MICROCRACKING DURING TRIAXIAL DEFORMATION OF POROUS ROCKS MONITORED BY CHANGES IN ROCK PHYSICAL-PROPERTIES .1. ELASTIC-WAVE PROPAGATION MEASUREMENTS ON DRY ROCKS

We present results from two series of triaxial deformation experiments performed on ''dry'' samples of two sandstones (Darley Dale and Gosford) carried out at confining pressures from 25 MPa to 200 MPa. Over this pressure range the mode of failure in both these sandstones passes from localized brittle failure with a clear through-going fault to distributed cataclastic flow. During these experiments, stress, strain, compressional-wave velocity (V-p) and shear-wave velocity (V-s) measurements were made simultaneously in the direction of the maximum principal compressive loading axis. Initial application of the hydrostatic confining pressure causes both V-p and V-s to increase, and upon raising the axial stress above the confining pressure both velocities increase further at first (generally by only a few percent), but then decrease as dilatant crack growth commences. During dilatancy, V-s decreases proportionately more than V-p, and this decrease is generally of the order of 10-15%. These velocity measurements allow changes in rock physical properties to be calculated along with the axial and transverse crack volume density parameters, epsilon(X) and epsilon(Z). The results from two selected tests are analysed in detail. These tests were chosen because they exhibit; (a) typical brittle shear failure, and (b) typical ductile cataclastic flow, respectively. The full interrogating elastic waveforms were also recorded during testing, and these have been used to calculate the seismic quality factors Q(p) and Q(s). To our knowledge, this is the first time this has been reported for rock samples undergoing triaxial deformation. The changes in Q values generally exhibit similar trends to those observed in the velocity measurements, but the percentage changes in and are an order of magnitude greater, suggesting that this parameter is a more sensitive measure of dilatant crack damage. The measurements on dry rock samples reported here provide the basis for comparison with measurements of changes in complementary physical properties made on water-saturated rock samples under the same experimental conditions, and reported in a companion paper in this issue (Read et al., 1995).