Settlement behavior of coal mine waste in different surrounding rock conditions

J. Cent. South Univ. TechnoL.(2008)15: 350-355DOI:10.1007/sl171-0080066zSettlement behavior of coal mine waste indifferent surrounding rock conditionsMA Chun-de(马春德),LIXi-bing(李夕兵), HU Bing-nan(胡炳南)2,CHEN Feng(陈枫), XU Ji-cheng(徐纪成)},LIDi-yuan(李地元)(1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;2. Centre Coal Research Institute, Beijing 100013, China)Abstract: In order to investigate the influence of complex conditions of in-situ surrounding rocks on the settlement behavior ofnubbly coal mine waste subjected to high gravity pressure, four kinds of loading chambers made of different similar materials withdifferent elastic moduli in experiments were used to simulate the deformation features of in-site rocks, including soft, moderatehardness, hard and extra-hard rocks. The results show that all the settlement-axial load(or axial strain-stress)curves obtained underfour different surrounding rock conditions present power-exponential function feature. The final settlement of coal mine waste underthe same axial load is closely related to the lumpiness gradations and the deformation behavior of chamber materials used to simulatebehaviors of different in-situ surrounding rocks. In the same surrounding rock condition, the final settlement under the samemaximum axial load decreases with the decrease of the proportion of larger gradation of coal mine waste, while for the samelumpiness gradation case, the settlement increases with the decrease of elastic modulus of simulated surrounding rocks and the lateralpressure induced by axial load increases with the increase of elastic modulus of loading chambers that are used to simulate differentsurrounding rocks. The test results also reveal that both the compaction curve and lateral pressure curve show a three-stage behavior,and the duration of each stage, which is closely related to gradations and the deformation feature of loading chamber materials,decreases with the increase of the proportion of the small size of coal mine waste and elastic modulus of the simulated rock materials.Key words: coal mine waste; settlement behavior; surrounding rock conditions; final settlement; lateral pressurelow cost and easily obtaining on the1 Introductionsuccessful experiences have been achieved in this fieldthrough lots of years'studies 8-10, However, no deepCoal, as one of the important energy sources, has study has been conducted on the settlement behavior ofbeen explored several hundreds of years, which left a coal mine waste in complex surrounding rockgreat amount of underground mined-out areas with conditions!l-15), such as the influence of size proportiondifferent scales, depths and complexity. Every year, of lane section to granular coal mine waste fragmentationseveral thousand million tons of waste materials are and the deformation feature of loading chamber on thegenerated during the process of clearing raw coal, which settlement. In their studies only rigid loadingwill not only occupy a great amount of agricultural land and small caliber loading chambers were used,and deteriorate environment due to its deposition in the not consistent well with the surrounding enviopen air, but also bring a huge potential danger to ground using coal mine waste as filling material. In fact, thearchitectures, highways, dams and railways. In some elastic modulus of surrounding rock has significantcases, coal mine waste hill collapse is prone to take place influence on the settlement. In order to treat theseif it is not properly piled up>. To deal with these problems and obtain more precise results of settlementproblems, a new green-filling technique is developed(6-7, behavior of coal mine waste, large caliber loadingby which coal mine waste, which is used to be lifted out chambers with four different elastic moduli, were used toof underground mine, can be filled directly into simulate different surrounding rocks in the present work.underground mining-out areas or abandon laneways to The obtained results may hopefully provide a moresupport underground structures, control ground precise theoretical prediction to the settlement of groundsettlement and collapsesurface settlement above the underground mined-out areaUsing coal mine waste as a backfill material and provide a practical settlement control mepossesses a lot of advantages, such as simplicity, facility,Foundation item: Project(50490274)supported by the National Natural Science Founda中国煤化工 the Hunan provincialNatural Science Foundation, ChinaReceived date: 2007-10-21; Accepted date: 2007-12-12HCNMHGCorresponding author: MA Chun-de, Doctor candidate; TeL:+86-731-8876593; E-mail:camaigJ Cent. South Univ. Technol. (2008)15: 350-355thick wall cylinders with 320 mm in inside diameter, 402 Experimental method and proceduresmm in wall thickness, and 300 mm in height, and thechamber in Fig. 1(d) used to simulate the extra-hard2.1 Sample preparationsurrounding rock was made of seamless steel pipe, withAll coal mine wastes used in the test were chosen 320 mm in inside diameter, 10 mm in wall thickness, andfrom the Gaokeng Coal Mine in Jiangxi Province, China, 300 mm in height. The elastic modulus of the similarwhich were hoisted currently from underground coal material was measured by the standard samplesmine. They was divided first into five granular size artificially made using the same material when thelevels from large to small using sieving method: Pl homologous loading chambers were moulded10 mm) and P5(<10 mm), and then different samples 2.3 Experimental design and procedureswere built up according to the lumpiness gradationsIn this work, four surrounding rock conditioncases listed in Table 1corresponding to soft, moderate hardness, hard and extrahard coal bed, were simulated to investigate theTable 1 Lumpiness gradations cases of contrastive samples settlement behavior of coal mine waste located at about800 m in depth underground mined-out areas where theProportion of each granular size levelgravity is about 20 MPa. All tests were conducted onSample NoINSTRON 1346 testing machine with the maximum load10611914318.544.7of 2 MN, under stress control mode with the loading rateof 0.0625 MPa/s. In order to measure lateral pressure,20.020020.020020.0circumferential strains were also measured during the0010.0tests using the strain gauges sticked on the mid-belowpart of the loading chamber's opposite surface. TheIt can be seen from the lumpiness gradations of circumferential strains can be isochronously measuredthree contrastive samples that the proportion of larger using DH3817 static-dynamic strain apparatus in thesize coal mine waste increases while that of the smallerding process.size decreases from sample A to sample CThe experimental procedures are as follows: firstlyighing the coal mine2.2 Experimental deviceproportion of the designed lumpiness gradations case andIn order to investigate the effect of surrounding rock mixing them uniformly, then putting them into loadingenvironment on the settlement of filled coal mine waste, chamber and pugging with a crabstick to eliminate somefour loading chambers with different elastic moduli were large caves among them. An important thing that shouldused to simulate different deformation features of be paid attention to is that the height of testing samplesurrounding rocks around the coal mine waste. The must be ensured to be the same(275 mm)in each test forelastic moduli of loading chambers made of reinforced the compaction feature of the testing results. Secondly,epoxy resin and concrete were about 3, 9, 24 and more loading via a steel disk with 50 mm in thickness on thethan 50 GPa, respectively, corresponding to the simula- surface of the sample, a small load was added first totion of the soft, moderate hardness, hard and extra-hard eliminate the interspace between the disk and sample andcoal bed. These chambers are displayed in Fig. 1. The make them contacted well. In the end, the controlchambers in Figs. 1(a),(b)and(c) were all made into software in the computer gave the instruction of slowlygeor中国煤化工CNMHGFig1 Loading chambers to simulate surrounding rocks with different moduli:(a)E-3 GPa; (b )E-9 GPa; ( c)E-24 GPa; d)E>50loading with a rate of 0.062 5 MPa/s, and the test surrounding rock conditions are calculated, and theformally began. At the same time, the lateral strain obtained compaction and lateral pressure curves areneasurement system started to logging the test data given in Fig. 2synchronously3.2 Analysis for test3 Experimental results and analysisIt can be found from Figs. 2(a(c)) that there exists3.1 Data processing methodaxial stress and strain, and the compaction curves3.1.1 Compaction curves and analysis of axial settlement obtained under four different surrounding rockconditions are almost thesame, which can beThe original load and displacement data recorded byapproximately divided into three stages.the corresponding transducers can be used to obtain theThe first stage is the initial linear stageaxial pressure-settlement relationship. The axial corresponding to the filling course of the space amongstress-strain curve (i.e. compaction curve) of eachthe coal waste particles. In this stage the axialsample can be drawn easily through a simple calculation, deformation (or strain) increases quickly as the axial loadand the axial settlement property of coal mine waste with(or stress) increases slowly, which reveals thedifferent gradations and surrounding rock conditions can deformation characteristic of loose media under lowerthen be analyzed3.1.2 Lateral pressure curves and analysis of lateralaxial load. But, due to the influence of distinct elasticmodulus of the simmaterials on the lateralpressure effect of coal mine wasteA lateral pressure curve of bulk material, namely adeformation, the duration of this stage under foursurrounding rock conditions is quite different, whichrelationship curve of axial stress and lateral pressure, can satisfies the following formulaaveragecircumferential strain value first from the four strain Iot >Imodnt >Thard >lektra-hardgauges sticked on the opposite sides of loadingchamber's surface, and then substituting these values where l denotes the length of the stage, subscript iinto formulas(1)and(2), which correspond to the denotes the hardness of the surrounding rock, and thealculation of thin and thick wall cylinder lateral superscript denotes the serial number of loading stage,pressure.for the first stage,产1It can be seen from formula (3)that with theEawhere hin is the lateral pressure of the f. (i) increase of the elastic modulus of similar material,thelength of the first stage becomes short, denoting that thewall clearance among the coal mine waste particles will becylinder; E is the elastic modulus of loading chambers; ee filled up in a short timeis the average circumferential strain; T is the thickness ofAt the end of the first stage, most part of thea thin wall cylinder; r is the radius of thin wall cylinder. clearance among the coal mine waste particles will befilled up. With further increasing the axial stress, a non-linear relationship between axial stress and strain can(2) clearly be observed, which follows the power-2exponential function law. These curves show that withwhere Thick is the lateral pressure of thick wall cylinder; the increase of the axial stress, the increasing rate of ther, and r2 are the internal and extermal diameters of the axial strain becomes slower than that in the first stage.Inthick wall cylinder, respectivelythis stage, the phenomena of clearance filling and blockThe thin and atick values calculated from formulas crushing are concurrent, while the latter is dominant.(I )and (2)can be used to draw the lateral During this period, a crepitant noise with a feature ofpressure-time curve, compared with the axial stress- weak-strong-weak-disappearing can be clearly heard. Intime curve, they possess the same time axis, and the axials stage, the loose coal mine waste material is almosttogether in the same figure. Therefore, the property of to that in the first sage. he length of this stage is similarstress and the lateral pressure curves can then be drawn completely compactedlateral pressure of coal mine waste with differentgradations under complex surrounding rock conditions 1 0a中国煤化工canbe analyzeIn this way, the axial stress, strain and lateralCNMHG hardness of theressure of samples A, B and C under different ment of coal mine waste during the compaction courseent. South Univ Technol. (2008)15: 350-355353a)-Soft surrounding rock. Moderate hardness surrounding rock(d).- Soft surrounding rockHardsurrounding rockExtra-hardsurrounding rocksurrounding rock▲一 Hard surrounding rockExtra-hardsurrounding rockAxial strainAxial stress/MPa. Moderate hardness surrounding rockSoft surrounding rock▲一 Hard surrounding rooModerate hardnessExtra-hardsurrounding rocksurrounding rockHard surrounsurrounding rock0.2Axial strainAxial stress/MPac).-Soft surrounding rock(· Soft surrounding rockModerate hardness surrounding rockModerHard surrounding rocksurrounding rock10∴二 surrounding rocAxial strainAxial stress/MPaFig 2 Compaction and lateral pressure curves of three kinds of samples under different surrounding rock conditions: (a) Compactioncurves of sample A; (b)Compaction curves of sample B; (c)Compaction curves of sample C;(d) Lateral pressure curves of sample A;(e)Lateral pressure curves of sample B; (f Lateral pressure curves of sample Cand under a harder surrounding rock condition the time curves reach this stage, the granular characteristic of coalto reach the stable settlement deformation is shorter than mine waste will almost vanish, and coal mine waste willthat under the soft onepresent the constitutive behavior of the intact elastomer.In the third stage the axial stress increases rapidly There中国煤化工 as follodeformation and the increscent rate of the axial stra n are ion 1CNMHGsmaller than those of the anterior stages. When theThe above results show that the deformation feature354J. Cent, South Univ. Technol.(2008)15: 350-355of the surrounding rocks is of a significant influence on the amplitude of lateral pressure is, especially under thethe final settlement of the coal mine waste. The filled hard or extra-hard chamber conditions. This trait alsocoal mine waste quickly reaches the final settlement appears in the third stage, but the shape of lateralunder the harder surrounding rock condition, which will pressure becomes linear. This is because the large axialbe beneficial to the filling process. The influence of stress has already eliminated the characteristics of coaldifferent surrounding rock conditions on the axial mine waste particles and made them become somewhat adeformation of coal mine waste can be clearly observed intact elastomer.om Figs. 2(a)-(c)). The compaction curves obtainedfrom the three samples with different lumpiness 3.3 Analysis for final settlement and maximum lateralgradations also show some interesting results, thepressurespacing of the compaction curves of sample A with aThe final settlement results and the maximumhigher proportion of small granule obtained under the lateral pressure are listed in Table 2four surrounding rock conditions, is obvious sparseThe test results show that the final settlement ofwhich is larger than that of samples B and C. It can be coal mine waste under the same axial load is closelyreasonably deduced that the surrounding rock condition related to the lumpiness gradations and the deformationcan produce a greater effect on samples with a higher behavior of chamber materials that are used to simulateproportion of small granule.behaviors of different in-situ surrounding rocks. TheFigs. 2(d(n)) show that the lateral pressure curve final settlements(or the axial deformation) of sample A,under the four different surrounding rock conditions can which are tested under four simulated soft, moderatethree stages, which are hard and extra-hard surrounding rock conditions, arecorresponding to the three stages of compaction curves. 92.51, 88.00, 86.50 and 77.98 mm, respectively; those ofIn the first stage, a short-time fluctuation nearby 0 of the sample B are 94.03, 90.80, 89.60 and 85. 24 mm,lateral pressure can be observed with the increase of respectively; and for sample C, they are 94.62, 91.10,axial stress. This is may be that the axial stress is mainly 90.75 and 89 73 mm, respectively. a visible rule canused to fill the clearance among coal mine waste reached that under the same surrounding rock condition,particles in this stage. In the second stage, the coal mine the final settlement due to the same maximum axial loadwaste enters into crushed course with the increase of the decreases with the decrease of the proportion of largeraxial stress, which will lead to the catastrophe and gradation of coalwaste, while for the samasymmetry of the lateral pressure. Most of lateral lumpiness gradation case, the final settlement increasespressure curves show the serrate fluctuation in this stage. with the decrease of elastic modulus of the simulatedThe figures also show that with the increase of the axial surrounding rocks.stress there is a rapid rise for the lateral pressure inIn addition, when appropriate similar materials areperiod, and the harder the surrounding rock is, the larger used to simulate soft, moderate hardness, hard and extraTable 2 Fimal settlements and maximum lateral pressures under different surrounding rock conditionsSimulated surroundingFinal settlement/ Maximum lateral pressure/rock conditionsimilar materials/ GPaSample NommA9251Soft94032.318800Moderate908043386.506915.72Extra-hard中国煤化工752CNMHG5.Note: Elasticus of similar material was measured by standard samples artificially made using the same material when moulding the homologous loadingchambers; the final settlement and the maximum lateral pressure all corresponded to the peak axail stress of about 20MPaJ Cent. 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