1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

McKinnon Residence Retaining Wall NGA File No 910614 1617 Edgemoor Lane December 2, 2014 Everett, Washington Page 4 layers that we interpreted as native glacial till deposits. Underlying the glacial till soils, we encountered dense to very dense, brown fine to coarse sand with silt and varying amounts of gravel that we interpreted as native advance outwash soils. Borings B-1 and B-2 were terminated within the native advance outwash soils at depths of 25.5 and 21.5 feet below the existing ground surface, respectively. In both of our hand augers, we encountered approximately 0.5 feet of surficial topsoil. Underlying the topsoil, we encountered medium dense to very dense, brown fine to coarse sand with gravel that we interpreted as native glacial advance outwash soils. Hand Augers HA-1 and HA-2 were terminated within the native glacial advance outwash deposits at a depth of 7.0 and 5.0 feet below the existing ground surface. Hydrologic Conditions We did not encounter groundwater seepage in our explorations. However, we anticipate that a perched water condition could likely occur within the undocumented fill and silty soils encountered within our explorations. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of underlying, less permeable soils. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of precipitation. We would expect the amount of perched water to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2012 International Building Code (IBC). Since mostly native medium dense/stiff to dense/hard soils are interpreted to underlie the site at depth, the site conditions best fit the IBC description for Soil Class D. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the competent glacial deposits interpreted to underlie the site at depth have a low potential for liquefaction or amplification of ground motion. The medium dense/stiff or better native glacial soils interpreted to form the core of the site slope are considered stable with respect to deep-seated slope failures. However, the loose surficial materials and NELSON GEOTECHN/CAL ASSOCIATES, INC.