Test analysis and modified SBSP model prediction for strength decay and healing of jointed soft rock

The strength of jointed soft rock changes in a certain geo-stress field around the tunnel. To explore the behavior, specimens with a single joint plane are triaxially compressed under different consolidation times and confining pressures. On the one hand, jointed soft rock shows strength healing. The residual strength, peak strength, and secant modulus of jointed soft rock increase as isotropic consolidation time increases. On the other hand, jointed soft rock shows special strength decay because of the joint plane, that is strain-softening curve of jointed soft rock is linear. The strength decay occurs for jointed soft rock under normal consolidation condition. To predict the strength decay and healing behavior of jointed soft rock, a modified structural bounding surface plasticity (modified SBSP) model is formulated by introducing an improved expression of elastic modulus for jointed soft rock. The modified SBSP model contains four parameters corresponding to linear strain-softening, compressive hardening, strength, and secant modulus increasing induced by consolidation. The modified SBSP model well predicts the strength decay and healing of jointed soft rock by comparing the results from modified SBSP model, SBSP model, and experiment. Especially, the modified SBSP model overcomes the limitation that compressive hardening and elastic modulus of jointed soft rock cannot be captured by bounding surface plasticity model like SBSP model. The results are important for reasonable supporting time and optimum supporting strategy.