Mgumbwa, J, Page, A, Human, L & Dunn, MJ 2017, 'Managing a change in rock mass response to mining at the Frog’s Leg underground mine', in J Wesseloo (ed.), Proceedings of the Eighth International Conference on Deep and High Stress Mining
, Australian Centre for Geomechanics, Perth, pp. 917-936.
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.
Keywords: seismicity, high stress, closure pillar, extraction sequence, ground support
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