1915 W MUKILTEO BLVD 2018-01-02 MF Import

Geotechnical Engineering Evaluation and Slide Repa rs 1915 Mukilteo Boulevard July 26, 2006 NGA File No. 743206 Page 9 We recommend that the pile be a minimum of 4-inch diameter galvanized steel pipe sections driven into place using a hydraulic hammer, with a minimum energy rating of 1,100 ft-lb. The piles should be spaced at a maximum distance of 4 feet, and should be embedded a minimum of 15 feet into the competent slope material. The maximum spacing and minimum embedment values are to provide additional reinforcement to the slope and lessen the potential for sliding. The piles should be battered back toward the core of the slope at an angle of 15 degrees from vertical. In addition to the minimum recommended embedment into the native material, the piles should be driven to a refusal criteria of less than one inch of movement during 15 seconds of continuous driving. The exposed portion of the wall should not exceed ten feet in height. Any backfill placed behind the wall should consist of clean 2-inch crushed rock. The tic -backs should extend a minimum of 20 feet into the slope aad should be battered at an angle of 15 degrees below horizontal. The tiebacks should be a minimum of 4-inches in diameter, but should be sized to provide resistance for an active pressure resulting from a fluid of equivalent density of 40 pounds per cubic foot (pcf), acting along the back of the wall. All active pressure should be picked up by the tie -backs and no passive resistance on the embedded portions of the piles should be utilized. This may entail using two rows (or more) of tie backs for this wall. The tie -backs should be tested after installation to ensure adequate performance. We can discuss tic -back test procedure with the contractor at the time of construction. The attached Figure 10 is a schematic detail showing the recommended wall. Slope Improvements and Drainage After the wall has been installed, the portion of the slope below the wall should be investigated and slope improvements performed to lessen the potential for future erosion and sliding below the wall. These conditions, Knot addressed, could eventually undennine the wall. These. improvements should include removing the vegetative cover and slide debris in certain locations, benching the slope, placing a layer of rock spalls to buttress the slope, and installing shallow drains to control surface and subsurface water flow. The actual extent of such improvements should be determined in the field at the time of construction. However, in general, areas of sloughing and slide debris would be over -excavated down to competent material and 4-foot wide benches created in these areas. A minimum 1-foot thick layer of 4- to 8-inch rock spalls would then be placed on the exposed surface. Shallow cut- off drains would be installed at strategic locations to divert water seepage away from sensitive areas. The NELSON GEOTECHNICAL ASSOCIATES, INC. I