Abstract
To study the effect of excavation unloading on hard rock failure, a series of true-triaxial compression tests, biaxial compression tests, and true-triaxial unloading compression tests (two different unloading rates) at different confining pressures was conducted on red sandstone cube samples. The strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were analyzed. Based on the test results, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were explored, and a reasonable explanation for unloading-induced spalling in hard rock tunnels was presented. The results show the stress-strain curve of highly stressed red sandstone exhibits a stress step during unloading, and the higher the unloading rate, the lower the stress level required for a stress step. The rock strength-weakening effect induced by unloading was confirmed. The mechanical properties of red sandstone become more unstable and complicated after unloading. After the red sandstone is unloaded to a two-dimensional stress state, with increasing confining pressure, the strength increases first and then decreases; the failure mode changes from a low-confining pressure tensile-shear failure to a high-confining pressure tensile failure; and the geometries of the slabs change from large thick plates and wedges to medium- and small-sized thin plates. At equal confining pressures, the higher the unloading rate, the lower the strength (i.e., the strength-weakening effect is more pronounced), the thinner the slab, and the lower the confining pressure required for the failure mode to change from tensile-shear failure to tensile failure. The unloading rate and confining pressure affect the strength and failure characteristics by affecting the crack initiation type and propagation direction in hard rock. For deep hard rock tunnels with high unloading rate and axial stress, neglecting the effects of unloading rate and axial stress will lead to a dangerous support design. For deep hard rock ore, if the maximal horizontal principal stress exceeds the critical confining pressure, the mining surface should be perpendicular to the direction of the minimal horizontal principal stress. The results of this study are of great engineering significance for guiding deep hard rock tunnel construction and mining.
Highlights
Deep rock mass is in the original three-dimensional (3D) geo-stress environment and exhibits generally a stable state without failure before excavation
The rock mass experiences the unloading of confining pressure. e confining pressure at the excavation surface is reduced to Advances in Civil Engineering zero, which results in a significant decrease in the strength of the rock mass and, eventually, its failure
The strength of rocks increases gradually with increasing confining pressure, indicating an evident positive correlation. e failure mode changes with the change in the confining pressure, thereby transitioning from a tensile failure at low confining pressure to a shear failure at high confining pressure [16,17,18,19]. e harder the rock, the higher its strength, and the more significantly the failure mode is affected by the confining pressure [20]
Summary
Deep rock mass is in the original three-dimensional (3D) geo-stress environment and exhibits generally a stable state without failure before excavation. The confining pressure parallel to the excavation surface and tangential stress affect simultaneously the deformation and failure of the surrounding rock. Experimental research results have shown that a confining pressure parallel to the excavation surface affects the strength and failure mode of the surrounding rock, such as the severity of the hard rock failure [33,34,35]. During the excavation process, the hard rock is simultaneously affected by the confining pressure parallel to the excavation surface and the unloading rate. Erefore, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were revealed, and a reasonable explanation for unloading-induced spalling in hard rock tunnels (or caverns) is presented in this paper A series of true-triaxial compression tests (TCTs), biaxial compression tests (BCTs), and true-triaxial unloading compression tests (including true-triaxial unloading compression tests at a low unloading rate (TLU) and at a high unloading rate (THU)) was performed on red sandstone cube samples to analyze the effects of the unloading rate and confining pressure on hard rock failure. rough a detailed analysis and summary of the test results, the strength and failure characteristics and their relationship for red sandstone unloading at different unloading rates and confining pressures were determined. erefore, the effects of the unloading rate and confining pressure on the strength and failure characteristics of hard rock were revealed, and a reasonable explanation for unloading-induced spalling in hard rock tunnels (or caverns) is presented in this paper
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