Analysis of Coal Surface Mining Equipment in Illinois

7/29/2019 Analysis of Coal Surface Mining Equipment in Illinois

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Analysis of coal surface mining equipment inIllinoisS.B. Bhagwat

Abs tract-E quipm ent capacities of a sample of Illinoissurface coal mines representing about 60% of surfaceproduction in the state were stud ied. Available equipmentcapacity w as compared with the required capaciry, as in-dicated by coal production and stripping ratios reported.Most mines sampled showed surplus equipment capacity,which if avoided, would result in substantial cost savings.The surplus capacity results mostly ffom market jluctua-tions that result in underutilization of equipment. Studyfindings underline the importance of market assessment atthe time of mine planning and of communication betweenmarketing and engineering arms of a coal firm.Introduction

In h e planning and economic analysis phase of surfacecoal mine development, the stripping ratio* plays adecisive role because equipment selection and costs ofcoal production are influenced by h e amount of earthmaterial that must be handled. Consequently, prospectivebuyers of coal or of a coal mine who are attempting todetermine the economic potential of a mine expect to gainsignificant insight by observing and comparing the strip-ping ratios. This approach, however, can sometimes bemisleading. A computerized model for cost estimation insurface mines by the Illinois State Geological Survey(ISGS) in 1983 (Bhagwat, 1982) indicated, for example,that for Illinois mines, a statistically significant correla-

0-10 15 20 25 30Stripping ratio

Fig. 1 - Cost of mining and cleaning coal vs. the strip-ping ratio*Stripping ratio indicates how much of soil and rock material must beremoved per unit of coal production. The ratio can be defined in variousways such as feet of overburden per foot of coal thickness or cubicyards of overburden removed per ton of coal or cubic meters of overbur-den per metric ton of coal. In this paper, it is defined as cubic meters ofloose overburden removed per metric ton of clean wa l.

tion between stripping ratios and overall cost of miningand cleaning coal could not be established (Fig. 1).Several reasons might account for h e lack of correlation.Part of this discrepancy could be explained by differencesin coal cleaning practices at different mines. This analysisof a sample of Illinois surface coal mines and the equip-ment used in these mines was conducted to shed greaterlight on the causes of h e lack of correlation between strip-ping ratios and cost of mining. To collect the necessary in-formation, we mailed a questionnaire to h e 20 surfacecoal mines operating in 1983. Fourteen mines respondedand 10 were selected for study; the remaining four wereomitted primarily because of their extreme low produc-tion in 1982.Th e sampled mines

The 10 sampled mines represent about 60% of surfacecoal production in Illinois and can be considered repre-sentative of the population of surface mines in Illinois onthe basis of their annual production distribution (Fig. 2).

SamoleAverage irnine

1 .O 2.0 3.0Annual coal production im~llionons)

Fig. 2 - ize distribution of mines in investigated sample(top) and in all Illinois surface mines (bottom)The average mine size in the sample is, however, some-what larger than the Illinois population average - 1.36 vs.1.13 Mtla.The sample also indicates industry preference for cer-tain sizes of equipments (Fig. 3) . The size distribution isS.B. Bhagwat, member SME, is head, Mineral Economics Sec-tion, I llinois Geological Survey, Champaign, IL. SME nonmeetingpaper 85-230. Manuscript October 1985. Discussion of this papermust be submitted, in duplicate, prior to Aug. 31. 1987.

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F r o r i t t n d L O ~ ~ P I S Id"1 r 1

B u c k e t s,zP i y d ' )H a u l a g e T r u c k s I , . /30 1 -

H a u l a g e ~ a p a c t y r u n s )

Fig. 3 - Equipment size and frequency distribution i nsampled minesmostly unimodal, indicating that despite the range ofequipment sizes available, there is a tendency for miningcompanies to prefer a particular size equipment. Thus,stripping shovels with a bucket size of about 50 m3 (65cu yd), loading ghovels of 8.5 m3 (11 cu yd), front-endloaders of 5.5 m (7 cu yd), and haulage trucks of 90- to110-t (100- to 120-st) capacity are most preferred. Bycomparison, the size distribution for draglines is bimodal,with modes located at about 7.5- and 84-m3 (10- and 110-

cu yd) bucket capacity because smaller draglines areoften used in reclamation. The dispersion around thesemost preferred sizes, however, is considerable, thus rais-ing questions about the propriety of using the mostpreferred equipment sizes for cost estimation models, asis often done. The sample does indicate that lllinois sur-face mines are among the users of the largest equipmentin the United States.In the following analysis, data from the sampled minesare used to investigate the relationship between availableand required equipment capacity of mines, to relate bothto the stripping ratio and to point out discrepancies thatmight exist.Methodology used

The methodology used to compare the available equip-ment capacity with needed equipment capacity of a mineis summ arized as follows:'The area of land disturbed annually was calculated,using the stripping ratios reported by the mining com-panies and actual annual coal production.The volume of topsoil to be removed and replacedby scrapers was determined, using the area of land dis-turbed annually. Topsoil and subsoil handling practicesdiffer considerably from mine to mine (Chugh and Hale,1982). Most mining companies commonly handle about30 cm (12 in.) of topsoil separately, but the amount ofsubsoil removed, stored separately, and replaced at dif-ferent mines varies greatly.The volume of overburden to be moved was es-timated on the basis of reported stripping ratios and an-nual coal production.'The volume of coal to be moved was calcula ted onthe basis of annual coal production figures.The capacity of mines to move topsoil, overburden,and coal and to haul the material was estimated on thebasis of the size and number of units of equipmentreported, theoretical bucket fill factors, availability ofequipment, number of shifts and days of operation, andcycle time of equipment.The cycle time for haulage trucks, a function of pitsize and shape, annual acreage disturbed, age of mine,and size of loading machines was estimated. The cycletime of stripping and loading machines was taken fromstandard literature references (Adler, 1970; NUS Corp.,1981; McLean Research Centre, 1980; Science Applica-tions, 1983).Thc haulage capacity was estimated on the basis ofnumber of haulage trucks and larger equipment, such asdraglines, shovels, and bucket wheel excavators, that ex-cavate and transport the overburden. 3Thc ratios of available equipm cnt capacity (m ort/a) LO thc required capacity werc calculated. These ratiosserved as thc basis for the analysis and discussion that fol-lows.

Results of investigationFigure 4 presents the results of this investigation. Theexcess cquipment capacity represented in Fig. 4 on a

mine-by-minc basis was calculated by dividing the avail-able capacity by the required capacity. Ideally, the equip-ment in use at the mines should be just large enough tohandle the amount of material that needs to be handled.

2 0 1 6 T r a n s a c t 1 o n s V o l 2 8 0 Soclety o l M l n l n g E n g ~ n e e r s f A lM E


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however, such an ideal match is impossible to achieve.The aim should, therefore, be to keep the equipmentcapacity margin as low as possible. The results of this in-vestigation show that 90% of the mines display con-siderable excess equipment capacity in at least one of thefunctional areas, many in the range of 100% to 350%.

E 200A r ~ t h r n e t i cm e a n------

Coa l

- . - . - . - .A . - . - . - - .-A = 1 s h ~ f t l d a y

Haulage

, O O jnn -

Fig. 4 - Estimated excess equipment capacities insampled minesScraping: About eight of the 10 mines studied havemore than adequate scraper capacity to handle all the top-soil. The arithmetic average of all sampled mines indi-cates a 70% excess scraper capacity. However, the avail-able data on amount of topsoil actually handled by mines

was incomplete. The calculations were based on the as-sumption that 30 cm (12 in.) of topsoil was handled.Therefore, the excess ratios could be due to the fact thatmines may be handling more than 30 cm (12 in.) of top-soil or that they may be using scrapers part of the time tohandle some subsoil. The estimates indicate that miningcompanies have the capacity to handle an average ofabout 20 cm (8 in.) more of top and/or subsoil than theyactually do handle. In one case, this excess capacity wasfound to be equivalent to more than 60 cm (24 in.) ofsoil. A partial explanation for the excess equipmentcapaciy existing today is the uncertainty about regulationsthat led mining companies to purchase equipment beyondtheir subsequent actual needs. In some cases, the use ofbucket wheel excavators in topsoil handling leads to over-capacity of scrapers, while in others the availability ofscrapers declines with age, resulting in an apparent over-

capacity. The age factor is generally valid for all miningequipment.Stripping: The average available equipment capacityfor stripping operations exceeds the needs by about 85%(six out of 10 mines have excess stripping equipmentcapacities ranging from 50% to 260%). One reasongenerally valid in the midwestem United States is theweather. Bad weather conditions force rehandling of

material, which requires equipment capacity that does notshow up in the calculations. Another obvious reason forexcess stripping capacity is the fact that coal mines in I1-linois have been operating at 70% to 80% of capacity,which would explain a 25% to 40% overcapacity. In thisregard, it is important to recognize the significance ofdemand forecasts at the time of mine planning. Any erroror uncertainty in projecting demand must result in excessor undercapacity at a later date. Similarly, changes in en-vironmental regulations can lead to loss of contracts be-cause of coal quality and cause excess capacity.On the other hand, the estimatcs of actual strippingcapacities were based on the number of days the minesreported to be operating. The sampled mines averaged260 days per year of operation. Stripping, however, istypically designed to operate round-the-clock on all daysof the year. Therefore, the actual stripping capacity is like-ly to average about 40% higher than our calculations indi-cate. Thus, the results indicate the existence of significantexcess capacity in stripping operations.

Coal loading: Two estimates of average capacity ex-cesses were made for coal loading operations becauseseven mines reported two-shifts-per-day coal operationsand three reported single-shift operations. For the lattermines, two estimates were made based on one and twoshifts per day. On the basis of two-shifts-per-day opera-tions in all 10 mines, excess capacity in coal-loadingoperations ranged from 10% to 360%, averaging about210%. When the three mines reporting single coal shiftsper day were included as reported, the average excesscapacity declined to about 140%. The decline in capacityuse due to market conditions would explain only a smallportion of this excess capacity.Haulage: Calculation of available haulage capacity ismore complicated than other capacity calculations be-cause part of the haulage work is done by draglines andbucket wheel excavators and part by haulage trucks. Also,total needed haulage capacity must include coal transpor-tation but exclude topsoil operations. Very large strippingshovels could, under certain conditions, serve the haulagefunction by dumping the overburden into its final posi-tion. In most cases, however, stripping shovels are usedto load the haulage trucks. This investigation was, there-fore, conducted under the latter assumption. As a result,the overall haulage capacity is only about 40% higherthan needed and could be explained by the prevailingcapacity underutilization at most mines.Summary ofjndings: When the results shown in Fig. 4are considered collectively, they confirm that significant

excess material handling capacities do exist in overburdenand coal operations but that neither topsoil operations norhaulage operations have serious overcapacity problems.The lack of significant excess haulage capacity is a strongSociety of Mi n~ ng ngineers of AIME Transactions Vol. 280-2017


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indication that the overcapacity in stripping and coaloperations is genuine.Possible economic implications

Significant economic consequences could result fromcarrying an excess equipment capacity over a number ofyears. Most importantly, if the needed and actual equip-ment capacity can be closely matched, the size of theequipment and number of units purchased could bereduced. The implications of overcapacity can be iden-tified by looking at the prices of equipment as a functionof size. Figure 5 represents the 1980 prices of draglines,coal shovels, and haulage trucks (NUS Corp., 1981). Ifstripping capacity excesses can be reduced from 85% lo2096, the investments in draglines and shovels could besubstantially reduced.For example, a mining company using one dragline ofbucket size 84 m3 (1 10 cu yd) could reduce inveslmentsfrom about $28 million to about $20 million by substitut-ing a 57-m3 (75-cu yd) dragline. Stripping shovel pricesare similar to dragline prices, and substantial savingscould also be expected there. As Fig. 5 shows, savingsper unit of coal shovels or haulage trucks will be muchsmaller than per unit savings for draglines or strippingshovels. However, because mining companies usually buy

C o a l shove ls1 4 1

Fig. 5 - 1980 equipment prices by size (excludingtransportation and installation)

L-_ !5 1 0 1 5 2 0B u c k e t s i r e ( m 3 )

T r u c k s081

more coal shovels and trucks than draglines or stripping

0 7 -0 6 -

2 0 5 --E 0 4 -

shovels, appreciable reductions in investments could

. .a * .

result here too.To demonstrate the potential cost savings that couldresult from reduction in excess equipment capacity, threemines (having a small, medium, and large annual produc-tion) were selected, and their mining costs were calcu-lated using the cost estimation model referred to in Bhag-wat (1982) - with the excess equipment capacity, thenwith a reduction of the excess stripping and coal loadingcapacity to about 20% above the needed levels (Table 1).The potential cost savings ranged from $0.35 to $5.65/t(about $0.30 to $5.15 per st).

V, 0 3 ; . ..1 1

2 5 5 0 7 5 1 0 0 1 2 5 1 5 0C a p a c ~ t y t o n s)

Table 1 - enefits of Reducing ExcessEquipment Capacity to 20%Cost per ton

PresentAfter capacityadjustment

Savings per ton

Mine A Mine B Mine C

Excess stripping 80% 20% 130%capacity

Excessma1- 200% 80% 120%loading capacity

Concluding observationsIn surface coal mines, the stripping ratios are an im-portant indicator of mining costs because they show theamount of earth material that must be moved for everyton of coal mined. Often, however, lower stripping ratiosdo not necessarily translate into lower costs. This dis-crepancy could result from equipment overcapacity. Thisinvestigation was conducted on a sample of Illinois sur-face mines to determine if such overcapacity in fact ex-isted and to estimate its economic impact.Study results confirm the existence in the sampledmines of significant equipment overcapacity that may beaffecting m ining costs adversely.The widespread equipment overcapacity could be at-tributable to a number of factors: (1) expectations ofhigher future production, (2) inability to secure contractsto match capacity, (3) loss of contract due to changing en-vironmental regulations, (4) bargain buys of used (butlarger than needed) equipment, (5) equipment purchases

in reaction to reclamation laws that proved to be exces-sive in some cases, (6) mines yet to reach capacityproduction, (7) mines phasing out of production, and (8)inaccuracies in forecasting demand at the time of mineplanning.'These estimates of equipment overcapacity must betempered, however, by the recognition that investigationsbased on partly simulated cost models such as this cannotadequately account for differences in soil and overburdencharacteristics and imponderables such as labor-relatedstoppages , weathe r-related rehandling of m aterials, and in-creased maintenance due to old a ge of equipment.The economic consequences of a failure to matchequipment purchases with needed equipment capacitiescould be costly because of increased cost per ton in yearsto come. However, it would not always be feasible for ex-isting mines to make the required capacity adjustments.

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The results highlight the imporlancc of specific S U N ~ Y eprint 1983A. National Symposium on Surface M ining. Hydrology, Sedimentd-ogy and Reclamation. Lexington, KY, ec. 5-10.demand forecasts in mine planning for Ihe Chugh, Y.P., nd Hale, J.E., 1482, "Reclam ation equipment performa nm charaderim-economic well-being of the Company.. tics in the Illinois Coal Basin.' Emcdhgs. National Sywosiurn on Surface Mining,Hydrology. Sedimentdogy and R eclamation. Lexington.KY.Dec. 5-10, pp.581-586.References McLe an Research Centre. Inc.. 1980. "Developme nt of sur lam mines cost eslimatingequations.' McCean. VA. for USDO E. Septembe r. . .Adler. N.. 1970. "Anatyzing ex ca va lin a nd handling equipment; ~ ~ , b ~ i ~3, NUS Corp.. 1981. 'Surface c oal mining ms t models," - Vols. 3Virginia Polylechnic Institule. Research Dw.. February. an d 4. for EPRI. January.Scienm Applications Inc.. 1983, 'RAMC Surfa m mining cost equations development.'Bhagwar. S.B.. 1982. "Cost of surface mining coal in Illinois." Illinois State Geological Wayne. PA, for Energy Information Administration. September.

Investigation of the causes of roof falls in adeep underground coal mineW.H. Su and S.S. Peng

Abstract - Extensive investigation has been conductedto determine the causes of a series of roof falls occurringin an underground coal mine located in southern WestVirginia. Underground observation, along withlaboratory testings and finite element analyses, were con-ducted to search for possible mechanisms for the rooffalls. Results of the investigation have led to a better un-derstanding of the mechanisms involved in the roof falls .Necessary modifications of mine layout and roof supportplan a re proposed for improving safety and profitabilityat the mine.

IntroductionThis paper presents the results of an extensive inves-tigation into the causes of a series of roof falls in a room-and-pillar working located in West Virginia. These rooffalls often interrupted production and posed serioushazards to the miners. Characteristics of the roof failuresuggest that other mechanisms besides a local highhorizontal stress field may also be responsible. Extensiveunderground observations were conducted to gain furtherinsight into the problem. Geological structure of the minesite was carefully studied. Physical and mechanicalproperties of the coal and the roof and floor rocks were

obtained in the laboratory. Three-dimensional finite ele-ment analyses were employed to simulate the mine work-ings to search for possible failure mechanisms responsiblefor the roof falls. As a result of this investigation, pos-sible mining alternatives were recommended for improv-ing safety and profitability at the mine.

DIRECTIONOF MINING

Fig. 1- ine layoutMining operation and geology

The mine is surrounded by old workings on threesides. Periodic roof falls across the crosscuts occurred W.HmSu and S.smPeng, members SME, are the Departmentwhen the main was in the northeast (N of Mining Engineering, West Virginia University. Morgantown. WV.56"E) direction (Fig. 1). Another main (1 East) 76.2 SME preprint 84-433, SME-AIME Fall Meeting, Denver, CO, Oc-(250 ft) southeast of this main did not experience the tober 1984. Manuscript October 1984. Discussion of this papersame problem. A slope is used to reach the coal seam. must be submitted, in duplicate, prior to Aug. 31, 1987.

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Analysis of Coal Surface Mining Equipment in Illinois