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REVIEWED ENG PLN RESUB 1-BLD2022-0527 Geotech LetterNELSON GEOTECHNICAL ASSOCIATES. INC. March 24, 2023 Mr. Robert Gregg Gregg Property and Associates, LLC Via Email: rrgregg@comcast.net Geotechnical Plan Review Letter Westgate Station Commercial Development 9601 Edmonds Way Edmonds, Washington NGA File No. 1156720 Dear Mr. Gregg: 17311-13511 Ave. N.E. Suite A-500 Woodinville, WA 98072 (425)486-1669 www.nelsongeotech.com ■-------------- Reviewed by City of Edmonds ; Planning Division '--------------- Reviewed by City of Edmonds Engineering Division RESUB Mar 29 2023 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT BLD2022-0527 This letter presents the results of our geotechnical engineering review of the civil plans for the proposed Westgate Station Commercial Development project located at 9601 Edmonds Way in Edmonds, Washington. INTRODUCTION We understand that the proposed development for the property will include the construction of a new commercial building within the central portion of the property along with associated paved parking areas and underground utilities. We previously prepared a geotechnical evaluation for the site dated April 14, 2020. In general, we concluded that the planned site development was feasible from a geotechnical standpoint. For our use in preparing this plan review letter, we have been provided with the following documents: • Civil plan set titled "Westgate Station," dated March 23, 2023 and prepared by CG Engineering. We have been requested to provide our plan review of the provided civil plans. In the following sections, we summarize the results of our geotechnical plan review. Geotechnical Plan Review Letter NGA File No. 1156720 Westgate Station Commercial Development March 24, 2023 Edmonds, Washington Page 2 PLAN REVIEW We have reviewed the geotechnical aspects of the provided civil plans and found the plans to be in general compliance with our recommendations as presented in our previous geotechnical report. In our previous report, we determined that traditional stormwater infiltration systems founded within the granular native advance outwash soils at depth were feasible within this site. We understand that stormwater within the site will be directed to flow into three rain -garden systems within the southern portion of the property and two gravel -filled infiltration systems within the western and eastern portions of the property. Each of the systems has incorporated an overflow component that is directed to flow into the existing storm system within Edmonds Way. It appears that the infiltration systems were sized and designed based on the long-term design infiltration rate provided in our previous report, with the exception of the Rain - Garden 1, which is the easternmost rain -garden. This rain -garden was sized with an infiltration rate of 2.88 inches per hour. This long-term design infiltration rate was based on the field measured rate of 8.0 inches per hour obtained from Infiltration PIT 2 performed within the approximate location of Rain - Garden 1 with a 0.36 correction factor applied to this rate, resulting in a long-term design infiltration rate of 2.88 inches per hour. In our opinion, utilizing the design rate of 2.88 inches per hour for the design of Rain -Garden 1 is feasible. The eastern infiltration trench will also include a sand filter at the base of the trench for water -quality treatment purposes. We recommend that all systems be fully founded within the native granular advance outwash soils at depth and NGA be retained during construction to confirm the soil conditions prior to backfilling the systems. All other recommendations provided in our previous report should be strictly followed. 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 Westgate Station Commercial Development Edmonds, Washington NGA File No. 1156720 March 24, 2023 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 2023 Khaled M. Shawish, PE Principal LSB:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.