Unloading Behaviors of Shale under the Effects of Water Through Experimental and Numerical Approaches

During tunnel excavations through shale formations in central and southwest China, engineering problems, such as large deformations and distorted arches, were encountered due to radial stress unloading and groundwater pressure. Few studies have been carried out on the issues exhibited by soft shale tunnels that have considered the comprehensive influence of unloading and water effects. To characterize the unloading behaviors, a series of conventional triaxial compression (CTC) tests and unloading confining pressure (UCP) tests were performed under drained conditions and compared. Based on theoretical and experimental analyses, deformation parameters from the UCP process were derived and compared with those from the CTC test. The stress-strain response, evolution of deformation and strength parameters, and postpeak mechanical behavior and failure modes were analyzed in order to reveal the unloading and water effects. To quantify the difference between the CTC and UCP behaviors in geotechnical practice, a tunnel excavation was simulated using the CTC and UCP models. The distributions of damage and displacement were analyzed in a numerical case study. The results showed the comprehensive dependency of shale behaviors on unloading and water effects. The numerical case revealed significant tunnel deformation and fewer plastic zones in the excavation when using the UCP model rather than the CTC model. To accurately simulate the deformation and damage distribution, this study provided a methodology for construction design using the UCP test and the model for unloading and water-affected geotechnical engineering.