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Geo letter-Updated Larsen SP Plan Review.pdfN Main Office 17311 — 135" Ave NE, A-500 Woodinville, WA 98072 (425) 486-1669 • FAX (425) 481-2510 May 6, 2019 Carl and Sharon Larsen 9832 Marine View Drive Mukilteo, Washington 98275 NELSON GEOTECHNICAL ASSOCIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS Geotechnical Plan Review Letter Larsen Property Short Plat 15729 — 75" Place West Edmonds, Washington NGA File No. 999017 Dear Mr. and Mrs. Larsen: Engineering -Geology Branch 5526 Industry Lane, #2 East Wenatchee, WA 98802 (509) 665-7696 • FAX (509) 665-7692 This letter presents the results of our geotechnical engineering review of the plans for your property short plat project located at 15729 — 75' Place West in Edmonds, Washington. INTRODUCTION We previously prepared a report for this property that included recommendations for underpinning support of the existing residence dated February 28, 2018. We also prepared plan review letters for the proposed improvements to the existing residence dated March 20, 2018, July 25, 2018, and August 23, 2018. We prepared a preliminary plan review letter for the proposed short plat development in a letter dated August 9, 2018. Project plans consist of subdividing the existing property into three separate properties. A new single-family residence is planned for the new upper eastern lot. 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. We have addressed all of these requirements in our previous report and previous supplemental letters. For our use in preparing this letter, we were provided with a set of plans titled "Larsen 3-Lot Short Plat," dated May 3, 2019, prepared by CG Engineering. Geotechnical Plan Review Letter NGA File No. 999017 Larsen Property Short Plat May 6, 2019 Edmonds, Washington Page 2 PLAN REVIEW AND CONCLUSIONS We have reviewed the geotechnical aspects of the most recent plans and found the plans to be in general compliance with our recommendations as presented in our previous geotechnical report. The proposed residence is to be located further than the recommended 60-foot minimum setback provided in our previous report and is planned to be supported on conventional shallow foundations. We understand that all stormwater runoff and sewer associated with this property is to be directed and tightlined to flow into the existing stormwater drainage and sewer system pipes located within the southern portion of the property. We understand that this existing system ultimately flows out and discharges to the west of the property. We recommend that the trenches be backfilled within native granular soils and compacted to structural fill specifications in accordance with our previous report. All disturbed areas as a result of the utility line installation should be revegetated and maintained until established. In our opinion, the proposed utility line installations should not adversely impact the existing slope stability conditions within the property. Temporary erosion control measures such as silt fences along the downhill side of the proposed development are incorporated into the plans. In our opinion, the proposed temporary erosion control measures appear to be feasible. Any disturbed soils remaining after the proposed development should be revegetated in accordance with our previous report. Permanent erosion control measures such as compost amended soils, ground cover, and permanent seeding and planting have also been incorporated into the plans. We understand that the 72' Avenue West along the upper eastern portion of the site is to be raised slightly and extended further to the south to provide future access to the proposed residence. Review of the plans indicates that the lower western side of the roadway will be supported with a permanent soldier pile retaining wall spanned with timber lagging. We were previously provided with soldier pile loading design parameters via email on October 30, 2018 from Dennis Titus with CG Engineering. At that time we confirmed that the design values were feasible for use in the shoring wall design. Review of the provided plans indicates that these values were indeed incorporated into the design of the proposed soldier pile shoring wall. We have included these design values along with additional recommendations for the proposed soldier pile shoring wall construction in the Soldier Pile Retaining Wall subsection of this letter. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter NGA File No. 999017 Larsen Property Short Plat May 6, 2019 Edmonds, Washington Page 3 As a part of the proposed grading associated with the roadway extension, we understand that approximately up to seven feet of fill will be placed to bring the roadway up to the proposed elevation. The proposed filling and roadway extension work will be located greater than 150 feet from the top of the steep west -facing bluff slope below the proposed development area. As a part of our plan review letter dated August 23, 2018, we provided slope stability analyses indicating relatively stable slope stability conditions for both the static and seismic cases near the top of the steep west -facing bluff slope. These analyses have been included in this letter as Figures 1 and 2. In our opinion, the proposed filling and grading associated with the proposed roadway extension should not adversely impact the existing slope stability conditions within the steep west -facing bluff area and nearby vicinity due to the relatively small amounts of fill being placed within the very eastern portion of the property and due to the proposed grading be located at least 150 feet away from the steep west -facing bluff. It is our opinion that the proposed residence development and roadway grading associated with the proposed short plat should not adversely impact or decrease the existing stability of the steep slope areas, neighboring properties, or critical areas; and is in compliance with the City of Edmonds Codes 23.80.060 and 23.80.070, provided our recommendations are incorporated into the proposed plans and followed during construction. We recommend that all of our recommendations provided in our previous report and letters, and this letter be strictly followed during construction. As discussed in our previous report, this site and the overall site vicinity lie within a known ancient landslide area. The site and vicinity have been relatively stable for a very long period of time, and development in the area has taken place in the form of single-family residences, roads, parks, retaining walls and underground utilities. Although the likelihood of the ancient slide to become active in the foreseeable future is very low, extreme environmental conditions coupled with inadequate human practices could, in theory, re -activate the ancient landslide. Such external factors could include severe and prolonged weather events and/or significant seismic activity. With proper site drainage and maintenance of the drainage systems and protection of steep slopes, we would estimate that the probability of failure on the subject site is likely to be less than 30 percent in 25 years. In our opinion, the proposed residence development and associated roadway grading within the eastern portion of the site does not increase this potential. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter Larsen Property Short Plat Edmonds, Washington Soldier Pile Retaining Wall NGA File No. 999017 May 6, 2019 Page 4 General: A soldier pile retaining wall will be utilized to support fills along the downslope western side of 72nd Avenue West to permanently support the eastern edge of the existing parking lot and driveway areas. The soldier pile wall heights will range between 2.0 and 7.0 feet in exposed height. Review of the wall design indicates the wall bas been designed in accordance with our recommendations. The shoring wall has been designed to resist a lateral load resulting from a fluid with a unit weight of 45 pounds per cubic foot (pcf) loading conditions. A uniform surcharge of 250 psf and a uniform surcharge of 8H have been applied to the wall design to account for traffic and seismic loading, respectively; "H" in this case, is the exposed height of the wall. A passive pressure of 150 pcf on the below grade soils has been utilized and applied on two -pile diameters under the excavation line. The upper one foot of pile embedment has been neglected when calculating the passive resistance. The below -grade portion of the wall is no less than 1.5 times the wall stick-up height. The above pressures assume that the on -site soils retained by the shoring wall are not significantly disturbed and that hydrostatic forces are not allowed to build up behind the wall. These values do not include the effects of any other surcharges than what is described above. The retained soils should be readily drained and collected water should be routed into a permanent storm system. Adequate gaps should be maintained between the lagging elements to allow for any potential water seepage buildup to flow through the wall. Shoring Wall Installation: The shoring wall should be installed by a shoring contractor experienced with this type of system. We anticipate that an open -hole drilling method may prove difficult to achieve for installing the soldier piles in the on -site soils, and therefore we recommend that the shoring contractor should have the capability of casing the holes as sloughing and/or water seepage will likely be encountered. It might be prudent to perform one or more "test" holes to confirm installation conditions prior to finalizing budget and work plans. Any sloughing or water that may collect in the drilled holes should be removed prior to pumping grout. Grout should be readily available on site at the time the holes are drilled and cased. If groundwater seepage is encountered, we recommend that water be pumped out of the holes and the concrete be tremied from the bottom of the excavations to displace the groundwater to the surface. Extra Portland Cement, or other additives, may also be placed in the excavations to reduce the effects of seepage. The spoils from the soldier pile excavations are expected to be moisture -sensitive materials and should be removed from the site. We should be retained to monitor on site activities during the shoring wall installation on a full-time basis. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Plan Review Letter NGA File No. 999017 Larsen Property Short Plat May 6, 2019 Edmonds, Washington Page 5 MINIMUM RISK STATEMENT Provided that the recommendations in this letter, the geotechnical report dated February 28, 2018 are followed during construction, the areas disturbed by construction should remain stable. Therefore, the risk of damage to the proposed development or to adjacent properties form soil instability should be minimal, and the proposed grading and development should not increase the potential for soil movement. 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 Larsen Property Short Plat Edmonds, Washington NGA File No. 999017 May 6, 2019 Page 6 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. Bcllah, LG Project Geologist Khalcd M. Shawish, PE Principal LSB:KMS:dy NELSON GEOTECHNICAL ASSOCIATES, INC.