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2017-0718 Krassner Peer Review.pdf"LM.A', HWAGEOSCIENCES INC. U Geotechnical & Penenzent Engineeting - Hydrogeologv - Geoenvironinental Seii,ices - Inspection & Testing December 29, 2017 HWA Project No. 2017-007-21 Task 300 City of Edmonds Public Works Engineering Division City Hall — 2 d Floor 121 5h Avenue N Edmonds, Washington 98020 Attention: Jeanie McConnell Engineering Program Manager Subject: VERTICAL ExCAVATION AsSESSMENT Peer Review — Proposed Residences — Lots 1 and 2 1811184" Avenue W Edmonds, Washington As requested, HWA GeoSciences Inc. (HWA) has reviewed excavation plans and the associated geotechnical reports for the two proposed residences on Lots I and 2. Lot I is located at the west side of the site and Lot 2 is located at the northeast comer of the site. The subject of this review concerns the proposed temporary unsupported slope cuts in the underlying dense to very dense glacial.advance outwash soils. This letter provides our comments based on this review. The following documents were provided in response to our peer review comments: • Excavation and Shoring Plans — Lot #1, dated November 2, 2017, prepared by Lucia Engineering Inc., for the New Residence, 18109 84th Avenue W, Edmonds, WA 98026. • Excavation and Shoring Plans — Lot #2, dated November 2, 2017, prepared by Lucia Engineering Inc., for the New Residence, 18111 84th Avenue W, Edmonds, WA 98026. • Geotechnical Recommendations, Proposed Residences — Lots I and 2, 18109 & 18111 — 841h Avenue West, Edmonds, Washington 98026, dated November 10, 2017 and prepared by Robert M. Pride, LLC. • Subsurface Exploration, Geologic Hazard, and Geotechnical Engineering Report, Edmonds Three -Lot Subdivision, 18111 84th Avenue West, Edmonds, Washington, dated July 30, 2008, prepared by Associated Earth Sciences, Inc. (AESI) for Mr. Steve Krassner. • Geotechnical Report Update, Single -Family Residence — Lot, 18109 84"' 21312 301 Drive SE Suite 110 Bothell, WA 98021-7010 Fax: 425.774.2714 www.hwageo.com Federal & State Certified DBE / NINVBE December 29, 2017 HWA Project No. 2017-007-21 Task 300 ,4venue W, Edmonds, Washington (Edmonds PermitNo. BLD2017-0717), dated September 25, 2017, prepared by Associated Earth Sciences, Inc. (AESI) for Mr. Dino Hass. 0 Geotechnical Plan Review, Proposed Residences —Lots I and 2, 18109111184" Avenue West, Edmonds, Washington, dated November 23, 2017 and prepared by Robert M. Pride, LLC. REVIEW COMMENTS According to the AEST report submitted in 2008, the site stratigraphy consists of approximately I foot of organic -rich sod and topsoil underlain by a medium dense to very dense, fine to coarse sand, generally containing trace to few amounts of gravel, interpreted to be advance outwash, to the full depth of the three exploration borings, all of which were terminated at a depth of approximately 31.5 feet. The report indicates that a static ground water table, represented by saturated soil and free flowing water, was not encountered in the three exploration borings advanced at the site in July 2008. However, scattered zones of wet soils were observed throughout the borings and indicate there are zones of perched ground water within the advance outwash. The plans provided propose several unsupported vertical cuts up to 9 feet tall with IH: IV slopes above to intersect the existing ground slope. The distance of the vertical cuts from the property line range from about 20 feet on Lot I to 7.5 feet on Lot 2. The excavation at Lot 2 will also entail a 19-foot deep cut within 19 feet of the ad acent house. In his report dated November 10, 2017, Robert Pride indicates that these temporary cuts can be performed as unsupported excavations. Although moist, dense to very dense advance outwash soils are capable of standing nearly vertical, the borings encountered wet soils and scattered zones of seepage. The presence of seepage significantly reduces the temporary slopes outwash soils can support. Based on these conditions, temporary excavations should be designed assuming the advance outwash soils will be wet at the time of excavation. Waiting until the excavation is open to identify that the advance outwash materials will not stand vertical represents a significant risk of slope failure and damage to the adjacent residences. The 2008 report by AESI recommended temporary shoring be implemented and provided parameters for temporary shoring design. Therefore, the applicant should revise the excavation and shoring plans to address slope stability concerns for the proposed temporary excavations. CONDITIONS AND LIMITATIONS We have prepared this review letter report for the City of Edmonds, for use in permitting considerations for the proposed project. It should be noted that HWA performed no subsurface explorations within the scope of our review and has relied solely on available information provided by others and existing within the public domain. Accordingly, our evaluations of site conditions and appropriateness of designs submitted is limited by any limitations in the Krassner Peer Review.doc 2 December 29, 2017 HWA Project No. 2017-007-21 Task 300 information available to us, and we cannot confirm that the actual geotechnical site conditions are as presented. We appreciate the opportunity to be of service. If you have any questions regarding this report, please do not hesitate to contact either of the undersigned at (425) 774-0106. Sincerely, HWA GEoSCIENCES INC. 9— —KAJI- Zakeyo Ngoma, P.E. Geotechnical Engineer MY W. z -,*A ON �Ofol i �,/� JoLyn Gillie, P.E. Geotechnical Engineer, Principal Krassner Peer Review.doc 3