Abstract：Viscoelastic properties during deformation are especially notable for porous rocks such as tuff and weathered rocks. The authors study the deformation characteristics of porous rocks，and propose a new rheological model based on constitutive equations previously proposed by the authors. The model consists of a spring and a dashpot. It is assumed that the constitutive equation described in a previous study can be applied to the spring. The viscosity of the dashpot is low before loading，and increases gradually with progressive loading. In creep testing at low stress levels，strain of the dashpot has close relationship with creep strain because the spring constant does not decrease significantly at low stress levels. The shape of the stress-strain curve and the loading rate-dependence of the peak strength can be explained according to the proposed rheological model. To compare the theoretical predictions with the test results，four kinds of rocks including muddy sandstone，Oya tuff，Tage tuff and Kawazu tuff are employed. The theoretical stress-strain curves are well consistent with the measured curves. The key result of this study is that even at low stress conditions，strain of the dashpot is considerably larger than that considered in previous studies. By this model，the difference of Young¢s moduli between air-dried and water-saturated conditions is well simulated where the difference in Young¢s modulus is assumed to be the difference in strain of the dashpot. Under water-saturated condition，strain of the dashpot increases more rapidly；and then Young¢s modulus is relatively small. The differences in the irrecoverable strain for wet and dry rocks are the reasons for the differences between the Young¢s moduli of wet and dry rocks.
