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April 29,2015 <br /> HWA Project No.2014-162-21 <br /> The IBC accounts for the effects of site-specific subsurface ground conditions on the response of <br /> structures in terms of site classes. Site classes are defined by the average density and stiffness of <br /> the soil profile underlying the site. The Site Class can be correlated to the average standard <br /> penetration resistance (NsPT) in the upper 100 feet of the soil profile. Based on our <br /> characterization of the subsurface conditions, the subject site classifies as IBC Site Class D. <br /> Table 1 presents the design spectral seismic coefficients obtained for this site, assuming the <br /> structure is not considered an essential structure. Based on the SDs and SDI values, the site is <br /> considered as Seismic Design Category D. <br /> Table 1. <br /> Design Seismic Coefficients for IBC 2012 Code Based Evaluation <br /> Spectral Spectral Design Spectral Design Spectral Site <br /> Site Acceleration Acceleration at Acceleration at Acceleration at Coefficients <br /> Class at 0.2 sec. 1.0 sec 0.2 sec. 1.0 sec. <br /> Ss,g S1,g SDS,g SDI,g <br /> Fa F, <br /> ED 1.406 0.534 0.938 0.534 1.000 1.500 <br /> Liquefaction <br /> The potential for soil liquefaction must be considered during the design of any soil-supported <br /> structure. Soil liquefaction is a phenomenon where loose, saturated, granular deposits <br /> temporarily lose strength and behave as a liquid in response to moderate to strong earthquake <br /> shaking. Primary factors controlling the development of liquefaction include the intensity and <br /> duration of strong ground motions, the characteristics of subsurface soils, in-situ stress <br /> conditions and the depth to ground water. Based on the fine grained nature of the soils <br /> encountered behind the wall and the very dense soils encountered at the toe of the wall, <br /> liquefaction will not be a design consideration for this project. <br /> Rockery Recommendations <br /> General <br /> Our evaluation indicates that the existing rockery walls were constructed with poor quality rock <br /> and poor craftsmanship. These two factors have led to the observed deterioration of the <br /> rockeries. In their current state, failure of the rockeries appears likley, even for the static <br /> condition. Failures will likely result from further deterioration of poor quality rocks and lead to <br /> rockfall onto the roadway and sidewalk. We recommend that the rockeries be replaced before <br /> collapses occur. <br /> 2014-162 Letter Report 5 HWA GEOSCIENCES INC. <br />