1617 EDGEMOOR LN 2016-11-10 - Laserfiche WebLink

McKinnon Residence Retaining Wall 1617 Edgemoor Lane Everett, Washington Subsurface Conditions NGA File No 910614 December 2, 2014 Page 3 Geology: The geologic units for this area are shown on the Geologic Map of the Everett 7.5-Minute Quadrangle, Snohomish County, Washington, by James P. Minard (U.S.G.S., 1985). The site is mapped as Advance Outwash (Qva) with Glacial Till (Qvt) mapped nearby. The advance outwash is described as clean, stratified sand and gravels. The glacial till is described as a non -sorted mixture of silt, sand, and cobbles. Our explorations generally encountered undocumented fill soils underlain by silt with varying amounts of sand and gravel and fine to medium sand with varying amounts of silt and gravel, consistent with the description of glacial till and advance outwash. Explorations: The subsurface conditions within the site were explored on November 7, 2014 by drilling two borings to depths in the range 21.5 to 25.5 feet below the existing ground surface using a portable, limited -access drill rig. We also performed two hand auger explorations on the slope below the residence. The approximate locations of our explorations are shown on the Schematic Site Plan in Figure 2. A geologist from Nelson Geotechnical Associates, Inc. (NGA) was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the explorations. A Standard Penetration Test (SPT) was performed on each of the samples during drilling to document soil density at depth. The SPT consists of driving a 2-inch outer - diameter, split -spoon sampler 18 inches using a 140-pound hammer falling 30 inches. The number of blows required to drive the sampler the final 12 inches is referred to as the "N" value and is presented on the boring logs. The N value is used to evaluate the strength and density of the deposit. The soils were visually classified in general accordance with the Unified Soil Classification System presented in Figure 4. The logs of our borings and hand augers are attached to this letter and presented as Figures 5 through 7. We present a brief summary of the subsurface conditions in the following paragraph. For a detailed description of the subsurface conditions, the boring and hand auger logs should be reviewed. In both of our borings, we encountered approximately 6.5 to 7.5 feet of very loose to loose dark brown to brown silty fine to medium sand with gravel and fine to medium sand with gravel and varying amounts of organics, interpreted as undocumented fill soils. Underlying the fill in the borings, we encountered approximately 5.0 to 13.5 feet of medium dense silty fine to medium sand with gravel and interbedded silt and sand layers and stiff to very stiff silt with varying amounts of sand and interbedded silt and sand NELSON GEOTECHNICAL ASSOCIATES, INC.