University of WollongongResearch Online

Underground Coal Operators' Conference Faculty of Engineering

1998

Dragline Digging Methods in Austra.lian StripMines - A SurveyH. MirabedinyUniversity of Wollongong

E. Y. BaafiUniversity of Wollongong

Research Online is the open access institutional repository for theUniversity of Wollongong. For further information contact ManagerRepository Services: morgan@uow.edu.au.

Publication DetailsThis conference paper was originally published as Mirabediny, H and Baafi, EY, Dragline Digging Methods in Austra.lian Strip Mines -A Survey, in Aziz, N (ed), Coal 1998: Coal Operators' Conference, University of Wollongong & the Australasian Institute of Miningand Metallurgy, 1998, 313-324.

Dragline Digging Methods in Austra.lian Strip Mines -A

SuJ.vey

H Mirabedinyl and E y Baafi1

ABSTRACT

Open cut mining in Australia is facing the greatest challenge in its hilstory in attempting to compete not only with otheroperations internationally, but also with underground operations domestically. Most flat dip and shallow depth surface-mineable coal reserves have been depleted during the last two decadc:s and new open cut operations must extract deepercoal deposits. As open cut coal mines move into deeper areas andl the stripping ratios increase, the relative cost ofoverburden removal also increases. It therefore becomes even more important to design the mine around the optimumoverburden removal scheme. The deeper mines are usually multi-sc:am operations with a more complex geology andwith more geotechnical and hydrological problems. Deeper mines are subjected to greater problems requiring moredetailed mine planning and design, such as selection of the optimUJm mining method and pit layout. In planning anddesign of such operations, the number of alternative methods which nl~ed to be considered is consequently greater.

Dragline productivity and its stripping capabilities are directly affect,~d by the selection of digging method, strip layoutand pit geometry .Every mine has a unique combination of geological conditions. The operating methods that work wellat one mine may not necessarily work at another site. Selection of an optimal stripping method, strip layout and pitgeometry for a given dragline must be considered with respect to the geological conditions of the mine. With increasinggeological complexity of Australian strip mines, it is becoming more: important to use sophisticated techniques such ascomputerised mine planning methods to assist in optimising dragline operations.

THE PROBLEl'rf

In the past twenty years the walking dragline has emerged as the dominant overburden removal machine in surface coalmining operations in Australia. There are now over 60 large walkiI1lg draglines operating in Australian open cut coalmines (Aspinal, 1992). Four new units were expected to be ordered in 1996 and possibly another four units in the nextfive years. The book value of these new draglines is about A$800 million (Hamilton, 1996). In NSW, there has been asignificant growth in using dragline operations compared with otJler mining methods since 1980 (Fig 1). Unlikeunderground mining, the productivity of Australian open cut coal mining has been disappointingly static during the lasttwo decades with the annual raw coal output per man employed remaining the same as it was in 1970nl (Wentworth,1988). Although there are several reasons for this steady status, the major factor is due to insufficient technicalimprovement in mining methods as the geological conditions become more complex.

Overburden depths at many mines have already reached depths whic]l1 draglines alone cannot handle without additionalpre-stripping equipment. Many Australian mining companies are currently faced with the decision either to continuestripping to increasing depths or to commence underground mining operations. These specific conditions require anextensive analysis of each dragline's working method to establish:

.the operating limits for the machine;

.the productivity during chop cut and rehandling operations; and

.the efficient sequences of different mining activities.

Department of Civil, Mining and Environmental Engineering., University of Wollongong, NSW, Australia

COAL98 Conference WoUongong 18- 20 February 1998 313

Fig. 1- Comparison of coal production by principal mining method in NSWz

A review of several case studies of stripping operations by Atkinson et al (1985) clearly indicated that the strippingcapabilities of the draglines used in Australian open cut coal mines were not fully utilised, resulting in low operatingefficiency. There are several ways to increase the efficiency of overburden removal operations, such as improved designof dragline components. However, dragline productivity improvement through the modification of the digging method isthe most cost effective and usually the most efficient mean!; (Pippenger, 1995). The feasibility of significantimprovement in dragline performance (up to 20%) through modifications to the digging method has been reported byseveral mines. The idea of modifying the digging method becomes increasingly more attractive as stripping ratio

increases during mine life, particularly in multi-seam operations.

In order to improve the efficiency of a dragline operation it is necessary to have a thorough understanding of thecharacteristics of the digging method and the sequencing of the excavation operations. There is no comprehensive studyevaluating the various digging methods currently in use by Australian open cut coal mines. Very limited information canbe found describing innovative digging methods and most of them are internal and confidential mine reports. Most of theavailable literature describe basic dragline digging methods applied to the US coal fields. Australian dragline minesgenerally have greater overburden and to some degree have more c;omplex geological conditions than US and Europeanstrip mines. Small draglines are rarely used and no tandem dragline operation currently exists in Australia. ManyAustralian dragline operations are using innovative digging methods to cope with these more difficult geologicalconditions and to increase dragline capabilities such as maximum reac;h and dump height. Because of the deeperoverburden, most Australian strip mines have wider pits, typically 60-80m versus 40-50m pit width overseas, to reduceoverall rehandle, dragline walking time and avoid both spoil and highwall failures.

A study was conducted to highlight the current status of the use of dragline in Australian coal mining. As the fIrst step aquestionnaire was prepared and sent to twenty eight open cut coal mines with a total of about sixty large walkingdraglines as the major overburden removal units. The questionnaire: sought information about general geology of the coaldeposit, the mine's dragline(s) and other major equipment specifications and details of the pit geometry with a particularreference to the dragline digging methods. A number of site visits was also undertaken to directly observe and evaluate

current dragline operations.

Of the twenty eight mines. twenty one mines (75% ). covering fifty one dragline operations responded to thequestionnaire. One mine has stopped using its dragline. The remaining 25% did not respond because of either lack of

314

2 NSW Coal Industry Profile, 1996

COAL98 Conference WoUongong 18- 20 February 1998

operational data or the company did not have personnel available to !~ather the requested data. The information providedby the mines was classified according to the mine geology. The deurils included number of dragline passes, number oflifts per pass, dragline positions, whether or not a throw blasting tec:hnique is used, and cut and spoil procedures. Theresults of the questionnaire have been summarised in Table I.

RESULTS

Various sizes of draglines are in use in Australian mines. The bucket size of the current draglines varies over a widerange of 12 to 103 mJ. Normally smaller draglines are used to remove the shallow depth interburdens (less than 30 m).Most of the recently ordered draglines or those under contract have larger stripping capacities when compared with theold generation of draglines (Seib and Carr, 1990). The dominant form of dragline ten years ago was a medium sizedragline such as BE l370W or Marion 8050 with bucket capacit)r around 47 mJ (Atkinson et al, 1985). The newgeneration of draglines in Australian mines have an average bucket c:apacity around 75 mJ. Contributing factors towardthe very large draglines are the increasing overburden depths, the need to increase stripping capacity of the mine toreduce unit stripping cost, and advances in dragline manufacturing te,chnology. Fig 2 shows the changes in dragline sizeand its stripping capability during the last two decades.

Ideally the digging method which results in the highest coal exposurl~ rate should be adopted for a particular operation.The choice of strip geometry is mainly governed by the selected stripping method and the size of dragline. Seven diggingmethods were identified to be representative of most of the Australian dragline operations. The common strippingmethods were:

simple side cast;

standard extended bench with an advance bench;

split bench (deep stripping);

.chop cut in-pit bench

.extended key cut;

single highwall and double lowwall multi-pass; and

.double highwall and single lowwall multi-pass.

In the last ten years as shown in Fig 3 there has been a significant tendency towards digging techniques with higherproductivity such as extended key cut and in-pit bench methods. There are a variety of reasons for modifications to theconventional techniques, including:

.changes in geology such as significant increases in overburden depths;

introduction of more efficient cast blasting techniques;

.development of multi-seam operations; and

.requirement for closer control on production costs.

COAL98 Conference WoUongong 18- 20 February 1998 315

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Fig. 3- Changes in dragline digging methods used by Australian open cut coal mines over the last decade

The selection of the best digging method depends on a combination of geological conditions, dragline size andcharacteristics, and management planning targets. The nature of the coal deposit and geological conditions such as thenumber of seams, overburdenlinterburden thickness and coal thickness are among the most important factors governingthe choice of a digging method. Other factors such as geotechnical conditions, spoil stability , blasting techniques, materialstrengths and engineering and operator's experience are also important in the selection of a digging method. Thecombination of various factors results in a wide variety of methods at strip mines. Shared experience among different sitesof a company owning various draglines is an important factor in the selection of a digging method. For example, BHP-Utah Coal Limited (BUCL) operates 35 draglines of varying sizes across the Bowen Basin of Central Queensland (Hill,1989). The four common methods used by the BUCL group are:

COAL98 Conference Wollongong 18- 20 February 1998 321

standard extended bridge;I.

deep prestrip (split bench);2.

extended key cut; and3.

in-pit bench.4.

The stripping operations commenced with box-cuts on the shallow area at depths of 15 to 25m. The depths have increasedover the years and average overburden depths now are around 50 to 55m in single seam operations. In many casesadditional waste stripping is occurring ahead of dragline operation. In some instances, draglines are being used to digdepths as much as 70 metres.

Unlike overburden depth which is mainly governed by the geology, strip width is an operating factor which can be variedwithin a practical range. Variations in strip width affects dragline productivity. Pit geometry, especially the strip width,must be evaluated in conjunction with the digging method adopted by the mine. Wide strips (greater than 60m) are morepreferable for methods such as the standard extended bench method due to the reductions in rehandle, while narrower pitsare more productive for methods using a cast blasting technique, such as extended key cut or in-pit bench method. Thestrip widths currently employed ranged from 40 to 90 metres with an :average of 60 to 70 metres.

Computer simulation and digging method selection

Draglines move waste at the lowest cost per unit volume only when 'they work within their normal range. Both efficiencyand productivity of a given dragline drop off dramatically with changes in its effective operational factors. To improve theperformance of a dragline, its mode of operation and influencing parameters must be fully understood and analysed.Finding the normal working ranges for a given dragline and optimising its operation requires repetitive arithmetic andanalytic solutions. This problem is ideally suited to the application of computer aided simulation methods. Better mineplanning and mining method selection through computer simulation has been successful in many cases and this has beenstrongly recommended for Australian operations (Atkinson et al, 1985; Hill, 1989; Wentworth, 1988; Aspinal, 1992;Sengstock, 1992). A computer simulation model which can simulate different mining methods (particularly the innovativeones) is a useful means for selection of the optimum dragline digging method for a given geology.

Computer simulation of dragline operation has the potential for rapid, low cost analysis of different mining scenarios.Simulation of the dragline operation enables an operator to test the logic of how the machine should be used, and thedesign of optimum operating methods for the varying mining conditions. Such an application may also be used as atraining simulator or to evaluate dragline performance with a given set of geological and operational conditions. Computersimulation can also be used for evaluating proposals for modifications to existing operations and is also useful incomparing the performance of different types of new draglines which are being considered for purchase (Hill, 1989).

Due to the variety of digging methods currently used by open cut mines, a more general approach was necessary forsimulation rather than using standard digging methods such as extended bridge. As a result of this study, a draglinesimulation model has been developed which can be used in evaluation and optimising different dragline operations. Ahighly flexible simulation language "DSLX" was used to program different dragline digging scenarios in the model. Suchan approach provided a library of various dragline digging techniques. The results from the simulation stage are thenaggregated with time study data to estimate productivity and costs of the operation. The final decision then can be madebased on either the highest production rate or the lowest unit cost from various digging techniques. An example of such acomparison for a single seam dragline operation is shown in Fig 4. The process of the modelling and results of differentcase studies have been discussed in detail in previous papers (Baafi, Mirabediny and Whitchurch, 1995; 1997).

322COAL98 Conference WoUongong 18- 20 February 1998

Fig. 4 -Unit cost for various digging methodls with their components

CONCLUDING REMARKS

Productivity and efficiency of walking draglines can be improved by modifications in dragline digging methods. To selectthe most suitable digging method and working parameters for dragline operations, the first step is to analyse characteristicsof various digging methods. A survey was conducted with the objective of evaluating the effects of various diggingmethods currently used by Australian dragline operations. The survey was conducted through sending a questionnaire totwenty eight mines covering more than sixty dragline operations. The questionnaire sought infonnation about the generalgeology, major equipment specifications, digging methods and pit geometry .The surveyed showed that with the naturalincrease in overburden depths and complex geology, most strip mines have introduced various innovative dragline diggingmethods and larger draglines.

All the possible options can then be tested on a specific set of geological and mining conditions via a computer simulationmodel. Such an approach has been developed and applied to several j\ustralian open cut coal mines both in NSW and

QLD.

ACKNOWLEDGMENT

The authors are grateful to ECS International Pty Ltd for assistance and support provided to this study. The strip mines inthe Hunter Valley (NSW) and the Bowen Basin (QLD) are acknowledged with thanks for permission to visit theiroperation and for providing data during the digging method survey.

REFEREN CES

Aspinal, T 0, 1992. Use of Draglines -Where to in the 21st Century, in Proceedings on Third Large Open Pit MiningConference, pp 25-32 (The Australasian Institute of Mining and Metallurgy, Mackay).

Atkinson, R G, Burdon, R, Frost, N, Godfrey, A and Reczek, C R, 1985. Technological Opportunities and ResearchPriorities, in Advanced Surface Mining Technology, Project report, pp 189-205 (The Warren Centre for AdvanceEngineering, The University of Sydney, Sydney).

Baafi, E Y, Mirabediny, H and Whitchurch, K, 1995. A Simulation Model for Selecting Suitable Digging Method for aDragline Operation, in 25th International Symposium on the Application of Computers and Operations Research inthe Mineral Industry, pp 345-348 (The Australasian Institute of Mining and Metallurgy, Brisbane).

COAL98 Conference Wollongong 18- 20 February 1998 ~?~

Baafi, E Y, Mirabediny, H and Whitchurch, K, 1997. Simulation of Dragline Operations, International Journal of Surface

Mining and Reclamation, No.11 (March),7-13.

Hamilton, J, 1996. Draglines Back in Town, The Australian MINER, (July), 21-26.

Hill, K I, 1989. A Review of BHP-Utah Coal Ltd Dragline Stripping Operations, in Proceedings on Second Large OpenPit Mining Conference, pp 295-308 (The Australasian Institute of Mining and Metallurgy, Victoria).

NSW Coal Industry Profile, 1996. Joint Coal Board Statistical Supplement, pp 4-6 (NSW Department of Miner~

Resources).

Pippenger, J G, 1995. Competing with the Big Boys: Productivity and Innovation at the Freedom Lignite Mine, MiningEngineering, (Society of Mining Engineers, AIME, New York), April, 333-336.

Seib, Wand Carr, G, 1990. Maximising Coal Recovery Using a Dragline at Callide Mine, in Open Cut Mining: CoalMarkets & Mine Planning; Rehabilitation & Recent Operational Practices, pp 1-13 (ACIRL 's Seminar Proceedings,

Moranbah).

Sengstock, G W 1992. Blast Profile Control for Dragline Productivity Optimisation, in Proceedings on Third Large OpenPit Mining Conference, pp 269-274, (The Australasian Institute of Mining and Metallurgy, Mackay).

Wentworth, D, 1988. The Production Cha11enge Open Cut Production, The Coal Journal, No 20,31-37.

COAL98 Conference Wollongong 18- 20 February 1998 324