1815 HOLBROOK AVE Geotech Report 2025-03-06

June 9,2022 Page 6 of 15 Geotechnical Evaluation The proposed construction can be completed in a manner to reduce the risk of erosion and soil movements through proper grading, benching, and creation of new foundation walls and lower magnitude cut slopes. We recommend deepening foundation elements for the new structure in order to create a minimum effective setback of 7 feet from the steeper slope areas (magnitudes of 4o percent or more). In addition to this setback, we recommend that foundation elements closest to the east property line be embedded to a depth to create a minimum 1H:1V envelope from the base of the footing to the toe of the slope. This may result in deeper footings that what would be necessary to meet the 7 foot setback. This should be provided in pictorial form on the architectural plans for review. Additionally,we recommend re-grading the steeper slopes to a 2H:1V slope(or flatter)in medium dense native soils. These areas should be vegetated and surfaced upon completion to reduce erosion potential. One or more retaining walls could be utilized in order to reduce slope magnitudes in this area.We can provide additional input once more detailed plans are prepared. We should review the final plans to verify that our recommendations are implemented. We should also be on site during construction to confirm suitability of the mitigation work, effective setbacks,drainage placement,wall and/or slope creation,and final erosion control systems. Once completed,we should be able to verify that the systems are`safe'as designed. Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is underlain by Indianola loamy sand (15 to 30 percent slopes) and Alderwood-Everett gravelly sandy loams(25 to 70 percent slopes). These soils would have a moderate to very severe erosion potential in a disturbed state depending on the slope magnitude. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season,with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the International Building Code (IBC). A Site Class D applies to an overall profile consisting of stiff/medium dense soils within the upper 10o feet. The site class reflects the native soils below the fill. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss,S,,F., and Fv. The USGS website includes the most updated published data on seismic conditions. The following tables provide seismic parameters from the USGS web site with referenced parameters from ASCE 7-16. www.cobaltgeo.com (206)331-1097