9512 EVERGREEN WAY 2022-08-29 - Laserfiche WebLink

May 4, 2017 HWA Project No. 2017-036-21 3.5.1 Seismic Hazards Seismic hazards are defined as areas subject to a severe risk of earthquake damage as a result of seismically -induced ground rupture, liquefaction, ground spreading, and settlement. A seismic event generates ground movements that are horizontal, vertical, or some combination of the two. It can result in permanent ground damage, directly by ground rupture or indirectly from ground shaking. Ground shaking can damage underground structures; trigger slippage in areas susceptible to landslides; or cause liquefaction, ground spreading or settlement. Ground rupture is the general term used to characterize an area where fault movement results in a distinct offset at the ground surface, or possibly cracks or fissures. The nearest known fault considered to be active by the United States Geological Survey is the South Whidbey Fault, which is located approximately 1.0 mile south of the subject site. Therefore, damage to structures on this site caused by rupture along this fault zone is considered unlikely. Liquefaction occurs when loose, saturated, and relatively cohesionless (clean sandy) soil deposits temporarily lose strength because of earthquake shaking. Locally, the site soils consist of glacially over -consolidated silty sands and gravels that are not subject to liquefaction. 3.5.2 Settlement Hazards Settlement occurs during an earthquake when shaking or liquefaction resultants in densification of loose soils sensitive to vibration effects, particularly thick peat deposits, granular alluvial soils, and improperly constructed fills. Typically, areas mapped as seismic hazards associated with liquefaction coincide with areas of settlement hazard. In consideration of the predominant dense to very dense materials encountered on site, and assuming the recommendations of this report are followed; however, the likelihood of structural damage due to settlement is considered negligible. 3.5.3 Erosion Hazards As discussed previously in Section 3.4, the local surficial soils mapping by the NRCS indicates that site soils pose only a slight erosion hazard. The highest potential for erosion will occur during development after the site has been cleared of vegetation and the soil is directly exposed to the elements. However, imposition of appropriate best management practices (BMPs), as outlined in the Everett Municipal Code, for handling of stormwater runoff during construction will largely mitigate adverse effects associated with soil erosion on site. 3.5.4 Landslide - Steep Slope Hazards Typically, landslide hazards are defined as those slope areas potentially subject to mass earth movement based on a combination of geographic, topographic, and hydrologic factors, with a vertical height of 10 feet or more. In accordance with Section 19.37.080 of the City of Everett Final Geologic Assessment 5 HWA GeoSciences Inc.