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.

SynopsisThe South Deep mine is a deep-level mine that is actively mining between 2600 m and 3000 m below surface, with expectations to mine to 3400 m depth. The The orebody lends itself to a fully mechanized mining method. The main geotechnical challenges for successfully mining the South Deep orebody were to destress and then cost-effectively extract the massive, low-grade orebody. Since Gold Fields acquired South Deep in 2007, several mining methods have been used to date, but destressing was done conventionally using traditional South African narrow-reef gold mining methods. In 2015, the mine moved to high-profile (5.5 m high) horizontal destress development with mechanized installation of ground support, and crush pillars were replaced with yield pillars. This has resulted in a safer working environment with industry best-practice support standards and less seismic energy release, while still allowing appropriate productivity rates This paper outlines the geotechnical processes used to overcome issues as they were encountered, including ground support, seismicity, and rock mass conditions, and highlights the key leanings of a deep-level massive mine's evolution over time.

The South Deep mine is located approximately 45 km southwest of Johannesburg in the Far West Rand goldfield of the Witwatersrand Basin. It is a deep level mine that is actively mining between 2600 m and 3000 m below surface with expectations to mine to 3400 m depth. South Deep is situated in the geologically unique and renowned Witwatersrand Basin, which is the world's premier gold region. The South Deep ore body gradually increases in thickness to the west, from approximately two metres at the sub-crop to approximately 120 metres in thickness. The geometry of the Upper Elsburg Reef package, which is the primary economic target, lends it to a fully mechanised mining method. The main geotechnical challenges to successfully mine the South Deep orebody were to introduce a mechanised massive mining method at depth to destress and then extract the extensive orebody. The destressing method then had to allow a productive method for economic extraction of an essentially low-grade bulk volume orebody. Several variations of different mining methods have been used to date but all rely on a destressing method to reduce the in situ stresses. Originally the destressing was done conventionally (traditional South African narrow reef gold mining methods). Since Gold Fields acquired South Deep in 2007, the push for further mechanisation has seen four mining method changes, including: mechanised, low profile, apparent dip destress mining; the introduction of a low profile, horizontal destress method with backfill. The year 2011 saw the introduction of low profile, horizontal destress with 2m wide crush pillars And in 2015, the mine moved to high profile (5.5m high) horizontal destress development with mechanised installation of ground support. Crush pillars were replaced with yield pillars.

The initial determination of the rock mass characteristics and its behaviour has been a key parameter to ensure that a safe and economical extraction can be achieved at Granny Smith Gold Mine (GSGM). The objective of this paper is to outline the geotechnical and planning approach that has been followed to ensure that the intended stoping performance is reached. The geotechnical analysis involves the use of geological and structural analysis, empirical design methodologies and numerical analysis/simulation to determine the effects of induced stresses on the overall stability of the open stopes as well as to the adjacent excavations. Stope and pillar design, size and placement are optimised with the aim to maximise extraction without compromise to personnel safety and global mine stability. The reinforcement requirements for accesses and adjacent excavations to the stoping areas are determined at the planning stage with the aid of numerical modelling. https://papers.acg.uwa.edu.au/p/1511_10_Machuca/ Geotechnical approach to stope and pillar optimisation at Granny Smith Mine L Machuca et al.

The mining methods used at South Deep started with conventional mining (pre-1998), which later evolved into mechanized mining with various layouts for drifting and benching, low-profile horizontal destressing with crush pillars and long hole stoping (LHS). The mining method was then changed during 2015-2016 to the current high profile destress with LHS. Pillars in the destress cuts are designed to yield and not pose a significant rock burst risk. High profile development was initially conducted using smaller pillars (4.5 m x 10 m and 6 m x 10 m) which completely yielded however, as these were prone to excessive scaling and extensive rehabilitation requirements. Numerical modelling was conducted by Lilly, (2016), to optimise the destress pillar dimensions. The study concluded that a larger yield pillar is viable in the high profile destress cuts. The new pillar design was still not proven in situ owing to lack of information regarding to yielding, closure rates, convergence and fracturing through the core as well as the transition between the old and new mining layout. As a first approach, borehole camera surveys were conducted to assess the pillar conditions. Ground penetrating radar (GPR) technology was then used to determine fracturing around the yield pillars and where possible, the results where compared to the borehole camera surveys. A Zeb Revo scanner was then used to obtain accurate closure and pillar yield performance. This data was used to calibrate actual vs model-predicted displacements and to calibrate numerical models, reconciling pillar and also pro-actively identify areas that were undergoing extensive deformation. This paper will describe the outcome of the above-mentioned monitoring and reconciliation programmes and the interpretation of the results.