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<br />GEOTECNIA <br />Project Number: 060805 <br />2888 Darius Way, San Leandro <br />October 27,2006 <br /> <br />Page 4 <br /> <br />26 to 38) have a high to very high potential for expansion. For purposes of this report, we refer <br />to the clay soils at the site as "highly expansive" clays. <br /> <br />Approximate PI Ran2e <br /><12 <br />12-15 <br />15-25 <br />25-35 <br />>35 <br /> <br />Expansion Potential <br />Nil <br />Low <br />Moderate <br />High <br />Very High <br /> <br />Expansive soils tend to swell with increases in moisture content and shrink with decreases in <br />moisture content. These moisture fluctuations typically occur during armual and seasonal <br />variations in precipitation; but can also occur from irrigation, changes in site drainage, or the <br />presence of tree roots. As the soil shrinks and swells, improvements supported on the expansive <br />soils may fall and rise. These movements may cause cracking and vertical and horizontal <br />deformations of the improvements. <br /> <br />There are several alternatives for reducing the damaging effects of expansive soils. Foundations <br />should consist of drilled piers extending below the zone of significant moisture fluctuation and <br />designed to resist the alternating uplift and downdrag forces from the expansive soils (and highly <br />weathered, soil-like bedrock). Subgrade soils should be moisture conditioned to at least 4 <br />percent wet of optimum moisture content so that swelling takes place before improvements are <br />placed. Exterior concrete flatwork may be thickened and constructed with additional <br />reinforcement to reduce cracking. It is also prudent to keep trees away from the foundations to <br />reduce the moisture fluctuation in the soils due to the presence of tree roots. <br /> <br />Potential for Slope Creep <br /> <br />When expansive soil behavior occurs on slopes, such as at the site, there is a component of <br />movement parallel to the downslope direction. Slope creep is a slow process, typically involving <br />a small fraction of an inch per year; however, this movement accumulates over the years and can <br />result in several inches of lateral movement over the life of a structure, in addition to the <br />differential vertical movements. <br /> <br />The drilled-pier-and-grade-beam foundation system recommended below is intended to resist the <br />alternating swelling and shrinking behavior of the in-situ highly expansive clays (and bedrock <br />since the clays were derived from the gabbro bedrock) as well as the lateral slope creep forces. <br /> <br />Geolo!!ic Hazards <br /> <br />General <br /> <br />It is our opinion that the potentials for liquefaction, lateral spreading, and seismic compaction are <br />low at the site because no loose, saturated (i.e. below the groundwater surface) granular soils <br />