2.2 Testing system and procedure
All of the triaxial cyclic loading-unloading compression tests were
performed in a RTX-4000 high-temperature and high-pressure rock triaxial
testing system at the state key laboratory for geomechanics and deep
underground engineering. The maximum axial loading capacity is 4000 kN,
and the maximum confining pressure is 140 MPa. The triaxial tiered
cyclic experiments were carried out under different confining pressure
(σ 3) of 0, 10, 20, 30 and 40 MPa, and consisted
of the following three steps. First, the confining pressure was applied
at a rate of 0.4 MPa/s. Second, the specimens were loaded to the
predetermined axial strain value at a controlled axial displacement rate
of 0.04 mm/min, and then unloaded the deviatoric stress to zero at a
rate of 40 MPa/min through controlling axial stress. In each subsequent
cycle, the second deviatoric strain level was increased, and then
unloaded it to zero again. Finally, the stress cyclic was continued in
that way until the specimens enter the residual strength phase.