Abstract: In many practices of underground engineering, rock mass is subjected to neither pure creep nor pure relaxation. In fact, both stress and strain change with the lapse of time, and thus rock mass has time-dependent property, which eventually lead to fracture of rock mass. It is hard to exhaustively explain this phenomenon by creep and relaxation. This study comprehensively describes the characteristics of generalized stress relaxation and the significance of engineering practices. Non-linear Maxwell model is applied to solve the exact solutions of variable compliance type constitutive equation under the generalized relaxation condition. In particular, we obtain the accurate value of parameter-n1 which governs the time-dependent behavior in the pre-failure region by alternately loading rate and loading-unloading tests. Generalized relaxation tests are carried out under 70% and 90% stress levels, and the time-dependent behavior under different stress levels and different direction coefficients are also discussed. Finally, experimental results of generalized relaxation under 70% and 90% stress levels are numerically simulated by the variable-compliance-type constitutive equation based on the non-linear Maxwell model. Calculated results are good agreement with experimental data, and the properties of generalized relaxation of Kawazu tuff are well explained.

Key words: Kawazu tuff, constitutive equation, loading rate-dependence, generalized relaxation, numerical simulation