Vertical excavation stability and mitigation of strainburst risk using preconditioning blasting in deep mining environments

Abstract

Preconditioning blasting is used as a means of managing strainburst risk in high stress development headings. Many of the published designs of preconditioning blast are used across varying lithologies and multiple mine sites without accounting for changes in the local rock strength, geological conditions, or stress regime. There are no guidelines for the design of a preconditioning blast or for evaluating the effectiveness of the blast. In this thesis, an in-depth field study as well as a numerical study of preconditioning blasting in a shaft sinking operation are presented. First, the rock mass response behavior of vertical bored raises excavated in brittle rock is examined. Data were collected from two in-line raises excavated in rock (with an average UCS of 225 MPa) at depths from 1150 to 1915 m. Through this work, the mechanisms of deterioration that occurred in the raises are examined along with the depth of failure around the excavations. The importance of the disturbance or damage zone around an excavation is highlighted because, similar to preconditioning blasting, the damaged zone around the raise can provide confinement, suppress spalling, strainbursting, and associated seismicity. However, when the damaged rock is removed, the exposed rock wall becomes unstable, leading to more damage, spalling, and bursting. Second, using FLAC3D, a Finite Difference Method (FDM) code, the observations that were made in the field relating to deterioration/growth in the 3-m-diameter bored raises are replicated. Demonstrating the ability to capture the deterioration observed in the raises, specifically the importance of the damaged zone around an excavation for promoting stability, is important for understanding the potential effect of preconditioning blasting at the larger 7.2-m-diameter shaft scale. Third, using observations made in lateral development, the bored raise excavations, and numerical modeling, a preconditioning blast design is proposed for the conventional blind shaft sink that occurred below 1915 m and extended to a depth of 2635 m. The proposed preconditioning blast design is trialed in the Onaping Depth shaft blind sink, and with the use of a microseismic system, the preconditioning blast is quantitatively evaluated to show the success of the blast. Fourth, the preconditioning blast design for the conventional shaft sink is simulated in a FLAC3D numerical model using scaled-up material properties based on the bored raise model. The damage generated from the preconditioning blast is simulated with shells of damage, representing the hole, the crush zone, and fracture damage radiating away from the blast hole. The shells are an elongated oval shape, which are in line with the field observations. The modulus and cohesion of the individual shells are reduced to represent the softening of the rock associated with the damaged zone resulting from preconditioning blasting. Based on these blind sink models, the preconditioning blasting design can be modified to examine the resulting stress accumulation and shedding that occurs ahead of the bench face. Finally, a framework for preconditioning blasting design in shaft sinking operations is presented. This approach is tested against the observations made in two main lithologies that were present in the blind sink – norite and gneiss. The work developed in this thesis provides a novel approach to developing a preconditioning blast design to suit the conditions that are encountered in the field. This work is important for understanding the stress conditions in deep shaft sinking operations, evaluating the effectiveness of a preconditioning blast, and safeguarding operators working in this environment.