954 NORTH PARK DR 2023-10-31 - Laserfiche WebLink

GeoTest Services, Inc. September 6, 2019 Proposed Swimming Pool — 954 North Park Drive, Everett, WA Project No. 19-0361 are those with "slopes greater than forty percent in all other geologic units." The slopes in the northern portion of the property exceed forty percent, and the Qtb unit was identified near the surface in the proximity of these slopes. This corresponds to a "very high" landslide hazard per the EMC. The City of Everett Critical Areas Map 2 (Goodwill, 2006) supports the "very high" landslide hazard designation for this property. Large scale global instability, consisting of deep-seated rotational failures, can extend down into the subsurface to substantial depths. These failures typically leave geomorphic evidence of their existence on the slope. Typical indicators are recessional and sometimes nested head scarps, tension cracks, sag ponds, seepage zones, hummocky ground surfaces, and slump blocks. It should be noted that no amount of engineering can completely mitigate or prevent slope instability. The Pacific Northwest is seismically active and it is difficult to predict how the slope(s) at the subject site may behave during a large earthquake. GTS's review did not find evidence of historic, large scale slope failures on or below this project site. Although large scale failures are feasible, the more likely method of slope failure would be via small scale "skin" slides. Evidence of this recent near -surface slope instability was noted on the existing northern slope as previously discussed in the Aerial Imagery Review section of this report. Small scale landslides are typically failures of variable widths and shallow depths. Although these types of failures can be visually impressive, in most instances these failures only have limited impacts to the slope. Horizontal, top -of -slope setbacks and vegetation maintenance is commonly used to mitigate small scale landslide potential. Mitigation is intended to make the risk posed by the slope that is present on site less. It should not be interpreted that mitigation will eliminate any and all risk that might be present on the site. It is assumed that the property owner is adequately informed of the risk associated with the steep slope on this property, and that the recommendations in this report will be followed for the design and construction of the proposed pool. Seismic Hazards and Liquefaction Potential Liquefaction is the precipitous loss of shear strength experienced by loose, wet soils during cyclic loading (earthquakes). The risk of liquefaction -induced settlement is generally greater for saturated, granular soils compared to drier or finer -grained soils. EMC Section 19.37.080.A.2 specifically identifies seismically hazardous areas as those "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 [OR] 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." The Dames and Moore inventory does not classify the slope as a seismic hazard, and the referenced Washington DNR map classifies the project area as "very low to low" liquefaction susceptibility. A narrow strip of land at the base of the slope is rated at 11