3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27, 2006 Page 12 Seismic Loads Horizontal accelerations during seismic events will momentarily increase lateral earth pressures against walls. We recommend using an equivalent seismically induced earth pressure with a rectangular distribution equal to FH psf, where F. depends on the magnitude of the ground acceleration and H is the unsupported wall height in feet. The resultant seismic force would act at 0.5H above the base of the wall. The seismic earth pressures are in addition to the static earth pressure and lateral surcharge pressures, and should be considered in the design of retaining walls. The anticipated peak ground acceleration (pga) at the site during the maximum credible earthquake on the Hayward Fault is estimated to be about 0.65 g. Using an estimated pga of 0.5 g at the site, the value ofF would be 23. For smaller seismic events, the value ofF would be lower. For a typical design ground acceleration of 0.2 to 0.3 g for residential structures, the value ofF would be between 7 and 10. The choice of the value ofF to be used for retaining wall design depends on the level of risk accepted by the designer and owner. If the walls are not designed for the appropriate seismically induced earth pressures, consequences during strong earthquake loading might include lateral movement, distress, or failure of the walls. The magnitudes of the seismically-induced earth pressures above were calculated based on the simplified procedure developed by Seed and Whitman (1970) and incorporated a reduction factor on the order of 20 percent to judgmentally account for possible effects of wave scattering or passage, the transient nature of earthquake ground motions, and possible wall-soil interaction effects. Concrete Slabs on Grade If typical 3- to 4-inch-thick concrete slabs with little to no reinforcement are used for new exterior walkway slabs and patios, they should be underlain by at least 36 inches of compacted baserock or gravel to reduce the potential for cracking. This assumes that the downslope sides of the walkways/patios have been stabilized by retaining walls designed to resist the lateral creep forces as discussed in a previous section of this report. Preferably, exterior slabs should be at least 6 inches thick with one grid of#4 steel reinforcement spaced at 12 inches in both directions (in the middle of the slab) to help reduce the potential for cracking. Ifmoisture penetration through interior floor slabs (garages) would be objectionable, slabs should be underlain by a capillary moisture break consisting of at least 4 inches of clean, free- draining crushed rock or gravel graded such that 100 percent will pass the I-inch sieve and none will pass the No.4 sieve. Further protection against slab moisture penetration can be provided by means of a moisture vapor barrier membrane, placed between the drain rock and the slab. The membrane should be covered with at least 2 inches of damp, clean sand to protect it during construction.