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May 4, 2017 <br />HWA Project No. 2017-036-21 <br />the soil profile underlying the site. The Site Class can be correlated to the average standard <br />penetration resistance (NSPT) in the upper 100 feet of the soil profile. Based on our <br />characterization of the subsurface conditions, the subject site classifies as IBC Site Class C. <br />Based on the SDS and SDI values, the site is considered as Seismic Design Category D. <br />Table 1. Design Seismic Coefficients for 2015IBC Code Based Evaluation <br />Spectral <br />Spectral <br />Design Spectral <br />Design Spectral <br />Site <br />Site <br />Acceleration <br />Acceleration at <br />Acceleration at <br />Acceleration at <br />Coefficients <br />Class <br />at 0.2 sec. <br />1.0 sec <br />0.2 sec. <br />1.0 sec. <br />Ss, g <br />Si, g <br />SDs, g <br />SDI, g <br />C <br />1.435 <br />0.553 <br />0.956 <br />0.480 <br />1.00 <br />1.30 <br />4.4 WALL PRESSURES <br />The site grading necessary to expand the building westward and accommodate an entryway into <br />the building along the west side will require that a short wall (<5 feet high) be constructed to <br />retain a portion of the Holly Drive embankment. <br />Assuming the walls are free to deflect slightly, they should be designed for lateral earth pressures <br />equivalent to a fluid weighing 35 pcf for static conditions and 55 pcf for seismic conditions. For <br />design purposes, an allowable passive earth pressure equivalent to a fluid weighing 300 pcf may <br />be assumed for properly compacted fill placed against the sides of the foundations. An allowable <br />bearing capacity of 3,000 psf should be used for wall foundation design. <br />Sliding resistance may be determined on the basis of sliding resistance coefficient of 0.45 <br />between the base of the footing and undisturbed glacial till or properly placed structural fill. <br />Factors of safety of at least 1.5 and 1.1 are recommended to be applied to the passive and sliding <br />resistance loads in determining allowable restraint loads under static and seismic conditions, <br />respectively. <br />It is also assumed that the walls will be backfilled with free -draining granular material and will <br />be provided with a perimeter foundation drainage system to ensure that the wall backfill does not <br />become saturated. <br />4.5 PAVEMENT CONSIDERATIONS <br />We recommend that the pavement section for the new parking area consist of at least two (2) <br />inches of Class `/2-inch HMA (PG 64-22) over four (4) inches of Crushed Surfacing Base Course <br />(WSDOT Standard Specifications Section 9-03.9(3)) and at least six (6) inches of Gravel Borrow <br />Final Geologic Assessment 8 HWA GeoSciences Inc. <br />