1215 MADRONA AVE 2016-08-24 - Laserfiche WebLink

Retaining Wall Installation Final Letter NGA File No. 924115 Gustafson Residence Retaining Wall February 10, 2016 Everett, Washington Page 2 also observed erosion contro] installation, drainage improvements, and revegetation of the steep slope area below the subject concrete rubble walls. After these improvements were made, we were then retained to evaluate the steep slope area and existing rubble walls, and provide mitigatio» measures to stabilize the top of the steep slope and rubble walls. We prepared a geotechnical report titled "Geotecl�nical E�lgineering Evaluation — Gustafson Residence Retaining Wall — Everett, Washington," dated June 12, 2015. As part of that study, we recommended a pin pile cantilever retaining wall be constructed along the down11i11 side of the concrete rubble walls to stabilize the affected area. We also recommended dr�ai�lage i�nprovements, as well as covel•ing the slope within the vicinity of the new wall with erosion control material and vegetation. We have since monitored the wall installation. Site Observations We monitored wall installation activities on a part-time basis from October 27, 2015 through December 30, 2015. Our observations during wall installation at this project were documented in Field Reports 1 through 13, which have been submitted to you. The new wall was located along the lower portion of the existing rubble walls. We observed the installatio�� of l0, 2-inch diameter schedule 80 piles that were driven with a 140-pound air hammer to a total embedment depth in the range of 8.0 feet to 15.4 feet below ground surface along the wall alignme�lt. Al] of the piles were driven to the recommended refusal rate of less than 1-inch oi' movement in 60 seconds of driving. In our opinio��, the pin piles were installed in accordance with our recommendations and the approved plans, and should support the wall. We observed the installation of four helical anchor tieback anchors along the wall alignment. The helical tiebacks consisted oi�triple-flight 8-, 10-, and 12-inch helix anchors with 1-3/4-incl� solid square shafts. The anchors were driven to depths in the range of 23.8 to 27.7 feet from the face of the retaining wall back into the slope. The anchors were driven until the max torque of the drive head used to drive the anchors was reached. This torque was measured at approximately 2,500 to 2,600 PSI at the drive head. The anchors were installed at an approximate angle of 20 degrees below horizo��tal. After ii�stallation, we returned to conduct a load test on one of the helical anchors to verify tlle load carrying capacity. We conducted the test on Anchor H1. The anchor was tested in 50% increments of the design load to a maximum load of 200% of the design load of 7.0 kips. We obse�ved mini�l�al movement of the anchor NELSON GEOTECHNICAL ASSOCIATES, INC.