The determination of the dynamic elastic moduli of rock masses by geophysical methods

Abstract

AbstractDynamic moduli values for rock masses are calculated from corresponding compressional and shear wave velocities and in situ bulk densities using standard equations based on elastic theory. These moduli values can be used for the design of foundations for vibrating machines to avoid resonance during operation. Static moduli values can be assessed from the dynamic moduli values if the rock mass quality has been determined either directly or from the ratio of νfield to νlab. Where possible, the results of these procedures should be correlated with the results of in situ static tests, such as large scale plate bearing tests.Geophysical surveys need to be carefully designed to ensure that the correct seismic event is being recorded and wave velocities are being measured accurately. There are several field techniques in common usage, which include seismic refraction surveying, cross-hole shooting and a variety of along-hole methods. In each case, careful consideration should be given to timing errors resulting from inadequate coupling of energy sources and receivers. Enhancement of shear-wave events is usually required to avoid ambiguity and misinterpretation, particularly in layered ground. For fractured rock masses the wavelength of the seismic events may also be significant for both compressional and shear wave events.The results of ground investigations at the proposed sites of generators for the Akjoujt Power Station in Mauritania, West Africa, are presented to illustrate the advantages and limitations of geophysical techniques for assessing rock mass deformation moduli. Compressional and shear wave velocities were determined by cross-hole, along-hole and surface seismic refraction surveying techniques for the very weathered metamorphic rocks. Dynamic values of Young’s modulus were obtained for 1 m intervals to a depth of 15 m.The results of this investigation are reviewed in the light of recent developments in equipment, field procedures and data processing techniques.