1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

Retaining Wall Installation Final Letter NGA File No. 9106B 15 Mckinnon Residence Retaining Wall June 15, 2016 Everett, Washington Page 2 Site Observations We monitored wall installation activities on a part-time basis from August 3, 2015 through August 31, 2015. Our observations during wall installation at this project were documented in Field Reports 1 through 9, which have been submitted to you. The new wall was located along the top of the steep slope immediately below the existing yard area. We observed the installation of 28, 3-inch square schedule 80 piles that were driven with a 140-pound air hammer to a total embedment depth in the range of 7.0 feet to 12.0 feet below ground surface along the wall alignment. All of the piles were driven to the recommended refusal rate of less 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 wall. We observed the installation of eight helical anchor tieback anchors along the wall alignment. The helical tiebacks consisted of double -flight, 10-, and 12-inch helix anchors with 1-1/2-inch solid square shafts. The anchors were driven to depths in the range of 11.5 to 19.0 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 and the anchor shafts began to twist. This torque was measured at approximately 1,800 to 2,300 PSI at the drive head. The anchors were installed at an approximate angle of 15 degrees below horizontal. After installation, we returned to conduct a load test on one of the helical anchors to verify the load carrying capacity. We conducted the test on Anchor 4. The anchor was tested in 50% increments of the design load to a maximum load of 200% of the design load of 10.0 kips. We observed minimal movement of the anchor during the testing and the hold periods, and the tested anchor appeared to perform well. In our opinion, all of the anchors were installed in accordance to the plans and our recommendations. We returned to the site on June 9, 2016 and observed that yard and drainage improvements had been completed. We were provided with detailed pictures and descriptions of the installed drainage improvements within the property. We understand that a series of french drains were installed within the yard and driveway area along with drainage lines from the roof drains that lead to a main catch basin located directly behind the retaining wall. After the installation of the pin piles and helical anchors, we observed that a retaining wall 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 leading into the main catch basin behind the retaining wall. A new 6-inch HDPE pipe connected to the main catch basin was NELSON GEOTECHNICAL ASSOCIATES, INC.