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June 13,2023 <br />HWA Project No. 2022-188-2 1 <br />4.4.1 Bearing Capacity for Footings <br />The proposed structures are located in areas in which the site explorations indicate the presence <br />of 3 feet to 10 feet of very soft to stiff, potentially compressible, organic topsoil and fine-grained <br />alluvial soils below the existing ground surface. These fine-grained alluvial soils will not be <br />suitable for support of the proposed structures. We therefore recommend over-excavating these <br />soils to expose underlying medium dense to dense coarse-grained alluvial soils. For footings <br />founded on the coarse-grained native materials, or properly compacted structural fill placed <br />directly on undisturbed native coarse-grained soils, we recommend using a net allowable bearing <br />capacity of 3,000 pounds per square foot (psf). For short-term wind and seismic loading <br />conditions, this allowable bearing pressure may be increased by 33 percent to resist increased <br />pressures on the edge of the foundation due to over-turning moments, Strip footings should have <br />a minimum width of 18 inches, and column footings should have a minimum width of 24 inches. <br />Footings should bear at least 2.5 feet below finished grade to be below the anticipated frost <br />penetration depth of 24 inches at the site. <br />4.4.2 LateralResistance <br />Wind, earthquakes, and unbalanced earth loads will subject the foundation system to lateral <br />forces which will be resisted by sliding (base) resistance. The frictional force between the native <br />coarse-grained soils or structural fill and mass concrete can be estimated as 50 percent of the <br />normal force (a frictional coefficient of 0.5). The friction coefficient is an ultimate value, and we <br />recommend applying a factor of safety of 1.5 with its use. <br />4.4.3 Settlement <br />For footings placed on structural fill placed in accordance with these recommendations provided <br />in Section 4.12.2, and having subgrades prepared as recommended in Section 4'l2.l,the <br />foundations would experience up to about % inch of differential settlement over 40 feet where <br />structural fill depths below the footings are approximately equal. Where the thickness of <br />structural fill below the footings varies, the differential settlement may be greater. For a <br />difference of about 10 feet in fiIl depth, differential settlement should be taken as an additional <br />%-inchand for a difference of about 20 feet of fillheight, an additional I inch should be <br />assumed. The largest potential for both overall settlement and differential settlement is due to <br />inadequate preparation/cleaning of the subgrade, and improper backfill and compaction of <br />structural fill, which is not accounted for in these estimations. <br />A method to limit differential settlement of smaller buildings placed over structural fill, would be <br />to cut the base of the excavation to be relatively level and the fill pad built up evenly across the <br />building footprint. The depth of excavation needed for these structures would likely be <br />determined during construction once the unsuitable soils have been completely excavated. <br />2022-188-21KCTS - Final Geotechnical Report 10 HWAGEoSCIENCES INC