It is well known that water accelerates the deformation and failure of rock and hence deteriorates the stability of rock structures on and under the ground. Previous studies have reported that both the strength and stiffness of rock are lower in wet conditions than in dry conditions. Water also accelerates the time-dependent behavior of rock; for example, the creep lifetime is shorter in wet conditions than in dry conditions at the same stress ratio to strength. However, most of the previous studies on the deformation and failure of rock examined the results for just two conditions, dry and wet, and did not address various water-saturation conditions. Since water-saturation conditions in the rock masses vary with place and time, it is not sufficient simply to investigate their deformation and failure in dry and wet conditions. In this study, the authors investigated the effect of water saturation on the deformation and failure of Sanjome andesite through compression and tension tests. The authors first prepared the rock specimens of six water-saturation conditions with varying the time the rock specimens were submerged in water and then dried. The water saturation, which is defined as the volume of water divided by the volume of pore in a specimen, ranged between almost 0 in oven-dried condition and 1 in water saturated condition. Then, Brazilian tension tests were conducted with these specimens under constant water-saturation conditions and compared with the previous compression tests. The results showed that both the compressive and tensile strengths decreased linearly to the logarithm of the water saturation. The effect of water on the failure criteria that are represented by the envelope curves of the Mohr's circles was clarified based on the test results.
Compressive and tensile characteristics of rock are two important considerations for the design and stability analysis of underground constructions. During excavating a cavity in the underground rock, as the increasing in excavation distance, the confining pressure as well as the crustal stress of surrounding rocks decrease. On the other side, in the process of excavation, not only compressive but also tensile stresses occur in the vicinity of a crack in the rock. In addition, since the water in the rock is gradually discharged from the cavity wall, the moisture condition of the rock changes with the excavation process. Therefore, it is necessary to understand the compressive and tensile strengths and deformation characteristics of rocks in various water saturated conditions.