Study of "3-Step Mining" Subsidence Control in Coal Mining Under Buildings

joumal of China University of Mining& TechnologVol 17 No. 3Availableonlineatwww.sciencedirect.comSCIENCEDIRECT·J China Univ Mining Technol 2007, 17(3): 0316-0320Study of“3- Step mining” SubsidenceControl in Coal Mining Under buildingsGUO Guang-li, ZHA Jian-feng, WU Bin, JIA Xin-guSchool of Environment Spatial Informatics, China University of Mining Technology, Xuzhou, Jiangsu 221008, ChinaAbstract: Mining subsidence damage is the main factor of restricting coal mining under buildings. To control or easeeffectively the degree of mining subsidence and deformation is essential to resolve this problem. Through analyzingboth advantages and disadvantages of some technologies such as mining with stowing, partial extraction and grouting inseparated beds of overburden, we used the principle of load replacement and propose a3-step mining"method, a newpattern of controlling mining subsidence, which consists of: strip mining, ie. grouting to fill and consolidate the cavingzone and retained strip pillar mining. The mechanism of controlling mining subsidence by using the3-step miningpattern is analyzed. The effect of the control is numerically simulated. The preliminary analysis shows that the 3-stepmining"can effectively control ground subsidence and deformation. By using this method, the ground subsidence factorcan be controlled to a value of about 0. 25. Coal recovery can reach 80%-90%. Coal mining without removing surfacebuildings can be realized and the economic loss resulting from ground subsidence can be greatly reducedKey words: subsidence control; strata movement; ground movement; mining under buildingsCLC number: TD1 Introductionrials to fill the caving zone formed after mining, in-cluding hydraulic sand-stowing and refuse-stowingThe history of coal mining under village buildings The effect of ground subsidence control depends onhas been plagued by difficult problems affecting coal the degree of filling the caving zone. The groundThe problem is especially serious in eastern China hydraulic sand-stowing to 60% for pneumatic re-because of its large population and densely populated fung. This method is costly and complex. Itvillages. Coal reserves under village buildings amou- has seldom been used in China during the last tennted to 4. 4 billion tons in Shandong Province in 2001, yearsaccounting for 53% of the minable coal reserves. It isThe method of grouting in separated beds of over-of vital importance to take some reasonable counter- burden proposed recently is a new method of groundmeasures to resolve the problem of mining under vil-subsidence control. Essentially the method is to groutlages. For a long time, the main methods of coal and fill the separated beds of overburden in order tomining under buildings was to remove the buildings prevent the mining-induced voids and fissures fromor to mine only a part of the reserves. Removing vil- upward transfers and to prevent or ease the continuallages costs money and is therefore hard to realize. subsidence of overburden beds, achieving the aim ofMoreover it is difficult to choose new sites for vil- easingground subsidence. Some data obtained fromlages. Although strip mining does less damage to vil- some working faces where this technology is usedlages, its recovery is quite low, causing large amounts show that the ground subsidence factor ranges fromof coal resources to become unusable reserves, re- 0.20-0.32. Literature shows that, in Poland, groundsulting in a great waste of resourcessubsidence decreased by 20% to 30% when using theOther technologies of controlling and easing the method of grouting in separated beds of overburdenstowing and grouting in separated beds of overburden, mo ever, when the method of grouting in separatedburden of ground subsidence include mining with Howe中国煤化工well controlled andetcMining with a stowing method uses foreign mate- it isYHCN MH Gund subsCorrespondingauthorTel+86-13615104142:E-mailaddressguogl65@126.comGUO Guang-li et alStudy of"3-Step Mining" Subsidence Control in Coal Mining Under Buildingsusing this method in a large area.mining with low recovery. The width of mining stripsPartial extraction methods include room mining should be controlled to about 1/5-1/10 of the thicknd strip mining, which uses rectangular pillars or ness of the overburden bed and the area recoverystrip pillars to support the overburden beds to control should be controlled below 35%. The width of re-ground subsidence. Strip mining is the main technol- tained strip pillars should be limited to about 1/3-1/5gy in controlling ground subsidence in mining under of the thickness of the overburden bedbuildings and is therefore widely appliedFirst step: the stripped working face is first minedThe theory of strip mining is that the coal seams according to the strip mining patten. In this case, thre divided into normal strips to be mined, which characteristics of overburden bed movement and de-means that a strip is mined and the next strip is kept formation after mining is equivalent to that of regularunmined as pillars to support the overburden bed, and strip mining with low area recovery. The immediateconsequently to ease the subsidence of overburden roof collapses to fill the caving zone, and the mainbeds and to control ground movements and deforma- roof fractures form block beams or a naturally baltion. Since 1967, strip mining has been used in many anced arched structure. According to experientialmining districts such as Fushun, Fuxin, Jiaohe, Feng- analysis of ground subsidence control in strip mining,feng, Hebi, Pingdingshan, Xuzhou and Zibo obtain- the ground subsidence factor can be controlled belowing great social and economic benefits and achieving 0.15many good scientific results. In situ observations Second step: the caving zone is filled and the colshows that strip mining can effectively control lapsed and fractured rocks in the caving zone areground subsidence. The subsidence factor of strip consolidated. By doing this, the caving zone andis 4.8%-26.8% of that of full extraction overburden fractured rock mass are consolidated intods, with a variation of 0.024-0. 206, leaving the a regenerated abutment pillar to restore its load-carrydamage to ground buildings controlled within grade l. ing capacity. This is the key step for this method toThe greatest disadvantage of ground subsidence con- control ground subsidence. The load-carrying capactrol in strip mining is that the permanent loss per- ity of the regenerated abutment pillar should reach orentage of coal is quite high. Although some scholars be close to the practical load-carrying capacity of thehave proposed and begun experiments in which blot original pillaror cable anchors are used to consolidate coal pillars Third step: the retained strip pillars are mined. Theand reduce the pillar size, successful examples of purpose is to guarantee the stability of the regeneratedstrip mining show that in most areas recoveries from abutment pillar and ease the effects of grouting andworking faces are only 30%-50%, which is a bot- caved wastes on coal quality and to be convenient fortle-neck restricting strip mining to be widely applied gateways driving. Two residual strip pillars, 5-10 min ground subsidence controlin width, should be retained on both sides when theretained pillars are mined. The regenerated abutment2 Principle 2pillar in which the carrying capacity has been restoredis used to support the overburden bed. So the loadTaking advantage of strip mining and mining with replacement of the overburden bed is realized and thestowing to control bed movements and ground subsi- secondary strip mining pattern, supported by regenerdence, the"3-step mining "sully uses the control of ated abutment pillars enwrapped by residual striprock mass structures over strata movement to disposeillars, is formed. The combination of strata move-of leap-frog mining on the strip working face, the ment and deformation resulting from the second stripgrouting to fill and consolidate caving and fractured mining with those from the first strip mining makeszones in order to restore its load-carrying capacity, the ground form a state similar to that of a gentle sub-he principle of load replacement is applied to control sience in a large area, greatly reducing ground sub-overburden beds. The basic idea of this method is that sidence deformationthe mining area is altenatively divided into miningThe process of 3-step mining for overburden bedstrips and retained strip pillars according to stripand ground subsidence control is shown in Fig. 1Coal seCoal seCoal seanCoal seamA化(a) Before mining (b)Fractured and movement characteristics (c)Fi中国煤化工 ement after miningof overburden bed in the first strip miningavingFig. 1 Diagram of 3-step mining for groutYHCNMHGJournal of China University of Mining TechnologyVol, 17 No. 33 Grouting and Consolidation3.2 Borehole grouting3.1 Pipe transport system to fill caving zoneBorehole grouting to fill and consolidate the cavingzone is technically simple and highly reliable. BeThe second step, filling the caving zone and cocause the grouting zone is relatively closed and itssolidating the collapsed and broken rocks in order to fractured overburden is the primary strip workingmake the coal pillar reach its original load-carrying face, surrounded by pillar bodies and integrativecapacity, is the key for the 3-step mining to control rocks, it is basically a closed zone, quite favorable forsuccessfully ground subsidence. The backfill of cav-grout diffusion and for controlling the amount ofing zones can be realized by a pipe transport system grout and pressure. So it is helpful to have a goodor borehole groutingfilling and consolidating effect and restore its load-It has taken a long time to adopt the pipe transport carrying capacitysystem to fill caving zones, including hydraulicThe backfill materials of grouting can be thesand-stowing and paste backfill. The hydraulic sandlow-cost compound grout, such as fly ash with high-stowing and the paste backfill have been successfullyater cement, or sand with high-water cement, or flyapplied in coal mines and metalliferous ore deposits, ash(or powdered coal waste)-cement. The non-respectively. But most of this backfill material cannot pressure backfill grouting is used during the initialbe cemented and cannot meet the demand of resum- stages while the low-pressure consolidation groutinging the load-carrying capacity of the caving and fracis used at later stages, with cement being the maintured zonematerial in the grout.Dr. Zhou Huaqiang of China University of MiningAccording to practical experience of grouting toTechnology has developed a new PL and SL series fill and consolidate the caving and fractured zone inof paste backfill materials composed of solid wastes, both coal mines and iron mines, the load-carryingsuch as coal gangue and fly ash, which can be trans- capacity of the caving zone and its overburden can beported to working faces to fill caving zones via a pipe restored to a great extent by grouting and filling.Theline system. The results of this preliminary experi- uniaxial compressive strength of the backfilled rockmentshow that the two composition materials can set body can reach 3. 39-8 24 MPa for different groutingvery quickly and have a high early strengthparameters and the triaxial compressive strengthThrough a uniaxial fully stress-strain test, the compressive strength of PL paste backfill material is 0.5 quickly increases with the increasing surroundingstress, with the measured strength of the wall rock ofMPa with an average modulus of elasticity of 565 drilling being 20-55 MPa 5I. Fig. 2 shows the con-MPa. The strength of the paste backfill body may trasting drawings of apparent parameter contours inreach 1.4-1.7 MPa after 28 days. The deformation of one section of the Huainan Xinzhuangzi Coal Minebackfill material is clearly affected by lateral subefore and after grouting, which was obtained by usrounding stresses. The characteristic of strain hardning is good. But when a pipe transport system is ing a high density resistivity method b). It can be seenthat the fractured rock masses in the grouting andused to fill and consolidate the caving zone, the paste consolidation zone have been consolidated into anbackfill material is hard to fill and consolidate thefissures on the block beam of the main roof in the entire rock bodyupper position of the caving zone50(a)pa parameter contour before groutingb)pa apparent parameter contour after grouting中国煤化工CNMH(c)Apparent contour before groutingig 2 Contrast drawings of apparent parameter contours in one section before and after grouting 6JGUO Guang-li et alStudy of "3-Step Mining"Subsidence Control in Coal Mining Under Buildings319The regenerated abutment pillar for which the factors on ground subsidence, i. e the recovery of thed-carrying capacity has been restored, is used to original strip mining, the strength of the backfill bodysupport the overburden bed, while the collapsed rock and the width of separated pillars on two sides, areof the immediate roof as well as that of the main roof studied. Fig. 3 shows the numerical simulation modelof the new strip caving zone is used to fill the caving The simulation result shows that the strength of thezone, keeping the regenerated abutment pillar under a backfill body and the recovery of the original strip3D stressed state. This stable state can be maintained mining have a large effect on ground subsidencefor a long time. The development of the caving and When the recovery and the ratio of mined/retainedfractured zone will naturally stop when it has devel- remain unchanged, the ground subsidence and horioped to a certain height to form a balanced structure zontal deformation will decrease with increasingof a natural arch in which two arch abutments lie on strength of the backfill body. The area recovery of thethe regenerated abutment pillar, or a balanced struc- original strip mining should be controlled at aboutture of the major stratum supported by the regener- 40%. When the strength of the backfill body is equalated abutment pillar. The ground subsidence shows a or close to the strength of the coal body and the widthflat subsidence basin with a strip pattern. Even when of the separated pillar on two sides is 5-10 m, theit is overlapped with the gently subsided ground ground subsidence can be controlled below 20%formed after the original strip mining, the degree of given the experience of strip mining in China. Thesubsidence can still be controlled within an accept- value of the horizontal extension of the ground is <2able rangemm/m and the degree of damage to be controlled canbe guaranteed within the range of grade I. Fig. 44 Numerical Simulationshows the ground subsidence curve after the first stepand the third step of 3-step miningThe FLAC40(Fast Lagrangian Analysis forContinuum, two-dimension, version 4.0) developedTop soilby the Itasca Company in America is used to carrySand stonethrough a numerical simulation. The software can beused to simulate the mechanical behavior of fracturesor plastic flows when the geological materials reachI.me stonethe limit of their strength or their yield point, to ana-lyze the gradual fracturing and loss of stability and tosimulate large deformations of mining subsidenceFig 3 Numerical simulation modelThrough numerical simulation, the effects of three1002003004005006007008009001000(a) First step of strip mining(b) Third step of mining the retained strip pillarsFig. 4 Ground subsidence curves after the first and the third step of 3-step miningTable 1 shows the contrast of ground deformation ground deformation of 3-step mining is greater thanmong full mining, strip mining and 3-step mining of that of regular strip mining, the degree of ground de-a horizontal coal seam, 250 m in depth and 2 m in formation degree can be still controlled within thethickness, obtained from numerical simulation.range of grade l(horizontal deformation <2 mm/mTable 1 Contrast of ground deformation among fulland guarantees the safety of buildings)mining, strip mining and 3-step miningSubsidence Maximum horizontalArea5 Conclusionsdeformation(mm/m) recovery(%)a countrvProducing coal in large quan40titie中国煤化工 problem in that3-step mining 0.198grCNMHGings, railwayswhen this occursIt can be seen from Table 1 that the recovery of under the village buildings. The 3-step coal mining3-step mining is lower than that of full mining, but under village buildings works fairly well in decreas-much higher than that of strip mining. Although the ing ground subsidence and is high in coal recovery,oumal of China University of Mining& teVol 17 No.3keeping the mining induced damage to village build- conventional methods of mining with stowing andings within the range of grade I. The 3-step mining is grouting into separated beds of overburden in backfilla new, challenging method of subsidence control in technology and in controlling the quality of consoliprotecting the environment and decreasing miningdation. It is also superior to conventional strip miningin coal recovery and superior to the method of mining2)Combining the advantages of strip mining and with stowing in that it benefits the mine.mining with stowing, the 3-step mining is superior toReferen[l] Su Z J, Liu W S. Study and application of new grouting technology for separated strata to slacken the surface subsidenceChina Safety Science JoumaL, 2001, 11(4): 21-24[2] Guo G L, Wang Y H, Ma ZG A new method for ground subsidence control in coal mining Joumal of China University ofMining& Technology, 2004, 33(2): 150-153.[3] Zhou H Q, Qu QD, Hou CJ, et al. Paste backfill study for none-village-relocation coal mining. In: Mining Science Tech-nology. London: AA Balkema Publishers, 2004: 91-94.[4] Zhao Z Zhou H Q, Qu Q D, et al. Preliminary test on mechanical properties of paste filling material. Journal of ChinaUniversity of Mining Technology, 2004, 33(2): 159-161J Cao S G Liu C Y Grouting reinforcement technique in fault broken roof in working face with individual props. Journal ofChina Coal Society, 2004, 29(5): 545-549]Guo G L, Den K Z, HeG Q, et al. Grouting consolidation and detection of fractured rockmass foundation over abandonedmine goaf. 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