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GEO LETTER Larsen Residence Garage Addition Updated Landau ResponseNELSON GEOTECHNICAL ASSOCIATES. INC. October 22, 2020 Carl and Sharon Larsen 9832 Marine View Drive Mukilteo, Washington 98275 Updated Geotechnical Peer Review Comment Response Letter Larsen Residence Garage Addition 7329 —158`h Street SW Edmonds, Washington NGA File No. 999017 Dear Mr. and Mrs. Larsen: 17311-1351h Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com This letter presents the results of our updated geotechnical peer review comment response for your planned residence garage addition project located at 7329 — 1581h Street SW in Edmonds, Washington. INTRODUCTION We previously prepared a report for this property that included recommendations for the proposed short plat and underpinning support of the existing residence dated February 28, 2018. We also prepared a plan review letter for the proposed garage addition dated July 10, 2020. Project plans consist of constructing a new garage addition along the eastern side of the existing residence near the toe of steep west -facing slope above. The proposed garage addition will be supported on 4-inch driven pin piles. Lateral loads on the eastern portion of the structure are to be restrained by helical anchors. The site falls within "Zones B through D" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds. This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017 Larsen Residence Garage Addition October 22, 2020 Edmonds, Washington Page 2 We have addressed all of these requirements in our previous report and previous plan review letter. We recommend that our previous report and letter be thoroughly reviewed. The conclusions and recommendations provided in this letter should be considered supplemental to the conclusions and recommendations provided in our previous report and letters. For our use in preparing this letter, we were provided with a set of plans titled "Larsen 3-Lot Short Plat," dated July 31, 2019, prepared by CG Engineering. LANDAU PEER REVIEW COMMENTS AND OUR RESPONSES As a part of the City of Edmonds review of this project, Landau Associates performed an updated peer review of the proposed project and our previous geotechnical reports and letters in accordance with Edmonds Community Development Code Chapter 19.10. Landau Associates updated comments and opinions were presented in a technical memorandum dated September 30, 2020. In the following section, we summarize the relevant geotechnical concerns raised by Landau Associates, followed by our response. Comment 1: NGA's revised Geotechnical Plan Review Letter provides lateral earth pressures that can be used to design the recommended debris catchment wall. NGA recommends a lateral earth pressure value of 100 pounds per cubic foot for the active condition. Catchment structures must be designed to withstand the impact of debris and to contain the anticipated volume of debris. A site -specific study should be performed to determine the size of the landslide, the zone of debris runout, and the velocity of the landslide debris at the proposed catchment location. In addition, the height of the catchment wall must be sufficient to prevent overtopping, and the wall must be capable of withstanding impact loads. Based on the recommendations in the revised Geotechnical Plan Review Letter, it is not clear that NGA considered impact loads or the anticipated volume of debris. NGA should confirm that its design recommendations include impact loads and the anticipated volume of debris. Response: The proposed four foot tall concrete debris catchment wall on the upper eastern side of the garage addition is intended to protect the structure against shallow surficial debris generated on the steep west -facing slope above and to the east. As a result, we utilized a wedge shape shallow surficial landslide with an overall depth of five feet and height of ten feet extending up slope for our analysis. Our analysis is only evaluating a one foot wide section of the slide to confirm design assumptions for the debris catchment wall. With the above slide dimensions, a total volume of 25 cubic feet of material for a one NELSON GEOTECHNICAL ASSOCIATES, INC. Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017 Larsen Residence Garage Addition October 22, 2020 Edmonds, Washington Page 3 foot wide section will be generated at the landslide initiation point. Due to the length of the slope, and dense vegetation and mature trees located within the steep west -facing slope above the proposed garage addition, we have estimated that approximately 25 percent of the material from the landslide initiation point will reach the toe of the steep slope and the debris catchment wall, for a total of 6.25 cubic feet per linear foot. With an assumed overall density of 125 pounds per cubic foot (PCF), the total mass of the debris reaching the debris catchment wall is 781 pounds. We estimate an approximate velocity of the flow of 20 miles per hour (29.33 ft/sec). The overall momentum of the debris material reaching the wall is 22,906 pound feet per second. We have assumed that the momentum of the debris reaching the wall would be slowed down and captured by the catchment wall over a span of 30 seconds which results in an overall force of 763 lbs. Utilizing the resulting force at the four foot tall wall as a result of the debris impact, a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 95 pcf will be experienced. We have recommended that the wall be designed for any active earth pressure of 100 PCF. The proposed debris catchment wall will be located approximately 4 to 12 feet from the toe of the steep slope. Based on review of the plans, we estimate approximately 272 square feet of area between the toe of the slope and the debris catchment wall. With an overall height of four feet above the finished ground surface for the debris catchment wall, we estimate approximately 1,088 cubic feet (40 cubic yards) of total storage volume for debris between the debris catchment wall and the toe of the steep slope which in our opinion should be adequate for the design landslide event. The debris flow will not be contained on the north and south sides of the debris catchment wall and debris will be allowed to flow around the garage and debris catchment wall to the north and south. Comment 2: To maintain the effectiveness of the wall, debris must be cleared following a landslide event. The location and configuration of the wall should be included in the plan sets to verify that an excavator can access the catchment wall and remove debris that has collected behind the wall. Additionally, a drainage layer should be included on the upslope side of the wall to promote dewatering of landslide debris. Inclusion of a drainage layer will reduce static loads on the wall and facilitate excavation of debris. Response: Based on review of the plans, we understand that the proposed debris catchment wall will be located approximately 4 to 12 feet from the toe of the steep west -facing slope and the finished ground surface between the toe of the steep slope and the catchment wall will be relatively level. Based on this overall NELSON GEOTECHNICAL ASSOCIATES, INC. Updated Geotechnical Peer Review Comment Response Letter Larsen Residence Garage Addition Edmonds, Washington NGA File No. 999017 October 22, 2020 Page 4 distance, it is our opinion that debris removal after a landslide event can be performed by both smaller tracked equipment along with mini -excavators. A qualified earthwork contractor should be contracted to perform the debris removal immediately after a landslide event. Due to the debris not being contained on the north and south sides of the structure, we anticipate that hydrostatic pressure would be able to be partially relieved around the catchment wall. We recommend that any debris behind the wall be removed immediately and as a result, we anticipate that hydrostatic forces behind the wall will be minimal. Also, a drain is shown behind the wall, which will further reduce hydrostatic forces on the wall. 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. 4 NELSON GEOTECHNICAL ASSOCIATES, INC. Updated Geotechnical Peer Review Comment Response Letter NGA File No. 999017 Larsen Residence Garage Addition October 22, 2020 Edmonds, Washington Page 5 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 Project Geologist V�o'V WA 9 35215 Iw 41 ,SI0NAL E� Khaled M. Shawish, PE Principal LSB:KMS:sg NELSON GEOTECHNICAL ASSOCIATES, INC.