Numerical analysis of the destruction of water-resisting strata in a coal seam floor in mining above

Mining Science and Technology( China)21 (2011)537-541Contents lists available at Science Direct氵5Mining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcNumerical analysis of the destruction of water-resisting strata in a coal seam floorin mining above aquifersJiang ZhihaiState Key Laboratory of geomechanics and Deep Underground Engineering School of Resources and Geoscience China University of Mining 8 Technology, Xuzhou 221008, ChinaARTICLE INF OABSTRACTArticle history:With the increase in mining depth many mining areas in China have entered a period necessitatingReceived 25 December 2010ining above aquifers. Production safety in coal mines in northern China is under serious threat fromReceived in revised form 22 January 2011Ordovician karst water on coal seam floors. In order to analyze the destruction of water-resisting strataAccepted 20 February 2011Available online 22 July 2011in floors of coal seams being mined and to achieve safe mining above deep aquifers, we established anumerical model of water-resisting strata, considering the structural characteristics and mechanicalproperties of a floor layered with hard and soft rock We simulated the distribution characteristics olMining above aquiferdeformation, failure and seepage using the analytical module of fluid-structure interaction of FLAc.fater-resisting strata in floorsWe also obtained the corresponding stress distribution, deformation and flow vectors. Our results indi-umerical simulationcate that: (1)the advance of the working face causes water- resisting strata in goaf floors to form a deepdouble-clamped beam, subject to homogeneous loading at the bottom; (2 )the two sides of the rock beamStress fieldare subject to shear failure: (3)both sides of the rock seam at the bottom of the water-resisting strata aresubject to tension and the greater the working face advance, the more serious the failure; 4)the originalbalance of the stress and seepage fields are broken and redistributed due to mining activities, especiallyhe interaction of the abutment pressure in both sides of the goaf the lateral pressure on the goaf floorand the water pressure on the floor of the aquifer promote floor heave and shear failure on both sides ofthe floor forming a water-inrush passage. Our study results can provide references for the mechanism ofwater-inrush on mine floorse 2011 Published by Elsevier B.V. on behalf of China University of Mining& Technology.1 Introductionpermeability channel, water rock stress, 'zero position destruc-tion and in-suit fracture 'key strataand down three zones'haveWith the increase in mining depth and intensity, many coal improved understanding of the mechanism of water inrush onmines, especially those in northern China, will mine above aquifers floors from various aspects, providing some positive guiding rolesand a number of mines are mining coal seams under carboniferous for safe production in mines [3-14. since the process of watersystems. Because coal seams under carboniferous systems are inrush on floors is complex and similar tests do not always showclose to Ordovician limestone, the threat of water-inrush from coal the coupling effects of confined water and simulation materials,seams to floors caused by karst water is significant. Currently, it is difficult to understand the failure process and the variablesmore than 150 billion ton of coal reserves of the major coal mines involved in water inrush [15, 16]. Thus, in order to analyze thein the north are under threat of deep karst water from carbonifer damage inflicted on goafs in water-resisting strata (WRS)givenous systems. this karst water constitutes a largely hidden problem the effect of mining working faces as well as that of the confinedfor safe production of coal mines in northern China. However, water, we used numerical simulation to study changes in stressmining above aquifers is one of the more effective methods of coal and seepage fields in WRS in mine floors, summarized the rulesseam exploitation under threat of water-inrush to floors [1, 2]. For of failure and water inrush in floors, predicted troubles in floorsdecades, some successful exploration of coal seams, subject to caused by water and formulated timely preventive measures,water-inrush on floors, has been demonstrated by investigators which may be of great importance to ensure safe production andboth at home and abroad, with many advantages [1-16]. For improvement in coal mine productivity.xample new theories such as 'water inrush coefficient, 'strongWe will discuss the difference in strength and mechanical properties of the strata in floors and establish a mechanical model ofWRS in a mine above an aquifer flacd was used to simulate事Tel:+8651683885205the deformatioE-mailaddress:jiangzhihai@cumt.edu.cnle aquifer中国煤化工 deformation and1674-5264/s-see front matter o 2011 Published by Elsevier B V on behalf of China University of MiningCNMHGdoi:101016/mstc.2011J Zhihai/Mining Science and Technology(China)21(2011)537-541respectively stands for the vertical stress and shear stress, MPa.Table 2 shows the peak stress in the floor of the goaf at differentadvancing distances. These figures show that:(1)given the effectof the overlying strata and the lower confined aquifer, there iscertain level of tensile stress concentration in the roof and floorof the goaf; if the stress in the floor is the same as in the roof, therewill be a certain amount of crushing in the floor of the goaf as theworking face advances; the tensile strength o, of the rockmass isalways very low, which means that this part of the strata has poorFig 1. Model of mining above an aquiferwater insulating effect;(2)the goaf hanging area becomesenlarged as the working face advances, the stress concentrationflow vector distribution on the basis of which we studied the pos- concentration range extends to the depth of the roof and Nlevel at both ends of the goaf increases continuously and the stresssibility and factors affecting water inrush on floors [17].especially the extension in the floor which is the main factor2, Numerical calculation model and schemethe formation of a water-inrush passage from the continuingexpansion of the fractures which, in the end, break the wrS.Table 2 shows the peak stress in the floor at different advancingIn order to analyze the effect of confined water on its insulation distances, indicating that the peak vertical stress at the top ofwRSeffect on WRS in floors, we established a model on mining above in the floor increases by 2-3 MPa and the peak shear stress byaquifers, shown in Fig. 1. The model is 400 m long and 200 m wide, 0.6-1.8 MPa, for every 10 m advance of the working face. Thisincluding the WRS, a coal seam, overlying strata and a lower aquiimplies that the peak vertical stress increases about 17 MPa andfer. The WRS lie in the middle of this model, which is composed of the peak shear stress about 4.8 MPa, when the working facethree 6 m thick hard rock and two soft rock seams, also 6 m thick. advances from 40 m to 100 mhe coal seam is 3 m thick, the immediate mudstone roof 2 m andthe overlying stratum, 85 m. Below is an 80 m thick limestoneaquifer. Both sides of the model, as well as the bottom, are simply 3.2. Deformation feature of WRS infloorgarded as fixed constraints while the upper boundary is subjectto an equal, distributed load g which represents the weight ofTo some extent, the deformation of wRS in the floor may reflectthe rock base. Obviously, the load g is related to the thickness of itsits damage. Fig. 6 shows the vertical displacement of the top wRSthe rock base. if the density of this base is 2000 kg/m, then the in the floor changing with the advance of the working face, whereock base is 600 m thick and 12 MPathe horizontal axis L is the distance from the initial working face,During simulation, we kept the lithology parameters of the soft m: and the vertical axis h is the vertical displacement of the topand hard strata unchanged in the wRS, given the same thickness ofWRS in the floor, mm. Table 3 shows the vertical displacement ofoth the soft and hard strata, established the step of the working the top Wr in the floor at different working face advancingface advance as 10 m and the total number of excavation steps distances. Fig 6 and Table 3 show that: (1)given the effect of thewas 10, i.e the entire working face advance is set for 100 m. The lower confined aquifer, the goaf floor shows obvious floor heavespecific physical parameters of each layer are presented in Table 1. and with the advance of the working face the goaf heave areaenlarges and floor heave becomes increasingly more serious, withthe maximum floor heave in the middle of the goaf;(2)whenthe working face had advanced 40 m, the vertical displacement3. 1. Stress distribution of WRS in floorof the WRS in floor was 37. 8 mm, the deformation increased gradually as the working face advanced; when the working face hadFigs 2 and 3 show, respectively, the vertical stress and shear advanced 80 m, the vertical displacement had increased 62% andstress distribution in the floor at different advancing distances of reached 61. 2 mm; (3) when the working face had advancedthe working face. Figs. 4 and 5 show the curves of vertical stress 90 m, the floor heave increased rapidly to 122.3 mm, i.e., 224% ofand shear stress, changing with the advancing distance of the the floor heave when the working face had advanced 40 m, whichworking face, where the horizontal axis L is the distance from means the WRS in the floor may have become unstable at thisthe initial working face, m: and the vertical axis dy and t moment; (4) when the working face had advanced to 100 m, theMechanical parameters of surrounding rock in coal face.NameThickness Elastic modulus CompressionPoisson's Density p FrictionSeepagePorosity Pore pressureE(GPa)rength as(MPa)tatoμg)ange中(°) coefficientμ(mn(‰) coefficient kOverlaying00010352.5000o10010Mudstone252.001151mestone0352.500010Ordovician 8035中国煤化工CNMHG. Zhihai/Mining Science and Technology( China)21(2011)537-5415010000YY-stress contours120160200120160200240280a)Working face advance 40 m(b) Working face advance 60 m00E+015o Contour interval=5.00E+0600009000120160200240280240280(c) Working face advance 80(d) Working face advance 100 mFig. 2. Normal stress distribution of rock surrounding the working face.19000120160200240280120160200240280(a) Working face advance 40 m(b)Working face advance 60 m.80E+07170 Contour interval=3.00E+ooO9n00900019000120160200240120160200(c)Working face advance 80 m(d) Working face advance 100 mFig 3. Shear stress distribution of rock surrounding the working face中国煤化工CNMHGFig. 4. Vertical stress in floor with working face advancing.Fig. 5. Shear stress in fioor witn working lace advancing.1 Zhihai/ Mining Science and Technology( China)21(2011)537-541Table 2These figures show that: (1)the plastic zone in the surroundingMaximum stress of floor with advancing working face.rock first appears at both ends of the goaf; when the working faceorking face advancinghad advanced 40 m the plastic zone started to appear in the floordistance L(m)and enlarged with the advancing of the working face: (2)theVertical stress oy ( MPa) 39.37 44.82 46.5250135623plastic zone increased slowly in the direction vertical to the goafShear stress t(MPa)10.1811811235123513.111492floor and when the plastic zone had increased to a depth of about12 m, the plastic zone did not develop along the direction verticalto the goaf floor; (3)when the working face had advanced 60 m,both ends of the wRS in the floor appeared the plastic zone. withthe advancing of the working face the plastic zone increased andextended towards the lower part; (4)when the working face hadadvanced to 100 m, the plastic zone at both ends of the WRS inthe floor integrated and formed a water-inrush passage3.4. Flow vector distributionL(m)A flow vector is the flow rate per unit length of the model and isFlg. 6 Vertical displacement of WRS with working face advancing.mainly used to measure the rate of water flow. Fig. 8 shows thedistribution of flow vectors in the rock surrounding the goaf atdifferent working face advancing distances. Table 4 shows themaximum flow vectors of the wRs in the floor at different workingMaximum displacement of floor with working face advancing.face advancing distances. this figure and Table 4 show that: (1)theadvance of the working face damaged the balance of the initialseepage field; when the working face had advanced 40 m, underVertical displacement h(mm) 37. 8 47.7 52.6 61.2 122.3 148.6the effect of mining, both ends of the goaf floor showed a regionwith local damage caused by shear failure, the fissure water inthe wrs collected in this region and the water in the aquifer wentvertical displacement in the floor was 148. 6 mm. the rate of defor- advancing distance the shear failure in the Wrs in the floormation had slowed down, the floor was seriously damaged and the became increasingly more serious, the focus of the fissure water lestress released. The large floor heave may have caused the tensile vel increased and ran through the weak plane in the upper stratafailure in the middle of the WRS which will become one of the along the left shear failure region, the flow vector increased slowlywater-inrush passages from the floor.from 29.27 um/s when the working face had advanced 40 m to35.82 um/s; when the working face had advanced 80 m, the3.3. Plastic area distribution of wRS in fioorwater-inrush passage had still not formed between the two bottomends of the WRS: however, under the effect of the confined aquiferFig 7 shows the distribution of the plastic zone in the rock sur- at the bottom of the WRS, some fractures appeared, the confinedrounding the goaf at different working face advancing distances. water started to push into the fractures which caused a further120160200240280At yield in shear or vol.20160200240280(a)Working face advance 40 mElastic, yield in past(b)Working face advance 60m120160200240中国煤化工(c) Working face advance 80 mYHCNMHGFig. 7. Distribution of plastic area in rock surrounding the working faceJ Zhihai/ Mining Science and Technology(China)21(201 1)537-5411709000120160200240280120160200240280(a) Working face advance 40 m(b)Working face advance 100Fig 8. Flow vector distribution of wRS in floor at different advancing distances.ble 4of the goaf and the confined water pressure from the aquiferMaximum flow vector of wRs in fioor atof the slope. In the end, a water-inrush passage fage at both endstogether caused floor heave and local plastic darAdvancing distance L(m) 40flow vector292760 70 80 90water-inrush on the floor(um/s)This work was supportthe National Basic Researchexpansion of the fractures;(3)when the working face had ad- Program of China( No. 2007CB209400)and the National Naturalvanced 90 m, the zones of mining fissure and of progressive guide Science Foundation of China(Nos. 50634050, 50834004rise fissures, connected and a water-inrush passage formed; the 50874103 and 50904065)and the Young Scientists Fund of theconfined water in the floor ran into the coal seam through thewater-inrush passage, causing the water-inrush on the floor witha flow vector of 54.50 um/s; when the working face had advanced Referencesto 100 m, the flow vector in the floor had increased significantly toSeepage theory of mining rock63. 12 um/s under the effect of flushing.[1] Miao x Liu wQ Che[2] Feng MM. 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