Short-term solutions to squeezing ground at Agnew Gold Mine, Western Australia

Abstract

The Waroonga Underground Complex (Agnew Gold Mine) utilises the longhole open stoping method to extract ore from a quartz breccia zone. The Lode 1 ore zone dips west at approximately 65° and sits between a geological hanging wall (HW) made up of a sandstone unit and geological footwall (FW) made up of altered sandstone/ultramafic rock units. During 2014, Lode 1 transitioned from a V-shaped mining front to an M, or inverted double V, mining front. No deviation from the existing ground conditions was observed during transition from V to M front. When production began at 1,180 m below surface reduced level (RL) (Level 1), a higher degree of deformation was observed in the FW and HW shoulder of this level and the four levels below (Levels 2-5) than had been previously seen. This study outlines the monitoring, modelling, testwork and short-term strategies put in place to ensure that potential impacts to personnel, safety and production are minimised while longer-term solutions to squeezing ground are examined. Monitoring includes extensometer pins installed over two levels, with profile survey points also included in the lowermost level. Photographic monitoring stations are established in all active development/production levels. The initial numerical modelling undertaken included 2-dimensional finite element and kinematic analysis to ensure that the ground support used would achieve the required Factor of Safety (FS). Preliminary observations indicate that regional faults in the ore zone play a significant part in level of FW deterioration with the majority of squeezing occurring between the 'Thresher' fault and 'Sandbar' fold in the northern part of the ore drives. Testwork commissioned on core samples from the affected ore drives focused on mineralogy testing. Alteration mineral type and abundance in FW rock units are hypothesised to be influencing squeezing behaviour. Where deformation has occurred in levels ahead of stoping, they have been signed off and rehabilitation is conducted just prior to production requirements. It is believed that further modelling and trials of ground support systems/elements, possibly in conjunction with changes to extraction sequence, are required to mitigate squeezing conditions in the longer term, with ground support being varied according to likelihood of squeezing. However, it is believed that the current strategy of monitoring and signing off affected areas before just-in-time rehabilitation is an adequate short-term solution.

Managing the onset of accelerated deformation in capital development at Agnew Gold Mine

“…This drive deformation was previously being observed within the oredrives (Woolley & Andrews 2015). Since the stoping fronts have continued to retreat over multiple levels the deformation has begun occurring in long-term capital development specifically hanging wall access drives and Kim Decline below the retreating level.…”

“…The lower areas of Kim Lode are intersected by three major fault structures known as Thresher, Spinner and Mako Faults. The Thresher Fault is a reactivated structure, which tends to increase deformation within oredrive development (Woolley & Andrews 2015). All of these faults are sub-vertical, tightly healed and did not cause stability issues during initial development.…”

Design tools for squeezing ground conditions in hard rock mines

“…This index can provide a first indication for the potential of squeezing and the long term strain level based on ranges for the foliation spacing and the stress to strength ratio as shown in Figure 2. The squeezing index has been successfully used so far by the Wattle Dam mine in Western Australia (Marlow & Mikula 2013), the Waroonga underground complex (Woolley & Andrews 2015) and the Westwood mine in Canada (Armatys 2012). Mercier-Langevin and Wilson (2013) demonstrated that the index predicted effectively the strain in a variety of conditions from no squeezing to extreme squeezing in the Lapa Mine.…”

Evaluation of ground management in underground excavation design

“…Deformation of underground openings sometimes reach tens of centimetres in squeezing ground conditions. In this type of ground condition, stopping deformation is not generally accepted and the focus should be to control and manage ground movement (Woolley & Andrews 2015). Evaluation of rock mass stability is the main concern of squeezing problems due to the deformation of rock mass which may happen over a few days.…”