Evaluation of the excavation damage zone associated with the mining methods for underground mine safety

This paper was developed as part of an effort by the National Institute for Occupational Safety and Health (NIOSH) to identify risk factors associated with bumps in the prevention of fatalities and accidents in highly stressed, burst-prone ground conditions. Self-initiated rockbursts, also known as strainbursts, occur when the stresses near the boundary of an excavation exceed the rock mass strength, and failure proceeds in an unstable or violent manner. The stresses generally increase as a result of nearby mining. In addition, the rock mass strength may degrade with time or with loss of confinement, leading to sudden failure. In either case, the rock mass strength-to-stress ratio reaches unity and the rock fails. The severity (depth of failure or lateral extent of failure) is often related to the maximum tangential-stress-to-intact-rock-strength ratio. In a drill-and-blast excavation there is often an annulus of damaged rock in which both the strength and the modulus are reduced relative to the original values, the reduction being dependent on the quality of blasting. Hence, it is possible that the blast damage creates a more favorable state with respect to bursting, particularly in comparison to the undamaged wall of a mechanized excavation. In the 2002 version of the Hoek-Brown criterion, the influence of blast damage on the near-surface rock mass properties is considered via a damage factor D. This factor depends upon the degree of disturbance due to blast damage and stress relaxation. In this study, a numerical study is conducted to estimate whether a strainburst potential could be influenced by the choice of mining methods. We found that the disturbance factor is normally about 0.6 in drill-and-blast excavations and the depth of disturbance ranges from 0.5 m to 1 m. Because the strainburst criterion is a function of the maximum tangential-stress-to-intact-rock-strength ratio, the change in rock mass strength is not directly incorporated and the stress reduction is directly translated into a reduction in strainburst potential. The lower strength contributed to yielding in the rock mass and a further stress reduction. A mitigating factor to this increased burst potential is the rate at which the stress change is imposed. The numerical investigation in this study also showed that the amount of kinetic seismic energy released upon excavation is proportional to the excavation rate. Consequently, all other factors aside, mechanically excavated openings are relatively at greater risk of strainbursting compared to openings excavated by drilling and blasting excavations.