Laboratory study of deformation and failure of soft rock for deep ground tunnelling with TBM

The TBM excavation in deep stratum is a process of smooth and quasi-static unloading of high initial confining pressure. In order to reveal the deformation and failure characteristics of soft rock at deep ground under the condition of TBM tunnelling, triaxial unloading tests on sandy mudstone with different unloading rates were carried out. The pre-peak stress-strain curves under the condition of smooth and quasi-static unloading of confining pressure were found close to the ones obtained from the conventional triaxial compression test. At the yielding stage upon unloading, the specimens exhibit damage dilatation, the lateral deformation grows accelerated, resulting in a turning of the volumetric strain from shrinkage into dilatation. After the peak strength, the specimens slide along the fracture surface already formed to display the modest brittle drop of strength once or twice. Then a linear strain softening stage accompanied by multistage micro fractures occurs on the stress-strain curve with a line segment of a small slope as the confining pressure is continuously and slowly unloaded. The complete deformation process of unloading is composed of the elastic deformation, the damage dilation, the post-peak brittle drop, the linear strain softening and the residual stress period. The specimens display composite shear-tension failure macroscopically accompanied with axial splitting cracks during the process of smooth unloading. Many split cracks link and cross each other to form a shear band of a certain width. The rock in the shear band is squeezed and wears into the fine particles and the powder as a result of the interaction of axial extrusion and the shear stress along the shear plane. ©, 2015, Academia Sinica. All right reserved.