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Geotechnical Engineering FtLpUrt 1rerracon <br /> SN4924—Silver Lake North Cell Tower Everett, Washington <br /> October 24, 2014 Terracon Project No. 81145043 <br /> displace drilling fluids or groundwater and prevent re-entry of bentonite slurry or groundwater <br /> into the tremie pipe. <br /> 4.3.3 Engineered Mat Foundation <br /> The proposed tower may alternatively be supported on an engineered mat foundation bearing <br /> on native soils a minimum of 3 feet below ground surface. Design for uplift and overturning <br /> forces will be required. Thickening the mat or increasing embedment depths would provide <br /> additional weight to resist these forces. <br /> Description Values <br /> Net allowable bearing pressure' <br /> Foundation bearing directly on 3,000 psf <br /> brown silty sand soils < 5 ft BGS <br /> Net allowable bearing pressure' <br /> Foundation bearing directly on grey 5,000 psf <br /> silty sand soils :! 5 ft BGS <br /> Mat dimensions Minimum 10 feet <br /> Embedment below finished grade for 18 inches <br /> frost protection 2 <br /> Approximate total settlements Less than 1 inch <br /> Coefficient of sliding friction 0.4 (ultimate) <br /> Coefficient of subgrade reactions "k," 4 150 pci <br /> 1. The recommended net allowable bearing pressure is the pressure in excess of the minimum <br /> surrounding overburden pressure at the footing base elevation. <br /> 2. To reduce the effects of seasonal moisture variations in the subgrade soils. <br /> 3. The foundation settlement will depend upon the variations within the subsurface soil profile, the <br /> structural loading conditions, the embedment depth of the footings, and the quality of the earthwork <br /> operations. <br /> 4. This value is representative of a 1 foot by 1 foot footing and should be scaled appropriately for a <br /> larger foundation. The coefficient decreases as the width of the foundation increases. The following <br /> equation by Das' (1990) or other appropriate relationship may be used to scale the coefficient of <br /> subgrade reaction. <br /> k = k, [(B+1)/ (2B)]2 <br /> k, = coefficient of subgrade reaction (1 foot X 1 foot foundation) <br /> k = scaled coefficient of subgrade reaction <br /> B =foundation width <br /> Compacted native soils or compacted select fill placed over the embedded mat are anticipated <br /> to have moist unit weights on the order of 110 to 120 pounds per cubic foot (pcf), respectively. <br /> ' "Principles of Foundation Engineering", by Braja M. Das, 1990, p. 241. <br /> Responsive■ Resourceful■ Reliable 8 <br />