Influence of intermediate principal stress on rockburst in deep cavern under true triaxial condition with single face unloading

Abstract

Rockburst disaster has become a major safety hazard for deep cavern construction worldwide. Using the new true triaxial rockburst test system, an indoor physical simulation test of “single face lateral unloading-three direction and five faces stress-vertical continuous loading” under different intermediate principal stresses was conducted. The ejection failure process, failure mode, acoustic emission characteristics, fragment characteristics and ejection kinetic energy of rock samples under different intermediate principal stresses were compared and analyzed. Based on the three-dimensional discrete element theory and the polycrystalline modeling technique (randomly generated Voronoi mineral grains), the entire process of rockburst inoculation-occurrence-development-failure as well as the energy evolution characteristics under true triaxial single face unloading conditions were studied. The results indicate that the ejection failure process of rockburst under different intermediate principal stresses can be summarized as four stages: grains ejection, rock spalling into plates, rock shearing into fragments, and rock fragments ejection. Among them, the rock spalling into plates is an inevitable failure phenomenon in the process of rockburst. The failure modes of free face of rock samples have similar multi-zone failure characteristics. That is, the splitting cracks dominated by tensile failure are generated near the free face, and the penetrating shear cracks dominated by shear failure are generated away from the free face. The evolution process of AE hits can be divided into four stages: relatively stable, rapid rising, overall failure stage and quiet period. As the intermediate principal stress increases, the ejection kinetic energy of the fragments increases. Rockburst fragments is mostly coarse-grained, medium-grained, and fine-grained fragments. With the increase of intermediate principal stress, the mass of fine particles is basically unchanged, while the mass of fine, medium and coarse particles shows a steady increase trend. The entire process of rockburst inoculation−occurrence−development−failure is successfully simulated using the crystal scale fine model (CSFM) considering the grain mineral composition. The variation trend of the stress-strain curve of the rock sample is basically consistent with the indoor test results and the curve is in good agreement. The evolution curve of elastic strain energy can be roughly divided into the early growth rate increasing stage, the approximate linear growth stage, the growth rate decreasing stage during the yield platform period and the post-peak sharp decline stage.

Keywords

• deep cavern
• rockburst
• intermediate principal stress
• true triaxial test
• discrete element method