Abstract

To reveal the failure conditions and mechanisms of deep hard rock, the deformation and failure characteristics of hard rocks under true triaxial compression are reviewed. For the constant σ3, there is a non-symmetrical variation on the failure strength which first increases and then decreases with an increasing intermediate principal stress. But the bearing capacity of rock samples under the unloading condition is higher than the loading test. With increasing σ3 or decreasing σ2, the rock failure gradually changes from brittle failure to ductile failure. The failure in engineering fracture zone is mainly induced by tensile failure and the rigid joint also has great influence on rock failure which can induce different failure modes. Furthermore, there is an obvious time-dependent failure behavior on hard rocks under true triaxial compression condition.

1 Introduction

The typical failure of hard rocks such as deep cracking, collapse, rock burst and spalling has been the greatest challenge in deep rock engineering (Li et al. 2012; Feng et al. 2017; Liu et al. 2017). It is urgent to study these failure conditions and mechanisms of deep hard rocks. In general, deep hard rocks are in a general stress state (σ123) with high stress condition. Under the influence of excavation disturbance, great changes have taken place in the stress state of the surrounding rock mass with a loading or unloading adjustment.

For evaluating the bearing capacity of hard rocks, a good understanding on the true triaxial failure strength is significantly essential. Early attempts have been made to examine the influence of σ2 on failure strength by some researchers such as Mogi (1971) and Haimson (2006). However, these studies on failure strength are at a limited range of σ2, which can't cover all the stress state. Based on the in situ monitor such as borehole camera and wave velocity measuring, the surrounding rock mass in excavation damage zone will be of different failure extent induced by excavation (Feng et al. 2018a). Meanwhile, the damaged rock often undergoes a post-peak failure response. Furthermore, the time-dependent deep fracturing of hard rocks is also found (Feng et al. 2017), indicating that the failure of hard rocks is time-dependent. For the long-term stability of the deep tunnel, the time-dependent fracture needs to be investigated.

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