A Laboratory Study of Strain Burst and the Effect of Relief Holes Using Dumbbell Specimens

The laboratory study aimed to demonstrate strain rock burst and the effect of relief holes on the reduction in the magnitude of the burst. A special dumbbell shape was adopted to the rock specimens to achieve stress concentrations in the specimens. The specimen is a regular tetrahedron with two semicircular holes on the opposite sides. Two types of specimens, i.e., without and with relief holes, were used for the tests. Relief holes were drilled on the sidewalls of the two semicircular holes when needed. The specimens were made from a white granite and a red granite. The tests showed that strain burst occurred in the specimens without relief holes, and V-notches were generated in the sidewalls of the semicircular hole after burst. The mass of ejected rock from the more burst-prone white granite specimen WG-1 approximately two times the mass from the red granite specimen RG-1. The ejection velocity of the white granite, back-calculated from ejection trajectory of rock fragments, was less than 5.5 m/s. No or significantly light strain bursts occurred in specimens with relief holes. In such specimens, the failure was progressive and the failure predominantly was in the relief holes. Both the tests and the numerical modelling have confirmed that relief holes reduce the magnitude of the strain bursts through transferring the stress concentration area in a greater depth in the sidewalls of the semicircular holes.

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