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■ The building addition can be supported on conventional spread footings bearing on: (1) suitable <br />undisturbed dense to very dense glacial till deposits, (2) structural fill extendingto the dense glacial till <br />deposits, or (3) at least 2 feet of structural fill. Footings supported on undisturbed dense to very dense <br />glacial till deposits may be designed using a maximum allowable bearing pressure of 5,000 persquare <br />foot (psf). Footings supported on structural fill that extends down to undisturbed dense glacial deposits <br />or is supported on at least 2 feet of compacted structural fill may be designed using a maximum <br />allowable bearing pressure of 3,000 psf. The allowable bearing pressures may be increased by <br />one-third for short duration loads such as wind or seismic events. <br />■ Imported gravel borrow should be used as structural fill under all building addition elements. <br />■ We recommend that perimeter footing drains be installed around the exterior of the building addition. <br />The perimeter drains should be installed at the base of the perimeter footings. <br />■ The building addition floor slab and at -grade pavement areas may be supported on existing soils <br />provided that the upper 12 inches of exposed subgrade is evaluated and recompacted (if needed) to <br />at least 95 percent of the maximum dry density (MDD) per ASTM International (ASTM) D 1557. <br />■ Earthworkshould be performed in the typical dry season (June through September) to reduce earthwork <br />costs and to provide the best opportunity to reuse on -site soils during construction. <br />Our specific geotechnical recommendations are presented in the following sections of this report. <br />4.1. Earthquake Engineering <br />We evaluated the site for seismic hazards including liquefaction, lateral spreading and fault rupture. Our <br />evaluation indicates the site does not have liquefiable soils present and, therefore, also has little to no risk <br />of liquefaction -induced ground disturbance including lateral spreading. There are no mapped faults in the <br />immediate vicinity of the site, although the Southern Whidbey Island fault zone is mapped approximately <br />9 miles southwest of the site. Because of the thickness of the Quaternary sediments below the site, which <br />are commonly more than 1,000 feetthick, the potential forsurface fault rupture is considered remote. <br />4.1.1.2018 IBC Seismic Design Information <br />We recommend the use of the following 2018 IBC parameters for Site Class, short period spectral response <br />acceleration (Ss), 1-second period spectral response acceleration (Si) and seismic coefficients (FA and Fv) <br />for the projectsite as shown in Table 1 for design. <br />TABLE 1. IBC SEISMIC PARAMETERS <br />2018 IBC Parameter <br />Site Class <br />Recommended Value <br />C <br />Short Period Spectral Response Acceleration, Ss (percent g) 134.3 <br />1-Second Period Spectral Response Acceleration, S1 (percent g) 47.6 <br />Seismic Coefficient, FA <br />Seismic Coefficient, Fv <br />1.000 <br />1.820 <br />GEOENGINEERi.r,P <br />December6,2021 Page3 <br />File No. 24 59-003-00 <br />