A Laboratory Investigation of Acoustic Emissions Associated with the Brittle Fracture of Rock Under Dry and Wet Conditions

We have carried out a series of laboratory experiments on AE monitoring to investigate the fracture behaviour of some dry and water-saturated rock samples of south-east Gujarat and Maharashtra which have been experiencing prolific seismicity in recent years in India. The results obtained from the study of Pavgadh rhyolites and Godhra granites tested to failure under triaxial compression are presented and discussed in this paper. The AE monitoring was carried out using a 2-channel Spartan AE system during the deformation and failure of rock at confining pressures ranging between 15 MPa and 60 MPa. The tests were carried out using a Hoek cell and a 150Ton MTS Servo-controlled Testing Machine. The stress-strain data and statistical parameters of Acoustic Emissions (AE) were monitored and recorded concurrently during each test using PC-based systems. The time-histories, load-based plots and distribution functions of AE have been analyzed using Mistras software. The results show that at 60 MPa confining pressure, the triaxial compressive strength of these rocks reach values of nearly 3 to 3.5 times the UCS. The water–saturated rhyolite showed a lower compressive strength at 60 MPa confining pressure than the dry rock by nearly 50-60 MPa. Correspondingly, the AE statistics (occurrence rate, energy counts etc) of water-saturated sample were found to be less than those of the dry sample. Whereas the water-saturated fine-grained Godhra granite showed a marginal increase in triaxial compressive strength at 60 MPa confining pressure with reduced AE statistics. The AE signatures in general, and b-value in particular, have indicated that the rate at which the crack damage accumulates is functionally dependent on the fluid-rock interaction and the linkage of cracks during the inelastic deformation and failure of the rock.