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HomeMy WebLinkAboutC_Geotech_Response_Letter_Meadow_Ave_220307_v115365 N.E. 90th Street, Suite 100 • Redmond, WA 98052 • (425) 449-4704 • FAX (425) 449-4711 Earth Solutions NW LLC Geotechnical Engineering, Construction Observation/Testing and Environmental Services March 7, 2022 ES-7126.01 Mr. Huy Nguyen 15400 Southeast 155th Place, Unit 99 Renton, Washington 98058 Subject: Geotechnical Review: Infiltration Trenches Proposed Short Plat 3804 Meadow Avenue North Renton, Washington Reference: Earth Solutions NW, LLC Geotechnical Engineering Study, ES-7126, dated March 12, 2020 Encompass Engineering & Surveying Preliminary Short Plat Plans, Drawing No. 20591, dated March 7, 2022 2017 City of Renton Surface Water Design Manual (2017 Renton SWDM) Greetings, Mr. Nguyen: As requested by Encompass Engineering & Surveying (Encompass), Earth Solutions NW, LLC (ESNW) has prepared this letter for the subject project. Based on our understanding of site conditions and review of COR Maps, a high erosion hazard (EH) area is present along the eastern margin of the subject site. The City of Renton (City), pursuant to the requirements of the referenced 2017 Renton SWDM, requires that a geotechnical professional review the referenced plans because the infiltration trenches are sited within 200 feet of an EH area. As summarized in the referenced geotechnical report, the site is underlain generally by one-and- one-half to four-and-one-half feet of fill, with underlying native recessional outwash deposits. Previous infiltration testing within the recessional outwash (at TP-2, approximately three-and- one-half feet below grade) resulted in a recommended long-term infiltration rate of 5 in/hr for the tested location and depth. In our opinion, the previous infiltration testing indicates the native recessional outwash possesses moderate to high capacity to function as a suitable receptor soil for infiltration systems. Mr. Huy Nguyen ES-7126.01 March 7, 2022 Page 2 Earth Solutions NW, LLC ESNW recommends the infiltration trenches be sited within the well-drained, native recessional outwash at depth. Topsoil, existing fills, silt-rich soil, or otherwise unsuitable material must be completed advanced through such that the infiltration trench bottoms are sited entirely within the native recessional outwash. Over-excavations, as recommended by ESNW at the time of construction, should be restored using washed drain rock or gravel backfill per City Standard Plan 225.20. Based on our review of the referenced plans, these recommendations have been incorporated into the infiltration trench designs. Provided the recommendations outlined in the referenced geotechnical report and this review letter are incorporated into construction, it is our opinion the infiltration trenches are feasible, from a geotechnical standpoint, as depicted on the referenced plans. Considering cumulative impacts from the project and surrounding area under full built-out conditions, it is our opinion the proposed infiltration trenches will not adversely affect the site and surrounding area. ESNW should be contacted to observe and document infiltration trench installation. Supplementary recommendations may be provided at the time of construction based on observed conditions. We trust this letter meets your current needs. Please call if you have any questions about this letter or if we can be of further assistance. Sincerely, EARTH SOLUTIONS NW, LLC Keven D. Hoffmann, P.E. Associate Principal Engineer cc: Encompass Engineering & Surveying Attention: Mr. Costa Philippides, P.E. (Email only) 03/07/2022