1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

McKinnon Residence Retaining Wall NGA File No 910614 1617 Edgemoor Lane December 2, 2014 Everett, Washington Page 9 Tiebacks General: If tieback anchors are needed for the wall design, we recommend that these systems could include drilled, grouted tiebacks, or mechanically installed anchors such as helical anchors. We recommend that one of the anchors be tested to a minimum of 200 percent of the design loads to confirm design values. We recommend that measurements be made by the contractor in the field at the time of tieback installation to verify that tiebacks do not encounter any existing structures or underground utilities. No -Load Zone: The anchor portion of all tiebacks must be located a sufficient distance behind the wall face to develop resistance within a stable soil mass. We recommend the anchorage be obtained behind an assumed no-load zone. The no-load zone is defined by a line extending horizontally from the base of the wall back towards the residence a distance of five feet. The line should then extend up from the base elevation at an angle from the horizontal of 60 degrees. We expect that some undocumented fill may exist beyond the no-load zone. We therefore recommend that the anchors be a minimum of 20 feet long. We recommend that we monitor soil conditions during anchor installation in order to evaluate adequate penetration into competent soils. Soil Design Values: The tiebacks will likely terminate in glacial soils below the walls. For use in design of the anchors, we estimate an allowable grout to soil adhesion of 1,500 pounds per square foot (pso. Load carrying capacities on the order of 10 kips or more could be achieved using 8- to 10-inch diameter helical anchors installed successfully. We recommend that we review anchor design and proposed installation methods. We should also observe anchor installation and testing. Tieback Installation and Testing: The contractor should be responsible for using equipment suited for the site conditions. We do not recommend the use of open -hole methods for the purpose of installing the tiebacks due to the potential for soil caving. Secondary grouting to increase soil adhesion may be used; however, if secondary grouting is used, the anchors should be tested using the methods outlined for the performance testing. In the case of the helical anchors, the contractor should determine the torque values required to achieve the desired capacity. However, the anchors should advance a minimum of 20 feet from the face of the wall and one anchor be tested to confirm design capacities. NELSON GEOTECHNICAL ASSOCIATES, INC.