MICROCRACKING DURING TRIAXIAL DEFORMATION OF POROUS ROCKS MONITORED BY CHANGES IN ROCK PHYSICAL-PROPERTIES .2. PORE VOLUMOMETRY AND ACOUSTIC-EMISSION MEASUREMENTS ON WATER-SATURATED ROCKS

We report results from a series of laboratory triaxial-deformation exp eriments performed on samples of Darley Dale sandstone at servo-contro lled constant strain rate and constant pore-fluid pressure. During def ormation, the volume of wafer either expelled from the samples during compaction or injected into the samples during dilatancy in order to m aintain a constant pore-fluid pressure was continuously monitored thro ughout each experiment. In addition, the parameters of differential ax ial stress, axial strain, and acoustic emission (AE) characteristics w ere also recorded. Complete AE waveforms were also captured and stored by means of a transient recorder, and Fourier analysis of these wavef orms was subsequently carried out. The reported experiments were all p erformed in a conventional, high-pressure, gas-medium triaxial cell, b ut with the pore-fluid pressure maintained constant using a new servo- controlled fluid pressure intensifier and pore volumometer. The AE and pore volumometry measurements on water-saturated rocks reported in th is paper are complementary to the measurements of elastic-wave propaga tion parameters made on dry rocks and reported in a companion paper in this issue. The two suites of data have been integrated to infer rela tions between crack density parameters calculated from elastic-wave ve locity measurements and the directly measured pore volume during defor mation. Our results show a distinct, positive correlation between chan ges in the axial crack density parameter (epsilon(X)) and changes in t he pore volume during deformation once fluid expulsion due to elastic pore collapse has been corrected for. These two parameters have then b een combined to obtain an estimate of changes in the mean aspect ratio of propagating dilatant cracks. The results suggest that, under moder ate confining pressures, axially aligned dilatant cracks are likely to bow open elastically prior to significant crack extension and growth.