5901 23RD DR W Geotech Report 2025-07-29

I October 22,2007 Project No.T-5922-1 Based on our observations of surface conditions on the slope, and considering the slope is underlain by dense to very dense glacially consolidated sediments, it is our opinion that the slope is currently stable with regard to deep-seated movement. It is also our opinion that the planned development will not result in adverse impacts to slope stability. This is supported by the results of stability analysis performed along a section line identified on Figure 2 as Section A-A' using the computer program WinStabl. Our analyses considered both static and pseudostatic (seismic) conditions for the post-development conditions shown on the project information provided to us. The soil parameters used for our analyses are based on field and laboratory data and our past experience with similar soils. These parameters are shown on the WinStabl analysis output text and profiles in Appendix B. The pseudostatic analysis used a horizontal earthquake coefficient value of 0.15g to model sustained ground motions expected from a severe earthquake. This value was based on current USGS seismic hazard maps of the site latitude and longitude for a seismic event having a 10 percent probability of exceedance in a 50-year period. The USGS map indicates the subject site is located within an area where the peak horizontal ground acceleration for this return period is expected to be about 0.3g. Our analysis considered a horizontal acceleration equal to one-half this value. Our analyses determined minimum factors of safety of 1.67 and 1.26 for the static and pseudostatic conditions, respectively. These safety factors are higher than the minimum safety factors considered acceptable for stable slopes by local geotechnical engineering practice. In addition, we evaluated stability along Section A-A' (static) where cohesion was excluded from the soil strength. The results of this analysis indicate a minimum factor of safety of 1.24. As expected, this factor of safety is slightly Iower than the minimum safety factor determined using cohesion (F.S.= 1.67). The results of the stability analyses indicate that the site is stable with respect to deep-seated failure under static and pseudostatic conditions. Based on the results of our subsurface investigation, our field observations, and our stability analysis, and provided BMPs for erosion prevention and sedimentation control are properly implemented and maintained in the planned development area,along with temporary and permanent drainage improvements, it is our opinion that the 25-foot setback from the crest of the steep slope to the planned development area will adequately mitigate the landslide hazard. 4.3 Seismic/Liquefaction Hazard Section 19.37.080(A)(2)of the EMC defines seismic hazard areas as: a. Those areas mapped as seismic/liquefaction hazards per the Dames and Moore Methodology for the Inventory, Classification and Designation of Geologically Hazardous Areas, City of Everett, Washington: July 1, 1991. b. Those areas mapped as high and moderate to high liquefaction susceptibility on the Liquefaction Susceptibility Map of Snohomish County, Washington, Washington State Department of Natural Resources,Palmer,Stephen,et al.,September,2004. I Page No. 6 I