1311 W MUKILTEO BLVD 2022-06-29 - Laserfiche WebLink

Geotechnical Engineering Evaluat. • NGA File No. 1295621 Tsang Slope Restoration October 21,2021 Everett,Washington Page 4 ie SENSITIVE AREA EVALUATION Erosion Hazard The City of Everett designates erosion hazards based on maps published in 2006 based on a methodology from 1991.The site is mapped as a 'medium'erosion hazard by the City. The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient,vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types,which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington, by the Natural Resources Conservation Service (NRCS), classifies the native soils within the upper portion of the site as Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes. Soils on the lower portion of the property are mapped as Everett very gravelly sandy loam, 0 to 8 percent slopes. The former soil is mapped as a severe erosion hazard, while the later is mapped as a moderate hazard. Based on our experience in the area and our observations in the field, it is our opinion that the native soils on the site would have a moderate to high erosion hazard for areas where they are exposed on steep slopes. It is our opinion that the erosion hazard for native site soils should be low in areas where vegetation is not disturbed. Landslide Hazard The criteria used for evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The City of Everett designates Landslide Hazard Areas in the Everett Municipal Code (EMC) Section 19.37.080(A)(1). Maps published in 2006 by the City suggest a'very high' landslide hazard based on a 1991 inventory, which is consistent with our interpretation of the code, since uncontrolled fill is present on the site. CONCLUSIONS AND RECOMMENDATIONS General In accordance with the guidance by the City of Everett, we understand the site must be restored to original grade and/or stabilized in accordance with geotechnical recommendations, including the removal of the ecology block wall. As such, we have made recommendations for permanent slope conditions and erosion control improvements in the remaining subsections of this report. We understand that at the time of production of this report,the retaining wall had been removed and the slope was regraded to an inclination of two units Horizontal to one unit Vertical (2H:1V) at our verbal recommendation. Further recommendations for long-term stabilization are presented hereon. NELSON GEOTECHNICAL ASSOCIATES, INC.