TranSystems Corporation - LGM Construction

certain conditions liquefaction can also occur in low-plasticity silts and clays (Seed et al.,
2003). Possible consequences of liquefaction at the site include sand boils, pavement
damage, disruption of buried utilities, and ground settlement.
The site is mapped as being susceptible to earthquake induced liquefaction per the
referenced State of California Seismic Hazard Zone Report for the San Leandro
Quadrangle (CGS, 2003). Groundwater is shallow at the site (5 to 10 feet deep). The
peak horizontal ground surface acceleration (PGA) with a 10 percent probability of
exceedance in 50 years for alluvial conditions is approximately 0.55g (CGS, 2003). The
modal earthquake for both the liquefaction susceptibility and lateral spread calculations
is the Mw = 7.1 seismic event on the Hayward Fault.
We evaluated liquefaction triggering potential using procedures presented by Youd et
al. (2001) and guidelines presented by California Division of Mines and Geology (1997)
and Martin and Lew (1999). Based on our analyses, very loose to loose sands of about
6 ft in thickness underlying the medium dense to dense clayey gravel fill are susceptible
to liquefaction in the event of the design earthquake. Depending on the liquefiable layer
thickness and composition, we estimated that up to 2.5 inches of liquefaction-induced
settlement can be expected at the site. We estimate up to one inch of differential
settlement across the site due to liquefaction settlement.
6.4 CYCLIC DENSIFICATION
Seismically induced compaction or densification of non-saturated sand (sand above the
groundwater table) due to earthquake vibrations may cause settlement. Most of the soil
above the groundwater table at the site consists of medium dense clayey gravel with
sand. We judge settlement of these soils due to cyclic densification will be low (less
than 1/4 inch).
6.5 LATERAL SPREADING
Lateral spread is defined as mostly horizontal movement of gently sloping ground (less
than five percent surface slope) due to elevated pore pressures or liquefaction in
underlying, saturated soils. Structures at the head of the slide are sometimes pulled
apart while those at the toe are subjected to buckling or compression of the foundation
soil. Linear infrastructure, such as utility lines and roadways, is particularly susceptible
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