Influence of mining activities on the reactivation of a footwall fault

Original Paper

Abstract

The presence of geological faults intersecting with steeply dipping orebodies is not uncommon in hardrock mining. More often than not, such a scenario leads to seismic activities as ore extraction approaches the intersection point. This is particularly true for deep mining. This paper examines the influence of mining direction, i.e., overhand (bottom-up) versus underhand (top-down) mining, on the reactivation of a footwall fault in a sublevel stoping system. To do so, the static and dynamic behaviour of a footwall fault intersecting with a steeply dipping orebody is investigated. A model parametric study is undertaken with respect to the dip angle of the fault and the location of the fault intersection point with the orebody. The static analyses demonstrate that the volume of ore extracted before mining activity reaches the fault intersection is an influential factor, which impacts the degree of unclamping of the fault. It is thus suggested that mining directions be assessed in such a way as to minimize the volume of mined ore prior to intersection with the fault. It is also shown that the effect of mining direction on seismicity along the fault significantly decreases as the fault dip angle decreases. A dynamic analysis is then carried out and it confirms the same trends as the static analysis, i.e., fault-slip occurs over a larger fault surface area as the volume of extracted ore prior to intersecting the fault increases. However, there is no noticeable difference in the intensity of near-field ground motion during fault-slip for the two mining directions. A case study of the Lucky Friday Mine is then reviewed, where bedding faults intersect with a steeply dipping vein from the footwall side. It is reported that shear movements and seismicity on the fault are not the direct cause of rockbursts that took place around a stope. Rather, the damage was caused by the stress change induced by the slip. In order to elucidate the mechanism, the development of deviatoric stress induced by the shear movement along the footwall fault is analyzed while comparing with the case at Lucky Friday Mine. In order to clarify the influence of shear movements, the deviatoric stress is compared in models with and without faults. It is shown that the shear movement significantly contributes to the increase in deviatoric stress around the fault, thus suggesting damage to the rockmass and generation of seismically active zones. The results are found to be in agreement with the case at Lucky Friday Mine.

Keywords

Fault-slip Footwall fault Seismic moment Bottom-up Top-down Sublevel stoping method 

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Copyright information

© Saudi Society for Geosciences 2017

Authors and Affiliations

  1. 1.International Research Organization for Advanced Science and TechnologyKumamoto UniversityKumamotoJapan
  2. 2.Department of Mining and Materials EngineeringMcGill UniversityMontrealCanada

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