Abstract:

Researchers at the National Institute for Occupational Safety and Health (NIOSH), Spokane Mining Research Division (SMRD) have developed a modeling framework, using commercially available software, for investigating the fracture and ejection of rock into underground mine openings caused by seismic loading and the restraint of this failed rock by the ground support system. The modeling framework presented here establishes the conceptual foundation and demonstrates the required capabilities for a model of a simple two-dimensional cross-section of a deep mine drift. The model has been used to investigate the effects of various wave parameters on the stability of underground excavations, both with and without ground support. Additionally, kinetic energy of ejected rock and energy dissipated by the ground support system are calculated and compared. Finally, the effectiveness of ground support, with and without containment, has been investigated. The main conclusion is that simplified empirical rockburst damage analyses based solely on volume of ejected rock are inadequate. The damage potential of a seismic event should be measured in terms of dynamic energy demand on the ground support. Further, peak specific power may be a better wave parameter to correlate with this demand. This research aids in the understanding of the effects of mining induced seismic loading on underground excavations and support systems, and may lead to improvements in miner safety.

Introduction

Continued advancement of computing technology and capabilities of commercially available numerical modeling software has made fully-dynamic numerical simulation of problems in geomechanics more practical than ever before. However, application of this technology is still not common place (as compared with static numerical modeling).

Numerical modeling in geomechanics is associated with unique and significant challenges because of the uncertainty involved with engineering of natural systems. In rock mechanics, this uncertainty stems primarily from inherent complexities associated with geologic materials. Often times, even for static or quasistatic problems, it is not feasible to incorporate these complexities into a model, resulting in solutions that have high margins of error; the problem is data-limited.

Solutions to dynamic problems in geomechanics are therefore hampered from the outset, because the dynamic solution is dependent upon the initial conditions, which are generally the static equilibrium state of the excavation, which is itself a data-limited problem. Additionally, there is uncertainty surrounding nearly every aspect of the dynamic solution. For a mining rockburst this includes dynamic behavior of intact rock, discontinuities, and ground support; source mechanism; the characteristics of the ground motion and the changes that occur as the wave propagates through the rock mass (path effects); and interactions between the seismic waves, excavation, and ground support.

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