1901 W MUKILTEO BLVD Geotech Report 2022-10-25

Scriven Residence Improvements-REVISED NGA File No 974316 1901 West Mukilteo Boulevard November 29,2016 Everett,Washington Page 13 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 (psf) can be used for grouted anchors. 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 two anchors be tested to confirm design capacities. 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 location should be determined in the field,based on soil conditions observed during anchor installation. All other tiebacks should be proof-tested to at least 130 percent of design capacity. Site Drainage and Slope Improvements We recommend that after the soldier piles are installed, a 4-inch perforated PVC pipe be placed behind the base of the wall and extended through the face of the retaining wall to aid in drainage through the wall. The pipe should be surrounded by a minimum of one foot of drain rock and the drain rock entirely wrapped in a heavy-duty filter fabric prior to the placement of the crushed rock backfill. The drain should be tightlined into the existing drain system or independently down to the base of the slope and adequately anchored to the slope. Extreme care should be used as to not disturb the tiebacks during fill placement. We should be retained to evaluate the crushed rock placement during construction. The areas along the top of the slope below the proposed wall should be improved by gently re-grading the over-steepened/loosened areas, removing the vegetative cover, compacting the exposed surface to a non- yielding condition,placing suitable erosion control systems on the prepared areas, and re-vegetating with deep-rooted drought-resistant plants. The actual improvement methods for these areas will be highly NELSON GEOTECHNICAL ASSOCIATES, INC.