5109 SOUND AVE 2017-08-02 - Laserfiche WebLink

July 21, 2014 Two Single-Family residences L&A Job No. 14-065 Page 7 formation and seep out of the slope or cut banks where the Whidbey formation is exposed. Prolonged seepage may cause erosion and soil sloughing, especially in winder and with vegetation cover removed. GEOLOGIC HAZARDS AND MITIGATION Erosion and Landslide Hazard The toe of the hillside in the southeast quadrant of the project site is underlain by a hard clayey silt deposit which is of extremely low permeabilit��. This deposit would perch stormwater infiltrating into the more permeable surficial soils. Rising perched groundwater in this area would saturate and weaken the surficial soils overlying the clayey silt deposit. Also, the perched groundwater may emerge out of the toe of the hillside. Groundwater seepage may cause soil erosion ii' the toe of the hillside is disturbed and the vegetation cover is stripped. Progressive erosion may cause slide to occur over the underlying hard silt deposit. To mitigate potential of erosion induced landslides on the ioe of hillside, a retaining wall may be constructed along the toe of hillside. Recommendations for such retaining wall are provided under the RETAINING WALL section below. Seismic Hazard The Puget Sound region is in an active seismic zone. The site is underlain at shallo��v depth by either hard silt within the toe of hillside area or by medium-dense to dense advance outwash deposit in the gently sloped area. The seismic hazard in the gently sloped area of the site should be minimaL The weak surficial soils overlying the hard silt LIU & ASSOCIATES, INC.