5109 SEAHURST AVE 2025-12-12 - Laserfiche WebLink

L�2COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EVERETT, WASHINGTON November 16,2018 i j Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. Due to very dense,fine-grained soil conditions,the site has a very low risk of liquefaction. 7.o DISCUSSION 7-m General The proposed single family residence and accessory structure(s)may be supported on shallow foundation systems bearing on medium dense or firmer native soils. We anticipate that suitable bearing soils will be encountered between 2.5 and 4 feet below existing site grades. Local overexcavation of fill and/or weathered native soils will likely be required below some of the new foundation elements depending on the proposed finish floor elevations. Infiltration of stormwater runoff is not feasible. The site is underlain by relatively dense, fine-grained native soils which are not conducive to infiltration of runoff. 8.o Recommendations 8.1.1 Site Preparation Trees,shrubs and other vegetation should be removed prior to stripping of surficial organic-rich soil and fill. Based on observations from the site investigation program, the stripping depth will range from 6 to 12 inches. Deeper excavations may be required below large trees, existing foundation locations, and in any areas where fill is present. The on-site soils consist of silty-sand with gravel. These soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. Soils with more than about 5 percent fine may only be suitable for use as fill during the summer months, as they will be somewhat moisture sensitive. These soils may degrade during periods of wet weather and under equipment traffic. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines(material passing the U.S.Standard No. 200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density,as determined by the ASTM D 1557 test method. 8.1.2 Temporary Excavations Based on our understanding of the project,we anticipate that the grading could include local cuts on the order of approximately 4 feet or less for foundation and utility placement. Any deeper excavations should be sloped no steeper than 1H:1V in medium dense to dense native soils. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 1.5H:1V,where room permits. 4 PO Box 82243 Kenmore,WA 98028 cobaltgeopgmail.com 2o6-331-1097