1903 W MUKILTEO BLVD Geotech Report 2022-10-25

Dr. Hong Li JN 12084 May 10, 2012 Page 4 that to adequately protect the existing house against a slope failure under significant seismic loading, a structure capable of retaining at least the upper 15 feet of soil would be needed at the top of the slope. This retaining structure would need to be constructed with large-diameter, heavily reinforced, concrete filled piles. These drilled piles would be spaced closely to adequately retain the upper soils in the event of a failure occurring downslope of the pile wall. Installation of this type of wall would require the use of large drilling equipment sitting in the limited space between the house and the steep slope. Further recommendations for design and construction can be found in the CLOSELY SPACED PILE STABILIZATION section of this report. The analysis of this structure assumes no additional fill will be placed at the top of the steep slope. We recommend against the placement of more fill at the top of the slope, as it would dramatically affect the design of any type of slope stabilization system. Alternatively, if a more economical/less robust retaining structure was desired, a smaller wall could be constructed atop the steep slope. This structure would only be intended reduce the chance for shallow failures encroaching further into the existing yard area, but would not be adequately designed for a large, deep-seated failure. This wall would be constructed using small-section steel -beams that are placed in drilled holes or driven into the ground. These beams would extend into the dense soils beneath about 20 feet below the top of the slope. The space between the piles would be lagged with wood lagging, with lagging added following any movement of the soil on the downslope face of the wall. Specific recommendations for design and construction can be found in the SOLDIER PILE WALL section of this report. As with the previous wall, the analysis assumes no additional fill will be placed at the top of the steep slope. Regardless of the type of retaining structure, removing the existing fill material and landscape walls from the top of the steep slope is recommended and would slightly increase the overall slope stability. Once removed, the existing fill should be taken away from the site, and not be placed on the slope or in the rear yard. Some erosion of the soils at the ground surface was evident at the top of the steep slope during our site investigation. As we recommended in our concurrent report to your Dr. Hong Li, we recommend that surface water be directed away from the slope where your two properties meet. Additionally, the north end of the footing for the existing western retaining wall needs to be resupported. There are several ways to accomplish this, with potential methods including filling the void under the wall with concrete, or supporting the undermined section with small-diameter pipe piles. We also observed that most of the downspouts from both homes have been piped to locations about 15 to 20 feet down from the top of the slope. It is important that these pipes remain intact and functioning, and thus should be inspected regularly. Stormwater collected from roof drains or surface drains should not be allowed to discharge atop the steep slope. We recommend extending the pipes further down the slope, to the more moderately-inclined portion. Also, grading and vegetation along the top of the steep slope should be maintained to prevent erosion gullies from forming due to concentrated surface runoff. We recommend including this report, in its entirety, in the project contract documents. This report should also be provided to any future property owners so they will be aware of our findings and recommendations. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site soil profile within 100 feet of the ground surface is best represented by Site Class Type D (Stiff Site Class). As noted in the USGS GEOTECH CONSULTANTS, INC.