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ENG GEOTECH LETTERNELSON GEOTECHNICAL ASSOCIATES. INC. September 17, 2024 Michael Koh Beng Chye cc: Nazim Nice Motionspace Architecture and Design VIA Email: nazim@motion-space.com Bill Grayum DLH, Inc. VIA Email: bill@dlhinc.com Wet Season Grading Extension Letter Koh Residence Development 15604 — 75th Place West Edmonds, Washington NGA File No. 136081324 17311-135th Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com Dear Michael: This letter presents our opinions and recommendations regarding wet -season earthwork construction at the Koh Residence Development project located at 15604 — 75th Place West in Edmonds, Washington. We understand that grading activities are anticipated to be needed during the wet season (October 1 through April 30), and therefore, you plan to apply for a wet season grading extension. We visited the site on September 3, 2024, to discuss the proposed construction schedule with the general contractor, as well as observe the current site conditions and erosion control measures. The TESC plans were provided to us in a plan set dated January 15, 2024. These plans consist of covering on site stockpiles, temporary cuts and slopes with plastic, maintaining a silt fence around the perimeter of the site, a stabilized construction entrance, and permanent slopes to be covered with erosion control matting. During our recent site visit, we observed the residence foundations had been constructed and that erosion control measures have been installed in general accordance with the project plans and have performed adequately thus far based on part-time construction monitoring during the initial earthwork and drilled pier installation. Wet Season Grading Extension Letter NGA File No. 136081324 Koh Residence Development September 27, 2024 Edmonds, Washington Page 2 Based on discussions with the general contractor, we understand the expected work to be performed within the wet season will likely include casting the foundation retaining walls and installing perimeter foundation drain and downspout infrastructure, minor/shallow utility installation. The backfill materials to be utilized for these activities will consist of weather resistant granular soils. We have reviewed the provided TESC plan, and the updated proposed wet -season grading activity schedule and concluded that the proposed scope for dry -season grading extension is appropriate for this project. Based on our review of the site TESC plan and grading activity description, we understand that all exposed cuts will be covered with plastic if present, all construction equipment will remain on site, all exposed surface soils will be covered with straw or similar erosion control material if applicable, and that silt fences and other erosion control materials will be installed and regularly maintained to minimize soil and or muddy water movement from the site. Based on the grading activity schedule provided by you, we understand that grading activities within the site may be needed approximately 4- to 8-weeks after a wet season grading extension is granted. We anticipate the erosion control measures currently in place should provide adequate site protection during the wet season, along with best management practices during any further earthwork. NGA should be retained to evaluate erosion control measures and site stability, especially during significant rain events in order to and provide recommendations for corrective action, as necessary. In our opinion, provided the planned construction activities are completed in general accordance with the project plans and our daily recommendations during construction, extending grading activities beyond September 30, 2024, for this project should not result in adverse impacts or environmental harm to the surrounding vicinity. • • • NELSON GEOTECHNICAL ASSOCIATES, INC. Wet Season Grading Extension Letter Koh Residence Development Edmonds, Washington NGA File No. 136081324 September 27, 2024 Page 3 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please contact us. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. 24015933 cz, �1y ��GISTER�0 �w I0NAL � Alex B. Rinaldi, LG, PE Project Geologist 9.27.2024 Khaled M. Shawish, PE Principal RJB:ABR:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.