Structural motion of water-resisting key strata lying on overburden

Available online at www. sciencedirect.comJOURNAL 0F CHINA UNVERITYOFScienceDirectMINING &盂TECHNOLOGYELSEVIERJ China Univ Mining & Technol 18 (2008) 0353 0357www.elsevier.com/locate/jcumtStructural motion of water-resisting key stratalying on overburdenPU Hail'2, MIAO Xie xing", YAO Bang hua', TIAN Mu-jun''School of Sciences, China University of Mining & Technology. Xuzhou, Jiangsu 21116.6 China'State Key Laboratory for Geomechanics & Deep Underground Engineering, China Universiry of Mining & Technology,Xuzhou, Jiangsu 221008, China'Abstract: Water-preserved mining is one of the important parts of the 'Green Mining' technological system. The purpose of wa-ter-preserved mining is to prevent water from bursting out in coal mines and thus to protect water resources. The principle of wa-ter-resisting key strata (WKS) is proposed to establish a model capable of guiding and developing water-preserved mining technol->gy. The experimental model of the WKS is constructed following requirements of the Data Ilmage Correlative Method (DICM).Five experimental schemes are designed according to different combined patterms of the WKS. The water-resisting performance ofthe WKS is analyzed from observation of structural stability. Al1 of them provide referential value for wate-preserved mining.Key words: water-preserved mining; water-resisting key strata (WKS); digial image correlative method (DICM); water-inrush incoal mine1 Introductionpreserve water resources, is a very important part ofthe 'Green Mining' technological system!-4.The direct economic loss resulting from wa-ter-inrush accidents in China is much more serious2 Principlesthan from all other kinds of mining accidents. There-fore, preventing such accidents has become an im-It is well known that coal-bearing strata have aportant target for engineering mining security.layered nature. Some layers are hard, with excellentOver the past 20 years more than 250 mines haveload bearing ability, while others are weak, with goodbeen submerged by water causing a direct economicwater-resisting ability. Water-resisting key strataloss of more than 35 billion Yuan. In recent years the(WKS) are ilustrated in Fig.1. In Fig. 1 it is seen thatnumber of water-inrush accidents in coal mines keepsthere are generally several rock strata between theincreasing. The number of water-nush accidents inwater-inrush working face and the water sources.the last 5 years is more than the total during the pre-However, the strata are different in their characteris-vious 10 years. Moreover, this number will tend totics and positions in the mined rock mass and they areincrease because of complex geological and hydro-different in their water-resisting ability. Rock stratalogical conditions in the mines. In addition, 5.6 bil-through which water can not penetrate are calledlion m' of water have to be pumped from coal minesWKS. There are two ways for water to break throughevery year in China to prevent water flooding of theWKS: 1) natural passages, and 2) passages inducedmine. Only 26% of that water is utilized leading to aby mining. A water-inrush accident will take placeshortage of water resources and damage to the eco-whenever any one kind of these passages is created.system in the mining area.The structural character of coal-bearing strata af-Because of the severity of water- inrush disasters,ects the possibility for water-inush. From the modelas well as the damage to water resources and the en-it can be inferred that: First, if thick and weak wa-vironment in the mining areas in China, the develop-ter-resisting strata obviously exist between the waterment of water-preserving mining technologies, whichsource(s) and the working face (like thick surface soil)can not only prevent water-inrush accidents but alsothen中国煤化Irdly likely to hap-Received 11 February 2007; accepted 16 June 2008YHCNMHG.Prvjects 2007CB209400 supprted by the National Basic Research Program of China (973), 50574090 and 5U634050 by the Natonal Natural Science Foundation of ChinaCorresponding author Tel: +86-516-83885205; E-mail address: hai_ pu@263.net354Joumal of China University of Mining & TechnologyVol.18 No.3pen. Second, assuming that either: 1) the aquifer isIn determining and controlling the structural stabil-above both the coal seam and the structural key strataity of the WKS, we should have a mining plan that(SKS)S-6, or 2) the aquifer is under both the coalproperly arranges the mining sections and determinesseam and the SKS and the SKS remain unbroken af-the size of the working face. In addition, we shouldter mining, then the SKS can resist water and theanalyze possible failures of the SKS by using SKSWKS is the SKS, itself. Third, if the SKS was brokentheory, numerical simulation and physical simulation.by mining activity but water-inrush passages have notIt will be safe to implement water-preserved mining ifformed because mining induced cracks were filled bythe SKS in the mined rock mass is stable.weak strata, then the SKS and the weak strata com-In determining seepage stability and for controllingbine to form a composite WKS. Therefore, we canthe WKS we should use electrical prospecting toprotect the integrity of the WKS to prevent wa-identify the geological structure and water sourcester-inrush accidents and to preserve water re-around the working face, to analyze the seepagesources7-81.properties of the mined rock mass and to estimate thepossibility of forming seepage paths. It will be safeWater resourcefor mining if the seepage conditions are stable.厂LIn controlling to generation of new seepage path-WKSways we should further improve the mining plans tochange any failure pattern of the SKS in the minedrock mass so as to prevent possible seepage pathsNaturalMinedfrom forming.[Water-innush passages3 Experimental model3.1 TechniquesThe test model was composed of overlying strata, anFig. 1 A sketch of the WKSaquifer, a WKS, an immediate roof, a coal seam andthe floor. The horizontal and vertical lengths wereThe principle of the WKS in water-preserved min-240 m and 120 m, respectively. The entire mininging is ilustrated in Fig. 2. The figure shows that wa-process typical for a 3 m thick coal seam at a depth ofter-preserved mining has four steps: 1) determining580 m was used (Table 1).the position of the WKS; 2) determining and control-The total thickness of the WKS was 40 m, andling the structural stability of the WKS; 3) determin-each of the hard strata was 10 m thick, these are con-ing and controlling the seepage stability of the WKS;stant. The thicknesses of the weak strata wereand 4) controlling the development of seepagemuta-changed, forming five different experimental combi-tion passage. ,nations (Table 2).Table 1 Key mechanical parameters of the strataElectrical prospecting,used in the modelposition of WKSgeological condition, etcElastie Compressive .Deadweight ThicknessLithologymodulusStrength(10 SN/mm)Stable Determining and controlling.Mining design,(GPa)(MPa)structural stability of WKS theory of SKS analysis, etcOverlying strata202(2.140Determining and cortoling." Seepage mutation theory,Aquifer2:2.10seepage sabili of WKSalecriealnrnsneatine eWKWeak strata 2Control of seepageStope designs optimiztion,Immediate roofmutation passagesL grouting reinforcement, etcCoal seamloor7Safe miningFig. 2 Principle of the WKS in wate-preserved miningTable 2 Design schemes of the model(m)Scheme Scheme Scheme Scheme SchemeIn determining the position of the WKS, electricalprospecting should be used to detect geologicalstructures and the distribution of water bodies. In ad-中国煤化工10dition, the hydrological and structural conditionsfYHCNMHGshould be analyzed to determine the positions of theHard stratum_ 10SKS and the WKS in the mined rock mass.PU Haiet alStnuctural motion of water-resisting key strata lying on overburden553.2 Model preparationand deformation nephograms. PostViewer2007 alsoThe size of the model shelf is 127.8 cm long, 171has the function of querying, statistic analysis, datacm high and 8.5 cm wide. The main body is con-extracting, batching and outputting.structed of welded channel and angle steel. Both sidesof the shelf are fixed by a template with excellentrigidity. The size of the model itself is 120 cmx60cmx8 cm (Fig. 3).品Contolled datum marksOverlying strataMeasured pointsAquifer 5 cm工Kws 20 cmFloor 8.5 cmiMining distance Coal plla'30 cm60cm30cmFig. 4 Analysis interface of the DICM programFig. 3 Sketch of model manufacturingResults and discussionSix“+”were used as datum marks on the fourcorners and on the middle points of the upper andThe basic mode of structural failure of the WKSlower boundaries. These allowed the use of the Datawas obtained by collecting high resolution photo-Image Correlative Method (DCIM) on the model. Agraphs of the model and analyzing the results of therow of fiducial marks was placed at 5 cm intervals infive experimental schemes using the DICM. The pa-the WKS to improve the correlation of sequentialrameters analyzed by the software include the imme-photographs of the model. Hence there were 4x21diate roof, the water- resisting key strata, the verticalcolored marks (nails) with a horizontal spacing of 5displacement field and the stress field. The picturescm. The first row of these measuring points was 3.75from experimental Scheme I are shown in Fig.1.cm below the aquifer.The movement of the overlying strata can beclearly seen in the test photographs (Fig. 5). As the3.3 DICM testing and data analysis systemworking face advances, the separation of layers be-The principles are the same for both the DICM andtween strata becomes increasingly evident as itthe digital speckle image correlative method9-1o. Theprogresses from lower areas into upper ones accom-basic measuring process is to record an object's twopanied by vertical fractures. When the working facespeckle diagrams, which reflect both the initial featuresadvances to 60 m the immediate roof collapses; whenand those after transformation or displacement, usingit advances to 70 m, the lower hard strata collapsea high resolution digital camera. The images are thenand the first weighting happens-subsequently, thetransformed into two digital speckle diagrams by a di-weak strata on the top collapse entirely; at 100 m ad-gitizing image card. Finally, correlative calculationsvance the hard strata on the top also collapse and theare done on the two digital gray level images to findfailure of the overlying strata eventually spreads intothe extrema. This allows displacement and deforma-the aquifer.tion values to be obtained.Fig. 6 shows the vertical displacement field of theThe, DICM described in this paper was proposedWKS, which is analyzed by using the DICM. Theby Ll1-12I. A“three step search algorithm" (3SS) ismovement of the strata in the field can be clearlysimultaneously used to simplify the deformations intoseen.two modes of translation and rotation based on tradi-The displacement is small before the immediatetional image correlative analysis.roof breaks. However, when the working face ad-The measurement software system for DICM in-vances to 60 m, there is an obvious vertical dis-cludes digital image analysis (PhotoInfor2007) andplacement caused by the rupture of the immediatevisual post processing of the results (PostView-roof. With further advancement of the working faceer2007). The main functions of PhotoInfor2007 (Fig.breaking of the strata becomes more obvious and the4) are the production of datum-mark documentsscope of breaking enlarges. Then the ruptured area isthe ordinal images, automatic matching, setting mea-gradually_ compressed as the position of maximumsuring points and analysis parameters, checking thelisp |中国煤化工vards.net of measurement points, examining the results andEmmediate roof col-outputting the deformation data. PostViewer2007lapsCNMHGvanced to 60 m. Atused for visual post processing to analyze images can80 m advance the lower hard strata collapsed and thecreate and display a deformation net, vector-graphs,top weak strata began rupturing at 90 m. When the356Journal of China University of Mining & TechnologyVol.18 No.3working face advanced to 110 m the top hard strataWhen the working face advanced to 60 m the col-collapsed, accompanied by a synchronous rupture oflapse of the immediate roof leads to an obvious dis-the weak strata, resulting in connection to the aquifer.placement of the overlying strata. In addition, as theIt can be found from a graph of the vertical displace-working face moves the displacement becomes morement field that the displacement of the overlyingobvious from the lower to the upper regions and thestrata is small before the immediate roof has broken.range of displacement becomes larger.(e) Working face advancing to 100mFig. 5 Movement of the strata for Scheme 1Fig. 6 Vertical displacement field for Scheme 1The Scheme 3 model immediately suffered roofvertical displacement and stress an obvious layeredcollapse when the working face advanced to 70 m.character.The first weighting takes place when the advance-In the model test of Scheme 4 part of the imme-ment length reached 80 m resulting in the breaking ofdiate roof collapsed when the working face advancedthe top weak strata. When the working face advancedto 60 m. The roof was fully collapsed at 70 m. Anto 110 m the collapse of the top strala may cause theadvance to 80 m caused the first weighting to takeweak strata above it to break thus allowing the rup-place and the lower hard strata ruptured, which leadsture of the overlying strata to spread into the aquifer.to the break of the top weak strata when the workingThe graph of the displacement field showed that theface advanced to 90 m. Finally, the top hard stratadisplacement of the overlying strata was small beforecollapsed at an advance of 110 m, the weak stratathe immediate roof collapsed. Then the lower strataunder the aquifer breaking at the same time. It can bewas fully broken by 80 m, which is obviously seen infound中国煤化工cement field thatthe graph. As the working face moves there is a largerthe toEplacement beforearea with clear displacement, making the collapsedthe imCNMHGtheworkingfacearea gradually compress and the position of maxi-advances to 70 m the break of the immediate roofmum displacement move upwards. This gives thecauses the lower hard strata to become displaced inPU Haietal357n arched shape. However, the lower hard strataing took place and the hard strata ruptured leading tobreak at 80 m advance, by which point big displace-the breaking and deformation of the top weak strata.ments are evident in the graph.Finally, the weak strata under the aquifer CollapseIn the model test of Scheme 5, part of the imme-when the working face advanced to 120 m, connect-diate roof collapsed when the working face advanceding the mine with the aquifer.to 60 m. Full collapse occurred when the advance-To present a clearer picture of the five schemes,ment length reached 80 m. At 100 m the first weight-some key data are compared and listed in Table 3.Table 3 Comparison of key data for the five schemes(m)Breaking distance for the Breaking distance for the Breaking distance for the Breaking distance for theTotal breaking distanceSchemeimmediate rooflower hard stratumlower weak stratumtop hard stratumof the WKS070900100s0901011030121205 Conclusionsand mechanic analysis of water-proof key strata in wa-ter-keeping mining. Journal of China Coal Society, 2007,32(6): 561- -564. (In Chinese)1) The WKS is the underground pressure (me-2] Qian M G Xu J L, Miao X X. Green technique in coalchanics) model for basic theoretical research intomining. Joumnal of China University of Mining & Tech-water-preserved mining and water-inrush accidentnology, 2003, 32(4): 343 -347. (n Chinese)prevention. Although the WKS is different from the3] XuJL, Zhu w B, Lai W Q, et al. Green mining tech-SKS of strata control, these two concepts have someniques in the coal mines of China. Jourmal of Mines,internal relationships. There are three meanings forMetals and Fuels, 2004, 52(12): 395- -398, 409.the water-resisting key strata: water resisting litholo-4] Qian M G Miao X X, XuJ L. Resources and environ-gy for weak strata; water resisting structure for hardment harmonics (green) mining. and its technologicalsystem. Journal of Miring & Safety Engineering, 2006,strata, and water resisting by flling of passages.23(1): 1-5. (In Chinese)2) The behavior of the immediate roof is not great-5]QianMGMiaoXX,Xu儿,etal.TheoryofKeyStrataly different in the five schemes. It collapsed to dif-in Strata Control. Xuzhou: China University of Miningferent extents in every scheme when the working face& Technology Press, 2003. (In Chinese)had advanced to 60 m. It is the hard strata that play6] Qian M G Miao X X. Advance in the key strata theorythe most important role in load bearing. The weakof mining rockmass. Journal of China University ofstrata can bear only limited loads and so will be bro-Mining & Technology, 2000, 29(1): 25 -29. (In Chinese)ken along with the hard strata.7]MaoxB,MiaoxX,QianMGStudyonbrokenlaws3) The distance of the first weighting, or the firstof key strata in mining overlying strata. Joumal of ChinaUniversity of Mining & Technology, 1998, 27(1): 39 42.rupture distance of the lower hard strata, increases(In Chinese)gradually with decreasing thickness of the weak strata8] Miao X X, Liu W Q, Chen Z Q. Seepage Theory ofbetween two hard strata. The rupture distance of theMined Rock Mass. Beijing: Science Press, 2004. (Inwhole water-resisting key strata increases graduallyChinese)from scheme 1 to 5, showing that the distance needed9] RuiJ B, Jin G C, Xu B Y. A new digital speckle correla-to get access to the aquifer increases gradually fromtion method and its applcation. Acta Mechanica Sinica,Schemes 1 to 5 only in terms of the primitive struc-1994, 26(5): 599- 607. 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