A Micromechanics-Based Model for Creep Behavior of Rock

Recently, various ideas on underground development have been proposed and associated technical problems have been studied. One of the issues of concern is the prediction of the long-term behavior of rock, such as creep phenomena and fatigue. The mechanical behavior of rock is known to be greatly affected by temperature, confining pressure, pore fluid pressure and pH. It is necessary to establish a prediction method for creep deformation and creep failure of rock in order to ensure the long-term safety of the underground structures such as vaults for nuclear waste and power stations. Studies with the scanning electron microscope (SEM) revealed that the mechanisms of creep deformation and creep failure is the growth of microcrack nucleated at a pre-existing defect. Under compression below the failure strength, the microcrack gradually grows and the rock specimen fails after a certain time. The mechanism of time-dependent crack growth is understood as the stress corrosion at the crack tips. The objective of this study is to establish a prediction method of creep behavior. It is necessary to understand the governing mechanism of phenomena and to build a model for the reproduction of creep behavior. In the present study, an analytical model of microcrack growth under compression on the basis of micromechanics is proposed. The analytical results of the proposed model are compared with the experimental results. It appears that the experimental data are reproduced by the model. Moreover, a constitutive equation is derived from the proposed micromechanical model and is implemented into a finite element program to analyze the creep behavior of underground structures. As an example, a problem of elliptical excavation under hydrothermal conditions is analyzed and a crack length field is predicted as a function of time at different temperatures. It is concluded that the results of the finite element analysis indicate the possibility that rock may fail due to the effect of high temperature.