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 Gosfo rd) carried out at confining pressures from 25 MPa to 200 MPa. Over th is pressure range the mode of failure in both these sandstones passes from localized brittle failure with a clear through-going fault to dis tributed cataclastic flow. During these experiments, stress, strain, c ompressional-wave velocity (V-p) and shear-wave velocity (V-s) measure ments were made simultaneously in the direction of the maximum princip al compressive loading axis. Initial application of the hydrostatic co nfining pressure causes both V-p and V-s to increase, and upon raising the axial stress above the confining pressure both velocities increas e further at first (generally by only a few percent), but then decreas e as dilatant crack growth commences. During dilatancy, V-s decreases proportionately more than V-p, and this decrease is generally of the o rder of 10-15%. These velocity measurements allow changes in rock phys ical properties to be calculated along with the axial and transverse c rack volume density parameters, epsilon(X) and epsilon(Z). The results from two selected tests are analysed in detail. These tests were chos en because they exhibit; (a) typical brittle shear failure, and (b) ty pical ductile cataclastic flow, respectively. The full interrogating e lastic waveforms were also recorded during testing, and these have bee n 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 samp les 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 dilatan t crack damage. The measurements on dry rock samples reported here pro vide the basis for comparison with measurements of changes in compleme ntary physical properties made on water-saturated rock samples under t he same experimental conditions, and reported in a companion paper in this issue (Read et al., 1995).