Laboratory Tests of Strain Burst in Boreholes of Four Types of Rocks Under Biaxial Loading

Abstract Biaxial compression tests were carried out on four types of rocks to investigate the relationship between the magnitude of strain burst and the intrinsic burst energy of the rock. The specimens were in the shape of cube with a circular hole in the center. They were loaded by σ 1 and σ 2 in the plane perpendicular to the hole axis. Under such a loading condition, V-notches were generated in the sidewalls of the hole of all specimens. However, the failure process was different in hard and soft rocks. Strain burst occurred in the hard and strong Jinzhou and Guangxi granites, while the failure was progressive spalling in the relatively soft and weak Beishan granite and Shandong sandstone. Back-calculations to some of the ejected debris in the Guangxi granite specimens showed that the ejection velocity varied from 2.33 to 4.75 m/s which was consistent with the ejection velocity of 2.98 m/s estimated from the intrinsic burst energy of the rock. The numerical modeling indicated that the rock surrounding the burst pit contributed little energy in a strain burst event. Both experiments and numerical modeling confirm that the magnitude of a strain burst is mainly attributed to the intrinsic burst energy of the rock. Based on the numerical modeling, the minimum side length of the cubic specimen must be four times the hole diameter such that the deviation of the stress distribution in the specimen is negligibly small from the analytical solutions in an infinite mass.