2D numerical analysis of rock damage induced by dynamic in-situ stress redistribution and blast loading in underground blasting excavation

Abstract

When underground cavities are created in initially stressed rock masses by the drill and blast method, an unwanted excavation damage zone (EDZ) is induced around the cavities due to the combined effects of in-situ stress redistribution and blast loading. During rock fragmentation by blasting, the in-situ stress on blast-created excavation boundaries is suddenly released. The in-situ stress redistribution is a dynamic process that starts from the transient release of stress and reaches a final static stress state after excavation. For a circular tunnel that is excavated underground by full-face millisecond delay blasting, 2D finite element simulation is performed to investigate the rock damage induced by the dynamic in-situ stress redistribution and blast loading. The critical peak particle velocity (PPV) for the initiation of blast damage in pre-stressed rock masses is also numerically studied. The results show that the transient stress release generates additional stress waves, resulting in a larger damage zone compared with that following quasi-static stress redistribution. The effect that the additional stress waves have on rock damage becomes more obvious as the in-situ stress levels and excavation dimensions increase and as the stress release duration decreases. Blast-induced tensile stress in the circumferential direction of a tunnel is neutralized by compressive in-situ stress. In deep-buried or high-stressed tunnel excavation, dynamic stress redistribution is responsible for the formation of EDZ; the critical PPV for the initiation of blast damage first increases and then decreases with an increase in the in-situ stress. Therefore, in underground blasting excavation, the factors that affect the level of in-situ stress such as tunnel depths should be considered with respect to the blasting vibration standards and damage criteria.