Underground pressure characteristics analysis in back-gully mining of shallow coal seam under a bedr

Mining Science and Technology( China)21 (2011)23-27Mining Science and Technology( china)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcUnderground pressure characteristics analysis in back-gully mining of shallowcoal seam under a bedrock gully slopeWang Xufeng,", Zhang dongsheng Fan Gangwei, Zhang ChengguosTate Key Laboratory of Coal Resource and Mine Safety, China University of Mining S Technology. Xuzhou 221008,ChinaARTICLE IN FOA BSTRACTWe studied underground pressure and its mechanism during back-gully mining in a shallow coal seamI April 2010nder a bedrock gully slope, by means of physical simulation, numerical modeling and field monitoring. TheJune 2010esults show that the intensity of underground pressure is relatedrelative position at the coalface. Thederground pressure is intensive and the support resistance reaches a maximum when the coalface is ate bottom of the gully, whereas the underground pressure is moderate and decreases gradually when theShallow coal seamdirection to the slope dip during back- gully mining and form an unstable. multilateral block hingedtructure, due to slipping. The subsidence of multilateral blocks is considerable when the block fragmen-rmous changes in the underground pressure with an increase in the mass ofhe block body, the block displacement will be reduced in conjunction with an increased clamp effect byboth the unbroken rocks and broken rocks in the goaf, resulting in a decrease of the underground pressure.Copyright o 2011, China University of Mining Technology. All rights reserved.1. Introductionunderground pressure and reveal its mechanism. The preliminarystudies have shown that the composition of gully slope materialThe undulating surface topography features are the result of and the advancing direction of a coalface have a large effect on thenew characteristics of underground pressure during long-wall pressure at underground coalfaces. Hence, we introduce themining in shallow coal seams in northwestern China. Increasingly,advance of coalfaces under back-gullies as the main miningmore attention is paid to this phenomenon with its adverse effect method, based on the geological conditions of bedrock gullieson safety and efficiency in coal production. Zhong, Zhang et alKang, Kang et aL. and Wang et al. have pointed out the particular 2. Shape and material composition of bedrock gully slopesphenomena of underground pressure and surface subsidenceduring coal mining under mountains in western areas of ChinaBy bedrock gully slopes, we refer to slopes entirely composed of11-5). In addition, we carried out a series of studies on the features weathered bedrock, or slopes covered with a thin unconsolidated layerof gully slope activities and their classification for mining under under conditions which can be ignored for the entire slope(category l).slopes in mining areas of western China with wide gullies 6-8Their shapes and characteristic material composition are shown in Fig 1.These investigations have provided the technical support forAs part of the dongsheng coalfield, the Yitai coalfield is mostlyground control, as well as enriching ground control theories of covered with material from the quaternary. A subject to the impactofshallow coal seam mining in this area. However, special studieswater erosion, the region is full of criss-crossed gullies, with exposedthe characteristics of ground pressure during shallow coal minirbedrock and the variation in surface height is relatively large. Besides,the under gully slopes are just at an initial stage and the achieve- the varied morphology of the overlying gully slopes is that of typicalments of these studies have not yet been systemic or thorough. hilly erosion of a plateau. Within this mining area, bedrock gullies aremining practices under gully slopes will become more commonostly eroded slopes with dual-structured material composition.The upper part is a loess layer of different periods of the quatenary,Therefore, it is necessary to analyze the characteristics ofusually 2-3 m thick but may be as much as 10 m. Thexposed in the lower part, where weathering is very serious. TheslopeCorresponding author. Tel: +86 15950672601.中国煤化工 generally2045. DitchesE-mail address: wangxufengecumteducn(W. Xufeng).CNMHG1674-5264/s-see front matter Copyright e 2011, China University of Mining Technology. All rightdoi:10.1016/msc201012002w. Xufeng et aL/Miming Science and Technology( China)21(2011)23-27is only one thick and hard rock in the bedrock which controls theoverburden movement and either lies in the lower part of the frac-tured zone or in the caved zone3. Undergroback-gully mining under bedrock gully slopes3.1. Physical simulation of slope activities during back-gully miningunder bedrock gullies3. 1.1. PhysicalA physical model was established basing on the geology of thelayewusu gully at the Nalinmiao coalmine with dimensions5 m x 0.3 m x 0.95 m. The major similarity coefficients are asa category Ifollows: geometric ratio ar- 100; bulk density ratio ar=167 andintensity ratio ar =aI x ay= 167. Sand is used as an aggregatematerial and calcium carbonate together with gypsum as cementing material. The corresponding material ratios of the various rockswere determined basing on a comprehensive consideration of thecharacteristics of rock deformation 9-11]. In order to improve onthe limitations of our plane stress model and reduce the test errors.a transparent plexiglass was placed at the bottom of the model,preventing excessive lateral deformation and fragmentary leakageIn order to analyze the features of slope gully mining activitiesunder different conditions of slope angles without changing thegeological conditions, two physical models with slope angles of 20,30 and 45 were established. Fig. 2 shows the model of a simulatedvertical gully, 73.5 m deep with a slope angle of 30b3. 1.2. Slope activitiesFig. 1. Bedrock gully slope shape and features of material composition.The slope activities of the bedrock gully slopes with different slopeangles are shown in Fig 3. As seen from this figure. in the process of thecoalface advancing, breaks appear in the immediate roof, the mainGiven the combination of the appearance revealed during mining roof does not shed any broken layers, but forms tensile cracks againstoperations and the experience of engineering projects in this coal- the direction of the slope. The cracks extend from the surface of thefield, the geological conditions which affect underground mining are slope to the coalface. A block is cut by the cracks and slope rocks in thesummarized as follows. o the coal seam is from 2 to 6 m thick with direction of the goaf and with the support of the action of the rockdip angles of 1-3 and a simple coal geology: the bedrock is thin, mass forms a stable, a multilateral hinged structure. The ability of theless than 50-60 m and the coal seam is less than 100 mdeep: O there block structure to pass the level of a multilateral force becomesFis. 2. Original model of the experiment中国煤化工b slope angle30°CNMHGw Xufeng et aL/Mining Science and Technology( china)21(2011)23-27overburden rock collapse with a low angle, where the instability ofUDEC(Version 3.10)the block structure lags the coalface by a specific distance as shownin Fig. 3a. When the slope angle is increased to 30. the multilateralblock appears to be in a more relatively obvious phenomenon ofleaning with a larger overburden collapse at a larger angle, as则HI shown in Fig. 3b. When the slope angle becomes 45, the multileans to the goad 30, as shown in Fig. 3c. The experiment indicates that, when thope angle is 30, massive dumping occur once at every 40-45 mon average. This interval is reduced to about 30 m when the slopeangle is increased to 45. We conclude that the frequency ofmassive dumping increases with the slope angle3. 2. Underground pressure characteristmulation of the groura model foundationAfter mining, the support suffers a certain amount of pressureaused by deformation and collapse due to the overburden. when the035pressure on the roof is greater than the support resistance. thege 10slope angle 15to be reduced which to some extent reflects roofslope angle20°一X- slope angle25°pressure. We used numerical calculation to simulate the characteristics of ground pressure on a coalface under an advancing back-gullyThe reduction in support resistance is regarded as an indicationtofunction of differ0.20The support unit in UDEC(Universal Distinct Element Codesed to simulate the suppothe data. The established model of gully slope under an advancingack-gully is shown in Fig 4a[12-16. The model is 290 m longwith a maximum overlying thickness of 95 m and a minimumthickness of 21.5 m under the gully bottom. The simulated reduccoalface mining length /mtions of support under the same support resistance(7000 kN)givendifferent slope angles(10°,15°,20°,25°) are shown in Fig.4b.From the simulation results shown in Fig 4b, the shrinkage of0 60 120 180 240 300 360 420 480 support under the advancing back-gully is small prior to theb Support deformationweighting of the coalface. However it will increase tremendouslyafter the weighting of the coalface. during the advance of mining,the shrinkage of support decreases slowly due to the increase of theUDEC (Version 3.10)thickness of overlying strata. It can be seen that during the advancemovement of the back-gully, the multilateral block( Fig. 4c)formedwheke miningInducedby the entire segment has a great impact on underground pressureThe shrinkage of support reduces and stabilizes after the coalface姚passes the slope segmentThe #2602 mechanized coalface of the Yitai Coal Group istypical bedrock gully slope coalface, with a ground surfaceelevation of +1228-+1294 m, an inclination 180 m long anda strike length of 735 m. An aeolian surface sand is largely found inthe lowland and mountain slopes, generally less than 5 m thick,next to the bedrock. according to drilling of the nearby coalface andthe exposed coalface the distance between the roof and the shal-lowest part of the surface is about 35 m in the open-off cut. Givenour comparison and field measurements, the surface sC Mining-induced slope activity characteristicsangle ranges from 8 to 25 under the advancing back-gully,average angle of 20. The type of coalface support is aFig. 4. Back-gully mining simulation results under bedrock slope6800 kN, and a mining height between 1.4 and 3. 1 m. The results ofour observations for the support resistance at the upper, middlerelativand as a result the structure is likely to fall in the face and lowre shown in Fig. 5at theunstable fulcrum.中国煤化工 r of the supporthat, the features of slope activities vary with changes resistalgle. when the gully slope angle is 20, a multilateral appeC MH Gart is large, botheblock seems to be in a relatively obscure phenomenon of a small small". The support resistance reaches its maximum at 6550 kN, ieW. Xufeng et al Mining Science and Technology( China)21(2011)23-276000upper partmiddle part→ lower partblock 31000block 4a Small block fragmentFig. 5. Characteristics of coalface support resistance.96.3% of the rated resistance, at the middle part when mining is underthe downstream gully. Toward the end, the maximum support resis-tance decreases when the face advances and the depth increases.4. Underground pressure characteristics analysis duringbedrock gully slope miningblock 5 block 6slopes to form an unstable multilater lt is easy for mining-inducedcoalface support resistance decreases gradually and then stabilizes.hile the distance between the gully and the coalface increases. Inrder to analyze the effective mechanism of the multilateral blockb Big block fragmenthinged structure on the coalface strata, we built a special physicalsimulation unit.Fig. 7. Characteristics of the multilateral block hinged structural movements.4. 1. Establishment of multilateral block hinged structural model maximum height of 80 cm. Eleven monitoring points, consisting ofseven vertical displacement points near the surface of the seaml andslope is presented in Fig 6a. Fig 6b shows a physical model to analyze vation unit 5 cm long and an excavation interval of 10 mi9aGiven the results of our physical simulation and numerical anal- four horizontal displacement points, are established and numberedis, a structural model of a long-wall mining under a bedrock gully sequentially. The total excavation length is 225 cm with each exca-the changes in multilateral block motions and articulated features.The model is composed of seven different sized multilateral 4.2. Mechanism analysis of the underground pressure affected byblocks, with a total length of 245 cm, a width of 20 cm and a the movement of multilateral block hinged structures4.2.1. Characteristics of the multilateral block hinged structuralFrom Fig. 7, it is seen that, the characteristics of the multilateralblock hinged structure motion change with the block fragmenta-tion. when the fragmentation is small (e.g. block 4), the block isrst aumection of the goaf∵.B.k, slowly sinkgoaf. And finally obtains a balance with the combined action oftunbroken rock, goaf stabilized rock, and the immediate roof with itssiLIEssupport. The structural characteristics are shown in Fig. 7aa Structural model of mining under bedrock gully slopeWhen the size of the block increases(e g, block 5). the spacebetween rocks becomes relatively small due to the compression ofrock blocks and the gradual filling of the gaps between rock blocksby broken rocks. The intact subsidence of block 5 decreases withthe effect of increasing compression of the broken block and4.2.2. Block displacement characteristicsThe vertical and horizontal displacements of the blocks in theworking face advancing process are shown in Fig 8a and b Fig 8ab Complete model with monitoring points中国煤化工mal. multilateral blocksd horizontal displace-Fig 6. Foundation of structural and multilateral block model under bedrock gullyCNMH Glarge blocks accelerateslope mininw. Xufeng et aL/Mining Science and Technology( China)21 (2011)23-27block fragmentation is small. Along with increasing fragmentation of the multilateral block, the clamping force generatedpalace mming leagueby the unbroken rock and the broken rock in the goaf willc一Kincrease and results in a decrease of the block displacement.AcknWe acknowledge the financial support for this work?"51004101SitE the National Natural Science Foundation of China(Nos. 51004101and 50904063 ), the Science Foundation for Young Scholars of Chinaa vertical displacementUniversity of Mining Technology(Nos. 2008A003 and 2009A001)the graduate Student scientific Research Innovation in the univer-sity of Jiangsu Province(No, CX07B-_ 149z) The Program for Intro-duction of Talents of China University of Mining Technology is alsogratefully acknowledged.Reference[11 Zhong XC Overlying strata movement in shallowalley. Xuzhou: china University of Mining an0函⊥A[2]Zhang XB, Fan KG. Zhang GS. Strata behaviors of fully mechanized coalfaceShandong Coal Science and Technology2009:(1)}122[3] Kang JR Analysis of effect of fissures caused by underground mining oration Chinese Joumal of Rock Mechanics and8. Displacement curves of blocks as a function of the distance of an advancing14) Kang JR. He WL Hu HE. Analysis of the mountain surface deformation andund mining. Beijing: China Science and Techfragmentation increases gradually, the overall displacement of the (sl Wang P Gao x Liu c wines LHigh precision slope deformation monitoringblock decreases with the increasing clamp effect of the unbrokenrock mass and the broken rocks in the goaf, resulting in a decreased 6l wan g e, in igg-vngruowd s wpd e minin gm echanixmhnd ts on uon oeray wfragmentation easily produce significant subsidence and largesoil gully sology 2010: 38(6): 18-22 In Chinesedisplacements at high speed of deformation. This results in a large (8)wang XF. Zhang DS, Zhai DY Fan Gw. Zhang CG. Analysis of activity char.impact on the coalface with the mass of the block body increasing.the displacement of blocks and their speed reduce because of thely slope in shallow coal2010 internaincreasing clamp effect by the unbroken lower rocks as well as thece on mine hazar prevention and control Paris: Atbroken rocks in the goaf, leading to a decreasing impact on coalface [9] u HC. Similar simulation experiment on mine. Xuzhou: China University ofI10] Gu DZ Equivalent material and similar models. Xuzhou: China University of5. ConclusionsWe arrive to the following conclusions:dia earth and planetary Science 2009 1(1/160- west of h hnique and[11] Zhang DS. Ma1121 Hang Y. Zhang GL Yang GY. Numerical simulation of dewatering thickof the coalface. Its intensity and support reaches a maximum 13) Hao y Un BO. Wu HL. Meng ,, Relationships bet ween gas reservoir and thewhen mining activity reaches the bottom of the gully, thesupport resistance will gradually decrease once the coalfacepasses the gully.[14] Wang XF, Zhang DS, Wang HS Qui T2)The results of our simulation show that the mining-induceddated aquifer. In: Proceedingslope rotates toward the reversed direction of the slope dip andm题p123eIndustry Publishing House:slipping. The smaller the multilateral block fragmentation, the 5I Zhang s Fan W Mau tfac e an ciu env Honrmenty of ltrcgton inmore the block rotates, subsides, and the more intense thein Shendong mning area, northwest of China. Iunderground pressure.3)The multilateral block subsides considerably. Its displacement[161 Wang XF, Zhang DS. Ma LQ Liu YD. Feng JQ. Numerical analysis on distrib-and speed of deformation are both relatively large when theMining Research and Development 2008: 28(5): 61-3 [in Chinesel中国煤化工CNMHG