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603284-001 <br />adjacent foundations after reviewing details of the planned basement walls in <br />relation to existing foundations. <br />Backfills for retaining walls should be compacted to a minimum of 90 percent <br />relative compaction (based on ASTM Test Method D1557). During construction <br />of retaining walls, the backcut should be made in accordance with the <br />requirements of Cal/OSHA Construction Safety Orders.Relatively light <br />construction equipment should be used to backfill retaining walls. We also <br />recommend using at-rest pressures for design of walls supporting settlement- <br />sensitive structures. <br />Earth pressures used in the design of the walls should be indicated on the <br />retaining wall plans. All retaining wall designs and plans should be reviewed by <br />the project geotechnical consultant to confirm that the appropriate soil <br />parameters are used. <br />5.5 Seismic Design Parameters <br />Seismic design values based on the mapped acceleration parameters and the <br />site-specific procedure per the 2010 CBC (based on ASCE/SEI 7-05) were <br />developed. Due to the liquefaction hazard at the site, the site type is classified <br />as "F", which would typically trigger a site-response analysis. However, based <br />on our discussions with the project structural engineer, the proposed buildings <br />are deemed short-period structures with fundamental periods of about 0.5 <br />second or less. Accordingly, the exception under Section 20.3.1 was used to <br />develop site-specific seismic parameters. <br />The justification for the short period exemption is that during an earthquake, the <br />soil profile will respond differently before and after the onset of liquefaction. <br />Ground motions may be considerably higher for short-period structures before <br />the onset of liquefaction. Before the onset of liquefaction, ground motions will be <br />characteristic of the behavior modeled by Site Classes for non-liquefied soils. <br />For non-liquefied soils, it is expected that the short period spectral accelerations <br />will be relatively high. After the onset of liquefaction, the soil will become much <br />softer and result in a reduction of the short period ground motions and spectral <br />accelerations. The longer period spectral response, however, may increase <br />dramatically in terms of greater displacements. This approach is believed to be <br />conservative for determining the design base shear for short period (T<0.5 <br />-25- <br />Leighton