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Geotechnical Engineering Evaluation Miller Property Slope Stabilization Everett, Washington NGA File No. 1333322 July 1, 2022 Page 14 Tiebacks General: These systems should consist of drilled, grouted tieback anchors. We recommend that two of the anchors be performance 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 shoring wall back towards the residence a distance of 10 feet. The line should then extend up from the base elevation at an angle from the horizontal of 60 degrees. 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 native, competent soil below the residence. For use in design of the anchors, we estimate an allowable grout to soil adhesion of 1,500 pounds per square foot (psf) be utilized for anchors terminated within the competent native soils. This value should be verified through two performance tests prior to ordering the production anchors. Tieback Installation and Testing: The contractor should be responsible for using equipment suited for the site conditions. We do not recommend the use of an open -hole method 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. All anchors should be installed at an approximate inclination of 20 to 25 degrees below horizontal. Two anchors should be performance -tested to 200 percent of the anchor design capacity. The performance test should consist of cyclic loading in increments of 25 percent of the design load, as outlined in the Federal Highways Administration (FHA) report No. FHWA/RD-82/047. The test locations should be determined in the field by NGA, based on soil conditions observed during anchor installation. All other tiebacks should be proof -tested to at least 130 percent of design capacity. continued on next page NELSON GEOTECHNICAL ASSOCIATES, INC.