1903 W MUKILTEO BLVD Geotech Report 2022-10-25

Dr. Hong Li JN 12084 May 10, 2012 Page 3 only at the locations tested. The relative densities and moisture descriptions indicated on the test boring logs are interpretive descriptions based on the conditions observed during drilling. CONCLUSIONS AND RECOMMENDATIONS GENERAL THIS SECTION CONTAINS A SUMMARY OF OUR STUDY AND FINDINGS FOR THE PURPOSES OF A GENERAL OVERVIEW ONLY. MORE SPECIFIC RECOMMENDATIONS AND CONCLUSIONS ARE CONTAINED IN THE REMAINDER OF THIS REPORT. ANY PARTY RELYING ON THIS REPORT SHOULD READ THE ENTIRE DOCUMENT. The steep, northern slope is most likely to experience periodic future episodes of shallow slides that affect the uppermost few feet of weathered, looser soil. These should be expected, and slow recession of the top of the steep slope will occur over time. These slides would most likely occur following extended wet weather or during an earthquake. In order to evaluate the potential for a future,failure of the sloping ground north of the residence, we completed a slope stability analysis using the WinStabl computer program. This program was developed by the University of Wisconsin. Using a Modified Bishop's analysis, we analyzed the slope's stability both under static and earthquake conditions. The existing slope configuration was used in the analysis; however, it should be noted that no detailed site topographic survey was available at the time of our study, and topographic information including slope angles, and the location of structures were approximated based on field observations .and measurements, and topographic information available on Snohomish County's GIS system. Therefore, the results of the analysis are approximate, and should be used as a general indication of slope stability. Of primary concern from a geotechnical engineering standpoint is the potential for a failure near or at, the interface between the medium-dense upper soils and the underlying dense soil, assuming perched groundwater near this interface. Multiple situations were analyzed: 1) a large-scale, deep- seated landslide occurring near the toe of the upper steep slope (near the interface of the looser upper soils and the underlying denser soils), and 2) a shallower, surficial failure occurring on the surface of the slope within the looser upper soils. These two scenarios where analyzed for failure surfaces that terminated in the rear lawn area of the home, as well as the potential for a failure to extend beneath the existing house foundation. These analyses were conducted for both static and seismic conditions, with an earthquake magnitude of 0.34g being used. As the attached results of the computer analysis show, for a larger, deep seated failure occurring at the location of the existing residence, a static safety factor of 1.5 and a seismic (dynamic) safety factor of 0.9 were calculated. Typically, it is desirable for new construction to have static and dynamic safety factors of at least 1.5 and 1.1 to 1.2, respectively. The safety factor against shallower slope movement occurring near the location of the existing house was shown to be the same. Considering that the foundation of the house in this location bears at least 4 feet below the existing grade, these safety factors would be slightly increased when considering potential slope failures beneath the existing foundation. The results of our borings and analyses indicate that the current conditions are adequate when considering a deep-seated failure occurring beneath the existing house foundation under static conditions. However, during a large seismic event, it is possible that a slope failure could occur that extends beneath the existing house foundation. Protecting the structure against this eventuality is a choice that the owner can make for an existing home. Our slope stability analysis indicated GEOTECH CONSULTANTS, INC.