Abstract: During tunnelling works with the method of drill and blast, millisecond delay blasting technique is always employed to reduce charge weight in a single fire for safety. Under the repeated blasting loads, cracks are propagated in quantity and extent, or connected gradually to main cracks or collective cracks, leading to irreversible accumulation of damage in remaining rock masses. However, considerable numerical calculations are directed towards a single-hole charge and a single blasting, and they rarely get involved in the repeated blasting. In the present study, the millisecond delay blasting in a circular tunnel is simulated to find out the effect of repeated blasting on the rock mass damage, where in-situ stress in surrounding rock masses is also considered. The numerical simulation is implemented by embedding a statistical damage model into the commercial software LS-DYNA through its user-subroutines. According to the simulation results, rock masses subjected to the repeated blasting loads in a blasting footage result in relatively excessive damage than a single blasting. The peak particle velocity (PPV) threshold for initiation of damage is reduced by 12% due to repeated dynamic loading. The blasting load mainly induces tensile damage to surrounding rock masses. However, this tensile effect is very easily inhibited by the in-situ stress in surrounding rocks. When the in-situ stress reaches a magnitude of 2 to 10 MPa, the accumulated damage extent is significantly reduced with the increasing of the stress, and the PPV threshold for initiation of damage increases by 24% to 57%.

Key words: rock mechanics, blasting excavation, tunnel, accumulated damage, numerical simulation