specs

Geotechnical Investigation South River Place, Gasser Drive, Napa 29 December 2010
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with a liquid limit less than 37%, a plasticity index less than 12, and an in-situ moisture content
that is greater than 0.85 times the liquid limit, are also vulnerable to liquefaction-like effects.
According to Special Publication 117 by the Division of Mines and Geology, the assessment of
hazards associated with potential liquefaction of soil deposits at a site must consider translational
site instability (i.e. lateral spreading, etc.) and more localized hazards such as bearing failure and
settlement.
The data used for evaluating liquefaction potential of the subsurface soils consisted of the in-situ
Standard Penetration Resistance values (N 1 ) 60 values, the unit weights, gradations, in-situ
moisture contents, the groundwater level, and the location of the site to the nearest active fault
and the predicted ground surface acceleration. The near surface fill materials consist of soft to
stiff sandy clay, firm sandy silt, and loose to medium dense clayey and silty sands. The native
soils below the fill consist predominately of stiff to very stiff sandy clay underlain by medium
dense to very dense clayey and sandy gravel. However, discontinuous layers of clayey sand
were encountered in Boring 3 between depths of 27 to 33 feet and in Boring 6 between depths of
20 to 33 feet. The native clays beneath the site are generally considered to have a liquid limit
greater than 37%, a plasticity index greater than 12 and an in-situ moisture content less than 0.85
times the liquid limit. Therefore, the clayey soils that comprise a majority of the subsurface soils
are not considered liquefiable. The medium dense to very dense gravels and sands encountered
below the clays are sufficiently dense to preclude liquefaction. The discontinuous layers of
medium dense native clayey sand encountered in Borings 3 and 6 have between 36.4 and 49.0
percent fines. Due to the relative density and high fines content of these materials, it is our
opinion that the liquefaction potential of the localized layers is low.
However, loose to medium dense sand and firm silt layers were encountered in the fill in Borings 2
through 5 that are either currently saturated or could potentially become saturated. A liquefaction
analysis was performed using the recommended analysis methods of the NCEER report 7 . These
layers were determined to be potentially liquefiable with factors of safety less than 1. The potential
liquefaction induced total settlements were calculated for these layers 8 . The results indicate
between 1.0 to 2.9 inches of total settlement may occur during a seismic event. Since these layers
are non-uniform across the site, we estimate that the differential settlements may equal the total
settlement values. In addition, due to the shallow nature of the potentially liquefiable materials,
sand boils may occur. The identified potential for liquefaction induced settlements and sand boils
can be mitigated by over-excavation and replacement as engineered fill or by in-situ densification as
recommended in the following sections of the report.
7 Youd, T. Leslie and Idriss, Izzat M., 1997, “Summary Report from the Proceedings of the NCEER Workshop on
Evaluation of Liquefaction Resistance of Soils,” Technical Report NCEER 97-0022, December 31, 1997
8 Zhang, G., Robertson, P.K., and Brachman, R.W.I., “Estimating Liquefaction Induced Ground Settlements from
CPT for Level Ground”
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