4800 DOGWOOD DR Geotech Report 2022-10-25

Subsurface Exploration, Geologic Hazards, View Ridge Community Church Addition and Geotechnical Engineering Report Everett, Washington Geologic Hazards and Mitigations parameters for the analysis included slope geometry, geology, soil strength parameters, and seismic conditions. For evaluation of slope stability under seismic conditions, a horizontal ground acceleration of 0.30g was used for our analysis; this value is equivalent to half of the peak horizontal ground acceleration. Soil strength parameters used for our analysis were assumed based on typical published values for similar materials and our prior experience. The values assumed for our analysis are shown on the Slope/W profiles included in Appendix B. The slope geometry used for our analysis was based on a topographic survey of the church property. Cross -Section A -A' shown on Figure 2 was selected for our analysis. The stability of a slope can be expressed in terms of its factor of safety. The factor of safety is the ratio between the forces that resist sliding to the forces that drive sliding. For example, a factor of safety of 1.0 would indicate a slope where the driving forces and the resisting forces are exactly equal. Increasing factor of safety values greater than 1.0 indicate increased stability. Factors of safety below 1.0 indicate conditions where the driving forces exceed resisting forces and landsliding is imminent. We recommend minimum factors of safety equal to 1.5 for static conditions and 1.1 for dynamic (seismic) conditions. Factors of safety equal to 3.3 and 1.4 were calculated for deep-seated failure at the setback buffer for the proposed addition under static and seismic conditions, respectively. Minimum factors of safety of 2.6 and 1.3 under static and seismic conditions, respectively, were calculated for critical deep-seated failure surfaces for the steep slope area. Copies of the Slope/W profiles, the soil strength parameters used for our analysis, and the calculated minimum factors of safety are provided in Appendix B. Given the lack of indications of historical landslide activity on the site and the calculated factors of safety for deep-seated failure near the proposed addition exceeding the minimum values, it is our opinion that the risk of damage to the subject site under either static or seismic conditions is low. Consequently, we consider a minimum of 25-foot slope setback buffer for the proposed addition to be adequate. 6.0 SEISMIC HAZARDS AND MITIGATIONS Earthquakes occur regularly in the Puget Lowland. The majority of these events are small and are usually not felt by people. However, large earthquakes do occur, as evidenced by the 1949, 7.2-magnitude event; the 2001, 6.8-magnitude event; and the 1965, 6.5-magnitude event. The 1949 earthquake appears to have been the largest in this region during recorded history and was centered in the Olympia area. Evaluation of earthquake return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely every 20 to 40 years in the Puget Sound area. August 29, 2016 ASSOCIATED EARTH SCIENCES, INC. SKL11d - KE160351A2 - ProjectsJ20160351JKEJWP Page 6