An experimental study is conducted on sandstone under different confining and pore pressures through triaxial creep tests The creep properties of sandstone under different conditions are compared and analyzed The test results show that: in the case of same confining pressure, pore pressure can increase the instantaneous elastic deformation and creep deformation but reduce the strength of sandstone. Furthermore, confining pressure has an impact on creep deformation while maintaining a constant pore pressure. Based on the classical creep model and the application of effective stress theory, a new nonlinear viscoelasto-plastic rheological model for sandstone is established, which can reflect the effect of pore pressure on creep properties and induced damage in the process of creep. The comparison between the results obtained from the modeling and those from the experiments shows that this model can describe well primary attenuation creep, steady creep and accelerated creep, and a good accord is obtained.
Rheology is the inherent nature of rock material; especially most of the engineering rock masses are in the generalized rheological deformation (Wang & Li 2008). Engineering practices show that rock deformation increases with time under a long-termed load leading to cracks or even failure on structures. Jun (2007) has systematically summarized the progress in rheological mechanics and its engineering application in recent years. However, most of these researches focused on the effect of stress field on the rheological properties. In fact, the mechanical properties become more and more complicated with the diversification of structure form and the expansion of construction scale in rock engineering. At the same time, engineering activities, geological effect and the change of environment have a direct or indirect effect on the long-term stability of rock engineering. Experiments and engineering researches have shown that water has important effect on rock rheological properties.
Recently, a large number of experimental results reported in the literature were obtained from the tests carried out under different water contents. (Li & Zhu et al. 2003, Zhu & Ye 2002, Yang & Xu et al. 2006). As have been pointed out by these experiments, rock creep properties with certain water content are more significant than those in air-dry state, and tend to reduce the strength of rock through the physical and chemical effects. Actually, the properties of rock mass in engineering such as tunnels and underground oil storage are not only affected by the surrounding rock stress, but also related to pore pressure which allows a rock-fluid interaction resulting in a negative influence on the long-term stabilities of engineering. However, the coupled effect of creep and pore pressure is rarely considered for studying the rheological properties of rock. Yan et al. (2010) compared the rheological properties of porous limestone through triaxial creep experiment conducted under different water pressures and deviatoric stresses. She et al. (2010) designed three types of load modes in which a high pore pressure is applied to study the influence of pore pressure on creep properties of marble. However, few experimental studies on rock creep are carried out under different confining pressures and pore pressures.