A Novel True Triaxial Apparatus for Studying the Time-Dependent Behaviour of Hard Rocks Under High Stress

The study introduced a true triaxial testing apparatus for conducting time-dependent experiments on hard rocks under high stress. The apparatus consists of two-rigid loading platens to apply the maximum and intermediate principal stresses, and a flexible loading system with which to apply the minimum principal stress, under constant-temperature control. It includes a simple sample-installation system, an acoustic emission acquisition system, data acquisition system, and a software interface. The novel apparatus overcomes the long-term eccentric loading problem faced when testing rock samples using a biaxial double-closed-loop servo-control mode. In addition, using an integrated structural design, the stiffness of the apparatus was increased, which was conducive to improving the data precision obtained in small-scale deformation experiments involving hard, stiff, or strong, rock samples. It also provides the possibility of studying the post-peak time-dependent behaviours of hard rocks under high stress. To verify the advantages of this apparatus and investigate the influence of intermediate principal stress on the time-dependent behaviour of hard rock, this study demonstrated the experimental methods and results for pre- and post-peak time-dependent of hard rock under true triaxial compression. The experimental results showed that intermediate principal stress exerted a significant influence on the time-dependent behaviour, and long-term deformation and strength, of hard rocks: post-peak fractured rocks still exhibited a certain time-dependent strength. The results also validated the reliability of this true triaxial time-dependent apparatus.