Influence of Intermediate Principal Stress on the Strainburst Characteristics of Beishan Granite with Consideration of End Effect

An experimental study on the strainburst characteristics of Beishan granite under different intermediate principal stress (σ2) loading conditions was conducted using a servo-controlled true-triaxial rockburst test machine equipped with an acoustic emission (AE) monitoring system. The primary objective is to investigate the influence of end effect under loading in the σ2 direction on the strainburst behaviors of the rock. Rectangular prism specimens with dimensions of approximately 200 × 100 × 100 mm3 were prepared and divided into two groups. For the first group, the specimens were in direct contact with platens during loading. For the second group, the specimen’s two surfaces loaded by σ2 were daubed with a layer of anti-friction agent to reduce the end friction of the platens. In the test, a loading mode, which kept one specimen surface free and applied loads on another five surfaces of the specimen, was used to simulate the stress state of a rock element on the excavation boundary. A high-speed video camera was used to capture the failure process of the specimens. The experimental results indicate that regardless of the use of the anti-friction agent or not, strainburst characteristics of the specimens depend on the magnitude of σ2. With increasing σ2, the degree of violence during rock failure presents an increasing trend and the dynamic failure process of the free specimen surface changes from local rock ejection to full-face burst. However, the σ2-dependent crack initiation, peak strength, failure mode and energy release characteristics of the lubricated specimens differ significantly from those of the non-lubricated ones. With increasing σ2, rock strengths increase to a peak value and then decrease, but the peak strength of the lubricated specimen is lower than that of the non-lubricated specimen under a given σ2. In addition, the peak strength difference between two groups increases as σ2 increases. Meanwhile, large end effect suppresses crack initiation of the specimens, causing crack initiation of the specimens to occur at higher stress levels, leading to higher peak strengths. Moreover, when σ2 = 1 MPa, the failure modes of the non-lubricated and lubricated specimens are similar. When σ2 is greater than 1 MPa, the number of the fractures in the surfaces loaded by σ2 for the lubricated specimens is larger than that for the non-lubricated specimens. In addition, the weight and the associated kinetic energy of ejected rock fragments from the lubricated specimen are higher than those from the non-lubricated specimen. The difference of the weight and the kinetic energy of the rock fragments between the non-lubricated and lubricated specimens increases with increasing σ2.