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LETTER 9289B16 Construction Final Edmonds.pdf March 13, 2018 Leif Bjorback, Building Official City of Edmonds, Development Services 121 Fifth Avenue North Edmonds, Washington 98020 Construction Completion Summary Niesel Retaining Wall Project 18500 Olympic View Drive Edmonds, Washington Application No.: BLD20170068 NGA File No. 9289B16 Dear Mr. Bjorback: This letter summarizes our consultation and construction monitoring services for the retaining wall and slope stabilization improvements that were constructed near the top of asteep slope below the Niesel residencelocated at 18500 Olympic View Drive in Edmonds, Washington. Introduction Nelson Geotechnical Associates, Inc. (NGA)completed a site evaluation at the subject property and presented our findings, conclusions, and recommendations in “Geotechnical Engineering Letter – Niesel Residence Settlement–18500 Olympic View Drive –Edmonds, Washington,”dated July 14, 2015. Although the focus of that evaluation was toward stabilizing portions of the existing house foundation with pin piles, we noted that existing railroad-tie retaining walls located immediately along and below the western portion of the residence were in serious disrepair and that it was possible the condition may be contributing to some of the observed settlement issues. NGA recommendedthat these walls be further evaluated with the expectation that they would likely be replaced with an engineered wall system. Construction Completion Summary NGA File No.9289B16 Niesel Retaining Wall Project March 13, 2018 Edmonds, WashingtonPage 2 In 2016, Landwork Enterprises Inc. of North Bend, Washington was retained to design and construct new mechanically stabilized earth (MSE) retaining walls to replace the failing timber-tie walls. The scope of the construction consisted of replacing the existing creosote-treated wood railroad ties with the MSE walls along with associated site preparation, earthwork, drainage, and site restoration. The project was nearing completion when the City of Edmonds informed the Niesels that a geotechnical engineering evaluation of the retaining wall design was needed before their final inspection. The Niesels retained NGA to complete the geotechnical evaluation, results ofwhich wereprovided in a letter dated November 9, 2016. Subsequently, the Niesels requested that NGA develop drawings and specifications covering the work as the Engineer-of-Record. The project documents completed by NGA were developed to correspond with the as-built conditions. Direct observations by NGA, that were recorded with photographs, field notes, and slope profile mapping,plus features that were inferred from conversations with the contractor and the Niesels, along with construction progress photographs provided by the Niesels,were relied uponto produce the as-built plans. Site Observations NGA visited the site on August 31, 2016 to observe and evaluate the construction work. Backfill compaction and minor grading to stabilize the site for theday were in process during our visit.While on site, we did not observe adverse impacts to the slope, or any safety concerns, as a result of the minor grading activities and temporary cut excavations associated with wall construction. NGA returned to the site on October 17, 2016 after the project was substantially complete. The only task to be completed at that time was installing permanenterosion control. Temporaryerosion and sediment control in place at the time of our visit consisted of crushed rock left over from construction intermingled with topsoil. Two features that were installed but not shown on the drawings consist of some yard drains installed within the gravel pathwaythat encircles the western portion of the residenceand a footing drain installed behind the upper MSE wall. The yard drains were installedin conjunction with replacing and re-routing rain water leaders from roof downspouts that were damaged during earthwork or conflicted with the new construction. The rainwater leaders discharge into an existing storm drain line that conveys water to the base of the steep slope. We consider this work to be incidental to the permitted work. The wall footing drain line connects to anew rain water leader at the northern end of the new wall.The connection point islocated immediately adjacent to a stairway that leads to a small bench just above the steepest slopes. The footing line consists of 3-inch diameter, perforated plastic pipe. The southern end of the footing drain terminates with a surface cleanout located immediately adjacent to the end of the wall. NELSON GEOTECHNICAL ASSOCIATES, INC. Construction Completion Summary NGA File No.9289B16 Niesel Retaining Wall Project March 13, 2018 Edmonds, WashingtonPage 3 NGA was not aware that a footing drain was in place when the plans were being developed and the lack of one was not a significant concern to us based on the followingrationale: Wall backfill consists of relatively clean sand and gravel that is considered to be relatively permeable, allowing water to percolate down to the base of each tier and disperse into native soils. Numerous small gaps between individual concrete units forming the wall facing do not permit water to build up within the backfill and allow water to pass through. The previouslyexisting timber tie retaining wall was constructed without a footing drain and no obvious signs ofsurface erosion were observed during our site reconnaissance visits in 2015. The site is located on the nose of a small spur ridge creating a convex, fan-shaped slope that naturally increases the width of the surface area available for dispersion or infiltration. Conclusions Based on limited part-time observations and our understanding of the construction process, it is our opinion that the retaining wallsand slope improvement measures were completed in general accordance with the approved plans and our recommendations. Fromour site observations, the slope appears to not have been adversely impacted by construction activities. Closure We should stress that steep slopesareprone to sloughing and shallow sliding events due to the inclination of the slope, the nature of the near surface soils, and wet weather conditions during winter months. Such events, if they do occur, should not immediately impact the stability of the wall system. However, erosion and sloughing events on the slope directly below the wall, if not addressed, can ultimately impact the wall. If any signs of distress are observed, we should be contacted immediately. At that time, we would provide recommendations, as warranted. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this letter was prepared. No other warranty, express or implied, is made. o-o-o NELSON GEOTECHNICAL ASSOCIATES, INC. Construction Completion Summary NGA File No.9289B16 Niesel Retaining Wall Project March 13, 2018 Edmonds, Washington Page 4 We appreciate the opportunity to be of service on this project. Please contact us if you have any questions regarding this letter or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Timothy D. Huntting, PE Senior Engineer TDH:dy cc: Chet and Nancy Niesel NELSON GEOTECHNICAL ASSOCIATES, INC.