ultra-deep mining: the increased potential for squeezing d.f. malan* and f.r.p. basson ......

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  • 353The Journal of The South African Institute of Mining and Metallurgy NOVEMBER/DECEMBER 1998

    Introduction

    The depletion of shallow ore reserves is forcingthe South African gold mining industry toexploit reefs at ever-increasing depths. It isforecast that by the year 2010 some 30% ofthe South African gold production will comefrom depths greater than 3000 m (Johnsonand Schweitzer1). Of significance are recentdiscussions of planned ultra-deep miningoperations at depths between 3500 m and5000 m (Grtunca2, Diering3, Johnson andSchweitzer1, Schweitzer and Johnson4). As theestimated vertical virgin stress at these depthswill vary between 95 MPa and 135 MPa(assuming a rock density of 2700 kg/m3),there is no doubt that the challenges facing theindustry are immense. As these excavationswill be the first attempt ever in the world tomine at these great depths, there is noguarantee that the current rock mechanicsknowledge and associated empirical rules willensure viable ore extraction. The need forresearch into suitable technologies and miningmethodologies is now probably more pressingthan ever. To address these issues, a collabo-rative research programme called Deepminewas recently established (Grtunca2). This is a

    joint effort between CSIR Division of MiningTechnology (Miningtek), the gold miningindustry, government, tertiary educationinstitutes and the Foundation for ResearchDevelopment (FRD). Diering3 discussed anumber of critical technologies needed forviable mining operations at these depths.Rapid horizontal access development andsupport of these tunnels so that they remainopen for their planned life-time are two of thecrucial issues. A phenomenon that is relativelyunknown in the South African gold miningindustry is the occurrence of squeezing rockconditions in tunnels. In some civilengineering tunnels, squeezing conditions leadto severe problems that require specializedexcavation techniques and expensive supportinstallation. As will be discussed in this work,there is an increased potential for squeezingconditions in ultra-deep mining that may putadditional strain on the profitability of thesemines. This paper will describe thephenomenon of squeezing, discuss examplesof this behaviour at Hartebeestfontein mineand explain why it may become moreprominent in ultra-deep mining. Existingsupport methodologies in squeezing conditionsare also reviewed.

    Squeezing behaviour

    Changes in the local stress state due to miningactivity perturb the stability of the rock masssurrounding excavations. The subsequentreadjustment of the rock towards a newequilibrium does not occur instantaneously butas a time-dependent process. This process caninclude two types of inelastic deformation,namely slow creep-like movements and

    Ultra-deep mining: The increasedpotential for squeezing conditionsby D.F. Malan* and F.R.P. Basson

    Synopsis

    This study reviews the concept of squeezing (large time-dependentdeformation) in rock and investigates the possibility of thisphenomenon becoming more pronounced in the planned ultra-deepmines of the South African gold mining industry. Althoughsqueezing behaviour is not commonly found at the current depthsof the gold mining industry, it is observed at HartebeestfonteinMine due to a combination of weak rock and high stresses.Calculations based on the average strength of the Witwatersrandquartzites and the competency factor used to identify squeezingconditions in civil engineering tunnels, indicate that there is anincreased risk of squeezing behaviour in ultra-deep mines. This willadversely affect the profitability of these mines and highlights theneed for further research into the time-dependent behaviour ofrock. Finally, the current support strategies in squeezing conditionsare reviewed.

    * Division of Mining Technology, CSIR, P.O. Box91230, Auckland Park, 2006, South Africa.

    Hartebeestfontein Division of Avgold, Private BagX800, Stilfontein, 2551, South Africa.

    The South African Institute of Mining andMetallurgy, 1998. SA ISSN 0038223X/3.00 +0.00. Paper received Jul. 1998; revised paperreceived Aug. 1998.

  • Ultra-deep mining: The increased potential for squeezing conditions

    rockbursts. Depending on the rock type and stress,excavations can show a propensity towards either of thesetwo phenomena. Although it is an active field of research, theconditions associated with the transition from stabledeformation to rockbursting are not yet fully understood. Inrelation to mine safety, it is preferable that all deformationoccurs in a non-violent fashion. Excessive time-dependentrock movement is however undesirable as it strongly affectsthe long-term stability of underground openings. Significantcreep-like movement of the rock is termed squeezingbehaviour. This results in large tunnel convergence withsevere support difficulties. The importance of understandingthis phenomenon was underlined by the ISRM whoestablished a commission on squeezing rocks (Barla5). Thedefinition of squeezing as proposed by this commission is:

    Squeezing of rock is the time-dependent largedeformation which occurs around the excavation, andis essentially associated with creep caused byexceeding a limiting shear stress. Deformation mayterminate during construction or continue over a longperiod.

    When a tunnel is driven into soft squeezing rock (such assoft clays or mudstone), the ground advances slowly into theopening without visible fracturing or loss of continuity(Gioda and Cividini6). Squeezing can however also involvedifferent mechanisms of discontinuous failure of thesurrounding rock. Possible mechanisms are complete shearfailure in rock if the existing discontinuities are widelyspaced, buckling failure in thinly bedded sedimentary rocksand sliding failure along bedding planes (Aydan et al.7). Themagnitude of tunnel convergence, the rate of deformationand the extent of the failure zone depend on the geotechnicalconditions and the magnitude of stress relative to rock massstrength. Of interest is that rockbursting is not associatedwith squeezing (Barla5). In this context rockburstingprobably refers to strain bursting and not to major damageresulting from large shear type events on geologicalstructures. The creep deformation acts as an efficient redistri-bution mechanism of stress, preventing the build-up of largestresses close to the excavation and thereby reducing theincidence of strain bursting. In a mine subjected to squeezingconditions, larger shear type seismic events may howeverstill be possible due to global stress changes affecting thestability of geological structures such as faults and dykes.

    Squeezing conditions at Hartebeestfontein Mine

    An example of squeezing rock conditions in the SouthAfrican gold mining industry can be found at No 6 shaft,Hartebeestfontein Mine in the Klerksdorp area (Malan andBosman8). Mining operations at Hartebeestfontein Minemainly expose the quartzitic members of the Main Bird (MB)Series which forms part of the Witwatersrand Supergroup.The uniaxial compressive strength of some of these memberscan be as low as 130 MPa. The overall competency of therock mass and its ability to provide stable miningexcavations for prolonged periods appears to be greater inthe quartzites encountered in the vicinity of VentersdorpContact Reef and Carbon Leader mining operations in otherparts of the Witwatersrand basin. These quartzites are lessbedded and substantially more siliceous than those present

    at Hartebeestfontein Mine. As a result of this greaterstrength, the rock in other mines would seem to be moresusceptible to strain bursting under high stress conditions.

    The combination of weak quartzites and high stress atHartebeestfontein Mine leads to appreciable time-dependentmovement of the rock. This is most notable in serviceexcavations located in highly stressed ground where the rockbecomes progressively more fractured with time. Tunnelclosure rates in the order of 50 cm per month have beenobserved, leading to severe support difficulties. In stopes,movements of up to 6 cm per shift can be observed. Theproblems caused by the time-dependent rock movements atthis mine were highlighted in a research study (Piper andWagner9) to evaluate the long-term stability of anunderground refrigeration complex at No 6 shaft of the mine.It was suggested in this study that the long-term stability ofthe refrigeration chamber could not be assured even with theupgrading of the support system. Further measurements andextensometer data showed an acceleration of chamberclosure as the No 6 shaft pillar became progressively moreisolated (Roberts and Jager10). It was, however, unclear atthat stage how much of this behaviour was caused by truetime-dependent behaviour and how much by an increase instress due to far-field mining activity. With deterioratingconditions, the refrigeration chamber was eventuallyrelocated to overstoped ground.

    The squeezing mechanism at Hartebeestfontein Mine isconsidered to be a combination of time-dependent failure ofthe intact rock and sliding along bedding planes. Thequartzites of the Main Bird Series are well bedded withbedding thickness in the range of 0.1 to 1.3 m (King et al.11).Appreciable shear slip and dilation of the bedding planeshave been noted (Roberts12). The buckling of layers close tothe excavation may also play a role.

    Figure 1 illustrates the typical bulging of the haulageprofile caused by the slow time-dependent processes in therock. Significant footwall heave is also noticeable. Withfurther deformation, the support eventually fails leading tothe destruction illustrated in Figure 2. This necessitatesfrequent rehabilitation of the tunnels. If the time-dependentprocesses could be better understood, optimum strategies

    354 NOVEMBER/DECEMBER 1998 The Journal of The South African Institute of Mining and Metallurgy

    Figure 1Haulage conditions experienced i

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