Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its applicati

International Journal of Mining Science and Technology 24(2014)349-352Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstfracture mechanics model of fully mechanized top coal caving of shallowcoal seams and its applicationZhang Jiangong, Miao Xiexing, Huang Yanli li mengSchool of Mines, China University of Mining and Technology, Xuzhou 221116, ChinaKey laboratory of deep Coal Resource Mining, Ministry of Education of China, Xuzhou 221116, ChinaARTICLE INFOA BSTRACT SArticle historBased on break characteristics of roofs in fully mechanized top-coal mining of thick shallow coal seams, aReceived 10 October 2013fracture mechanics model was built, and the criterion of crack propagation in the main roof was derivedReceived in revised form 15 November 2013Accepted 8 December 2013ng the fracture mechanics theory. The relationships between the fracture length of the roof and theAvailable online 29 April 2014yorking resistance of the supports were discovered, and the correlations between the load on the over-lying strata and the ratio of the crack's length to the thickness of the roof were obtained. Using a workingface of Jindi Coal Mine, Xing county Shanxi province as an example, the relationships between the frac-Super-thick shallow coal seamture length of the roof and the working resistance of the supports were analysed in detail. The results givea design basis in hydraulic top coal caving supports, which could provide useful references in the pracMain rooftical application. On-site experiment proves that the periodic weighting step interval of the caving faceFracture mechanics modelois 15-16 m, which is basically consistent with the theoretical analysis results, and indicates that themechanized caving hydraulic support is capable of meeting the support requirements in the mining ofa super-thick but shallowly buried coal seamc 2014 Published by Elsevier B V on behalf of China University of Mining Technology1 Introductionto long-term geological tectonic movement, different sizes of cran-nies and joints, and even faults inevitably exist in the rock, destroyThe wall rock of a working face in coal mines moves under the ing the integrity of rock beam and playing a key role in the fracturegravitational effects of the overlying strata after active coal mining, of main roof [8, 9]. In the western mining areas of Shendongrhich exerts stress on the hydraulic support systems, and that is Pingshuo and Xingxian, the coal seams are not buried deep, andthe root cause of mine pressure [1. Signs of pressure are specific the roof bedrock is thin, but the earth surface is thick aeolian sandappearance of the activities of the surrounding wall rocks, and to or loess. Especially in the weathered upper rock, fractures and jointsstudy these signs, it must firstly study the roof activity through are well developed, affecting the integrity of the rock mass to a cer-monitoring and assumptions to the roof structure and the pattern tain extent the pressure caused mining with hydraulic shears canof activities[2 Studies in China have concluded the beneficial re- lead to cut-down of the roof bedrock along the face [10, 11sults on strata movement and pressure control in long-wall minBased on analyzing the characteristics of fully mechanized toping. Qian established the masonry beam theory in which he coal caving mining of super-thick shallow seams, a fracturestablished the condition of stability between the key block ma- mechanics model for the main roof was established in this studysonry beam slide and rotary deformation, namely the S-R condition The relationship between fracture length of the main roof and4. Shi proposed two structure forms in terms of the character- the reasonable working resistance of top coal caving supportsistics of roof caving in shallow coal seams, namely the "short block was deduced. This study takes the working face of the Xingxian Jinstructure of voussoir beam"and the"step rock beam, and he used di Coal Mine 1113 in Shanxi as an example, and presents a designthe theory of rigid body equilibrium to analyze the stability of the of hydraulic supports for thick coal seams. the pattern of minekey blocks in the main roof [5]pressure behavior was measured and analyzedCurrent roof structure models generally assume the rock beam ashomogeneous continuum medium, and analyze its stability andfracture all based on the basic theories and methods in theoretical 2. Break characteristics of the main roof with fully mechanizedmechanics and material mechanics of materials (6, 7]. In fact, due top coal caving mining in shallow coal seamCorresponding author. Tel. +86 13905207498For shallot中国煤化工 erally focus on themailaddress:huangyanli6567@163.com(Y.Huang).surrounding roccyHCNMHGpressure, but thehttp://dx.doiorg/10.1016/j.ijmst.2014.03.0112095-2686 2014 Published by Elsevier B V on behalf of China University of Mining Technology50J. Zhang et al/International Journal of mining Science and Technology 24(2014)349-352break characteristics and strata behavior have rarely been studiedDuring the fully mechanized top-coal caving of a super-thick shal-low coal seam, as the mining height increases sharply, the caving ofthe immediate roof cannot fill the goaf. When the mining heightreaches a certain value, the cutting after support appears to therock beam of the main roof, and leads to roof rock cut down alongthe face, as shown in Fig. 1 [ 12]. Meanwhile, when the main roofbreaks. it cannot form a bonded beam structure because there islittle extrusion force. Therefore the main roof can be regarded asclamped or cantilever beam, breaking at the clamped endFig. 2. Fracture mechanics model of main roof.3. fracture mechanics model of main roof and its solutionWhen the first or periodic weighting of the main roof occurs insuper-thick shallow coal seam, the overburden stress g can be ob3.1. Establishment of the fracture mechanics modeltained by using the equilibrium arch theory [20 It can be calculated by eq (1)The shallow coal seams in Shendong, Pingshuo, Xingxian andother mining areas in western China are shallowly buried with thin 28 tan proof bedrock, and the surface layers are thick aeolian sands orloess. The bedrock is especially affected by weathering, and the where r is the average bulk density, the coefficient of lateralfracture and joint development affect the integrity of rock mass stress; and o the angle of internal frictionto a certain extent. In the mining process, fractures and joints de-T has extruding effects on cracks and causes Type I stress intenvelop easily in part due to the pressure of the hydraulic shearer, sity factor, as followsand in turn increase the overall instability of the rock. A main roof K =(-T/h)Fr(a/h)VIanized top-caving mining of super-thick shallow coal seams[13]. whereHowever, due to the overlying layer of thin bedrock and saabove the coal seam, an"arch"damage of the main roof can easilyFr(a/h)=112-0231(ah)+10.55a/h)2-21.72(a/h3occur during the first weighting and periodic weighting. If this+3039a/hdevelops, the load on the key blocks of the roof is not the entireof the overburden 14 In addition, the load on the supportq and p have shearing effects on cracks and cause Type ll stresswas treated as a triangularly distributed load in the model, instead ntensity factor, as followsof a uniformly distributed load [15]Kn=(2qL-pF4a/h)/vπaTherefore, in shallow coal seam mining, with the main roofsfirst weighting or periodic weightings, the main roof can be consid-ered as a clamped beam or cantilever beam structure in order tolculate the interval of weightings. However, since the breaks Fg(a/h)=[130-0.65(a/h )+0.37 (a/ h)?+0.28(a/ h)1/V/1-a/hare at the ends of the clamped beam, the main roof above the goafBending moments are caused by overburden stress. Its stressCracks of the main roof beam are composite compression-shear intensity factor can be calculated by Eq(4)cracks under complex loading, not simply I open type, Il slide type, KM=3qL FM(a/h)vra/h2Ill tear type. Fig. 2 shows the main roof fracture mechanics modelHere, g is the load of overlying strata; T is the extrusion pressure: p whereis the support force: L is the periodic weighting interval: is the FM(a/h)=1.122-140(@/h)+733(a/h)2-13.08(a/)3roof thickness [18, 19]+140a/h)The stress intensity factor of a cracked tip of the main roof is3. 2. Solution of the fracture mechanics modelequal to the addition of the above three simple loadings, asfollowsAs shown in Fig. 2, stress intensity factor of the main roof can betreated as a composition of horizontal stress, overburden stress∑K=3q2FMa/h)√mah2-TFra/h)√ma/hand bending under composite loadings∑Kn=(2qL-pFa/h)/√aCompression-shear fracture criterion of rock and concrete is inthe following form [ 21]A∑K+∑Km=Kain roorwhere i is the coefficient of compression-shear ratio; and Kc theBy substituting Eqs. (1)and(5)into Eq. (6), the criterion can be中0x门H史任Fig. 1. Break characteristics of roof during fully mechanized top-caving mining insuper-thick shallow coal seam.J. Zhang et al /International Journal of Mining Science and Technology 24(2014)349-352176Table 117Technical parameters of hydraulic support.Number ItevaluedeZF10000/23/352300/3500234567dth(mm)1430/1600Centerline spacing(mm)7754Supporting intensity(MPa)41.61.82.02.Intensity ratio between base and roof (MPa) 3.2ement distance(mm)9000Operate on itselfFig 3. Relation between periodic fracture length and support intensity11Pump station pressure(kg/cm2)ding to fieldthe periodicwhich is basically consistent with the theoretical analysis result. the valve of thesupport at the fully mechanized caving face did not open during weightings, indiating that the mechanized caving hydraulic support is capable of meeting thesupport requirements in the mining of a super-thick but shallowly buried coal seam.a/ h increases to a certain number, the periodic fracture length becomes stable. The main reason is that: with the increases of a/ h, itis difficult for the fracture to develop when it is under complexload, and the main roof will be easier to cut down, resulting inthe decrease of the periodic fracture length101214161820224.2. Engineering applicationFig. 4. Relation between periodic fracture length and ratio of crack length to mainBased on mining pressure measurement of an adjacent workingoof thicknessface, the average periodic fracture length is identified as 16 m anda/h as 4-8% for the studied working face. According to coal seamwhere Ke is the fracture toughness; p the working resistanceconditions and the results of the above theoretical studies ithydraulic support: q the overburden stress; and a h the ratio of crack Should be ensured that the support strength will not be less thanlength to thickness. q and a/h have important impacts on fracture1.0 MPa, and therefore the zf10000/23/35 top-calength of the main roof. This observation provides theoretical guid- nized support is chosen to hold the roof, whereas ZFG10000/25/37H top-caving fully mechanized support is chosen to hold theance for determining the working resistance of hydraulic supports. top and bottom of the head board as a transitional support Tableshows the main technical parameters of zf10000 /23 35 support4. Engineering example and application4.1. Engineering exampl5 Conclusionsa fully mechanized caving face of indi Coal Mine makes use of(1)Based on the break characteristics of main roof in fully mechfully mechanized top coal caving mining. The average seam thick-anized top coal caving mining of super-thick shallow coaless is 11.9 m with an average buried-depth of 175 m. the averageseams. a fracture mechanics model of the main roof was builtthickness of the immediate roof is 1.4 m, including sandy mud-and the criterion of the crack propagation and break in thestone, mudstone, siltstone and sandstone. The main roof is largelymain roof was derived using the fracture mechanics theorymade up of siltstone and fine sandstone, and has an average thick(2)The relationship between the fracture length of the mainness of 5 m The fracture toughness and compression-shear ratio ofroof, the working resistance of the top coal caving fully mechthe main roof are 1.0 MN/ m