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20141210132030290.pdfNELSON GEOIFECHNICAL, AssocIA""I"ES, INCO G E'0 I'MIiNICAL F."AGiNr-.Fs,,s & GEDI-cmuSTS main Offict, 17311 ­ I 3r0l Avo NE, A-500 5526 11)(Justry U,=, #2 Woodinville,, WA 98072 East WA 98802 (425)486 1669 ° FWX (425) 481-2510 (509) 665-7696 • FAX (509) 665-7692 November M 2014 Mr, John Richardson 7d,50 -­ 176"' Street SW Edmonds,'Washington 98026 Retaining Wall Installation Summary Letter Richardson Residence 7450 -.4761" Strect SW E'dinonds, Washingtox, NGA File No, 8802B 14. Dear Mr. Richardson - This letter summarizes our geotechnical construction monitoring services for the shotcrete retaining wall that was constructed in front of an existing timber retaining wall behind your residence located at "IA6T. lieu' Street SW in Edmonds, Washington. Emm The Project consisted of the installation of a new shotcrete retaining wall that was supported with pin CA I'liI piles and grouted tieback anchors to stabilize an existing un -engineered timber retaining wall along the western and northern sides of the residence. The residence is located along the top of a steep north- and west -facing slope. The timber retaining wall which supports fill associated with the backyard area is located along the top of the steep slope. We understand that, the timber retaining wall was installed without an engineered design. We were retained to provide as geotechnical engineering report and consultation services to provide rnit igation measures to stabilize the timber retaining wall. We previously prepared a geotechnical report for the project titled Retaining Wall Installation Final letter Richardson Residence Edmonds, Washington NGA File No. 8802B14 November I3, 2014 Page 2 "Geotechnical Engineering Evaluation — Richardson Residence Retaining Wall — 7405 1760' Street SW -- Edmonds, Washington," dated September 6, 2013. To stabilize the existing wall, we recommended a retaining wall be constructed in front of the timber wall and the sloping area below the new wall be covered with permanent erosion control material and vegetation. Site Observations We monitored wall installation activities on a part-time basis from July 29, 2104 through October 24, 2014. Our observations during wall installation at this project were documented in Field Reports 1 through 19, which have been submitted to you. The wall extends north to south along the western side of the residence and east -west along the northern side of the residence immediately in front of the existing timber retaining wall along the top of a steep slope. We observed the installation of 31 grouted tieback anchors throughout the wall alignment. The tiebacks consisted of 4 -inch diameter holes that were drilled to depths 23.5 to 31..0 feet from the face of the excavation. The anchors were installed at an approximate angle of 20 degrees below horizontal. After the holes were drilled, 10 -foot pvc pipe bond breakers were installed in the upper portion of the anchor along with threaded hollow metal anchor rods connected with threaded couplers to the depth of the drilled holes. Grout was pumped through the hollow anchor rods into the holes until the grout was observed flowing out of the drilled holes. In our opinion, all of the anchors were installed in accordance to the plans and our recommendations. We returned to the site to conduct load tests on three of the grouted tieback anchors. We conducted the tests on Tiebacks 213, 9B, and 16A. Each tieback was tested in 25% increments of the design load to a maximum load of 200% of the design load. We observed minimal movement of the anchors during the testing and the hold periods, and all of the tested anchors appeared to perform well. After the wall was constructed, 6 -inch square metal plates were placed at each anchor location and each anchor was locked off to the specified lock -off load. We observed the installation of 41, 2 -inch diameter schedule 80 piles that were driven with a 140 -pound air hammer to a total embedment depth in the range of TO feet to 14,0 feet below the current ground surface along the wall alignment. All of the piles were driven to the recommended refusal rate of less Retaining Wall Installation Final letter Richardson Residence Edmonds, Washington NGA File No. 8802B14 November 13, 2014 Page 3 than 1 -inch of movement in 60 seconds of driving. In our opinion, the pin piles were installed in accordance with our recommendations and the approved plans, and should support the planned loads. After the installation of the pin piles and grouted anchors, we observed that a foundation drain consisting of a 4 -inch perforated PVC pipe was placed behind the base of the retaining wall and covered with drain rock and filter fabric. The foundation drain was daylighted through the retaining wall within the northwest corner and within the central northern portion of the retaining wall. We observed that the foundation drain at the northwest corner of the wall was tightlined via a 4 -inch corrugated drain pipe to the toe of the steep slope, while the foundation drain within the central northern portion of the wall was tightlined via a 4 -inch corrugated pipe to approximately three quarters the way down the steep slope to the north of the retaining wall: The existing drain pipe within the northwest corner of the wall was also tightlined to the toe of the steep slope. We also observed that I -inch weep holes were installed in numerous locations through the base of the retaining wall. The two roof drains associated with the northern portion of the residence were observed to daylight within the sloping area to the north of the residence. We recommended to you that these roof drain pipes be extended down to discharge at the toe of the slope. We understand that this recommendation has been carried out. We also recommend that all drainage pipes be inspected periodically to verify they are working properly and any repairs if needed be performed promptly. We returned to the site on Friday, October 24, 2014 to observe the finished wall and erosion control measures. We observed that the gap behind the wall was completely backfilled with 2- to 4 -inch crushed rock to the top of the retaining wall. We also observed that the soils and slope below the wall had been covered with jute netting, and plants and grass/clover seed had been placed. In our opinion, these measures are adequate. We recommend that this area be continually monitored through the wetter winter months for any indications of instability until the vegetation is allowed to establish. Closure Based on our part-time observations, it is our opinion that the retaining wall and stabilization measures have been installed successfully and in accordance to our recommendations and the plans. It is also our opinion that the erosion control efforts are reasonable. Though the erosion control looks adequate, we recommend that you monitor the slope from time to time, especially after significant rain events. If any signs of distress are observed, we should be contacted. At that time, we would provide recommendations Retaining Wall Installation Final Letter Richardson Residence Edmonds, Washington NCA File No. 88021314 November 13, 2014 Page 4 for further repairs to the slope at that time, as warranted. We should stress that the steep slope below the wall will always be prone to sloughing and shallow sliding events due to the steepness of the slope, the nature of the near surface soils, and the wet weather conditions during the 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 below the wall can ultimately impact the pin piles if they lead to pin pile exposure. We therefore recommend that the slope be monitored an on-going basis and promptly repaired if any signs of soil instability are observed. MIM r �. Retaining wall installation Final Letter Richardson Residence Edmonds, Washington NGA File No. 8802B14 November 13, 2014 Page 5 We appreciate the opportunity to be of 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 Exp; MY 28, 2015 Khaled M. 5hawish, PE Principal LSBXMS;cja