International Journal of Rock Mechanics & Mining Sciences 42 (2005) 219–235 3D numerical modeling of longwall mining with top-coal caving N.E. Yasitli, B. Unver à Department of Mining Engineering, Hacettepe University, Beytepe 06532, Ankara, Turkey Accepted 8 August 2004 Available online 23 December 2004 Abstract There is a considerable amount of lignite reserve in the form of thick seams in Turkey. It is rather complicated to predict the characteristicsofstrataresponsetominingoperationinthickseams.However,acomprehensiveevaluationofgroundbehaviorisa prerequisite for maintaining efficient production, especially when the top-coal-caving method behind the face is applied. Top-coal cavingisthekeyfactoraffectingtheefficiencyofproductionatthick-coalseams.Duringproductionoftopcoalbycavingbehind thefacenotonlyasignificantamountofcoalislostinthegoafbutthecoaldrawnbymeansofcavingisdilutedconsiderablywith surrounding rock. Therefore, it is not possible to carry out an efficient production operation unless caving of top coal behind the faceisoptimized.Inthispaper,resultsof3Dmodelingofthetop-coal-cavingmechanismbyusingthefinitedifferencecodeFLAC 3D at the M3 longwall panel of the Omerler Underground Mine located at Tuncbilek (Turkey) are presented. According to the modeling results, maximum vertical abutment stresses were formed at a distance of 7m in front of the face. An analysis of the conditionsoftopcoalhasrevealedthata1.5mthicklayerofcoaljustabovetheshieldsupportsiswellfractured.However,a3.5m thicklayerofcoalabovethefracturedpartiseithernotfracturedorisfracturedintheformoflargeblocksleadingtoobstructionof windows of shields during coal drawing. It is concluded that, in order to decrease dilution and increase extraction ratio and efficiencyofoperation,topcoalshouldbeasuniformlyfracturedaspossible.Hence,anefficientandcontinuouscoalflowingbehind thefacecanbemaintained.Aspecialpre-fractureblastingstrategyjustsufficientenoughtoformcracksinthetopcoalissuggested by means of comparing with the results of numerical modeling. r 2004 Elsevier Ltd. All rights reserved. 1. Introduction Coal seams may be classified as thick, moderate and thin. Thick-coal seam categorization differs among countries, but a thickness of 4.8m is accepted as the lowerlimit [1].Thickseamscomprisehalfoftheworld’s coal reserve and 70–80% of thick-coal seams are produced by means of underground mining methods [2,3]. Production methods at the former Yugoslavia, Hungary,Poland,France,IndiaandChinaaregenerally similar; however, differences arise due to local condi- tions [4–8]. Thick-coal-seam mining is different from conven- tional single-slice coal mining in many aspects. Although there are operating longwall faces up to a heightof6minsomepartsoftheworld,inpracticeitis difficult to extract a slice of more than 4m. Depending onthelocalcharacteristicsofacoalseam,suchasseam, inclination, continuity, surrounding rock and coal seam mechanical properties and susceptibility of coal to spontaneous combustion, a face height of 2–2.5m is preferred. Production of thick seams having variable slope and thicknessmaypreventtheapplicationofslicing,hence,a face is located at the floor of the seam and the coal above the face is won by means of caving behind the face [7,8]. Production of thick seams by using top-coal caving is much simpler in comparison to slice mining and requires less development; consequently, the effi- ciency of production is significantly higher. The top- coal-caving method was first applied in the 1940s in Russia and then subsequently used in France, former ARTICLE IN PRESS www.elsevier.com/locate/ijrmms 1365-1609/$-see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2004.08.007 à Correspondingauthor.Tel.:+903122977600;fax:+903122992 155. E-mail address: unver@hacettepe.edu.tr (B. Unver).