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October 19, 2021 <br />Page 3 of 12 <br />Geotechnical Evaluation <br />Seismic Design Parameters (ASCE 9-16) <br />Site <br />Spectral <br />Spectral <br />Site <br />Design Spectral <br />Design <br />Class <br />Acceleration <br />Acceleration <br />Coefficients <br />Response Parameters <br />PGA <br />at 0.2 sec. (g) <br />at t.o sec. (g) <br />F, <br />F <br />SDS <br />SD, <br />D <br />1.376 <br />0.489 <br />t.o <br />Null <br />0.918 <br />Null <br />0.594 <br />Additional seismic considerations include liquefaction potential and amplification of ground <br />motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a <br />high groundwater table. The site has a low likelihood of liquefaction. For items listed as "Null' <br />see Section 11.4.8 of the ASCE. <br />Conclusions and Recommendations <br />General <br />The site is underlain by weathered and unweathered glacial fill which becomes denser with depth. <br />The proposed residential structures may be supported on shallow foundation systems bearing on <br />medium dense or firmer native soils or on structural fill placed on the native soils. Local <br />overexcavation or recompaction of loose weathered native soils may be necessary depending on <br />the proposed elevations and locations of the new footings. <br />Rain gardens and permeable pavements appear feasible provided there is adequate overflow to <br />City infrastructure or another approved conveyance. We should review the final plans and verify <br />soil conditions during construction. Any system must be lower than finish floor and slab <br />elevations or have cutoff drains placed between systems and structures to reduce the risk of <br />lateral groundwater migration. <br />Site Preparation <br />Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich <br />soil and fill. Based on observations from the site investigation program, it is anticipated that the <br />stripping depth will be 6 to 24 inches. Deeper excavations will be necessary in areas of existing <br />foundation systems (where present) and in any areas underlain by undocumented fill. <br />The native soils consist of silty -sand with gravel. Most of the native soils may be used as <br />structural fill provided they achieve compaction requirements and are within 3 percent of the <br />optimum moisture. Some of these soils may only be suitable for use as fill during the summer <br />months, as they will be above the optimum moisture levels in their current state. These soils are <br />variably moisture sensitive and may degrade during periods of wet weather and under equipment <br />traffic. <br />Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of <br />3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). <br />Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted <br />www.cobaltgeo.com (206) 331-1097 <br />