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Veritas Construction, Inc. <br />October 6, 2015 <br />ES-4011 <br />Page 2 <br />Based on review of the referenced geotechnical report, the subject site is underlain by dense <br />and competent glacial till deposits. Recent subsurface investigation performed by ESNW has <br />also confirmed the competent nature of the underlying geology. Subsurface data and related <br />Test Pit Location Plan are provided as an attachment to this letter. Consistent with the <br />findings of the subsurface explorations, the geologic mapping of the area (Minard 1982) <br />identifies glacial till (Qvt) deposits throughout the site and surrounding areas. Throughout the <br />descending sloped areas to the north and west, advance outwash (Qva) is identified, with <br />localized exposures of undivided till (Qtu). <br />Geologically Hazardous Areas — EMC 37.080 <br />As defined by the referenced City of Everett Municipal Code (EMC 37.080 — Geologically <br />hazardous areas), the footprint of the proposed vault structures will reside partially within <br />segments of designated landslide hazard area. Based on recent reconnaissance of the slopes <br />along the north and west margins of the property, existing stability within and beyond the <br />property limits is characterized as good. The slopes are heavily forested, and no indications of <br />large scale instability were identified. In this respect, the planned positioning of the vault <br />structures within portions of the designated landslide hazard area is feasible from a <br />geotechnical standpoint. The planned development activities within segments of the <br />designated landslide hazard area would be subject to the referenced municipal code and <br />provision for alterations (EMC 37.080C — Permitted Alterations). As part of the requirements <br />for allowing alterations within a designated landslide hazard area, an evaluation of slope <br />stability is required. <br />Analysis of Proposal (EMC 37.080C — Permitted Alterations) <br />Alterations within designated landslide hazard areas can only occur if the planned <br />development activities will not create a hazard to the site and surrounding properties. To <br />evaluate the level of hazard, a slope stability analysis was conducted with respect to the <br />proposed vault construction along the north and west margins of the property. <br />Stability Analysis — Vault Structures <br />Analysis of slope stability (static and seismic) was completed as part of our overall evaluation <br />of the proposed stormwater vault structures. As described above, the vault structures will be <br />located along the north and west margins of the planned development area, and will derive <br />support within the underlying dense glacial deposits. The attached site plan illustrates the <br />approximate location and alignment of the vault structures. The foundation for the vaults will <br />derive support within the dense glacial till deposits. Finish grades around the margins of the <br />site will be supported in part by the concrete vault walls. , To evaluate stability of the vault <br />structures and related fills, two representative cross -sections (A -A' and B-B') were developed <br />through the north and west slope areas of the site. The cross sections are illustrated on the <br />attached site plan. A limit equilibrium computer modeling program (Slope-W) was used to <br />model the static and seismic slope stability through the representative cross sections. The soil <br />strength parameters used in the analysis were based in part on the identified site geologic <br />conditions and engineering judgment. The following table provides the soil strength <br />parameters used for the slope stability analyses: <br />Earth Solutions NW, LLC <br />