Depth to bedrock

For use in response calculation, a knowledge of the depth to bedrock or rock-like material is essential. Beyond the ordinary borehole depth of 50-100 m, bedrock may be determined from geophysical refraction surveys, preferably checked by reference to information from geological refraction surveys, preferably checked in turn by reference to information from geological records, artesian water or oil boreholes where available. In areas of deep overburden, for seismic response purposes the depth at which bedrock or equivalent bedrock is reached may have to be defined fairly arbitrarily. For example, on some sites it may be reasonable to say that equivalent bedrock is material for which the shear-wave velocity at low strains (<0.001%) is vS > 760 m/s, where such material is not underlain by materials having significantly lower shear-wave velocities (see also Table 5.1). In California on a typical site, effective bedrock would be found within 30 m of the surface, while on virtually all Californian sites it would be found within 150 m depth. The order of accuracy of bedrock depth determination that should be realistic targets for seismic response calculations is as follows:

Bedrock depth (m)

Approximate accuracy (m)











The large errors permissible in the measurement of deeper bedrock reflect the great approximations made in soil response analyses at the present time.

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