3815 SHORE AVE 2020-05-20 - Laserfiche WebLink

Geologically Hazardous Areas: Geotechnical Letter Kennedy Single Family Residence 3815 Shore Avenue March 23, 2018 RN File No. 3279-001A Page 6 In our opinion, the proposed site development will not create any additional impacts to the erosion hazard risk relative to the existing conditions. Because stormwater will be collected and tightlined down the slope, the erosion risk due to groundwater either will not be impacted or may improve relative to current conditions. The proposed site development is located on the flat portion of the site and will not impact vegetation or slope gradient within the high erosion hazard area. The erosion hazard area presents low risk to the proposed single family residence because of the setback from the top ofslope. To further reduce erosion at the steep slope,and preserve the existing lawn as much as possible, we suggest placing geofabric to preserve the existing grade and revegetating the top of slope with short bushes that will develop a more expansive root system than the current sod. We also suggest the installation of a cutoff drain at the depth of the contact between the sandy soils and the transitional bed silts to reduce the flow of perched water out to the face of the steep slope. The cutoff drain may be functionally substituted by sufficiently deep footing drains, as discussed in the Drainage subsection of this letter. Seismic Hazard: It is our opinion based on our subsurface explorations that the Soil Profile in accordance with the 2015 International Building Code (IBC) is Site Class D with Seismic Design Category D. We used the US Geological Survey program "U.S. Seismic Design Maps Web Application." The design maps summary report for the 2012/15 IBC is included in this report as Appendix A. Additional seismic considerations include liquefaction potential and amplification of ground motions by soft soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table, The underlying dense transitional beds soils are considered to have.a very low potential for liquefaction and amplification of ground motion and seismically induced lateral spread. The project is mapped on Faults and Earthquakes in Washington State (Jessica L. Czajkowski and Jeffrey D. Bowman, USGS OFR 2014-05) as located within the Southern Whidbey Island Fault Zone. This is a class A fault and is considered to have a low potential for surface displacement because of the recurrence interval of up to 9,000 years, the age since the last suspected deformation of 2,700 years ago and its slip-rate category of approximately 0.6 mm per year. Site Preparation,and Grading The first step of site preparation should be to strip the vegetation, topsoil, or loose soils to expose medium dense or firmer native soils in pavement and building areas. The excavated material should be removed from the site, or stockpiled for later use as landscaping fill. The resulting subgrade should be compacted to a firm, non-yielding condition. Areas observed to pump or yield should be repaired prior to placing hard surfaces. The on-site glacial outwash or transitional beds likely to be exposed at shallow depths during construction are considered moderately moisture sensitive; the fill is also moisture sensitive and the surface will disturb easily when wet. We expect these soils would be difficult to compact to structural fill specifications in wet weather. We recommend that earthwork be Robinson Noble, Inc.