Evolution of anisotropy during sandstone rockburst process under double-faces unloading

Abstract

Rockburst is one of the major disasters in deep underground rock mechanics and engineering. The precursors of rockbursts play important roles in rockburst prediction. Strainburst experiments were performed under double-face unloading on sandstone with horizontal bedding planes using an independently designed rockburst testing facility. P-wave propagation time during the tests was automatically recorded by the acoustic emission apparatus. The P-wave velocities were calculated in both two directions to analyze their patterns. To find a characteristic precursor for rockburst, the dynamic evolution of rock anisotropy during the rockburst test is quantified by the anisotropic coefficient k, defined as the ratio of the two P-wave velocities in the directions vertical to and parallel to the bedding planes. The results show that rockburst occurs on the two free surfaces asynchronously. The rockburst failure occurs in the following order: crack generation, rock peeling, particle ejection, and rock fracture. In the process of rockburst under double-face unloading, the potential evolution characteristics of anisotropy can be generalized as anisotropy-isotropy-anisotropy. The suddenly unloading induces damage in the rock and presents anisotropic coefficient k steeply increasing departing from one, i.e., isotropy. The rocks with horizontal bedding planes will reach the isotropic state before rockburst, which could be considered as a characteristic precursor of this kind of rockburst.

Keywords

strainburst / bedding plane / P-wave velocity / anisotropy / rockburst prediction

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