Strength Reduction of Initial High-Stress Rock Pillars Under Different Triaxial Unloading Paths

The mechanical behavior of deep rock pillars is closely related to the coupling effect of the initial high stress and the excavation-unloading process. In this study, the failure and strength characteristics of rock pillars are studied under triaxial unloading paths of three-dimensional (3D) initial high stress followed by two-dimensional (2D) stress unloading and one-dimensional (1D) stress adjustment. Two different 2D stress unloading paths were used: horizontal bidirectional sequential unloading path (YU test), and horizontal bidirectional simultaneous unloading path (TU test). The 3D initial high stresses were σ 1 = 60 MPa, σ 2 = σ 3 = 10, 20, 30, 40, and 50 MPa, that were applied on cubic granite specimens (50 × 50 × 50 mm). Uniaxial compression tests are also conducted for comparison. Violent rockburst occurred in all specimens under the triaxial unloading paths. The results reveal that the peak strengths of the cubic granite specimens under the triaxial unloading paths are significantly lower than the uniaxial compressive strength, namely exhibiting a remarkable strength reduction effect, and the degree of strength reduction will increase with the increase of initial stress. Further, the specimen strengths in the TU tests are lower than those in the YU tests, which indicates that the weakening intensity effect of simultaneous unloading is greater than that of sequential unloading. The weakening mechanism of deep pillar under high-stress unloading can be expressed as that the 3D initial stress determines the strength level of the pillar, whereas the triaxial unloading path affects its weakening intensity. Highlights The rockburst failure and strength characteristics of rock pillars under the high-stress condition were studied from the perspective of triaxial unloading of 3D initial high stress + 2D stress unloading + 1D stress adjustment. The pillar strength reduction effect was observed and the triaxial unloading path has a significant influence on the strength reduction effect. The initial 3D stress determines the strength level of the pillar, whereas the triaxial unloading path affects its weakening intensity.