The effects of water on the strength of black sandstone in a brittle regime

The mechanical behavior of rock is significantly affected by water. To further study the influence of water on the mechanical behavior of rocks, uniaxial compressive loading experiments were performed on black sandstone specimens (collected at a site in Longchang, Sichuan Province, China) with different water contents. The water absorption was first tested to study the evolution of the water content in the specimens. Then, the mechanical behaviors of the specimens after different durations of soaking in water (from dry to saturation) were tested under uniaxial compression. The results showed that, in the stress-strain curves for the specimens, the initial slope and strength decrease with increasing soaking duration and that the curves are strongly nonlinear in the prepeak region. Furthermore, with increasing water content, the elastic modulus and uniaxial compressive strength decrease, while the Poisson's ratio increases. A comprehensive set of predictive empirical relationships is established between the mechanical properties and the water content/soaking duration for the black sandstone. As the soaking duration increases, the failure pattern gradually changes from tensile to shear-dominated failure. A novel observation is that, for two specimens with the same water content, the strength of a specimen with a homogeneous water distribution is lower than that of a specimen with a non-homogeneous water distribution. The results suggest that the water soaking duration, water distribution and loading-saturation sequence of a rock should all be considered in experiments to gain a deeper understanding of the mechanical responses of rocks with different water contents.