Measurement of in situ dynamic properties in relation to geologic conditions
Evaluation of nuclear power plant sites from a material stability standpoint requires measurement of in situ dynamic properties. These are derived from measurement of shear-wave propagation velocity in subsurface materials by utilizing either the seismic downhole or seismic cross-hole techniques, or both. The downhole technique is rapid and less expensive than the cross-hole survey but is limited in maximum survey depth and resolution of thin, higher velocity beds. The cross-hole test, although less subject to depth and resolution limitations, requires more boreholes and is therefore more expensive. This test is also more exacting because of borehole drift considerations and timing corrections that may or may not be required for the particular energy source used.
Actual field cross-hole examples for several sites of varied subsurface geology illustrate the variation of shear velocity versus material make-up and condition. High shear-wave velocities are characteristic of massive crystalline rock, whereas fractured rock and unconsolidated sediments show progressively reduced values.
The combination of the dynamic model with a proposed design earthquake event produces the basic data upon which site evaluation from an engineering standpoint may be made. An example from a deep sand sediment site shows the increased potential for liquefaction as the strength of the design earthquake is increased.