4801 HARBOR LN 2018-01-02 MF Import

Geotechnical Engineering i ; ort J-1280 Johnson Residence Addition 27 February 2002 Everett, Washington Page 5 the slope utilizing the computer program STABL 5. The following soil parameters were utilized in the analysis. We selected the parameters based upon the results of field and laboratory testing, published data, and our experience with projects of a similar nature in the site vicinity. Soil Parameters below Upper Fill and Loose Native Soil Medium Dense Native Granular Soil Dense to Very Dense Native Granular Soil Density c 90 110 120 Cohesion s 0 0 0 Phi Angle (degrees) 30 14 38 We evaluated the slope stability under static (existing) conditions, and also modeled the slope to evaluate the effects of a seismic event with a recurrence probability of 10 percent over a 500 year period. We utilized a peak horizontal ground surface acceleration of 0.14 g (gravity) in the dynamic analysis. The analysis indicates that the potential exists for development of a critical landslide slip surface that extends through the soft and loose fill and loose native soils during the design seismic event. The calculated dynamic case critical slip surface with a factor of safety equal to 1 is depicted on Figure 2. The static case factor of safety for the slope along Section A -A' with a factor of safety of approximately 1.5 is very similar to the , surface shown for the dynamic case. The calculated critical failure surfaces for both tht :'tatic and dynamic cases extend to the foundation of the existing dwelling. It should be noted that the static and seismic factors of safety would be reduced by saturation of the fill soils or underlying native soils. Our analyses assumed that drainage measures employed during construction are sufficient to prevent saturation of soils on the slope. If saturation occurs due to pipe breaks or Ieaks, heavy rainfall, or surface runoff, the slope stability would be reduced. Landslide Mitigation Mitigation of the risk of future landsliding could be accomplished by regrading the hillside below the dwelling. This process would entail excavation of the soft and loose fill and native soils, and reconstructing the hillside by compacting soil in place under controlled and engineered conditions. It may also be possible to ,.litigate the potential for future landsliding through structural means, such as by the construction of a large retaining wall. Detailed analysis of landslide mitigation methods was not within our scope of services. A reduction in the risk of potential adverse impacts of landsliding on the proposed residential additional could be accomplished by constructing the foundations such that they are founded in the denser soils below the theoretical critical slip surface. In order to reduce adverse impacts to the proposed residential addition in the event of future slope movement along the critical slip surface shown on Figure 2 we recommend that the new dwelling foundation extend into the denser native granular soils below the very loose to loose fill and native soils. Zipper Zeman Associates, Inc. 18905 33`u Avenue West, Suite 117 I.)llnwood, Washington 98036 (125) 77I •3301 iq