Project Summary

Shear wave velocity, Vs, was defined as a statistical function of SPT blow count, N60, and vertical effective stress using a data set collected at California bridge sites where N60 and Vs were recorded from the same borehole. Downhole suspension logging was used to record the Vs values. Statistical relations are proposed for sand, silt and clay soil types. The relation between Vs and N60 is shown to depend strongly on vertical effective stress, which has been nearly entirely neglected in previously-published relations. A random effects regression model was 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 30m, Vs30, was computed directly from the suspension logs and compared with Vs30 computed from the statistical relations. The relations were 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.


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