specs

Geotechnical Investigation South River Place, Gasser Drive, Napa 29 December 2010
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Based on the Interactive Probabilistic Seismic Hazard Map on the CGS website 6 , the peak
ground acceleration that has a 10% probability of exceedance in 50 years (475 year return
period) is 0.46g (alluvium). Using hazard deaggregation as performed on the US Geological
Survey website, the peak ground acceleration that has a 2% probability of exceedance in 50 years
(2475 year return period) is 0.74g (firm rock). The major contributing fault for the site is the
West Napa Fault with a 64% contribution. The next highest individual fault contribution is from
the Green Valley/Cordelia Fault Zones with a 23% contribution. All other faults contribute
approximately 2% or less to the seismic contribution.
Structures at the site should be designed to withstand the anticipated ground accelerations. Based
on our review of published maps and the probabilistic ground motion parameters from the CGS
website, the following 2007 California Building Code earthquake design criteria are applicable for
the site:
Site Class: D
Design Spectral Response Accelerations: S DS = 1.12 g; S D1 = 0.60 g
Fault Rupture
The site is not located within an Alquist-Priolo Earthquake Fault Zone. Based on our review of
geologic maps, no known active or inactive faults cross or project toward the subject site. In
addition, no evidence of active faulting was visible on the site during our site reconnaissance.
Therefore, it is our opinion that there is no potential for fault-related surface rupture at the
subject site.
Landslides
The subject site and immediate vicinity is relatively flat and therefore not subject to seismicallyinduced
landslide hazards.
Liquefaction
Soil liquefaction is a phenomenon in which loose and saturated cohesionless soils are subject to a
temporary, but essentially total loss of shear strength, due to pore pressure build-up under the
reversing cyclic shear stresses associated with earthquakes. Soils typically found most
susceptible to liquefaction are saturated and loose, fine to medium grained sand having a uniform
particle range and less than 5% fines passing the No. 200 sieve. However, fine-grained soils
6 U.S. Geological Survey and California Geological Survey, 2002, Interactive Probabilistic Seismic Hazards Map,
accessed 9 March 2009, from the CGS website:http://redirect.conservation.ca.gov/cgs/rghm/pshamap/pshamain.html
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