Shear wave velocity as function of standard penetration test resistance and vertical effective stress at California bridge sites

Shear wave velocity, Vs, is defined as a statistical function of SPT blow count, N60, and vertical effective stress, σ′vσ′v, using a data set collected at various California bridge sites. At each site, Vs measurements were recorded by suspension logging in the same borehole in which N60 was measured. Regression analysis was used to derive statistical relations for sand, silt, and clay soil types. The relation between Vs and N60 is shown to depend strongly on σ′vσ′v since Vs and N60 normalize differently with overburden, which has been mostly omitted in previously published correlations. A random effects regression model is used to separate the error into intra- and inter-boring terms. Inter-boring errors are shown to depend weakly on geologic age. The average shear wave velocity in the upper 30 m, Vs30, is computed directly from the suspension logs and compared with Vs30¯ computed from the statistical relations. The relations are shown to provide unbiased estimates of Vs30, with standard deviation of the error equal to the standard deviation of the inter-boring error term. Ground motion prediction equations require Vs30 as an input parameter, and the statistical relations may be useful for estimating Vs30 at sites where only penetration resistance data are available. The proposed relations should not substitute for more accurate geophysical measurements when predicted ground motions are sensitive to the uncertainty in Vs30, but may be useful for identifying whether geophysical measurements should be performed to better refine the Vs30 estimate.