Anisotropic Failure Strength of Shale with Increasing Confinement: Behaviors, Factors and Mechanism

Some studies reported that the anisotropic failure strength of shale will be weakened by increasing confinement. In this paper, it is found that there are various types of anisotropic strength behaviors. Four types of anisotropic strength ratio ([Formula: see text]) behaviors and three types of anisotropic strength difference ([Formula: see text]) behaviors have been classified based on laboratory experiments on nine groups of different shale samples. The cohesion [Formula: see text] and friction angle [Formula: see text] of the weak planes are proven to be two dominant factors according to a series of bonded-particle discrete element modelling analyses. It is observed that shale is more prone to a slight increase of [Formula: see text] and significant increase of [Formula: see text] with increasing confinement for higher cohesion [Formula: see text] and lower to medium friction angle [Formula: see text]. This study also investigated the mechanism of the anisotropic strength behaviors with increasing confinement. Owing to different contributions of [Formula: see text] and [Formula: see text] under different confinements, different combinations of [Formula: see text] and [Formula: see text] may have various types of influences on the minimum failure strength with the increasing confinement; therefore, different types of anisotropic behaviors occur for different shale specimens as the confinement increases. These findings are very important to understand the stability of wellbore and underground tunneling in the shale rock mass, and should be helpful for further studies on hydraulic fracture propagations in the shale reservoir.