Quasi-static analysis of soft rock failure characteristics targeted to cyclic direct shear test

Abstract In Japan, the disastrous effects of the 2011 Tohoku Earthquake has affected seismic design. Furthermore, there is an increasing need for dynamic nonlinear analysis that can consider large strain levels in assessing the seismic resilience of essential structures such as nuclear power plants. However, the applicability of dynamic nonlinear analysis methods to rock masses has not yet been fully evaluated. Their applicability needs to be confirmed by dynamic phenomena, such as shaking table tests. Herein, as a preliminary step, quasi-static analyses of cyclic direct shear tests were conducted to validate the constitutive model. Soft rock samples with relatively few fractures were used as the testing material. In the tests, the load was applied in multiple steps by increasing the loading amplitude at each step, with a loading frequency of 0.1 Hz. The quasi-static analyses revealed that, although an existing constitutive model generally reproduced the shear load–displacement relationship obtained in the tests from small displacement levels to peak strength, discrepancies emerged beyond peak strength. Therefore, we focused on the post-failure history damping characteristics of the soft rocks and added a damping constant after rock failure to the existing constitutive model. Consequently, the accuracy of the values of the stress history beyond peak strength was improved by more than 10%, and the reproducibility of the shear load–displacement relationship improved. Therefore, the applicability of the improved constitutive model was confirmed for conditions of quasi-static cyclic loading of soft rock with few fractures, from small displacement levels to beyond peak strength.