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1 <br /> ' May 28, 2007 <br /> Proposed Office Building <br /> L&A Job No. 7A053 <br /> ' Page 12 <br /> ' The detention vault should also be designed for seismic loading based on a 100-year seismic <br /> event. For seismic design of the detention vault walls, a pseudo static soil pressure diagram of <br /> inverted triangle from the finished ground level to the bottom of the foundations should be used. <br /> Based on the soil conditions in the detention vault area,we recommend the lateral soil pressure at <br /> the top of the triangle be 8.5H psf for a 100-year seismic event, where H is the height from <br /> ground surface to bottom of footings in feet. A one-third increase in the above recommended <br /> allowable soil bearing pressure may be used when considering the seismic loading condition. <br /> The above design parameters are unfactored ultimate values. Proper factors of safety should be <br /> applied for the design of the vault walls against sliding and overturning failures. <br /> I <br /> GROUNDWATER PICKUP BY DETENTION VAULT <br /> None of the test pits excavated on the subject site encountered groundwater. Also, footing <br /> foundations poured over fresh till soils of extremely low permeability is very difficult for <br /> groundwater to penetrate through. Therefore, pickup of groundwater by the detention vault will <br /> be negligible and need not be considered in the design of the detention vault. Also, due to the <br /> lack of groundwater encountered by the test pits excavated on the site, it is our opinion that <br /> groundwater in footing drains of the residences to be constructed on the site will also be <br /> negligent even during the wet winter months. Therefore, it is opinion that groundwater in the <br /> footing of the residences needs not be considered in the design of the detention vault. <br /> BUILDING FOUNDATIONS <br /> Conventional footing foundations may be used to support the office building to be constructed on <br /> the site. The footing foundations should be placed on or into undisturbed, very-dense, fresh till <br /> soils, or on compacted structural fill constructed over these competent basal soils. To reach the <br /> fresh advance outwash bearing soils, excavation depths of 3 to 4 feet below existing grade would <br /> be required. Water should not be allowed to accumulate in excavated footing trenches. <br /> LIU & ASSOCIATES, INC. <br /> p <br />