The conditions for rockbursts occurrence are traditionally identified as: high stress, high extraction ratio, strong brittle rocks, folding, faulting and unfavourable excavation geometry. Some rockbursts cannot be explained by any one or a combination of these factors. Salamon (1983) stated that a disconcerting feature of rockbursts is that they defy conventional explanation. Based on detailed review of case histories, this paper identifies oblique loading of orebodies by the major far field principal stress as a cause of rockbursts. Orebodies subjected to this loading condition are termed orebodies in shear. Orebodies in shear are subjected to compressive and shear loads. This paper shows it is risky to generalise that tabular orebodies have their axis perpendicular to the major far field principal stress. This study identifies characteristics of orebodies in shear and the consequences of not taking this loading mechanism into account in the planning, design and mining of such orebodies.
Part 1 of the paper defined shear loaded orebodies and showed through case histories that both pillars and excavations are at elevated risks of failure when mining these orebodies. Part 2 of the paper presents new knowledge on the behaviour of pillars and excavations when mining such orebodies. Numerical modelling is used to understand the behaviours of these structures in the orebodies. It is established that pillars in shear suffer confinement loss compared to their equivalents under pure compression. The confinement loss increases with increasing shear loading in pillars with width : height (W : H) §1. For pillars with W : H §1, the Lunder and Pakalnis (1997) empirical pillar design chart should be used with caution. For excavations in eccentrically loaded orebodies, passive and active high stress envelopes are created in the excavation process. The combined effect of the active high stress and tension zones often results in excavation surface sloughing.
Evolution Mining's Frog's Leg underground mine experienced an increase in the number of seismic events and rockburst occurrences during 2015. This was due to increased stress levels due to increasing mining depth and unfavourable mining geometry (mining the sole remaining diminishing pillar) as well as interaction with some seismically active crosscutting geological structures. At the time, the extraction sequence was transitioning from central access to an end-on retreat sequence with stopes in the diminishing pillar being extracted as triple lifts. Initially, a triple lift extraction methodology was implemented to eliminate the exposure of personnel and equipment to potentially seismically active ground as the closure pillar was extracted. All production activities for the closure pillar were conducted outside the oredrives from a drive situated in the hanging wall. Intense episodes of seismic activity occurred during the third and fourth quarters of 2015 as well as early in the first quarter of 2016 while the closure pillar was being extracted. During this period, two rockbursts occurred; after the second rockburst, management decided to temporarily halt production activities in the area pending geotechnical investigation and the development of a new extraction strategy. A Geotechnical Review Board (GRB) was formed to review and evaluate the situation to date and provide guidance on: the extraction sequence, ground support design and implementation, and seismic monitoring requirements. Following the GRB evaluation, a program of works was initiated including assessment of various extraction sequences and dynamic ground support design. Subsequently, these have been implemented and mining activities have resumed. This paper provides an overview of the mining practices that led to increased seismicity and rockbursts as well as measures that were implemented to mitigate the hazard associated with increased mining-induced seismicity and increasing stress levels.
La Mancha Resources Australia Pty Ltd's Frog's Leg Gold Mine is located approximately 20 km west of Kalgoorlie in Western Australia. The mine utilises long hole open stoping and cemented paste fill, to achieve full extraction of the orebody. The combined effects of high fines contents in the supplied reclaimed tailings, and very high water salinity, hampers the strength development and curing times of the paste fill, despite the high binder dosages. Given a decreasing gold price and a high operating cost environment, the need to identify areas where productivity and efficiency improvements can be made is paramount. Predominately due to the large volumes involved, paste backfill constitutes one of the largest costs to the underground operation, and therefore an area where even minor efficiency improvements can have a significant positive effect on mining costs. One of several improvement projects undertaken was to evaluate two paste additives, namely MM701 (supplied by BASF) and Acti-Gel® (supplied by Active Minerals) and their impact on paste flowability, compressive strengths, curing time and binder dosage rates.
This paper is an investigation report summarising a series of tests and observations conducted over a two year period as part of a feasibility study for replacing the tailings used in the cemented paste fill (CPF). The testworks were conducted at Evolution Mining's Frog's Leg mine, Kalgoorlie, which utilises undercut long-hole stoping with CPF. The project was initiated after the tailings reclaim agreement with a third party supplier failed to be negotiated, which compelled management to explore the possibility of harvesting tailings from Evolution's Mungari Gold Operations' (MGO) newly established tailings storage facility (TSF). The feasibility study includes testing tails for CPF compatibility and liaising with TSF engineer(s) for the best extraction of tails without hindering the dam lift cycle and stability. Access towards the centre of the TSF was an issue due to poor compaction of the surface near the decant point and this, in turn, restricted the amount of tails that could be extracted. Tailings with a fine particle size were required to reduce the void ratio in the CPF; however, these tailings had deposited towards the middle of the TSF, which was wet, resulting in the harvested tails containing excess coarse size particles. A combination of insufficient tailings available to meet the annual paste backfill requirements and the lack of fines in the tailings lead to the concept of adding screened oxide (overburden) to the extracted tails. This paper explores the blending of oxide with tailings for CPF mix at different ratios and its effect on the final CPF product. The key findings from the study concluded that the strength of the CPF will be inversely proportional to the ratio at which the oxide is added to the CPF mix.
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