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SC 130 Milbor-Pita <br />W� we <br />5.2.4 Ground Fault Hazard <br />No surficial evidence of active faulting or lineaments was observed during our activities <br />at the site. This, plus our review of the available literature on the site (Gower, et. At. <br />1985 and Brocher, Tom, 1999), did not indicate the potential presence of any active <br />known faults below or in the immediate vicinity of the site. <br />5.3 Building and Structure Foundations <br />5.3.1 General - Option Discussion <br />The compressible native clay deposits underlying the site are expected to undergo <br />consolidation when `;oaded over several 100 pounds per square foot (psf). The bearing <br />capacity is quite adequate for normal construction conditions but consolidation will occur <br />if the load is not distributed. Possible excavation of the 20-foot layer of clay would not <br />be economically feasible, except for a structure that had a complete basement. <br />Consequently, we recommend supporting the new major structure (the station building) <br />on deep foundations or spread footings that bear either on the underlying dense glacial <br />deposits or structural fill that is in direct contact to these deposits. <br />Digging to underlying glacial soil and backfilling with structural fill up to the footing <br />level is possible if the designers feel this is the less costly approach over deep <br />foundations. This will discussed further below. <br />Suitable deep foundation alternatives include drilled shafts, auger -cast piles, and driven <br />steel and concrete piles. In our opinion, auger -cast piles or drilled piers will be the most <br />suitable foundation types for this location. Driven piles may produce undesirable <br />vibrations and noise. Alternatively, properly constructed drilled piles :an be installed <br />with little noise and vibration. <br />Design recommendations, including allowable axial capacities, lateral load analyses, <br />settlement evaluations, construction considerations, and geotechnical monitoring <br />requirements, for each of these deep foundation types are provided below. <br />5.3.2 Deep Foundations <br />5.3.2.1 Allowable Axial Capacities for Dew Building Foundations <br />Analyses were performed to evaluate the allowable axial capacities for each <br />foundation alternative. We recommend all piles be embedded a minimum of 5 <br />feet into the dense glacial deposits, which were encountered between 18 and 20 <br />feet below the existing ground surface. The extent of embedment will depend on <br />the type of pile selected and capacity desired; piles driven into the dense glacial <br />soils will experience significant resistance at 5 feet of penetration, whereas drilled <br />piles can be installed much deeper and achieve much higher capacities. Deeper <br />pile embedment will provide greater skin friction resistance and result in greater <br />allowable capacities. <br />Table I presents preliminary results of our analyses including allowable pile <br />capacities and depth of embedment into glacial deposits. The approximate tip <br />Gmlethrdcal, Tunnel and tinvironmrntal tilgineers <br />3080 125m Ave NE Phone (425) 969-5778 <br />Dellcwe. WA 98005 Page 10 FU (425)861-0677 <br />