Mining-induced variation in water levels in unconsolidated aquifers and mechanisms of water preserva

Available online at www.sciencedirct.comMININGScienceDirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20 (2010) 0814 0819ww.eviec.comlocatejcumtMining-induced variation in water levels in unconsolidatedaquifers and mechanisms of water preservation in minesFAN Gangweil+", ZHOU Lei2'State Key Laboratory ofCoal Resource and Mine Safety, China University of Mining & Technology, Xuzhou 221008, ChinaZXuehu Coal Mine, Shenhuo Group, Yongcheng 476600, ChinaAbstracts: Phreatic water resources are widely found in thick unconsolidated surface layers in westerm China, where water levelsrespond sensitively and quickly to large- scale underground mining in conjunction with shallow coal seams. Longwall face #32201of the Bulianta Coal Mine, in the Shendong coalfield was selected as an industrial trail base, where field observations on ground-; face was below a water-rich area. The space time variation in the behavior of un-water!evels wereconductedwhentheworking tlace Wa.sobservedthe field trials.consolidated water levels in response to underground mining and its relation with of advance wereThe basic conditions for water preservation in mines are presented and the mechanisms of water preservation in mining analyzed,given the geological condition of two key strata and a severely weathered layer buried in the overburden. The field trails show thatwater preservation in mining shallow coal seams can be successful under suitable conditions, providing new technology for eavi-ronmental protection in the desert coalfields of northwestem China.Keywords: shallow coal seams; longwall coalface; water preservation in mines; water level1 IntroductionTo validate water preservation in mines and toanalyze its mechanisms, field trials were conducted atOverlying strata and ground surfaces respond sen-longwall face #32201, referred to as field observa-sitively and strongly to underground mining in shal-tions on water level variation when the working facelow coal seams. Along with mining-induced surfacewas below a water-rich area.subsidence, overlying aquifers will be affected oneven destroyed with a subsequent groundwater or2 Geo-mining details of an experimentalsurface water loss, which is detrimental to surfacecoalfaceenvironments, especially in the desert coalfields inthe northwest of China. The preservation of water andThe Shengdong coalfield, located in the south ofenvironmental protection, subjected to large-scalethe Inner Mongolia Autonomous Region, northerunderground mining, has become highly problematicShaanxi Province and the easterm part of thein these northwesterm coalfieldsh-9.Maowusu Desert, is one of the eight largest coalfieldsMany theoretical achievements and engineeringin the world and, as well, the largest modern coalfieldfeats have emerged since the introduction of the con-in China. The coal seams in this coalfield are shallow,cept of water preservation in mining. However, mostoverlain with a thin bedrock layer. The surface isof these were based on shortwall mining to keep aq-covered by thick sand and there is a valuable aquiferuifers from being destroyed by mining-induced acjust beneath the sandltivities"-14. In order to maximize the economic ef-fect and the recovery rate on the basis of safety and2.1 Hydrogeology conditionsenvironmental protection, a longwall water preserva-The longwall face #32201 of the Bulianta Coaltion mining technology in shallow coal seams shouldMine extracts coal from its #2~ 2 seam. There is abe developed for westerm coalfields of China, whichsandy aquifer above this seam with a flow rate ofought to recover groundwater levels after a temporarypermeability isdrawdown.1.04中国煤化工flows very slowlyalongdial direction. TheReceived: 20 March 2010; accepted 10 May 2010aquifMHCNMH Ga fre by the*Corresponding author. Tel: 86 13655203693Bulian dyke, a local water-rich zone within 2050 toE-mail addres: fangangwei@cumt.edu.cndoi: 10.1016/S1674-5264(09)60287-82550 m from the setup line of this longwall face,FAN Gangwei etal815formed in the aquifer, as shown in Fig. 1. The aquifer3.22 to 10 m thick (5.66 m on average) immediatelyis about 6 to 16 m thick, 10 m on average, as shownon the top of the bedrock.in Fig. 2. There is a severely weathered rock layer,0s 346.584385-46.00 Well#4Weliil11132-3(力WelRiy-5289)南4版Wel4 ILongwall face #32202948.15泰52123一 点47.80W上卢心41.92( Well #5Wel,20-为00f 50.16ψ57T77weln No Tudepoel”946.00Fig. 1 Water rich area located in longwall face #322011240longwall face is 3800 m long and 240 m wide, with1207.8m 1210.659m 1210.213m 1209.338 mthree developed gate roads, as shown in Fig. 1. A1200fully mechanized longwall mining method with a200.-2 2。117089Klarge mining height is used. The main assortment of曼.1160mining equipment used in the face is listed in Table 2. .The total power of the equipment used in the long-wall is 5561 kW. The rate of daily advancement isabout 21.6 m with bi-directional shearing. Daily coalCoal1080production of the face is 30000 t and annual coalFig.2 H section of overlying aquiferproduction about 10 Mt.Table 1 Generalized stratigraphic column of2.2 Details of overlying stratathe water-rich areaA generalized stratigraphic column of the water-LithologyThickness (m)_ Stalementrich area is shown in Table 1. The bedrock in thisUnconsolidatedUnconsolidated sandy layer 46.4water-rich area is 62.68 to 91.48 m thick, 78.08 m onaquifer at the baseaverage, predominantly of sandstone or siltstone. The.0Aquicludeimmediate roof of the #2~2 coal seam is a 6.55 mSilty mudstone12.0thick sandy mudstone. An 11 m thick medium-Medium-fined sandstone25.0Main key statumgrained sandstone layer acts as a sub key stratum andSily moadstone9.7a 25 m thick medium-grained sandstone layer acts as11.0Sub key stratumthe main key stratum.Silty mudstooe.6Coal 22.3 Introduction of the working faceFace #32201 is located in the 2nd panel of the Bu-lianta mine and is the first face in the panel. TheTable 2 Equipment in the longwall faceEquipmentTypeMain technical specificationDouble drum shearer SL500Design capacity 2800vh Drum web 865 mmCutting bheight2.7-5.4 m Drum diameter 2700 mmSupport resistanceSupport heightPower supportJOY8638-2.5/5.5Advance interval 865 mm Pull force 56.7t2x4319 kN2550-5500 mmAFCAFC10Capacity 3500hChain sped 1.4中国煤化工inopen centre chain3SLCapacity 3500 vhCNMHGednto19.8:1CrusberMaximum output size Rev 1489 /minChain intervyal 756 mm300 mmMining Science and TechnologyVol.20 No.63 Field observations on the water level ofcantly in the first 10 days after the longwall facethe unconsolidated aquiferpassed them. The water table of the wells in the mid-dle of the face, i.e., the #8 and #16 wells, stabilized3.1 Location of observation wellsquickly, while the water table of the #17 well, nearthe upper gate road, took at least 20 days to stabilize.In order to analyze the variation in the water levelThe water table of the #11 well, which was inside theof the unconsolidated aquifer when the working facecoal pillar of the upper gate road, fell relativelyis below the water-rich area, 21 observation wellsevenly with a small amount of water loss and took 30were placed around the middle and the two ends ofdays to become stabilized. The water table of the #18the face, as shown in Fig. 1. The observation wells,well inside the coal pillar of the lower gate road fell250 mm in diameter, ended at the top of the weath-slightly with lttle water loss and it took 20 days toered layer.start its slow recovery. In general, the water table ofThe retreat of the mining of face #32201 started onthe wells in the middle of the face stabilized rela-April 25, 2004 when the face was on the north side oftively quickly, the water loss was relatively small andthe Bulian dyke. By October 1, 2004, the face hadthe time taken for the recovery was relatively short,retreated to 2404.6 m from the setup line and suc-i.e., about 50 d. The water table in the wells near thecessfully passed the Bulian dyke and the water-richgate roads and inside the face stabilized relativelyzone.slowly, the water loss was relatively large and the3.2 Analysis of variation in water levelstime taken for the water table to recover was rela-tively long, almost 90 d. The water table of the wellsAffected by the underground mining, the overlyingoutside the gate roads stabilized quickly, the timestrata subsided unevenly and the storage equilibriumtaken to recover was short and the water loss small.of groundwater was subsequently disturbed as theThese observations are clearly shown in Figs. 3b toresult of a new flow potential. Due to the different3d. Fig. 3b shows the variation in the water table inlocation of the observation wells, the amounts of wa-the wells inside and outside the face. Fig. 3c showster drawn down, the water levels and water lossesthe variation in the water table of the wells in thevaried considerably. Fig. 3a shows the variation inmiddle of the face and Fig. 3d the variation of thewater levels in the wells along the face direction. Thewater table in the wells inside the coal pillars of theresults indicate that the water table of the wells insidegate roads.the face of the #8, #16 and #17 wells, fell signifi-11二8+←1610g亘901020304050607080901001102406050 100 T20T(d)(间) In the wells along the face line(6) In the wells inside and outside the face十9一11 +21到人h与76 20 4000 120T((C) In the middle of tbe longwall along the panel drection(d) In tbe wells inside and ouside the coal pllarFig. 3 Variation of water depth in different directionsOur observations indicated that the drawdown ofaquife中国煤化Iverlying bedrockthe water table occurred in all the wells and the fallbroke:YHinrush from thewas obvious within the first 10 days after the long-roofCN M H Gter the face hadwall face passed them. The water table then started topassed the wells seven weks earlier, the water tablefall slowly, indicating that part of the water in thein the wells started to rise gradually, indicating thatFANGangweietalMining-induced variation in water levels in unconsolidated ..17the. fractures in the broken bedrock were closed,1 mm/d. The results also indicated that this severelywhich prevented the water in the overlying aquiferweathered layer has a weak water-bearing capacityfrom draining off. The decline of the water table in alland very low permeability. Therefore, this severelythe wells was within 0.05 to 2.23 m, with an averageweathered layer can act as an excellent aquiclude.of1.45 m. ,Table 3 Compressive strength of weathered rock4 Mechanisms of water preservation inand viginal rockminingRockWeatheredWeahered VirginalLoss ratio4.1 Definition of water preservation in miningMudstone31.5043.2227.12Sandy mudstone29.7361.8948.04In the design and practice of water preservation inSiltstone37.2066.6155.85mines, the aquiclude is permitted to be broken tem-Fincly-grained sandstone 28.39porally, provided that the mining induced water-flowcracks can close after mining with the resultant re-Medium- grained sandstone 26.1548.0054.47covery of the water tableTherefore, the definition ofwater preservation in mines can be divided into theThe severely weathered layer has become clayeyfollowing three levels: level 1: underground miningand the plasticity increased significantly with a con-cannot afct the aquifer, level 2: the water table csiderably strengthened ability to resist deformation.recover in a short time after a drawdown; level 3:Once saturated with water, the weathered rock is eas-although the water table cannot recover to its originalily decomposed and has become muddy. Under thelevel, the normal water supply is not affected, or atgravitational force of its overlying layers, the struc-least the amount of phreatic water should be suff-tural fractures in the severely weathered layer closedcient for ecosystems.under compression; the porosity and permeability ofReferring to this arid or semiarid coalfield, waterthe weathered layer decreased, resulting in the loss ofpreservation in mining should prevent the onlyis ability to hold water and the weathered layer be-phreatic water resource or surface water from loss.came an aquiclude again. The weathered layer of a4.2 Mechanisms of water preservation in miningcertain level of plasticity keeps the mining-inducedand water-conducting. fractures from propagaing1) Severely weathered layers acting as aquicludesupwards, thereby. enabling the fractures to close,There is a severely weathered layer of about 5 m .which is beneficial for water preservation in mining.thick on the top of the bedrock. Compared with vir-2) Two hard rock layers acting ?s key strataginal rock, this severely weathered layer changedThere are two layers of comparatively hard rockssignificantly in mechanical strength, mineral compo-in the overlying strata of coal seam #22. These layersnents and other facets. The compressive strength ofact as key strata. The upper straum acts as a mainweathered rock and virginal rock are compared in key stratum and the lower as a sub key stratum. If theTable 3.sub key stratum is broken and the main key stratum isA high clay mineral content was found in the se-not, the weathered layer and the aquifer above theverely weathered rock through tests of mineral com-main key stratum would not be afcted by mining, asposition. In these clay minerals, kaolinite accounts forilstrated in Fig. 4a. If the main key stratum is bro-35% and montmorilonite for approximately 5%. Theken, the downward movement would make the min-results of water pumping tests, conducted in four wa-ing-induced fractures below to close, as shown in Fig.ter wells, indicated that the water flow rate in this4b. It should be noted that the stability of the roof inseverely weathered layer is less than 0.011 L(s-m),zone #1 and the fractures in zone #2 should be con-the permeability of the layer between 0.006 and 0.04m/d and the water conductivity of the layer less than中国煤化工细) Before main key statum is brokenYHCNMHGokenFig. 4 Overburden movement before and afier main key stratum is broken318Mining Science and TechnologyVol.20 No.64.3 Basic requirements of water preservation into recover was relatively long. The water table of theminingwells outside the gate roads stabilized quickly, th1) Rapid advancetime needed for recovery was short and the water lossIncreasing the rate of advance of the working face，small.can delay breaking of the main key stratum and pro-3) A severely weathered layer and two key stratalong the time for water resistance, in other words, theare the keys to water preservation in mining in thetime and spaceforthedevelopment of mining-case of the #32201 face of the Bulianta mine. Thenduced cracks will be reduced, which is beneficial forbasic requirements for water preservation in miningvater. preservation in mining. Two measurementsinclude a rapid advance of the longwall face throughshould be taken to achieve a rapid advance. The firstthe use of highly resistant hydraulic support and highis to select highly resistant hydraulic support in orderpowered face equipment and some special measuresto avoid subsidence of the step roof of the workingin localized areas through lowering mining height,face. The selected hydraulic support with a resistancebackilling and grouting. Our field trials havindi-of 8368 kN in this case met the requirement. Thecated that water preservation in mining can be suc-second is to select high powered face equipment. Incessful, given condition similar to those in facethis case, a total power of the face equipment was#32201 of the Bulianta mine.5561 kW and the equipment was capable of produc-ing 50000t of coal per day.AcknowledgementsThe more rapid the face advanced, the smootherthe overlying strata subsided and the smaller the wa-Financial supports for this work provided by theter loss. On September 3, 2004, the longwall faceResearch Fund of the State Key Laboratory of Coal#32201 stopped working and on the flliowing dayResources and Mine Safety (No.SKLCRSM08X2),water rushed into the face at a rate of 20 m'/h. Onhe Jjiangsu “333”High Qualified Talents, the Na-September 7, 2004, face #32201 advanced 5 m andtional Natural Science Foundation of Chinawater rushed into the face at 6.7 m/h the following (No.50904063), and the Scientific Research Founda-day. When the rate of advance ranged between 15 andtion of China Universty of Mining & Technology30 m/d, no water flowed into the face.These observa-(Nos.2008A003 and 2009A001) are gratefully ac-tions indicate that water preservation in mining canknowledged.be succesful at rapidly advancing rates.2) Special treatmentReferencesIn order to reduce the degree of development to alocalized region, such as a setup room, face ends, or[1] Qian M G XuJ L, Miao x x. 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