Applicable conditions for a classification system of aquifer-protective mining in hallow coal seams

Mining Science and Technology( china)21(2011)381-387Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcApplicable conditions for a classification system of aquifer-protective miningin hallow coal seamsLiu Yude a*, Zhang dongsheng b. c, Fan Gangwei b, Yan Shoufeng dCollege of Safety Engineering North China Insritute of science E TechBeijing 101601, Chinab School of Mines, China University of Mining 8 Technology, Xuzhou 221116,ChinaState Key Laboratory of Coal Resource and Mine Safety, China University of Mining 6 Technology, Xuzhou 221008, ChinDeparrment of Basic Courses, North China institute of Science 6 Technology, Beijing 101601, ChinaARTICLE INFOABSTRACTBased on the conclusions of domestic and foreign research, we have analyzed the collapse-fall characterReceived 16 October 2010istics of overlying strata and the mechanism of aquifer-protective mining in shallow coal seam workingReceived in revised form 15 November 2010Accepted 8 December 2010faces at the Shendong mine. We have selected the height of the water-conducting fracture zone in overAvailable online 12 June 2011lying strata as a composite index and established the applicable conditions of aquifer-protective miningin shallow coal seams with a multi-factor synthetic- index classification method. From our calculationand analyses of variance, we used factors such as the overlying strata strength, mining disturbing factorsand rock integrity as related factors of the composite index. We have classified the applicable conditionsDepth limits of shallow coal seaAquifer-protective miningof aquifer-protective mining in shallow coal seams into seven types by comparing the result of the heightComprehensive classification index analysisof water-conducting fractured zones of long-wall and short-wall working faces with the thickness of theWater-conducting cranny zonbedrock, the thickness of the weathered zone and the size of safety coal-rock pillars. As a result, we pro-Analysis of variancepose the preliminary classification system of aquifer-protective mining in shallow coal seams. It can pro-vide a theoretical guidance for safe applications of aquifer-protective mining technology in shallow coalo 2011 Published by Elsevier B V. on behalf of China University of Mining Technology1 IntroductionAquifer-protective mining is aimed at avoiding damage to thewater-bearing structure of aquiferous layers in the process of min-Northwestern China, covering about 1/3 of the country, abounds ing by applying appropriate methods. Even if there is damagein coal and attracts the world attention for its richness and thick- which may cause the loss of some water. the damage can be recov-ness of minable coal seams, its quality, simple structure and supe- ered within a certain period of time and should not affect the min-rior technological mining conditions. It has become the base of imal water-bearing level for the growth of surface flora[3-5]energy sources for our western and national economic develop-When the Shenfu-Dongsheng Coalfield was opened in the 1990s,ment 11 ]. But unfortunately, the largest part of northwestern China Chinese scientists carried out in large scale field measurements.lies in the Maowusu Desert and the transitional area of the loess their thorough research involved the basic mechanical propertiesPlateau, where water is scarce and the ecological environment of the major groups between coal seams and aquifer rocks. thifragileconsisted of the evolution of fractures, load transfer mechanismsFor example, the Shenfu-Dongsheng Coalfield covers 3124 km2 features of water-resisting and water-conduction and the relation-and its explored reserves are 223.6 billion tons. If normal methods ship between major related factors of mining. Various methods andwere used in mining, large quantities of surface water would be means were used, revealing the deformation and failure format oflost, resulting in environmental deterioration, which causes great different rocks, the roof breakage mechanism and the apparentharm to people's life, to the environment and to the sustainable ground pressure and determined the aquifer-protective action ofdevelopment of mining. Therefore, the greatest technological chal- water-resisting layers and the controllability of water-conductingpreserve as much water as possible during the process of safe coal classification of aquifer-protective mining. there is still much work2to be doneUsing Coal Seam 2-2 in the Shendong Mine as our researchobject, we analyzed the mechanism and key points of aquifer-Corresponding author Tel +86 15030621177protective min-lassified its applica-E-mail address: lydcumte 126 com(Y. Liu))ble conditionsYHa中国煤化工fication, which may674-5264/s- see front matter e 2011 Published by Elsevier B V on behalf of China University of MiningCNMHGdoi:101016mstc201105022Y Liu et al/ Mining Science and Technology(China)21(2011)381-3872.4. Key to aquifer-protective miningBefore it broke, the property of the strata was approximatecontinuum; after it broke, it assumed the property of a non-continuous. The movement within the strata from bottom to topis reflected in its loose bed, presented as equidistant surfacefracturesThe vertical fractures across the layers of the strata above theFig 1. Collapse and fall of overlying strata by similar material simulation. (0 the stope and developing only within a certain range. When theand(lll)closed cranny area.ately conduct aquifer, but depends on the bite and balance interac-tion relationship between the key blocksTherefore, the key point in aquifer-protective mining is toprovide some theoretical directions for building an ecological mine ensure that the fracture zone does not break over the aquifer. Ifit does, the fracture which can conduct water will be closed andremains in equilibrium by controlling factors under different natu2. Mechanism and key of aquifer protective mining in shallow ral conditionscoal seams2.1. Collapse and fall characteristics of overlying strata above the3. Classification method and index of aquifer-protective miningworking face of shallow coal seamsThere are a large number of factors affecting aquifer-protectivemining in shallow coal seams and it is hard to use only one physThe collapse and fall status of overlying strata by similar mate- ical quantity as a classification index. So, we use a single index ofial simulation is shown in Fig. 1. When the roof bedrock of a integrating a number of factors to classify the applicable conditionsshallow seam working face is whole cut down along its entire thick, of aquifer-protective mining in shallow coal seamsthe structure of the first broken pieces is asymmetric. The angle ofAccording to the key point of aquifer-protective mining, thethe broken bedrock is large and the break directly affected the sur- height of the water conducting fracture zone is used as the integraface. Along with the advancement of the working face, the separa- tion index of classificationtions and fractures of the overlying strata can be compacted andclosuredStudies have shown that the distribution of rock fractures andthe height of the water conducting fracture zone are the result ofintegration functions of many factors [10]. In turn, these depend2.2. Shallow limiton all kinds of uncontrollable natural factors and related controllable factors of production technical factorsTheories and practice have shown that when loose bed becomesthick(i.e, the base-load ratio Jz is reduced), the entire break of the 3. 1. Analysis of related factorsoverlying strata increases. According to elastic-plastic theorywhen the stress of strata is greater than or equal to its uniaxial 3. 1. Factors of aquifer-protective miningcompressive strength, rocks start to collapse and become unstable.Aquiver-protective mining is largely affected by natural factorsTherefore, in our study, Jz-1 is regarded as the critical point of and controlled factorshallow coal seams and the shallow limit hs can be calculated asfollows [10]The major natural factors include the height of bedrock, theheight of the soft strata, the height of the weathered zone, the heightH;≤2.R·k·kaηof the mudstone water-resisting layer, the distribution of bedrockfractures, the features of topographic and physiognomic and theIn this formula, ?1 refers to the volume weight of a loose bedThe major controllable factors include mining techniques,(kN/m).?2 to the volume weight of the bedrock(kN/m).k, to designing parameters and reasonable matching, which are com-the coefficient of rock integrity: ka is the mining coefficient, n the prehensively reflected in mining impact stress aspect.i long-term load coefficient and R refers to the comprehensivestrength of the strata(MPa)3.1.2. Analysis of the influencing factors by aNovAThe comprehensive strength of the overlying strata in theShendong mining area is commonly 25-45 MPa, so that the shal-low limit of coal seams should be 90-170 mOn ANOVA (analysis of variance)is one of the most effective wayssorting out the major factors [11-13]. The system of analysisvariance of factors affecting the height of water conducting fracture zone is shown in Table 12.3. Mechanism of aquifer-protective miningTable 2 shows the Fratios of seven single factors.when a=0.01 is used as the level of test, we check the table of FThe mechanism of aquifer-protective mining in shallow coal distribution P( Fnl n2)>F( n1, n2))=a, and obtain the followingseams is a water-resisting function of the strata in the weathered result:zone and closure of broken rock in a single key stratum, forming Fr(a-i, n-a)=Foo=4.43a stable structure of masonry beam. It may also involve interactiveclosure of major key stratum and sub-key stratum. when the subThe seven individual analyses show that three factors, i.e., rockkey stratum is broken, the major key stratum above it is still intact: intensity, rockhen the major key stratum is broken, the openings in the lower the movement中国煤化工 ng signicantly affectCre, the related factorssub-key stratum are pressed closed. These conditions can effec- of a compreheCNMH Gplicable technologicaltively stop infiltration of water from aboveconditions in aquirective mining are as followsY. Liu et aL/Mining Science and Technology(China)21(2011)381-38where kd refers to the mining coefficient (when the coeffi-Analysis of variance of a single factor.cient is less than 1. 1 is used). ky is the coefficient of rockoum tDegrees ofUnbiasedintegrity, a and b are coefficients of comprehensive rockintensity, R is the comprehensive intensity of cover rockFactor AMPa)and n the loading coefficient (i. e, reflecting the effectsMSE aSummation T STn-1(2)In short-wall mining, the height of the water conductingfracture zone in the cover rock is closely related to the min-ng system. Three arrangements of mining chamber are pro-posed for a short-wall working face [10-14], as shown inFig. 2Calculation results of F ratio(3) The height of the water conducting fracture zone is computed with the formula below:F ratioThickness of cover rockF1"3.60100·Hnkd·k·krF=3.20HLa·Hm+bRock intensityF134Thickness of weathered strata-3.70There k is the coefficient of residual coal. the other con-Fs=5.70stants and variables have been defined earlierEffect of bearing pressure on both sides4.333. 3. Deciding the parameters3.3.1. Mining heightThe range of comprehensive long-wall mining working face is(1)Comprehensive intensity (oc)of the overlying bedrock, i. e, from 2.5 to 6.0 m.the weighted average of uniaxial compressive strengthGiven that the equipment at short-wall continuous mining(MPa)of the cover rock in the working face can be obtained working faces is limited, the mining height should not be tooby laboratory standard rock compression testssmall; its range is usually from 4.0 to 6.0 m.orking face: kd is the correcoefficient of the comprehensive index and the advancebearing pressure from mining at the working face.3.3. 2. Mining coefficien(3)Exponent of rock integrity (I ): ki is the correlation coefficientWith data from experience as reference, the mining coefficientof the comprehensive index and the effect of the joints, frac- can be computed as follows:tures and stratification of the cover rock above theseam, defined by the geological features of the workingH3. 2. Computing formulasWhere Hi is the height of the direct roof is more than O m, and again,the other parameters as defined earlier.The formula of computing the height of a normal fracture zoneThe mining height of tHe Shendong Mine is generally 5-6 m andis not well applicable to shallow coal seams and should be modi- the height of the direct roof about 10 m. Hence, 1. 24 is taken as theied given the effect of other related factors. We therefore proposes mining coefficient kda method of computing the height of the fracture zone, using thecomprehensive strength of the rock as a variable and taking thecombined action of mining, rock integrity and the load on the rock 3.3.3. Coefficient of rock integrityWe have used related data about rock classification [15 as ref-asWhen H2 4Hm(H, is the thickness of the direct roof. Hm is the erence and the coefficient of additional damage to rocks as the dis-continuous rock face coefficient used in the soviet Union whenng height).he stability of rock was analyzed the rock integrity coefficient L.shown in Table 3, is decided on the basis of the regional geological(1)When the goaf is almost filled the function of the criticallayer of the basic roof is not visible and the height of the structure of the working face.The geological structure of the Shendong mine is simple, withwater conducting fracture zone is constant, the following an integrity coefficient of 1. 21formula is usedHL= 4. H( 2)When H, < 4Hm, the function of the critical layer of basic roof(cis visible(especially when the thickness of the direct roof isone third less than the mining height, or there is no directroof, the function of the critical layer of the basic roof is quite(1)In long-wall mining, the height of the water conducting fracture zone can be computed as followsH100·Hmkk1中国煤化工a=0.1075-0024In(Rn)b=0.155-00299ln(R7)Flg. 2. ArrangementsC MH King face. (a) singlehamber,(b)interval multi-chamber, and(c)continuous multi-chamberY, Liu et aL/Mining Science and Technology( China )21(2011)381-387Table 3Selection standards of rock integrity coefficientsCurvature radius rFaultage(distance rift zone300-100m4N-1NH+Hsducting fracture zone and the mining height are nearly linearlyelated, as shown in Fig. 3.where He refers to the computed height of the fracture zone(m), H,is the thickness of the protective layer prescribed in the sanxiaMining Regulations(m)and Hw is the thickness of weathered zone3. 4.2. Short-wall working faceThe computational results of the height of the water conductingfracture zone are presented in Tables 6-8.4. 1.1.1. Selection of the thickness of protective layer Hs. The SanxiaThe relations between the height of the water conducting frac- Mining Regulations prescribe that the thickness of the protectiveture zone at the short-wall continuous mining working face, the layer of waterproof coal rock pillars can be selected according toock intensity and the mining height is almost similar to those at existence or non-existence of loose beds and the thickness of itslong-wall working face, i.e., the height of the water conducting cohesive soil, from Table 10fracture zone and the mining height are nearly linearly relatedThe cover rock lithology of the Shendong Mine is soft-mediumhard and so(5-6) is selected, i.e., 27.5-33 m.4.1.1.2. Deciding the thickness of weathered zone Hw According tocoal coefficients at short-wall continuous mining working facesdrilling and geophysical prospecting data, we know that the gen-eral thickness of the weathered zone at the top of the bedrock is3.22-20.00 m, with an average thickness of 5.66 m From top toMulti-chamber interval arrangementbottom, it can be divided into a strong weathered zone and a weakSingle chamber arrangemenweathered zone. The rock in the strong weathered zone is looseand fragile, 2.70-4.89 m thick, with a 3. 13 m average the rock inHeight of water conducting fracture zone at long-wall working faceMining height(m)Comprehensive intensity of overlying rock(MPa)203033.5737.0245.03683826.6349.2976.1922.1642.985286329755.89306138中国煤化工9825.236.025.7533.0937,1141.48CN MHG678Y Liu et al/Mining Science and Technology(China)21(2011)381-387Table 6Height of water conducting fracture zone of multi-chamber continuous arrangement at continuous short-wall mining.Comprehensiv208122,7123.1700505022.2325.38275284029.7829673082326273636.473021366938.2140.65416533.32464750.30526661549594ble 7Height of water conducting fracture zone of multi-chamber interval arrangement at continuous short-wall mining.Aining height(m)intensity(MPa)3.0354.0505.560977104911081.56119712321235153116.19165413.73148316491767190816.78182119.3923.1418.5120.14226323.6025.17258222.51292534.194147444349.36Table 8Height of water conducting fracture zone of singde chamber arrangement at continuous short-wall mining.Mining height(m)intensity(MPa)205866.94398.468859.1810271060108111311.01109311.6413.113.5511.116.7724882962Thickness of the protective layer of waterproof coal rock pillars (not applicable toType Water level Thickness Unit water inflow Osmoticfully mechanized mining)(L(s m)coefficient(m/dRock lithologyH1≥H2H170 m), exceeding0.504030.2the shallow limit( 90-170 m). According to theory of aquifer-pro-35043.36001100tective mining, we know that it is easy to mine when water is000021.1243preserved at such a depth. Therefore, 70-80 m( weak aquifer),9001050130018003300and 75-85 m(strong or medium aquifer) of bedrock are defined971120.01400173324004400as easy aquifer-protective mining areas.1056121415001750216.73000550033.61650.5238.33300605.0When the thickness of the bedrock is between 50-60 m(weak1267145718002100260036006600aquifer)and 55-65 m(strong or medium aquifer), the base load ra06058565137.2157919502275281.739007150tio is larger than 0.4 and the depth usually less than 170 m, i.e.,7700within its shallow limits the overlying strata has shallow coal seam158.3 182.1 225.0 262.5 325.0 450.0 825.0 features of a single key stratum In the process of mining and going1689194.346.71794206425502975368351009350deeper the dynamic pressure becomes acute. Compared with thoverlying rock of the major and sub-key strata, it is difficult toThese are the lower and upper limits of normal aquifer-protectiveine with water preserved at the working face in this area.Bedrock thickness between 60-70m (weak aquifer) andmining at a working face65-75 m(strong or medium aquifer): between the easy aquiferprotective mining area and the difficult area, where the depth is4.1.2. Defining the methods of long-wall and short-wall miningabout 200 m i e. within the shallow limit the conditions featureThe results in Tables 5-8 about the thickness of the water con- that of a single key strata: beyond this shallow limit, the overlyingducting fracture zone have shown that in the Shendong Mine, the strata has the features of a major and sub-key strata.height of the water conducting fracture is about 45 m at the longwall working face at a 5-6 m mining height and 30-42 m at theshort-wall working face. When the thickness 5.6 m of rock in the 43. Re-dividing the aquifer-protective mining area at shallowweathered zone is considered, the aquifer-protective mining areas continuous short-wall working faceat long-wall and short-wall working faces can be divided on thethickness of the bedrock.Tables 6-8 show that the height of water conducting fracturesIn the strong or medium aquifer area, the limit of the bedrock is generally greater than 35 m at the continuous short-wall working face in a continuous multi-chamber arrangement and generallygreater than 15 m at the continuous short-wall working face in an45+56=50.6m(take55m)interval multi-chamber arrangement. Therefore, aquifer-protectivemining at short-wall continuous working faces can be re-dividedIn the weak aquifer area, the limit of the bedrock thickness is42+56=476m(take50m)(1)Continuous short-wall and continuous multi-chamber minwhen the bedrock thickness is less than 50-55 m, regular con-Ing areastinuous long-wall working face mining is not suitable and appro-In medium or strong aquifers, the bedrock is more than 40-priate safety measures should be taken or short-wall continuous55 m thick, in weak aquifers more than 35-55 m.mining conducted(2)Continuous short-wall and interval multi-chamber mining4.2. Re-dividing the aquifer-protective mining area at shallow longIn medium or strong aquifers, the bedrock is 20-40 m thick,wall working facein weak aquifers 15-35 m thick.(3)Continuous short-wall and single chamber mining areas:According to the base load ratios, the computed overallIn medium or strong aquifers, the bedrock is equal to or lessof the overlying rock, corresponding to the thickness of bedrock,than 20 m thick in weak aquifers it is equal or less thanare listed in Table 11Classification system of aquifer-protective mining in shallow coal seams.CassClass nameMain featurehe bedrock thickness is usually bigger than 80-85 m, which meets the demand for the thickness ofaterproof protection rock pillars prescribed in The Regulations of Coal Pillars Leaving and Mining in MainLanes under Butlding, Water Body, and RailwaysLong-wall easy aquifer-protective mining class The bedrock thickness is usually bigger than 70-75 m, the buried depth is over the shallow limit, and ithas the features of compound key strata In this case. aquifer-protective mining is relatively easyLong-wall moderate aquifer- protective mining The bedrock thickness is gLong-wall difficuit aquifer-protective miningcompound key strata. ying beween e asy a guitesprote ie ei dng hres ans hath he show imit andcassit has the features of single key strata. with mining at the working face, steps in the overlying rock caneasily go down as a whole, and the coming pressure becomes intenseShort-wall continuous water preservationThe bedrock thickness is usually bigger than 35-40 m, having the features of single key strata, and long-classwall mining is not suitablehort-wall interval water preservationThe bedrock thickness is usually less thaiontinuousingle key strata, and continuous miningYHa中国煤化工 aving the features ofort-wall single chamber water preservation The bedrock thickness is usually less thanCNMHGarrangement is suitablecontinuous mining classY. Liu et aL/ Mining Science and Technology(China)21(2011)381-38715 m: in these areas, discontinuous chambers are arranged Acknowledgmentsand overlying rock.Financial support for this work, provided by the research fundof the north China Institute of Science and Technology (No.4. 4. Classification system of aquifer-protective mining in shallow coal A09002) the National Natural Science Foundation of China(Noseams50834005)and the National Basic Research Program of ChinaNo, 2007CB209402), is gratefully acknowledged.According to the classification of the applicable water preservation conditions at long-wall and short-wall working faces, a classi- Referencesfication system about aquifer-protective mining in shallow coalseam has been established, as shown in Table 12[1] Cheng XB. General situation of water resources protection. Water Resour Prot2001:66(4):8-12[2] Wang A Manufacturing technique in modern billion-ton mine5. Conclusioarea. 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