Following the previous work about the relation between permeability evolution and rock creep deformation [1–2], triaxial creep tests of volcanic breccias under various seepage pressures were performed on rock specimens and their lateral deformation behavior in the accelerated creep phase volcanic breccias were also studied. Creep damage variable and related evolution equation in accelerated creep were proposed. Based on the relation of damage variable associated with lateral creep strain, the correlativity between axial permeability coefficient and lateral damage-induced strain under different confining pressures was analyzed, and then the relation equation of permeability coefficient as a function of damage-induced strain during the process of accelerated creep was obtained. An assumption was made that the axial permeability coefficient has close relationship not only with lateral creep strain, but also with creep damage-induced crack growth. The equation can be applied for for numerical analysis of coupling seepage and stress during the process of rock creep damage.
A distinct phenomenological feature of rock is not a constant, but a variable of the state stress and stress history. In this section, we examine the relationship between the damage-induced permeability variations and creep deformation of rock. In order to quantify the change in permeability during laboratory tests by measuring permeability in the axial load direction, and to research the relationship between stress and permeability, many experimental studies have been implemented. Furthermore, it is reasonable to think that permeability will increase, particularly for slightly impermeable rock, when the stress-state induces material damage by micro-crack opening and growth . Based on the results of extensive laboratory test, we conclude that there is a significant evolution of rock permeability which can be divided into four stages. First, the original voids and micro-cracks are compressed under low stress, so the permeability decreases as the stress increases. Then the rock goes into a state of steady elastic, in this stages, permeability varies not significantly. In the third stage, the permeability increases tempestuously. At last, after the stress increases beyond the peak strength, the permeability also decreases after a peak value.