Designing controlled pillar failure – crush pillar support .• Mining height and pillar size (w:h

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  • DESIGNING CONTROLLED PILLAR FAILURE CRUSH PILLAR SUPPORT 339

    IntroductionSafe mining practices are aimed at maximizing theextraction of a particular orebody. Mine stability is a keyconsideration and the type of layout (i.e. pillar type andspans) must be suitable for the prevailing rock massconditions. Crush pillar mining appears to be a methodunique to South African hard rock mines, with the pillarsystem being applied to shallow and intermediate-depthgold and platinum orebodies. It allows for a higherextraction than what can typically be achieved with aconventional rigid/elastic non-yield pillar system. The pillarsystem must, however, be used in conjunction with a barrierpillar system. The crush pillar dimensions are generallyselected to give a width-to-height ratio (w:h) ofapproximately two, (Ryder and Jager, 2002). This w:h ratiois selected to ensure that the pillars fail as they are being cutat the mining face. Once the pillar has failed in a stablemanner, the residual strength of the pillar contributes to therequired panel support by carrying the deadweight load tothe height of the uppermost parting on which separation isexpected to occur. Closely spaced support elements aretypically used between adjacent rows of pillars to provideadditional in-panel support.

    Ozbay and Roberts (1988) suggested that crush pillarsshould be implemented at a depth greater than 400 m belowsurface. This is based on the assumption that the averageface stress at this depth is large enough to enable crushingof the pillars.

    In contrast to stable pillar layouts, failure of crush pillarsis in fact desired as long as it occurs in a controlled manner.Pillar failure and the resulting load-shedding should ideallybe continuous to prevent accumulation of elastic strainenergy.

    Figure 1 is an illustration of the stress-strain relationshipof a typical pillar. The initial straight-line portion of thecurve up to the yield point reflects the elastic response ofthe pillar. The yield point indicates the onset of inelasticbehaviour, whereafter the pillar exhibits strain-hardeninguntil it reaches its peak strength. Load shedding follows

    until the pillar reaches its residual strength. Crush pillars aredesigned to function in this residual part of the pillar stress-strain curve.

    Historic use and design of Merensky crushpillars

    RPM (Rustenburg Section) was the first platinum minereported to have used crush pillars, (Ozbay et al., 1995).Crush pillars were implemented as early as 1974 on FrankShaft (now Khomanani Mine) and RPM (Union Section) in1977 (Korf, 1978). The pillar system was introduced toprevent back-breaks as a result of large spans created whenchanging the support method from initially stonewalls(1927) to stonepacks to crush pillars (1974) as miningprogressed deeper. Interestingly enough, none of theplatinum mine crush pillar sites investigated by Ozbay(1995) were making use of barrier pillars in conjunctionwith the crush pillars.

    Crush pillar layouts were initially designed using pillardimensions that were successful in other areas. The pillar

    DU PLESSIS, M and MALAN, D.F. Designing controlled pillar failure crush pillar support. The 6th International Platinum Conference, PlatinumMetalfor the Future, The Southern African Institute of Mining and Metallurgy, 2014.

    Designing controlled pillar failure crush pillar support

    M. DU PLESSIS* and D.F. MALAN*Lonmin Platinum

    University of Pretoria

    The aim of any mine design is to ensure that the excavations remain stable for the period it will bein use. Various pillar systems are used to ensure that underground stopes remain stable and thatmining activities do not impact on the surface infrastructure through either surface subsidence orseismicity.

    Intermediate-depth platinum mines make use of in-stope pillars designed to fail while the pillarsare being cut at the mining face. The pillar stress exceeds the loading capacity and the pillarscrush as a result.

    The aim of the paper is to provide an overview of in-stope crush pillars. This will include theapplication, behaviour, function, mechanism, impact, and design of a crush pillar system.

    Figure 1. Diagram illustrating the complete stress-strainbehaviour of a pillar (after Ryder and Jager, 2002)

  • PLATINUM METAL FOR THE FUTURE340

    dimensions and spacings were then adjusted until the pillarsexhibited the required behaviour (Ozbay et al., 1995). Thetypical range of w:h ratios of the crush pillars variedbetween 1.52.5. This accommodated the varying stopingwidths (0.92 m), the weak footwall rock in some areas,and structural weaknesses in the rock. An alternative designapproach was to cut the pillar at a w:h ratio of 2 and thenincrease or decrease the pillar width until crushing wasachieved.

    Ozbay et al. (1995) stated that the main purpose of thecrush pillars was to provide enough resistance to supportthe rock up to the highest known parting plane (i.e.Merensky Bastard reef contact at a height of 5 m to 45 m),and not to support the full overburden rock mass to surface.The load requirement of a crush pillar to function as localsupport can be established by determining the supportresistance required, which is dictated by the height of theprominent parting. Support resistance in the order of 1 MPais quoted (Roberts et al., 2005a) based on the back-analysisof backbreaks that occurred at Randfontein Estates andNortham, where the failures took place at 40 m and 30 minto the hangingwall respectively. Parting heights of 10 mand 20 m would result in a support resistance requirementof approximately 0.3 MPa and 0.6 MPa respectively.

    Typical crush pillar layoutsA typical mining configuration for a crush pillar layoutconsists of pillars being positioned either adjacent toraises/winzes (dip mining) or strike gullies (breast mining).The pillars are separated in the direction of mining by aholing to allow for either ventilation (vent holing) or toincrease extraction (pillar holing). Crush pillar layoutstypically consist of approximately 3033 m wide panelspans (inter-pillar) with slender pillars 2 m, 2.5 m, 3 m, or4 m wide and 3 m, 4 m, or 6 m in length. The pillars areseparated by 0.5 m to 3 m wide holings. In some instances asiding is mined adjacent to the raise or gully to ensure thatthe failed pillar material does not fall into the travellingway. These sidings are approximately 22.5 m deep and arecarried a maximum of either 3 m or 6 m behind the panelface (depending on the standard applied by the miningcompany). Figure 3 is an example of a typical up-dip crushpillar layout. An off-reef haulage links to the reef horizonvia a cross-cut and a travelling way.

    Uncertainty regarding pillar behaviour and design The measured and observed behaviour of a 2:1 Merenskycrush pillar is summarized in Figure 4 and Table I. Basedon stress measurements, Roberts et al. (2005b) determinedthat a crush pillar reaches its peak strength at between 310millistrains, then fails following a further compression ofapproximately 5 millistrains along an estimated negativepost-peak stiffness slope of 12 GN/m. Following furthercompression of the order of 5090 millistrains, it isassumed that footwall heave occurs as a result of the lateralconfinement of the foundation. At this point it is assumedthat the crushing of the foundation restricts the pillars loadcapacity as the pillar is reliant on the foundation, which isbelieved to be the limiting load-bearing component. Furthercompression could result in an increase in the contactfriction angle; the result is a squat effect, with the slope ofthe stress-strain curve becoming positive. This is assumedto occur when the vertical strain is > 0.4.

    The value of the peak pillar strength is unknown. Thevalues quoted above are based on estimates as described byRyder and Ozbay (1990).

    On most mining operations, the design of the crush pillarsis based on trial and error. As the pillar strength isunknown, the pillar sizes are adjusted to obtain the correctbehaviour. Several factors affect the behaviour of the crushpillars and in many cases satisfactory pillar crushing is notachieved. This results in a seismic hazard in many of themines using crush pillars. If pillar crushing is not initiatedwhile the pillar is being formed at the mining face, as themining face advances and the pillars move to the back areaof a stope, smaller pillars may burst while oversized pillars

    Figure 3. Typical layout (up-dip mining) for a narrow tabularreef mine using crush pillars plan view

    Figure 2. Photograph of a crush pillar in an underground trialsection at Lonmin

  • may punch into the footwall. There is therefore a point ofequilibrium which needs to be achieved to prevent thehazard of pillar-induced seismicity. If pillars are designedin such a way that they are fractured during cutting by theface abutment stresses so that the pillars will already haveyielded and reached their residual strength, furthercompression of the pillars will be associated with anincrease in load and stability ensured, (Ozbay and Roberts1988). The stiffness of the strata must therefore be greaterthan the post-peak stiffness of the pillar (Figure 5), orviolent pillar failure and hangingwall instability will occur(Figures 6 and 7). The pillar design should be aimed atdetermining pillar dimensions for which the post-peakcurve of the pillar is as flat as possible.

    There are many factors influencing the behaviour ofcrush pillars. These factors impact on the ability of thepillar to crush as well as the reaction of the strata inresponse to the pillar when entering a post-peak state. Someof the contributing factors are highlighted below:

    Mining depth (stress) Mining height and pillar size (w:h ratio) Stope layouts, including the position of the pillars and

    presence of a siding Strata stiffness and the influence of mini