents our geotechnical engineering comment response regarding the proposed residence development project located at 8475 Frederick Place in Edmonds, Washington. INTRODUCTION We understand that the proposed development for the property will include the construction of a new single-family residence and pool. We previously prepared a stormwater infiltration evaluation for the property for the previous owner dated February 2, 2018 and have since provided an updated memo dated February 21, 2023. We also provided a supplemental comment response letter dated March 1, 2024. We understand that the City of Edmonds has requested additional information regarding the geotechnical aspects of this project in a letter dated April 2, 2024. For our use in preparing this plan review letter, we have been provided with the following documents: • Civil plan set titled "Cochran Residence," dated February 23, 2024, prepared by CG Engineering. In the following section, we summarize the City's comments and our responses. Geotechnical Comment Response Letter Cochran Residence Development Infiltration Edmonds, Washington NGA File No. 10218623 May 2, 2024 Page 2 CITY OF EDMONDS STORMWATER REVIEW COMMENT AND OUR RESPONSE Comment 6: Per the Geotech report, the base of the infiltration trench shall be terminated within the granular glacial outwash soils. The Geotech report does not state the depth of the granular glacial outwash soils. Please provide a Geotech memo confirming the depth and update the trench dimensions if necessary. Our Response 6: Our previous explorations performed within the site generally encountered approximately 0.1 to 1.0 feet of surficial topsoil and/or undocumented fill underlain by red -brown to gray, fine to medium sand with varying amounts of silt and gravel, which we interpreted as native granular advance outwash soils to the depths explored. All of our explorations were terminated within the native granular advance outwash soils at depths in the range of 5.0 to 9.5 feet below the existing ground surface. Based on our explorations performed within the site, the native granular glacial outwash soils exist at depths in the range of 1.0 and 9.5 feet below the existing ground surface. Review of the provided plans indicates that the base of the infiltration trench will be approximately 7.7 feet below the existing ground surface. In our opinion, it appears that the proposed infiltration trench will be fully founded within the native granular advance outwash soils interpreted to underlie this site. We recommend that NGA be retained to observe the base of the infiltration trench during construction to confirm soil conditions will support the design infiltration rate. 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 Comment Response Letter Cochran Residence Development Infiltration Edmonds, Washington NGA File No. 10218623 May 2, 2024 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 Senior Geologist 02.2024 Khaled M. Shawish, PE Principal LSB:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.