Influence of Lithological Characters of Coal Bearing Formation on Stability of Roof of Coal Seams

Jum.2003Journal of China University of mining technologyVol 13 No. 1Influence of lithological Charactersof Coal bearing Formation on Stabilityof roof of coal seamsMENG Zhao- ping(孟召平), PENG Su-ping(彭苏萍),LⅠ Guo-qing(李国庆),HUANG Wei(黄为),LUJv芦俊), LEI Zhi-ymng(雷志勇)School of Resources and Safety Engineering, CUMT, Beijing 100083, ChinaAbstract: Lithology is one of the important factors influencing the stability of roof of coal seams. In order to investigatethis the phenomenon of underground pressure and distribution of pressure were studied by using the local observationand simulation test with similar materials. The observation results show that the distance of initial weighting andperiodic weighting of the mudstone roof is shorter than that of sandstone roofs. The sandstone roof with a high strengthhas a longer distance of initial weighting and periodic weighting the abutment stress on the working face is big and theheight of caving and fracture zone is high. The peak point of abutment stress in the sandstone roof is near to theworking face and the pressure bump is inclined to occur. The result is contrary to that in case of the mudstone roof witha low strength, While in the transition zone of nipped sandstone, roof rock-mass is broken and is poor in stabilitytherefore, it is difficult to hold the roofKey words: Coal bearing formation lithological characters roof stabilityCLC number: TD 82 Document code: A Article ID: 1006-1266 2003 1-0001-06including thickening, thinning or even pinching1 Introductionout laterally. Different depositional roof types willCoal-bearing formation or coal series is a suit of certainly affect the roof stability in some degree Icoal seams inter-layered with sedimentary rocks So the research on mechanics behavior and roofhich hmovement law of different types of lithologicalpaleogeographic environmentcharacters( lithofacies in coal mining is useful toconditions paleoclimate and paleobotany conditions the control of roof strata. The geological modelsare the main factors dominating the formation of coal with homogeneous continuous and equal thicknessseries.The formation of coal series begins with the of rock bed on which traditional theories offilling that happened at the edge of sedimentary underground strata and pressure controlwerebasin in wet climate conditions. The fillers areifferent from the realmainly composed of terrigenous sediment and coal conditions. With the development of sedimentologyeams, and the lithological characters mainly consist Elliott( 1974), Ferm( 1978), Horne, Ferm andof sandstone, siltstond clayHylbert 1977) and other scholars studied theby sedimentary environment during their formation, relation between sedimentation and roof stabilitythe lithological features of roof rock mass above the中国煤化工ced the theories ofoal seam and its thickness are variable andHCNMHdiscontinuous in the three-dimensional field.me domestic scholarsReceived date: 2002-11-15Foundation item: National Natural Science Foundation of China( No. 40172059)Biograpliy #*ZHaoping 1963-), male, from Miluo, Hunan province professor engaged in the research on mining geologyJournal of China University of mining technologyVol 13 No. 1began their studies with the sedimentary studied by means of local observation and simulationenvironment of coal-bearing formation. Thetest with similar material. Too obtainanalyzed the coal seam and the sedimentary parameters is very helpful for controlling the roofconditions under which the roof was formed. Based strata in coal miningon different sedimentary patterns they established a 2 Underground Phenomena of Roofs of Differentrelation between regional sedimentary pattern andLithological charactersty5 6]. The geomechanical modeling ofOn 1221 working face of Pansan mine fieldthe roof strata under discontinuous condition a the depth of coal seam 13-1 is 524 m and its averagegeological prediction techniques of the roof stability thickness is 3. 85 m. The immediate roof is awas discussed and the roof stability was successfullycompound roof composed of mudstone and coalforecasted 7 8l because of the lack of accurate dataseams. Its thickness ranges from 1. 0 m to 3. 5 mlithological characters of coal roofs and actuThe roof is brittle and easy to breakminIand the restriction of observationaccident often occurs. According to interpretation ofconditions there are some difficulties in the three-dimensional seismic data, the main roof ofcognition of roof stability. Simulation test withsandstone becomes thinned, but the mudstone in theimilar materials can obsne phenomenroof thickened along the mining direction. That iscan not be observed in the local and it can simulate the whole thickness of the sandstone decreases fromthe whole deformation and failure process of rocks in15-23 m to 5 m and finally thinning out On thecoal mining 9]. The distribution of undergroundcontrary the thickness of mudstone increases from 7pressure on working face and the roof stability of0-18mto25n(Fg.1)different lithological characters in coal mining is12E3 direction 1215Mudstone zoneSandstone zoneFig. 1 Cross section of 3D seismic interpretation of lithological characters ofthe 1221 working face in Pansan mine area, HuainanIn order to research the influence of lithological by the automatic pressure recorder on each line thecharacters on the distribution of underground chart of supporting resistance along the miningpressure, five surveying lines were set on the direction can be drawn to estimate the distance ofworking face and numbered 3 26# 49# ,72#, the main roof weighting( fig. 2 and Fig 3)93*. In each line there were two YTL-610 circleThe observation results show that in the roofdiagram instruments. They recorded the pressure around 26 on the working face 1221, the initialof front and back legs of psupport. weighting distance is about 41m the first perion terms of the original data, a chart was weighting distance is 18 m, and the second is 12.5drawn to show the distribution of setting load and msupporting resistance of the support and to display distH中国煤化工, the initial weightingCNMH Girst periodic weightingwhether pressure valve discharged the liquid distance is 19 m, and the second is 12 m otherapparently display the changes of roof and coal wall conditions were generally the same as above. Theand then estimate the periodic weighting distance of initial weighting distance is 41 m and themain rooofF A%ling to the original data recorded underground pressure lasts as long as about 24 hMENG Zhao ping et alInfluence of Lithological Characters of CoalThe underground pressure extends from the middle mudstone roof of coal seam 8 and 9 is 20-28 mto the two ends, and concentrates mainly on the and the periodic weighting distance is 10-15 middle part. When the underground pressure but in coal seam 5 and sandstone roof of coal seam 8occurs, the liquid is discharged obviously from the and 9, roof s initial weighting distance is 30-50 msafety valve and the side fall phenomena aggravate and the periodic weighting distance is 15-35 mand data recorded by the circle diagram instrument When the roof is extremely thick and stiffnessncreases rapidly. The maximum of the pressure especially the borderline between immediate roof andreaches the rated working resistance of the support. the main roof is not clear and have a similarThe periodic weighting distance changes with the lithological property the shock bump is easy toroof lithological property. The first periodic occur, such as on the 3652 andworkingweighting distance is 18-19m and the second is 12 faces. Through a further analysis of observation data12.5 m. The observation results show that the of the representative working faces in coal seam 12distance of roof periodic weighting in the sandstone seam 5, seam 8 and seam 9, it is proved that thezone is longer but shorter in the mudstone zone and distances of both initial weighting and periodicunderground pressure phenomena on working face is weighting of mudstone roof are obviously not asorded by circle great as that of sandstone roofdiagram instrument it can be seen that there are onlypartial side fall and roof fall phenomena within acontinuous 20 h. Fig. 2 and Fig. 3)distance to starting cul101723303743505763WD 41 mFPWD 19 m SPWD 12 m(Sandstone zone) (Sandstone zone)distance to starting cutWeighting distance/m04856647280Fig 3 Curve diagram of support resistanceIwD41 mFPWD 18m SPWD 12.5 min the 49th support(Sandstone zone)(Sandstone zone)(Mudstone zone)IWD. the initial weighting distanceFPWD, the first periodic weighting distanceFig. 2 Curve diagram of supporting resistanceSPWD. the second periodic weighting distancethe 26th supportIt is noteworthy that besides the lithologicalIWD. the initial weighting distanceFPWD. the first periodic weighting distanceproperty of the roof, the slice height of roof strataSPWD. the second periodic weighting distancealso influences on the roof stability significantlyAccording to the data of Tangshan Mine Generally the immediate roof is not thick enough toobserved for many years the underground pressurefill the gob area completely after breakage, solaw of the same coal seam varies in coal mining withUnderground pressure is obvious. But when thethe lithological property of its roof Table 1ediate mudstone roof is thick enough, theof is poor, and theunderground pressure would not be obtroof is broken away while mining such as the roof regard the roof strata as beams, the bending-of coal seam 12 and the partial mudstone roof of coal resistant ability ofhe overhangingdirectlseam 8 and 9, their weighting distances are shortand the periodic weighting is not obvious. Accordingthy中国煤化工 thickness, that is,theCNMH Gkely the immediate roofto some related statistics, the initial weightingis to be bent and deformed and the more stable itdistance of mudstone roof of coal seam 12 is 20-25 Sm, and the periodihting distance of it is 5-12m,thei数据 hting distance ofJournal of China University of mining technologyVol 13 No. 1Table 1 Observation data of underground pressure on working faceof roofs with different lithological propertiesCoalLithological HeightShearInitial Periodicproperty of property ofheightweighting weighting Approaching velocity ofdistance distance roof and floor/ mm h-I)Othersmmediate roofin roofsean/n2351 Gray mudstone Gray mudstone -610 2.0 19 9.6 Sudden change and with wicrange2453 Gray siltstone3112. 4 Great change and a wide rangeGray mid-fineSandstone-6502.14.5 14.3 Sudden change and a narrawange3652 Gray mid700Sudden change and a narraw ShockGravish brownrkLight gray2199.014-16 Maximum approachingmudstonefine sandstonevelocity of 6.05287N Grayish602.750.534elciMaximum approachingShock bumpmudstoneDark gray1227 Sapropegray500Little deformatio2221 sapropelicDark gray5.0No shock bump21Grayish-whitefine sandstone2.723.58-123 Modeling Analysis of Influence ofsecond layer of sandstone is stable abeLithology( lithofacies )on Roof Stability3. 6lm in the prototype which is 26. 05-29 87m3.1 Geologic modelawavfrom the 13-1 roof( Fig 4)In order to investigate the influence of 3.2 Analysis of the modelingsedimentation lithology on the roof stability simila3.2. 1 Failure laws of the roof rock masssimulation experiment is carried out. The physicalIn the experiment, the first layer of sandstonenodel of plane stress is designed with a dimension of in the roof is 1. 19 to 5.00 meters far away from the2. 5m X0.2m X1.8m. The real situation is in the 13-1 roof. And its thickness is unstable which isntral mining area of southern part of Xinji coal about 0-4. 66 m and pinches from the right side tomine, Huainan coalfield 8]. The main mining layer the middle of the prototype. The middle part of theis coal seam 13-1 of upper Shihezi group its average model is a transition zone which is characterized bydepth is 293 m and the workable thickness is 8.5 the pinch-out sandstone. When mining from left tom. The immediate roof is gray-black mudstone or right of the model the failure and the undergroundsandy mudstone with plentiful joints and fractures, pressure of roof rock mass is controlled significantlyblock-fractures structure. Its thickness varies by lithological characters of the transition zonelaterally in some areas even scoured away. And the where mining fractures are relatively matured thecoal seam contacts directly with sandstone. The roc中国煤化工 n caving and fracturemain roof is gray or gray-white fine or medium zonCNMHGquartz sandstone with plentiful beddings and itsIn the left part no first sandstone layer existsthickness is not steady. In the model the first layer the immediate roof is the thick mudstone and sandyof sandstone in the roof pinches out from right to mudstone with thimiddle oP ReR2Fodel. And the thickness of the breakage fill the gob immediately in the process ofMENG Zhao ping et alInfluence of Lithological Characters of Coalmining, so no obvious stress concentration and 3.2.2 Distribution of underground pressureunderground pressure pheIn order to determine the influence of the roofight part of the model because of the existence of lithology on the distribution of undergroundthe first sandstone, the periodic underground pressure several pressure sensors were set at the toppressure distance is 18.0 m. In the transition zone of the instable first layer of fine sandstone in the roofof nipped sandstone in the central section the rock mass in coal seam 13-1. The distributionperiodic weighting distance is 9 m. These laws character of underground pressure in the roof in coalaccord with the results of local observation perfectly. mining is shown in Fig. 5. It can be seen from FigMining direction5 that the point No. 1 is located in the roof rock2*-=--Fine sandstonmass without sandstone, the No. 3 located in thetransition zone of nipped fine sandstone and theNo 6 located in the fine sandstone roofSandy mudstoneNo 6Fig 4 A physical model of sedimentary facies variations6080100120Because of the difference in combinations ofDistance away from face s/medimentary rocks the failure characters of the roofFig 5 Distribution of underground pressure of roof strataare also different s. In the left part of the modelFrom Fig. 5 we can also know thatle leftfew sandstone layers result in a small offset angle of half of the model, the first sandstone layer isthe faults, from 50. 5 to 66.0 degrees. While in the missing, with the thick mudstone and sandyright part, there are many sandstone framework mudstone containing coal seam being its immediatelayers so the offset angle is big from 66.0 to 78.0 roof. The roof fall at any moment to fill the goaf anddegrees. The caving and breaking angle is 66.0 bear underground pressure resulting in no obviousdegrees above the starting cut and 68.0 degrees pressure concentration and periodical weighting. Soabove the stopping line. Below the main framework in front of working face the abutment pressurelayer in the left part of the model, because of the roof rock mass is relatively small While in theweak lithological characttransition zone of nipped sandstone, the miningcontinuously and quickly, leading to a large scale of fraell developed and increased relativeldeformation vertically. However, because of the under the action of abutment stress. The thin finecontinuous filling up of the mined-out area, the sandstone together with the mudstone layer belowdevelopment of the caving and fracture zone is may fall down during the coal mining and therestrained, so the caving and fracture zone is often periodical weighting distance is short. So the rooflow, about 73.5 m. While in the right part of the abutment pressure ahead of the working face is themodel, there are several sandstone framework layers smallest and the peak value position of maximumin the roof and the roof breaks discontinuously soabutment stress is about 17. 25 m away from thethe rock mass above he mining room is usually working face. In right part of the model, because ofinclined to have a tensile and shear strainnterlayer-slidingng along thedish中国煤化工 e periodical weightingCNMHG the roof abutmentdepositional structure faces which are called the pressure ahead of the working face is relatively theseparation layer failure. At the same time, themaximum and the peak value position of maximumheight of the caving and fracture zone is quite high abutment stress is about 12. 75 m away骤!us8g)worKing laceJournal of China University of mining technologyVol 13 No. 1than that of sandstone. In the case of one or more4 Conclusionlayers of framework sandstone the distance of initial1 ) Liothology is one of the important factors and periodic weighting is longer, and caving anddetermining the roof stability. Because of the variety fracture zone is quite highof lithological property the roof has different rock3) Because of the diversity of lithologicalmechanics characters which may result in different property and sedimentary combination of the roofunderground pressure and stability. Controlled by the abutment pressure and the peak position ahead ofdepositional environment the combined structure of the working face varies obviously. In the frameworkroof rocks behave as vertical cyclicality thickening sandstone roof with a high strength the abutmentor thinning up to pinch-out discontinuously pressure ahead of the working face is relatively highlaterally the stability of roof strata significantly the peak value point of abutment pressure is nearlydepends on the sedimentary facies. Certainly the the same as that of the working face. So the shockheight of the caving and fracture zone the distance bump is inclined to occur. But in the mudstone roofof main roof breaking, and other dynamical with a low strength especially in the thinning-outphenomena are closely related to the sedimentary transition zone of sandstone roof the roof can notresist the pressure of overlying strata as the2) In coal mining, both initial and periodic framework sandstone layer doesweighting distances of mudstone roof are shorterReferences[1 Elliott R e. The mine geologist and risk reduction J ]. The Mining engineer, 1974, (133): 173-184[2] Ferm JC. A study of roof falls in underground mines on the Pocahontas 3 seam Southern West Virginia and SouthwesternVirginia R ]. Bureau of Mines, Department of Interior, United States, Final Report on Contract No. H0230028, 1978[3] Horne JC, Ferm JC, Caruccio FT, et al. Depositional models in coal exploration and mine planning in Appalachian region[J] American Association of Petroleum Geologist Bulletin, 1978, 62(12): 2379-24114] Hylbert D V, Developing geological structural criteria for predicting unstable mine roof strata R ] Bumines Open File Rept9-78, Department of the Interior, United States, 1977[5] Peng S P. Sedimentary characters and patterns of complex delta J ]. Coal Transaction, 1994, 191): 89[6] Peng S, P. Strata control of roof stability in mining working[ A ] The Proceedings of the International Congress on miningScience, Applied Geology and Mining Technology[ C ]. St Petersburg, Russia ISBN 3-230-19519-3. 1993.85-91[7] Peng SP, Chugh Y P. Geological Moldering Techniques of Longwall Mining Roof Stability A Case Study[ A ]. Zhu DRProceedings of Rock Mechanics and Strata Control in Mining and Geotechnical Engineering. Beijing: A. A. Balkema press1995.232240[ 8] Meng Z P, Peng SP, Qu H L. Influence of stratigraphic facies variations on the roof stability by physical modeling study[AJ Xie H P, Golosinski. Mining Science and Technology'99 C]. Rotterdam AA. Balkema press, 1999H中国煤化工CNMHG