Water-weakening behavior of Hawkesbury sandstone in brittle regime

• Water-weakening of Hawkesbury sandstone is more pronounced at greater depths.
• Dilation mechanism is less significant at higher confining stresses.
• Micro-cracking begins at earlier stages of loading under higher confining stresses.

This paper investigates the effect of water on the mechanical behavior of intact Hawkesbury sandstone, a massive rock formation in Sydney, Australia. Two sets of cylindrical samples — air-dried and water-saturated — were tested under the confining pressures of 4, 10, 18 and 25 MPa. Water-saturated samples were tested under undrained conditions with initial pore-water pressures of 1, 4, 7.2 and 10 MPa, respectively.Initial pore-water pressure was observed to increase with increasing deviatoric stress during undrained tests and the maximum induced pore-water pressure was observed to increase with increasing effective confining pressure. Peak effective strength showed an increasing trend with increasing confining pressure for both sample sets and the strengths of dry samples were always greater. Peak strength drops of 13.36%, 25.27%, 34.70% and 38.12% were observed due to water at the confining pressures of 4, 10, 18 and 25 MPa, respectively. Results for the residual strength of tested samples displayed that it increased with increasing confining pressure for both sample sets. The volumetric strain response revealed that the volume reduction due to compaction increases with increasing confining pressure, and dilatancy-related volume increase close to sample failure was not significant at higher confining pressures for both sample sets. The samples tested under dry conditions showed a considerably higher volume reduction by compaction than that for the samples tested under undrained conditions. Analysis of failure mechanisms indicated that all samples failed mainly by shear localization, where the angle of the failure plane, measured from the minor principal stress direction, was varied from c.55° to c.45° for dry samples and from c.50° to c.40° for the samples tested under undrained conditions, at 4 and 25 MPa confining pressures, respectively. Fracture propagation behavior was studied using an acoustic emission detection system and the results demonstrated that the micro-crack initiation occurred very close to the failure stress under low confining pressures, leading to a more brittle sudden failure, w\hereas under higher confining pressures it occurred relatively earlier, compared to the failure stress, showing more quasi-brittle characteristics.