role of pillars in small ug mine

A Publication of the University of Miskolc Series A. Mining, Volume 63, (2003)

International Conference on Safety and Environmental Aspects of Mining

THE ROLE OF PILLARS IN SMALL

UNDERGROUND MINES

Prof . Dr. Horst W a g n e r Head Department of Mining Engineering

Montanuniversitt Leoben (Austira)

Summary

Alpine mineral deposits tend to be of limited extent and of complex geology. Historically pillar design was based on experience rather than broad based engineering facts. This differs substantially from the pillar design in extensive tabular deposits which, based on Salamon's pioneering work, has reached a high degree of proficiency. The basic differences in pillar design for small alpine mines and for extensive tabular deposits are discussed. It is shown that in the case of the former each case is treated on its merit and that there are no common design rules. The estimation of pillar strength constitutes a particular problem and examples of recent developments in this area are given. It is shown that discontinuities play an important role as far as the strength of pillars and the layout of pillar systems is concerned. The role of backfill is discussed and the effects of backfill on pillar behaviour are shown. Because of the limited lateral extent of most alpine pillar workings pillar failures tend to be stable but situations leading to unstable pillar failures can arise in the final stages of mining. Because of the vertical extent of some of the alpine mineral deposits pillar workings exist on a number of mining levels. This leads to interactions of workings in adjacent mining horizons. The resulting difficulties are discussed and the importance of superimpositioning of pillars is highlighted. A number of general conclusions complete the paper.

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1. INTRODUCTION

Pillars are essential elements of many mining systems. Pillars are employed to support stoping excavations, to protect surface and underground structures and to act as safety barriers. For many years the design of pillar systems was entirely experience based.

The tragic mining accident at the Coalbrook Colliery in South Africa which caused the loss of 437 lives, when several thousand coal pillars failed in 1960, marked the beginning of intensive research efforts into pillar systems. Through the pioneering efforts of Prof. M.D.G. Salamon and his colleagues at the Research Organisation of the Chamber of Mines of South Africa the foundations of modern pillar design principles based on probability theory were laid1-2 Based on these principles design guidelines for room and pillar workings were established and have found wide application not only in South African coal mines but also in other parts of the world3

The name of M.D.G. Salamon is not only closely linked to modern coal pillar design but also to the important question of pillar stability and instability and to the role of pillars in ameliorating the rockburst hazard in deep level mining4-5 There is no doubt that M.D.G. Salamon, more than anybody else, has made a lasting impact on the development of pillar mining systems.

Whereas Salamon's work on pillar mining system was concerned primarily, but not exclusively, with tabular mineral deposits of considerable lateral extent there are many situations where pillar mining is being carried out in mineral deposits of very limited extent. This is particularly the case in the Alps where rather small mineral deposits are being mined. In these situations some of the basic concepts such as the tributary area concept are no longer applicable. In addition the irregular shape of the deposit and in particular the variable thickness of many of the alpine deposits results in pillar systems which differ greatly from the classical coal mining or typical hard rock pillar mining situations found in tabular deposits.

In this paper a number pillar mining situations in small Austrian underground mines will be discussed and the differences between extensive and limited scale pillar mining applications highlighted. In particular some of the inherent dangers resulting from the application of limited scale mining experiences to larger scale applications will be addressed.

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The Role of Pillars in Small Underground Mines

2. FUNDAMENTAL DIFFERENCES BETWEEN LARGE SCALE AND LIMITED SCALE PILLAR MINING

The classical approach to the design of room and pillar workings is based on the safety factor concept and was proposed by Salamon in 19671 In this paper Salamon states:" The average pillar load is not easily predicted in the general case. If the area is not large or if it contains large intact portions of the seam, pressure p depends on many factors. Its maximum value can, however, be deduced in a simple manner, provided the pillars are reasonably uniform. Assuming that the whole weight of the overburden is carried by the pillars and assuming that the weight increases 2 500 kg/m2 for every metre of the depth*, then:

/?=(0.025H)/(l-e) (1)

where H Depth in m e Extraction ratio (lOOe = percentage extraction) p Pillar stress in MPa.

In the case of small alpine mineral deposits the difficulty mentioned by Salamon applies and the so called "tributary area" concept can not be used to estimate pillar load or pillar stress. This makes it extremely difficult to estimate the load acting on individual pillars. The problem is aggravated further by the irregular topography. The second difficulty arises from the fact that, because of the limited size of deposit and the complex geology of many of the alpine mineral deposits, there are usually insufficient numbers of representative mining situations available which can be used to apply the concept of back analysis which formed the basis of the formula for the strength of coal pillars developed by Salamon and Munro in 19672

As a consequence of these difficulties the probabilistic approach to pillar system design based on the concept of back analysis as proposed by Salamon in 1967 is faced with considerable problems when applied to small alpine deposits as there is usually insufficient field data available to follow this route. As a result pillar system design in the majority of Austrian mines has been done predominantly on the basis of practical experiences and judgement. Because of the limited extent of mining pillar failures, when they did occur, were mainly of local rather then a regional or global nature. There were however some large scale collapses such as the failure a whole stoping area in the lead-zinc mine Bleiberg in the 1970's which is now closed.

" Units converted from imperial to metric units.

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The basic differences between pillar systems employed in extensive tabular mineral deposits such as for example South African or Australian coal mines or the chrome mines of the Bushveld igneous complex in Southern Africa and the pillar systems found in many of the alpine mines are summarised in Table 1:

Property/parameter Overseas situations Alpine situation Geological situation regular complex Depth shallow to medium variable over short

distances Dip of deposit flat to slightly inclined irregular faulting slight to moderate moderate to extensive Thickness of deposit narrow to medium medium to large Lateral extent of workings

large limited

1. table: Differences between overseas and alpine pillar mining situations

As a consequence of the differences detailed in Table 1 pillar design in alpine deposits tends to be unique in each case whereas overseas differences between conditions in the mines operating a particular type of deposit tend to be rather small. As a result experiences gained in one mine can be applied in other mines.

From a pillar systems point of view the main differences are summarised in Table 2:

System property Overseas situation Alpine situation Pillar load based on tributary area

concept very variable, generally less than tributary area load

Pillar material very uniform irregular and extensively jointed

Pillar height 1 m-5 m 2 m -80 m Width/height ratio of pillars

usually between 2 - 8 often


3. MAJOR DESIGN PROBLEMS IN ALPINE P ILLAR MIN ING SITUATIONS

3.1 Pillar strength

The foremost problem encountered in the design of pillar systems for alpine mining situations is the estimation of pillar strength. For reasons detailed above back analysis of existing pillar workings is usually not possible. Consequently pillar strength has to be estimated on the basis of laboratory tests on rock samples and rock mass classification. The latter is used to down-rate the laboratory strength values to allow for the effects of jointing on the strength of pillars. In recent years the "Geological Strength Index" (GSI) which was introduced by Hoek in 19946has been used in conjunction with the generalised Hoek-Brown rock mass strength criterion' to determine the strength of the pillar material. The effect of the width to height ratio on the strength of pillars with W/H>1 pillar strength has been estimated using Salamon's pillar strength formula.

where C rock mass is the strength of the rock mass according to Hoek and Brown'

For pillars with W/H

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The results of laboratory tests and numerical simulations show that for very slender pillars the strength can drop to zero if the pillars are intersected by steeply dipping discontinuities.

Figure 2 shows that regular jointing of pillars can be a common feature in alpine mines and needs to be taken into account in pillar design. In the case of one set of discontinuities the most effective means of dealing with this problem is to use elongated pillars instead of square pillars on to orient the pillars in such a way that their long axis is aligned in the dip direction of the joints8

Figure 2 shows the beneficial effects of aligning the long axis of a pillar in the direction of dip of the joints (Angle=0 in Figure 2)

Dip angb of th Joint []

Fig.2: Effect of pillar elongation and orientation on strength of jointed pillars


Fig.3: Effects of regular system of discontinuities on pillar behaviour in an alpine gypsum mine.

3.2 Pillar load

The estimation of pillar load in alpine mines is made difficult by the uneven topography, the complex tectonic situation and the irregular shape of the mineral deposits. It is only since the availability of powerful computers and stress analysis software that reasonable estimates of pillar loads can be made. Even now the lack of knowledge of the primitive stresses acting in the rock mass makes it extremely difficult to determine pillar loads accurately. As a result of this uncertainty high nominal factors of safety have to be employed to ensure safe mine layouts.

A particular problem is the estimation of pillar loads in multi-level pillar workings. Such situations are not uncommon and have caused numerous problems in the past, A particular problem has been that for a variety of reasons pillars in different mining horizons have not been superimposed. As a consequence very unfavourable loading conditions have developed and in a number of instances resulted in local collapses. Figure 4 shows a typical example of layout problems in multi-level pillar workings.

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Fig. 4: Extreme example of layout problems in multi-level pillar mining. The example is from a marl mine in Tyrolia.

Figure 5 shows the results of a numerical simulations of typical multi-level pillar mining situations found in Austrian mines.

-100 melcr

Geometry: pillar width 4m pillar height 4m

Fig.5: Interaction between pillars in adjacent mining horizons

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The numbers in the diagram are stress concentration factors.

From Figure 5 it is apparent that the pillar load and the stress distribution in the sill pillars is very much dependent on the local circumstances. As can be seen from the diagrams the thickness of sill pillar between the individual extraction horizons is a critical parameter as far as the interaction between pillars in individual mining horizons is concerned.

The parametric studies have by and large confirmed the recommendations concerning the design of multi-seam room and pillar operations in South African collieries made by Salamon and Oravecz in 19769

3.3 Pillar behaviour and backfill

A typical mining situation in Austria is the extraction of lens type mineral deposits. Representative for this type of deposit are the alpine magnesite deposits. Typically these deposits have dimensions of 150m to 200m in length, 100m to 150 m in width and 50m to 100m in height. These deposits are usually extracted in ascending slices using room and pillar methods and backfill. In the final stages of extraction the magnesite pillars reach a height of 50m to 100m.

The pillars are situated in backfill which acts as the working platform for the mining equipment and provides lateral stability and confinement to the very slender pillars which typically have cross sections of 7m by 7m to 7m by 14m.

The backfill itself is usually uncemented waste rock which is compacted by the action of the mining equipment operating on top of the fill. Relatively little is known about the strength of these very slender pillars and the load acting on them. Experiences however show that the workings are stable even during the final stages of extraction that is when the ultimate height of pillars is reached.

Figure 6 shows the effect of compaction of backfill material as a result of operating equipment on top of backfill on the load deformation behaviour of uncemented backfill.

Laboratory tests on model pillars show that uncemented backfill has very little effect on the strength of pillars but is very beneficial from the point of view of post-failure behaviour of the pillars. Backfill is particularly beneficial in the

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presence of jointing in the pillars. These findings have been confirmed by the results of numerical simulations. Figures 7 and 8 show the results of numerical simulations of the effect of cemented and uncemented backfill in the behaviour of jointed and unjointed gypsum pillars of width/height ratio of 1. These simulations confirm the results of studies by Galvin10 on the effects of ashfill on the behaviour of coal pillars.

J / / J ^ Con paction 1 y W h M l .od*r

0 3 6 9 12 19 18 21 24 27 30 33 38

Varticai Dalormation [ % ]

Effect of Compaction by Wheel Loader an Vertical Stress and Deformation of Uncemented Backfill (Laboratory Scale Tests)

Fig.6: Effect of equipment operation on the load deformation behaviour of uncemented backfill

Figures 7 and 8 show that the load bearing capacity of pillar systems is increased only if either cemented backfill is used or the workings are almost completely backfilled. In all other instances the main advantage of using backfill is the improved post-failure behaviour of the pillars.

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Vertical Da formation [V.]

Effect of Cemented and Uncemented Backfill at Fill Heights of 50% on Pillar Stress

Fig. 7: Effect of backfilling 50% of the height of the pillar workings on the load deformation behaviour of pillars

Vert ica l De fo rmat i on [ % ]

Fig.8: Effect of filling the pillar workings almost completely with backfill

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3.4 Stable or unstable pillar behaviour

A feature of pillar systems in many of the small alpine mineral deposits is that as a result of the very limited extent of the pillar workings and the comparatively large thickness of overburden pillar failure where it does occur tends to be stable rather then unstable. Figures 9 and 10 show typical cases of stable pillar failures. This type of failure tends to be the exception in pillar workings of substantial lateral extent.

Fig.9 and 10: Examples of a failed stope pillars

Unfortunately the potential dangers associated with this pillar loading situation are often not fully appreciated by mining personnel who consider the situation as perfectly stable and can not envisage the possibility of sudden collapses as a result of changes in the mining geometry. The latter situation can arise at the final stages of mining when the extraction of remnant pillars separating pillar areas of low safety factors is contemplated. In these instances the mining spans are suddenly increased and the stiffness of the mining layout is greatly reduced.

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Typical examples are mines where the main development is situated in the central part of the mineral deposit. In the final stages of mining the pillars protecting the main development are often the only remaining mineral reserves and there is a considerable temptation to extract these pillars. Figure 11 gives an idealised view of this generic problem.

4. CONCLUSIONS

Pillars play an important role in the extraction of small alpine mineral deposits. Because of the irregular nature of the majority of these deposits, the complex geology and the limited lateral extent of the pillar workings there has been no systematic approach to pillar design.

The latter has been empirical in nature and not based on sound engineering principles. As a result of the limited extent of the pillar workings and hence the high stiffness of the pillar layouts pillar failures where they did occur have been usually of a stable nature thereby creating a false sense of security. Whereas pillar design in mines of significant lateral extent is based on the concept of tributary area pillar load this concept is not being applied to any significant extent in small alpine mines. As a result of the sub-critical mining spans pillars are subjected to only a fraction of the tributary area load. Pillar dimensions are often constant and independent of the depth of cover. Furthermore because of the generally low pillar

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loads and the often considerable thickness of the mineral deposits support pillars tend to have very low width to height ratios. Frequently these are well below one. As a result discontinuities in the rock mass play an important role in pillar design and the layout of pillar systems. Because of the considerable thickness of many of the deposits multi-level pillar mining and backfill play an important role in pillar mining. Until fairly recently the interaction of room and pillar workings in adjacent mining horizons has been ignored.

The role of backfill has been largely to provide a working platform for mining personnel and mining equipment and to a lesser degree to enhance the stability of the pillar workings. Lately the emphasis is changing and pillar stability is becoming a prominent feature of backfill systems.

The pioneering work by Salamon in the field of the design of pillar systems and the understanding of the conditions governing stability and instability of pillar workings has been of great value in the assessment of pillar systems in small alpine mines. This knowledge has led to a reassessment of pillar systems in many of the operating mines and in instances has resulted in substantial changes to the pillar layouts.

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References:

[1] Salamon, M.D.G. (1967) "A method of designing bord and pillar workings" J. S. Afr. Inst. Min. Metall., v. 68, pp. 68-78.

[2] Salamon, M.D.G. and Munro, A.H. (1967) "A study of the strength of coal pillars". J. S.Afr. Inst. Min. Metall. V 68 , pp. 55-67.

[3] Salamon, M.D.G., Galvin, J.M., Hocking, G. and Anderson, I. (1996 "Coal pillar strength from back -calculation" Res. Rep. 1/96, Department of Mining Engineering, Univ. New South Wales, 1996.

[4] Salamon, M.D.G. (1970) "Stability, instability and the design of pillar workings". Int. J. Rock Mech. Min. Sei., v. 7, pp. 613-631.

[5] Salamon, M.D.G. and Wagner, H. (1979) "Role of stabilising pillars in the amelioration of rockburst hazard in deep mines" Proc. 4th Congr., Int. Soc. Rock Mech., v. 2, pp.561-566, Balkema, Rotterdam, 1979.

[6] Hoek, E. ( 1994) "Strength of rock and rock masses" ISRM News Journal v. 2(2), pp. 4-16

[7] Hoek, E. and Brown, E.T. (1997) "Practical estimates of rock mass strength" Int. J. rock Mech. Min. Sei. v. 34, pp.1165-1186.

[8] Salamon, M.D.G. and Oravecz, K.I. (1976) "Rock mechanics in coal mining" Chamber of Mines of South Africa, Johannesburg, 1976.

[9] Galvin, J.M. and Wagner, H. (1982) "Use of ash to improve strata control in bord and pillar workings" Proc. Strata Mechanics, Newcastle-upon-Tyne, April 9-13, 1982

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sszefoglals

Az Alpokban egyre kevesebb az svnyi anyag lelhely, s azok geolgiai viszonyai is egyre bonyolultabbak. Trtneti okokbl a pillreket inkbb tapasztalatokra hagyatkozva, mintsem elmletileg megalapozva terveztk. Ez lnyegesen eltr attl a fejlett mdszertl, ami kiterjedt telepes svnyi lelhelyekre Dr. Salamon ttr munkja eredmnyeknt a rendelkezsnkre ll.

A tanulmny az Alpok kis bnyinak, illetve a kiteijedt telepes lelhelyek pillr mretezsi mdszerei kzti lnyeges eltrseket ismerteti. ltalnos tervezsi mdszerek nincsenek, minden esetben figyelembe veszik a mltbeli tapasztalatokat. Klns sly problma a pillrek szilrdsgi alapon val mretezse, nhny pldt bemutat a szerz e kutatsi terlet eredmnyeibl.

A diszkontinuitsok, amint arra az rs rmutat, fontos szerepet jtszanak a pillrek llkonysgban, s abban, hogy milyen elrendezs szerint kell visszahagyni azokat. Megmutatja a tmedkels szerept s a tmedknek a pillrek viselkedsre gyakorolt hatst is. Az Alpokban mkd pillrfejtsek kis horizontlis kiteijedsek, ezrt a pillrek ltalban llkonyak, de a bnyk bezrst meglelzen elfordulhatnak stabilitsi problmk. Az elfordulsok gyakran jelentkeny vertiklis kiterjedse tbbszeletes pillrfejtsek kialakulshoz vezetett. Ezrt a szomszdos szintek hatst gyakorolnak egymsra.

A tanulmny az ebbl ered nehzsgeket is trgyalja, s rmutat annak fontossgra, hogy az egyes szintek pillrei pontosan egyms fltt helyezkedjenek el. A cikket ltalnos kvetkeztets zrja.

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role of pillars in small ug mine

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