composite failure mechanisms in coal measures’ rock masses ... · composite failure mechanisms in...

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Introduction

Excavated slopes in open pit coal mines aredesigned to be as steep as possible, consistentwith stability and safety requirements. Slopefailures occur for many reasons, includingoversteepening. This paper is concerned withslope design and excavation experience in theBowen Basin coalfields of central Queensland(Figure 1), but it deals with many issues thatare common to open pit coal mining generally.After more than three decades of operationalexperience and technological advances in theBowen Basin mines, sudden rock slope failuresstill occur in circumstances where personneland equipment are at extreme risk.

The circumstances of a selection of thesesudden failures are reviewed in this paper, andsome concerning trends emerge. Classicalstructurally controlled slope failures occurquite rarely in the Bowen Basin, but rock massstructures appear to exert important controlson the sudden failures that are more widelyexperienced. The term ‘composite’ is used in

this paper to describe failures involvingcombinations of intact rock material fractureand shear movement on defects (Baczynski,2000).

Geological conditions and mining practicesin the Bowen Basin are reviewed andexperiences from a sample of suddencomposite slope failures are described. Fromthis information, the authors have identifiedsome myths inherent in traditional slopedesign for sedimentary rock masses, as well asthe reality that will have to be betterrecognized to achieve improved design andsafety performance.

Geological and geotechnical conditionsThe Bowen Basin of eastern Australia (Figure1) contains up to 10 km of largely clasticsediment from terrestrial and shallow marineorigin along with substantial volumes ofeconomic coals. Basin sedimentation anddeformation commenced in the early Permianand was most significant through the latePermian and earliest Triassic, withdeformation events during the Triassic andCretaceous and concluding in the Eocene(Fielding et al., 1995; Korsch and Totterdell,1995). There are five distinct groups ofcommercially important coal measures faciesas indicated in Figure 2.

The German Creek Formation (GCM), theMoranbah Coal Measures (MCM) and theRangal Group Measures (RCM) host most ofthe open pit and underground miningoperations.

Basin sediments were sourced fromemergent volcanic uplands, so clasts aregenerally lithic and form clay-rich weatheringproducts. The basal unit of the GCM is marineand devoid of coal, and overlain by a deltaplain coal-bearing sequence of siltstones, somesandstones, and interbedded, often

Composite failure mechanisms in coalmeasures’ rock masses—myths andrealityby J.V. Simmons* and P.J. Simpson†

Synopsis

Composite rock mass failures involve intact rock material as well asfracture and shear on existing defects. Experiences from open-cutcoalmining in the Bowen Basin of central Queensland are thatfailures of this nature occur suddenly with little warning, and aremuch more common than classical structurallycontrolled failures.Geological conditions and mining practices in the Bowen Basin aredescribed. The circumstances of a sample of composite failures aredescribed in detail in the paper. From this experience it appears thatseveral aspects of conventional geotechnical investigation anddesign practice have not contributed significantly to eitherpredicting or controlling the risks associated with sudden failures.These practices are described as myths for reasons outlined in thepaper. On the other hand, the reality seems to be that high initialstresses and stress changes caused by excavation lead by a processof extensional deformation to a critical stress condition within therock mass, and that sudden collapse follows when that criticalstress condition can no longer be sustained. It is concluded thatlearnings gained from underground excavations in rock may bemore applicable to open pit excavations than previously thought,and that the processes of prefailure deformation require morecareful study if we are to develop more reliable risk managementcontrols.

* Sherwood Geotechnical and Research Services.† BMA Coal Pty Ltd Central Queensland Office© The South African Institute of Mining and

Metallurgy, 2006. SA ISSN 0038–223X/3.00 +0.00. This paper was first published at the SAIMMInternational Symposium, Stability of rock slopesin open pit mining and civil engineering situations,3–6 April 2006.

459The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER JULY 2006 ▲

Composite failure mechanisms in coal measures’ rock masses—myths and reality

carbonaceous, siltstone and sandstone with coals. The MCMcomprises alluvial plain sediments overlying a shallowmarine shelf, with sequences of thick sheets of lithicsandstone and interbedded siltstone-sandstone generallyoverlying carbonaceous siltstone and coal. The basin-wideRCM sequence comprises sandstones, siltstones, interbedded

sandstone-siltstone, carbonaceous siltstones, and coals, andis overlain by a non-coal-bearing alluvial sequence ofmassive sandstone and siltstone. Several tuffaceous markerbeds occur, some of which are lithified while others form thinbands of high plasticity clay. In most areas there is a Tertiaryunconformity overlain by sheets of clay-bound alluvial orlacustrine sediments, and in some areas there are basaltflows.

Seam structure dips are generally in the range of 3° to 7°except in areas affected by thrust faulting or fault drageffects. Normal lithological joints have formed in response toburial and uplift, generally forming two orthogonal sets tobedding structure. Successive thrusting and extensionalfaulting events have overprinted the rock fabric with shearsand joints sub-parallel or conjugate to faults. Coal-seams ortheir weaker floor and roof horizons, and tuff bands, havebeen loci for bedding-parallel shearing. Intraformationalshearing is common in areas close to significant faults.Many apparent high-angle normal or reverse faults showpredominantly strike-slip slickensiding. In general terms, thedefect fabric is blocky with variable persistence acrossbedding surfaces being a distinctive feature (Figure 3).

Deep weathering cycles occurred during Tertiary time,and modern drainage courses include alluvial sequences todepths of 20 m or more. Weathering alteration includesleaching, laterization, and transformation to clay. Depths ofdistinct weathering to complete transformation range from 15 m to 45 m. In some cases where mining horizons aredetermined by machine specifications, remnant weatheredhorizons can drastically affect mining productivities.

Bowen Basin coal measures’ rock material strengthsrange from extremely low (UCS <2 MPa) to high (20 to 60 MPa) and generally average 10 to 25 MPa. Typicallymaterial stiffnesses fall into the low to moderate modulusratio range. Rock mass shear strengths have typically beendetermined by ‘guesstimation’ from back analysis of failures,

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460 JULY 2006 VOLUME 106 NON-REFEREED PAPER The Journal of The South African Institute of Mining and Metallurgy

Figure 1—Location of Bowen Basin open pit coalmines in easternAustralia

Figure 2—Major coal-bearing facies groups of the Bowen Basin (afterCallcott et al., 1990)

Figure 3—Typical defect fabric for Bowen Basin highwall rock mass

or from ‘gut feel’ relative to the shear strengths of rockmaterials. The advent of the generalized Hoek-Browncriterion in the readily available freeware RocLab(RocScience, 2002) has provided a means for estimatingmass strength which, with care, appears to provide realisticresults.

Table I is a summary of typical geotechnical parametersapplicable to open pit mining within the Bowen Basin.Obviously individual parameters vary widely with circum-stance, and one of the great challenges of any geotechnicalinvestigation is to assemble an appropriate, site-specific setof parameters.

Mining practices and rock slope designUnderground and more dominantly large-scale open pitmining has been undertaken in the Bowen Basin since theearly 1970s. Relatively flat topography and seam structuredips resulted in early development of dragline stripping inlong pits of typically 50 to 65 m width. As strips haveprogressed deeper, truck-shovel prestripping has beenincreasingly introduced. Truck-shovel methods have alsobeen widely developed in areas not suited to draglineoperations. Cast blasting and dozer assistance have addedfurther economic efficiencies for stripping.

Virtually all open pit stripping is characterized by verylarge production blasts. Pre-split blasting as a final facecontrol is preferred, but cost and scheduling constraints havelead to final face formation by ‘mid-splitting’ or ‘double-stitching’, which generally result in much greater back-breakand blast damage. Many operational safety issues arise forpit-floor operations because of rock wall damage.

For all forms of stripping, excavated rock walls arerequired to be as steep as possible for operational efficiency.In particular, optimized dragline stripping results in longstretches of highwalls with heights related to the dragline dighorizon and rehandle parameters. When there are multipleseams, uncover passes and sequencing may vary. Draglinehighwalls heights are typically 45 to 55 m high, but canrange to 75 m. Truck-shovel highwalls may have unbenchedbatter heights ranging from 30 m to 100 m.

Highwall batters are typically excavated at slopes of 37°to 70° in weathered horizons and 63° to 90° within freshmaterials. Constraints such as tail-room for drilling off-vertical holes, methodology for multi-seam mining, andrehandle and horizon limitations for draglines, may affect the

locations and dimensions of benches. Individual batter slopesare typically 70° or 75°. Vertical batters are rarely employedbecause of adverse experience with faces steeper than theaverage lithological joint orientations.

Coal is mined with or without seam blasting using aloader or an excavator. The delay between stripping andmining may range from days to months. Coalminingactivities take place within a narrow space below spoil dumpslopes up to 200 m high and excavated rock slopes up to 150 m high. While working to standard operatingprocedures, it is difficult under these circumstances toidentify unfavourably orientated rock structures, and retreatoptions are very limited if a pit wall failure develops.

Design practices typically follow precedents. Stabilityanalyses are rarely performed for routine design, and moreoften performed for feasibility studies when, ironically,knowledge of actual performance is minimal. Stress-deformation analyses are very rarely undertaken, and areoften considered impractical. There is a tendency for slopes tobe flatter in lower strength rocks, but the basis is oftenempirical or ‘gut feel’ rather than based on analysis.

There is general recognition that rock material strength isnot likely to be a limitation for open pit stability for depthsless than 300 m, which includes virtually all open pits. Thisimplies that rock mass defect fabric provides structuralcontrol of slope stability. However defect orientation analysisusing stereonets is rarely undertaken and almost universallyignored at mine sites, except sometimes when consideringwall stability in relation to major faults where orientation andlocation are well defined in advance.

Against this background, coal measures’ rock massesthat have been subjected to significant tectonic disturbancewill inevitably provide ‘surprises’ in the form of unpredictedslope failures. Unfortunately, operational constraints usuallymean that back analysis and/or detailed structural mappingare not undertaken, with geotechnical efforts being concen-trated on risk management for recovery operations. There iseven a belief in some quarters that opinions are morevaluable than rigorous observation, analysis, and technicalreview.

Observations of composite rock slope failuremechanismsTwo classes of rock slope instability mechanisms are usuallyrecognized: structurally controlled and rock mass failure

Composite failure mechanisms in coal measures’ rock masses—myths and realityJournal

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461The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER JULY 2006 ▲

Table I

Typical Bowen Basin coal measures’ material, rock mass shear strength, and stiffness parameters

Material Input parameters GHB strength and stiffness parameters Mohr-Coulomb (see Note) parameters

UCS (MPa) GSI (see Note) mi (see Note) D (see Note) mb s a E (MPa) UTS (MPa) c’ (kPa) f’ (deg) r (t/m3)

Tertiary sediments 0.6 22 16 0.0 0.987 0.0020 0.538 155 0.0 57 16.1 2.10

Weathered permian 9.0 60 18 0.7 1.999 0.0030 0.503 3465 0.014 266 39.5 2.30overburden

Fresh permian 35.0 70 18 0.7 3.463 0.0129 0.501 12160 0.131 708 53.1 2.48overburden

Coal 4.0 35 35 1.0 0.337 0.00002 0.516 420 0.0 88 19.7 1.50

Typical weak floor 2.0 80 5 0.7 1.666 0.0551 0.501 5170 0.066 176 25.8 2.40

Note: Generalized Hoek-Brown (GHB) parameters are described in RocScience (2002)


(Wyllie and Mah, 2004). Structurally controlled mechanismsfollow the existing structures or defects within the rock mass,and involve kinematic constraints. Typically planar sliding,wedge sliding, and toppling modes are considered. For thesemechanisms it is necessary to have an adequate knowledgeof defect shear strength and water pressure conditions inorder to make sensible predictions about stability. Rock massmechanisms develop through a combination of defects andintact rock bridges where the scale of the mechanism is muchgreater than the lengths of mobilized defects and the widthsof mobilized rock bridges. Typically, rock mass shearstrengths and groundwater pressure conditions must beadequately known and an appropriate method of analysismust be employed, using the tools developed for soilmechanics.

Within the Bowen Basin open pit mines, classical planarand wedge failure mechanisms occur but are comparativelyrare. Toppling mechanisms occur, but only under relativelyunusual conditions because defects of the appropriateorientation and persistence are not typical. Planar and wedgefailures can be predicted if the controlling structures arerecognized in advance, and are usually manifested during thedrill and blast or excavation stages of the mining cycle.Based on the authors’ experience, it is more often the casethat planar or wedge mechanisms are predicted but do notoccur due to limitations in continuity and persistence of thecontrolling structures.

Composite failure mechanisms, on the other hand, areusually very difficult to predict from prior defect data. Theauthors’ experience, which includes consultations with a verywide cross-section of mine workers, is that most compositefailures involve some combination of:

➤ Opening of joints in a manner not obviously associatedwith structurally controlled movement

➤ Sliding along one or more unfavourably orientated andnot previously recognized defects, typically planar orwedge mode

➤ More often than not during the coolest time of day, orimmediately following a rapid change in temperature

➤ Sometimes within several hours to one or two daysfollowing significant rainfall, but at other timesfollowing an extended period of dry weather.

In the aftermath of a composite failure, it is often the casethat adverse defect orientations are ‘discovered’. Even whenthis is so, it is difficult to obtain sensible strengths from backanalysis due to the wide range of unknowns and thedifficulties of identifying and then modelling the mechanismof failure. Despite efforts to simulate variabilities of rockmass and rock material attributes (e.g. Baczynski, 2000), ithas so far been impractical to gather reliable information forverification.

A summary of information from several selectedexcavated rock wall failures from the Bowen Basin isprovided in Table II. The locations and ‘ownerships’ of thefailures have been omitted, in order to focus on the technicalattributes. In assembling this information, the authors haveconcentrated on failures that were unexpected, in the sensethat stability and rockfall risk management controls were inplace and believed to be adequate for the identified hazards.The information is provided under several headings, whichare explained as follows:

➤ Overburden characteristics—a brief summary ofgeological conditions is provided. Characterization ofbedding unit thickness has generally been omittedbecause it is so variable, but some description isprovided where it was considered significant to themanner of the failure.

➤ Mining status at time of failure—there is stronganecdotal evidence from mine workers that failurestend to occur most frequently at particular stages of themining cycle, chiefly during coal removal.

➤ Preknowledge—for each of the failures, there is asummary of any preknowledge relevant to the actualfailure. This is highly significant particularly for riskmanagement because monitoring and miningprocedures are much more highly controlled whenthere is some preknowledge that failure is more likelythat usual.

➤ Tectonic overprint—coal measures’ rocks typicallydisplay lithological joints and bedding structure due todepositional conditions and the consequences of lithifi-cation, burial, and uplift. If subjected to tectonicdisturbance, there will be an additional overprint ofdisturbance-related defects. While knowledge ofcausality may be very limited, it is relatively straight-forward with experience to recognize tectonicoverprinting, which provides additional defect fabricfor mobilization of composite mechanisms.

➤ Length of wall affected, drop zone runout, and slopeheight—these attributes provide dimensions to thefailure process, and illustrate the wide range ofmechanisms that are generated in mining operations.

➤ Other comment—this information is provided to givemore of a sense of context and history to the examples.Typically, post-failure observations are critical forunderstanding the nature of the failure process andpredicting the post-failure response of the slope.

From the information in Table II, it may be concludedthat:

➤ Failure was relatively rapid, often without warning,and involved tectonic overprint structures

➤ Stand-up time prior to failure was highly variable fromimmediate to months, but in all cases failure occurredwhen coalmining, or roof or floor treatment, wasoccurring

➤ No obvious relationships exist between wall height orscale of failure and lithological or tectonic conditions

➤ The presence or significance of tectonic structures wasoften not recognized prior to failure.

Figure 4 is a sketch interpretation of a typical compositefailure mechanism, based on the experiences described inTable II. Investigations of incidents typically find that therewas some knowledge of rock mass structure, but that it wasnot possible to use this information to explain why a failureoccurred in one section of a highwall but not elsewhere inequivalent geological conditions. Even when there was afinding that more geological structure data should becollected, there was no sense that the data could be used toreliably predict future area of failure or non-failure. Backanalyses of composite failures have been attempted, but it isusually found that the meaningful rock mass shear strength

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The Journal of The South African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER JULY 2006 463 ▲

Tab

le II

Cha

ract

irist

ics

of c

ompo

site

failu

res

in b

owen

bas

in c

oal m

easu

res

rock

slo

pes

(see

not

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brev

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▲


Tab

le II

(con

tinue

d)

Cha

ract

irist

ics

of c

ompo

site

failu

res

in b

owen

bas

in c

oal m

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res

rock

slo

pes

(see

not

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brev

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Paper


Tab

le II

(con

tinue

d)

Cha

ract

irist

ics

of c

ompo

site

failu

res

in b

owen

bas

in c

oal m

easu

res

rock

slo

pes

(see

not

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brev

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▲


Tab

le II

(con

tinue

d)

Cha

ract

irist

ics

of c

ompo

site

failu

res

in b

owen

bas

in c

oal m

easu

res

rock

slo

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not

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Paper


Tab

le II

(con

tinue

d)

Cha

ract

irist

ics

of c

ompo

site

failu

res

in b

owen

bas

in c

oal m

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parameters cannot be matched to failure conditions. In theauthors’ opinion, one consequence of recent increases inemphasis on safety management has been a focus onimprovements in highwall monitoring rather than ongeotechnical analysis for prediction of instability.

Myths

The authors have concluded from the above experiences thatthere are several myths about geotechnical engineering andcomposite failure mechanisms.

➤ Myth 1—Traditional geotechnical pastimes such asgathering of defect orientation data either from faceexposures (preferably) or drill-hole data are necessaryfor prediction of failure.Experience has been that the data collected is usuallyinadequate. Even for oriented core data, no informationcan be obtained on continuity and persistence ofindividual structures. At many mine sites, availablestructural data indicates that wall failures should behappening regularly in pits where there have neverbeen any significant failures. It is believed that themain reason for this is limited information onpersistence and block size, and no analytical tools forrealistically using such information.Structural analysis by stereonets is of limited relevanceto a real rock mass. Stereographic analysis assumesthat defects are in 3D positions where they can fullyinteract. There are no tools presently available to evenvisualize 3D defect interactions. Geostatistical tools forsimulating spatial variability cannot presently accountfor the obvious anisotropy of bedded rock masses.

➤ Myth 2—Traditional 2D or less common 3D limitequilibrium stability analyses can be used to obtainfactors of safety or probabilities of failure for reliableidentification of failure.Back analyses are generally unsatisfactory, andidentified shear strengths corresponding to failureconditions are usually unrealistic. Much of this may berelated to the influence of tectonic structures on the

rock mass shear strength, but the situation is probablymore complex. Composite failures seem to be triggeredusually when coal is being mined. Coal is very differentfrom non-coal lithologies in having a much finer-scalefabric. The fact that failure occurs rapidly suggestsrunaway instability, which is more typical of brittlefailure than of shearing. Experience suggests that ifbrittle failure is initiated, stress transfer must occurrapidly and the obvious locus for such transfer wouldbe rock bridges adjacent to tectonic structures.However, the stress state that is likely to develop at thehighwall toe involves shear stress concentrations, buttectonic structures control the direction in which stresstrajectories may develop.Stress-deformation analyses provide important insightsinto the behaviour of rock slopes caused by excavation.Within the Bowen Basin, virgin rock conditionstypically involve highly directional principal stresseswith horizontal to vertical stress ratios typically greaterthan 1.5. Under such conditions the stress relief causedby excavation causes extensional deformation of thehighwall that is much larger than rebound deformationof the pit floor.

➤ Myth 3—Traditional rock tests and defectmeasurements can be used to obtain representativerock mass shear strengths suitable for rock slopestability analysis.UCS tests are most widely used to characterize rockmaterial strength. Triaxial rock tests are less regularlyundertaken, but provide useful information ondeformability as well as on material strength. Directshear tests are sometimes carried out on defectsurfaces. With all testing techniques there are issues ofsampling and preservation of fragile structures, leadingto concerns of data biased towards survivable samples.Downhole geophysical techniques are widely used forgeological exploration, and reasonable correlations canbe obtained with geotechnical parameters.No matter what material data is collected, there aremany complexities involved in characterizing rockmasses from such limited data. To start with,assumptions are always made about the representa-tiveness of drill-hole data. The authors have found thatreliable rock mass shear strength parameters can beobtained using the GHB model and RocLab, but this ismore by a process of back analysis than by forwardprediction by choosing GSI from borehole or mappingdata. There are current challenges with the meaning ofGSI in relation to bedded coal measures’ rock masseswith tectonic overprinting.

➤ Myth 4—The extent of failure and limitations ofcracking can be predicted from observations of rockmass conditions.Many of the observed composite failures appear tooccur in corridors between faults. Fault surfacesconstrain the orientations of principal stresses, andprovide stress dislocations. It is common experience forunderground coalminers to report adverse stress, roofinstability, and floor instability conditions in corridorsbetween certain faults but not other parallel structures.

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Figure 4—Sketch interpretation of Bowen Basin composite failuremechanism

The concept of such stress corridors challengesgeotechnical analysis, and to date there are noprocedures for recognizing or designing for suchconditions other than impractically intensive investi-gations. Some of the composite failures described inTable II involved dislocation of a single rock block,which appeared to act as a keystone, in the sense thatits movement provided releases for many other blockspreviously restrained by highly stressed defects.There is, accordingly, a suspicion that zones of highstress play a role in the initiation of failure through abrittle fracture process. Whether or not this is accurate,there is some evidence for removal of key blocks(either by dislocation, shearing, or brittle fracture)acting as a release for structurally constrained rockblocks. Interactions of high stresses, brittle fracture,and block dislocation are not well understood in rockslopes, and possibly better understood in the context ofunderground openings.

RealityTraditional geotechnical pastimes such as gathering of defectorientation data are unlikely to make more than a marginalimprovement to the prediction of failure. It appears thatsudden brittle failure occurs near the highwall toe, wherestresses may already be concentrated in controlled directionsby tectonic structures. Traditional approaches to rock massshear strength are unlikely to be able to predict failure undersuch conditions.

Initial stress conditions, and unloading caused byexcavation, both contribute to high concentrations of stressmagnitude and direction at highwall toes. Extensionalstraining is known to be associated with onset of tensilefracture (Stacey, 1981) and also with the onset of brittlefracture (Martin et al., 1999). There are indications thatextension straining provides an early indication of processesleading to large-scale slope failure (Stacey et al., 2003) butfurther research is required to determine how to use suchdeformation information for operational risk managementpurposes.

The advent of high-precision slope movement monitoringusing the Groundprobe slope stability radar (SSR) hasenabled unprecedented evidence to be gathered on themagnitude and rate of movement of rock walls in response toexcavation. Typical extensional movements develop at analmost linear rate, but this rate tends to increase by a factorof two or three at the stage when coal is being mined withinabout 10 m of the highwall toeline. In part this can beexplained by the development of stress concentrations, butthere is no current explanation for the rate being so muchhigher for coalmining in comparison to the rates ofmovement when overburden is being stripped.

There is also no current explanation for why a highwallcan remain stable for long periods, only to fail suddenlywhen coal is being mined. SSR data has also shown suddenstep changes in movement of sections of rock walls wellabove seam level. At one of the sites described in Table II,step changes of wall displacement occurred at locationsoutside of the initial failure zone. These locations werechecked against a high precision stereo photo model of thehighwall and found to correspond to specific joints orbedding surfaces. The step movements were interpreted as

stress-induced slip leading to stress redistribution in thatsection of the face, and were followed by unconditional localface stability. Similar slip movements may develop in coal orimmediate roof strata, perhaps related to load transfer causedby local brittle failure where stability cannot be achieved.

Summary and conclusionsThis paper has presented information about composite failurein coal measure rock slopes. In the authors’ experience, thesefailures challenge conventional geotechnical approaches torock slope analysis and design. Consideration of what havebeen termed myths leads to a view of reality, which requiresa change of approach.

It is impractical to monitor every rock slope closely, butthe evidence suggests that better understanding will be onlydeveloped from monitoring of rock slopes to determine inmore detail where movements occur, and how these relate tothe prevailing stress conditions and potential brittle fractureprocesses. It appears that learnings gained from modellingand monitoring underground excavations in rock may bemore applicable to rock slope behaviour than has previouslybeen thought or practised.

AcknowledgementsThe authors acknowledge permission from BHPBillitonMitsubishi Alliance (BMA) Coal to publish the informationthat forms a substantial part of Table II. The BMA Coalinformation has been supplemented by information madeavailable for training purposes by other mining operators inthe Bowen Basin. Discussions with technical colleagues havebeen instrumental in forming the views presented in thispaper, but these views are the responsibility of the authorsalone and do not reflect in any way the positions of anyBowen Basin mine operators on rock slope stability design.

ReferencesBACZYNSKI, N.R.P. STEPSIM4 ‘step-path’ method for slope risks. Proceedings,

GeoEng2000, an International Conference on Geotechnical and GeologicalEngineering, Australian Geomechanics Society, Melbourne, 2000. CDpaper reference SNES0213, 6 pp.

CALLCOTT, T.G., CALLCOTT, R., and QUINN, G.W. Review of coal characteristics ofthe Bowen Basin. Proceedings of the Bowen Basin Symposium, J.W. Beetson (ed.), Geological Society of Australia Inc (QueenslandDivision), Mackay, 1990. pp. 47–53.

FIELDING, C.R., STEPHENS, C.J., KASSAN, J., and HOLCOMBE, R.J. Revised paleogeo-graphic maps for the Bowen Basin, Central Queensland. Proceedings ofthe Bowen Basin Symposium, I.L. Follington, J.W. Beetson and L.H.Hamilton (eds.), Bowen Basin Geologists Group and Geological Society ofAustralia Inc Coal Geology Group, Mackay, 1995. pp. 7–15.

KORSCH, R.J. and TOTTERDELL, J.M. Structural events and deformational styles inthe Bowen Basin. Proceedings of the Bowen Basin Symposium, I.L.Follington, J.W. Beetson and L.H. Hamilton (eds.), Bowen BasinGeologists Group and Geological Society of Australia Inc Coal GeologyGroup, Mackay, 1995. pp. 27–35.

MARTIN, C.D., KAISER, P.K., and MCREATH, D.R. Hoek-Brown parameters forpredicting the depth of brittle failure around tunnels. CanadianGeotechnical Journal, 1999, vol. 36, pp. 136–151.

ROCSCIENCE. RocLab software for determination of rock mass shear strengthusing the Generalised Hoek-Brown criterion, 2002. www.rocscience.com.

STACEY, T.R. A simple extension strain fracture criterion for fracture of brittlerock. International Journal of Rock Mechanics and Mining Sciences, 1981,vol. 18, pp. 469–474.

STACEY, T.R., TERBRUGGE, P.J., KEYTER, G.J., and XIANBIN, Y. Extension strain—anew concept in open pit slope stability and its use in the explanation oftwo slope failures. Proceedings, Fifth Large Open Pit Mining Conference,Australasian Institute of Mining and Metallurgy, Kalgoorlie, 2003. pp. 259–266.

WYLLIE, D.C. and MAH, C.W. Rock Slope Engineering Civil and Mining 4thEdition, 2004. Spon Press, New York. Updated version of Rock SlopeEngineering, E. Hoek and J.W.S Bray, originally published 1973. ◆


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