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Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application
International Journal of Mining Science and Technology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application Zhang Jiangong, Miao Xiexing, Huang Yanli ⇑, Li Meng School of Mines, China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 8 December 2013 Available online xxxx Keywords: Super-thick shallow coal seam Fully mechanized top-caving mining Main roof Fracture mechanics model0
a b s t r a c t s Based on break characteristics of roofs in fully mechanized top-coal mining of thick shallow coal seams, a fracture mechanics model was built, and the criterion of crack propagation in the main roof was derived using the fracture mechanics theory. The relationships between the fracture length of the roof and the working resistance of the supports were discovered, and the correlations between the load on the overlying strata and the ratio of the crack’s length to the thickness of the roof were obtained. Using a working face of Jindi Coal Mine, Xing county Shanxi province as an example, the relationships between the fracture length of the roof and the working resistance of the supports were analysed in detail. The results give a design basis in hydraulic top coal caving supports, which could provide useful references in the practical application. On-site experiment proves that the periodic weighting step interval of the caving face is 15–16 m, which is basically consistent with the theoretical analysis results, and indicates that the mechanized caving hydraulic support is capable of meeting the support requirements in the mining of a super-thick but shallowly buried coal seam. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction The wall rock of a working face in coal mines moves under the gravitational effects of the overlying strata after active coal mining, which exerts stress on the hydraulic support systems, and that is the root cause of mine pressure [1]. Signs of pressure are specific appearance of the activities of the surrounding wall rocks, and to study these signs, it must firstly study the roof activity through monitoring and assumptions to the roof structure and the pattern of activities [2]. Studies in China have concluded the beneficial results on strata movement and pressure control in long-wall mining. Qian established the masonry beam theory, in which he established the condition of stability between the key block masonry beam slide and rotary deformation, namely the S-R condition [3,4]. Shi proposed two structure forms in terms of the characteristics of roof caving in shallow coal seams, namely the ‘‘short block structure of voussoir beam’’ and the ‘‘step rock beam’’, and he used the theory of rigid body equilibrium to analyze the stability of the key blocks in the main roof [5]. Current roof structure models generally assume the rock beam as homogeneous continuum medium, and analyze its stability and fracture all based on the basic theories and methods in theoretical mechanics and material mechanics of materials [6,7]. In fact, due ⇑ Corresponding author. Tel.: +86 13905207498. E-mail address: [email protected] (Y. Huang).
to long-term geological tectonic movement, different sizes of crannies and joints, and even faults inevitably exist in the rock, destroying the integrity of rock beam and playing a key role in the fracture of main roof [8,9]. In the western mining areas of Shendong, Pingshuo and Xingxian, the coal seams are not buried deep, and the roof bedrock is thin, but the earth surface is thick aeolian sand or loess. Especially in the weathered upper rock, fractures and joints are well developed, affecting the integrity of the rock mass to a certain extent. The pressure caused mining with hydraulic shears can lead to cut-down of the roof bedrock along the face [10,11]. Based on analyzing the characteristics of fully mechanized top coal caving mining of super-thick shallow seams, a fracture mechanics model for the main roof was established in this study. The relationship between fracture length of the main roof and the reasonable working resistance of top coal caving supports was deduced. This study takes the working face of the Xingxian Jindi Coal Mine 1113 in Shanxi as an example, and presents a design of hydraulic supports for thick coal seams. The pattern of mine pressure behavior was measured and analyzed. 2. Break characteristics of the main roof with fully mechanized top coal caving mining in shallow coal seam For shallow coal seams, available studies generally focus on the surrounding rock and the ways to control the pressure, but the
http://dx.doi.org/10.1016/j.ijmst.2014.03.011 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
2
J. Zhang et al. / International Journal of Mining Science and Technology xxx (2014) xxx–xxx
break characteristics and strata behavior have rarely been studied. During the fully mechanized top-coal caving of a super-thick shallow coal seam, as the mining height increases sharply, the caving of the immediate roof cannot fill the goaf. When the mining height reaches a certain value, the cutting after support appears to the rock beam of the main roof, and leads to roof rock cut down along the face, as shown in Fig. 1 [12]. Meanwhile, when the main roof breaks, it cannot form a bonded beam structure because there is little extrusion force. Therefore the main roof can be regarded as clamped or cantilever beam, breaking at the clamped end.
3. Fracture mechanics model of main roof and its solution 3.1. Establishment of the fracture mechanics model The shallow coal seams in Shendong, Pingshuo, Xingxian and other mining areas in western China are shallowly buried with thin roof bedrock, and the surface layers are thick aeolian sands or loess. The bedrock is especially affected by weathering, and the fracture and joint development affect the integrity of rock mass to a certain extent. In the mining process, fractures and joints develop easily in part due to the pressure of the hydraulic shearer, and in turn increase the overall instability of the rock. A main roof fracture mechanics model was established for the fully mechanized top-caving mining of super-thick shallow coal seams [13]. However, due to the overlying layer of thin bedrock and sand above the coal seam, an ‘‘arch’’ damage of the main roof can easily occur during the first weighting and periodic weighting. If this develops, the load on the key blocks of the roof is not the entire weight of the overburden [14]. In addition, the load on the support was treated as a triangularly distributed load in the model, instead of a uniformly distributed load [15]. Therefore, in shallow coal seam mining, with the main roof’s first weighting or periodic weightings, the main roof can be considered as a clamped beam or cantilever beam structure in order to calculate the interval of weightings. However, since the breaks are at the ends of the clamped beam, the main roof above the goaf can be regarded as a kind of beam structure with cracks [16,17]. Cracks of the main roof beam are composite compression-shear cracks under complex loading, not simply I open type, II slide type, III tear type. Fig. 2 shows the main roof fracture mechanics model. Here, q is the load of overlying strata; T is the extrusion pressure; p is the support force; L is the periodic weighting interval; l is the overhead zenith distance; a is the crack length; and h is the main roof thickness [18,19]. 3.2. Solution of the fracture mechanics model As shown in Fig. 2, stress intensity factor of the main roof can be treated as a composition of horizontal stress, overburden stress and bending under composite loadings.
q a h
T
L l
p
Fig. 2. Fracture mechanics model of main roof.
When the first or periodic weighting of the main roof occurs in super-thick shallow coal seam, the overburden stress q can be obtained by using the equilibrium arch theory [20]. It can be calculated by Eq.(1):
q¼
cL2
ð1Þ
2d tan u
where c is the average bulk density; d the coefficient of lateral stress; and u the angle of internal friction. T has extruding effects on cracks and causes Type I stress intensity factor, as follows:
pffiffiffiffiffiffi K I ¼ ðT=hÞF T ða=hÞ pa
ð2Þ
where 2
F T ða=hÞ ¼ 1:12 0:231ða=hÞ þ 10:55ða=hÞ 21:72ða=hÞ
3
4
þ 30:39ða=hÞ
q and p have shearing effects on cracks and cause Type II stress intensity factor, as follows:
pffiffiffiffiffiffi K II ¼ ð2qL plÞF q ða=hÞ= pa
ð3Þ
where
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 3 F q ða=hÞ ¼ ½1:30 0:65ða=hÞ þ 0:37ða=hÞ þ 0:28ða=hÞ = 1 a=h Bending moments are caused by overburden stress. Its stress intensity factor can be calculated by Eq. (4):
K M ¼ 3qL2 F M ða=hÞ
pffiffiffiffiffiffi 2 pa=h
ð4Þ
where 2
3
F M ða=hÞ ¼ 1:122 1:40ða=hÞ þ 7:33ða=hÞ 13:08ða=hÞ þ 14:0ða=hÞ
4
The stress intensity factor of a cracked tip of the main roof is equal to the addition of the above three simple loadings, as follows:
(P P
pffiffiffiffiffiffi 2 pffiffiffiffiffiffi K I ¼ 3qL2 F M ða=hÞ pa=h TF T ða=hÞ pa=h pffiffiffiffiffiffi K II ¼ ð2qL plÞF q ða=hÞ= pa
ð5Þ
Compression-shear fracture criterion of rock and concrete is in the following form [21]:
k
X
X KI þ K II ¼ K c
ð6Þ
where k is the coefficient of compression–shear ratio; and Kc the fracture toughness. By substituting Eqs. (1) and (5) into Eq. (6), the criterion can be further obtained as follows:
Fig. 1. Break characteristics of roof during fully mechanized top-caving mining in super-thick shallow coal seam.
h 4 pffiffiffiffiffiffi 2 pffiffiffiffiffiffi i q 1 l2 cL k 3 d tan u þ 2 F M ða=hÞ pa=h TF r ða=hÞ pa=h þ 3 pffiffiffiffiffiffi cL d tan u q1 l F q ða=hÞ= pa ¼ K c
ð7Þ
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
3
J. Zhang et al. / International Journal of Mining Science and Technology xxx (2014) xxx–xxx Table 1 Technical parameters of hydraulic support.
17.6 17.4 17.2
Number
Item
Value or description
1 2 3 4 5 6 7 8 9 10 11
Type Height (mm) Width (mm) Centerline spacing (mm) Setting load (kN) Working risistance (kN) Supporting intensity (MPa) Intensity ratio between base and roof (MPa) Movement distance (mm) Operation style Pump station pressure (kg/cm2)
ZF10000/23/35 2300/3500 1430/1600 1500 7754 10000 1.21–1.23 3.2 900/800 Operate on itself 315
L (m)
17.0 16.8 16.6 16.4 16.2 16.0 15.8 0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
p (MPa)
Fig. 3. Relation between periodic fracture length and support intensity.
According to field tests, the periodic weighting interval of the caving face is 15–16 m, which is basically consistent with the theoretical analysis result. The valve of the support at the fully mechanized caving face did not open during weightings, indicating that the mechanized caving hydraulic support is capable of meeting the support requirements in the mining of a super-thick but shallowly buried coal seam.
32 28 24
a/h increases to a certain number, the periodic fracture length becomes stable. The main reason is that: with the increases of a/h, it is difficult for the fracture to develop when it is under complex load, and the main roof will be easier to cut down, resulting in the decrease of the periodic fracture length.
L (m)
20 16 12 8 4 2
4
6
8
10
12
14
16
18
20
22
a/h (%)
Fig. 4. Relation between periodic fracture length and ratio of crack length to main roof thickness.
where Kc is the fracture toughness; p the working resistance of hydraulic support; q the overburden stress; and a/h the ratio of crack length to thickness. q and a/h have important impacts on fracture length of the main roof. This observation provides theoretical guidance for determining the working resistance of hydraulic supports. 4. Engineering example and application 4.1. Engineering example A fully mechanized caving face of Jindi Coal Mine makes use of fully mechanized top coal caving mining. The average seam thickness is 11.9 m with an average buried-depth of 175 m. The average thickness of the immediate roof is 1.4 m, including sandy mudstone, mudstone, siltstone and sandstone. The main roof is largely made up of siltstone and fine sandstone, and has an average thickness of 5 m. The fracture toughness and compression-shear ratio of the main roof are 1.0 MN/m3/2 and 1.0, respectively. The average bulk density of the rock above the main roof is 23 kN/m3 with an internal friction angle of 22°. The width of the workface, i.e., the support length, is 5.5 m and the horizontal extrusion pressure is approximately 0. With these values substituted into Eq. (7), the relation between the support strength p and the periodic fracture length L (relation curve shown in Fig. 3) can be obtained. In Fig. 4, the curve illustrates the relationship between the periodic fracture length L and a/h (main roof fracture length to thickness ratio) obtained with Eq. (7). From Fig. 3, we can see that: with the improvement of support strength, the periodic fracture length of the main roof decreases gradually, but the decrease of size increases. The main reason is that: with the improvement of support strength, the shear stress that the support imposed on the main roof increases, which makes it easier for top fall, causing the periodic fracture length to decrease gradually. With the increase of a/h, the periodic fracture length decreases gradually and the size shows a smaller decrease. When the value of
4.2. Engineering application Based on mining pressure measurement of an adjacent working face, the average periodic fracture length is identified as 16 m and a/h as 4–8% for the studied working face. According to coal seam conditions and the results of the above theoretical studies, it should be ensured that the support strength will not be less than 1.0 MPa, and therefore the zf10000/23/35 top-caving fully mechanized support is chosen to hold the roof, whereas ZFG10000/25/ 37H top-caving fully mechanized support is chosen to hold the top and bottom of the head board as a transitional support. Table 1 shows the main technical parameters of zf10000/23/35 support.
5. Conclusions (1) Based on the break characteristics of main roof in fully mechanized top coal caving mining of super-thick shallow coal seams, a fracture mechanics model of the main roof was built, and the criterion of the crack propagation and break in the main roof was derived using the fracture mechanics theory. (2) The relationship between the fracture length of the main roof, the working resistance of the top coal caving fully mechanized support, loads of overlying strata, and the ratio of the crack’s length to the thickness of the main roof was found. (3) According to the results, it was identified for the sample, a working face of Jindi Coal Mine, Xing county, Shanxi province, that the working resistance of the top coal caving fully mechanized support should not be less than 1.0 MPa, and the type of the hydraulic support is ZF10000/23/35. Good results have been obtained in practical applications.
References [1] Dou LM, Lu CP, Mu ZL, Cen CH. Control and monitoring of stope roof. Xuzhou: China University of Mining and Technology Press; 2009. [2] Yuan Y, Tu SH, Zhang XG, Li B. System dynamics model of the supportsurrounding rock system in fully mechanized mining with large mining height face and its application. Int J Min Sci Technol 2013;23(6):879–84. [3] Qian MG, Miao XX, He FL. Analysis of key block in the structure of voussoir beam in longwall mining. J China Coal Soc 1994;19(6):557–63.
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
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[4] Yan S, Bai JB, Li WF, Chen JG, Li L. Deformation mechanism and stability control of roadway along a fault subjected to mining. Int J Min Sci Technol 2012;22(4):559–65. [5] Huang QH, Qian MG, Shi PW. Structural analysis of main roof stability during periodic weighting in longwall face. J China Coal Soc 1999;24(6):581–5. [6] Jiang BS, Feng Q, Zhang Q. Stability criterion calculation of limestone rock roof of the 15# coal in Wozhuang mine. Eng Mech 2013;30(7):270–5. [7] Chang QL, Zhou HQ, Bai JB, Duan CR, Li YW. Stability study and practice of overlying strata with paste backfilling. J Min Saf Eng 2011;28(2):279–82. [8] Zhang YX, Tu SH, Bai QS, Li JJ. Overburden fracture evolution laws and watercontrolling technologies in mining very thick coal seam under water-rich roof. Int J Min Sci Technol 2013;23(5):693–700. [9] Chen P, Wang EY, Ou JC, Li ZH, Wei MY, Li XL. Fractal characteristics of surface crack evolution in the process of gas-containing coal extrusion. Int J Min Sci Technol 2013;23(1):121–6 [Original Research Article]. [10] Zhang ZQ, Xu JL, Zhu WB, Shan ZJ. Simulation research on the influence of eroded primary key strata on dynamic strata pressure of shallow coal seams in gully terrain. Int J Min Sci Technol 2012;22(1):51–5. [11] Zhang DM, Qi XH, Yin GZ, Zheng BB. Coal and rock fissure evolution and distribution characteristics of multi-seam mining. Int J Min Sci Technol 2013;23(6):835–40. [12] Xu XL, Zhang N, Tian SC. Mining-induced movement properties and fissure time-space evolution law in overlying strata. Int J Min Sci Technol 2012;22(6):817–20 [Original Research Article].
[13] Khanal M, Adhikary D, Balusu R. Numerical analysis and geotechnical assessment of mine scale model. Int J Min Sci Technol 2012;22(5):693–8 [Original Research Article]. [14] Zhang HL, Cao JJ, Tu M. Floor stress evolution laws and its effect on stability of floor roadway. Int J Min Sci Technol 2013;23(5):631–6. [15] Miao XX. Principle of underground pressure control in fully-mechanized coal mining with solid filling and force analysis of mining support. J China Univ Min Technol 2010;39(6):795–801. [16] Qian MG, Shi PW. Mining pressure and strata control. Xuzhou: China University of Mining and Technology Press; 2003. [17] Liu CL, Tan ZX, Deng KZ, Li PX. Synergistic instability of coal pillar and roof system and filling method based on plate model. Int J Min Sci Technol 2013;23(1):145–9. [18] Chen ZH, Feng JJ, Xiao CC, Li RH. Fracture mechanical model of key roof for fully-mechanized top-coal caving in shallow thick coal seam. J China Coal Soc 2007;32(5):449–52. [19] Wang JX, Wang JC, Chen ZH. Movement rule of shallowly-buried thick coal seams with hard top coal and roof. J Min Saf Eng 2006;23(2):228–32. [20] Lu ZF. Study on strata mechanical and support adaptability of shallow seam. Beijing: China Coal Research Institute; 2007. [21] Yu XZ, Qiao CQ, Zhou QL. Fracture mechanics of rock and concrete. Changsha: Central South University Press; 1991.
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application Zhang Jiangong, Miao Xiexing, Huang Yanli ⇑, Li Meng School of Mines, China University of Mining and Technology, Xuzhou 221116, China Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 10 October 2013 Received in revised form 15 November 2013 Accepted 8 December 2013 Available online xxxx Keywords: Super-thick shallow coal seam Fully mechanized top-caving mining Main roof Fracture mechanics model0
a b s t r a c t s Based on break characteristics of roofs in fully mechanized top-coal mining of thick shallow coal seams, a fracture mechanics model was built, and the criterion of crack propagation in the main roof was derived using the fracture mechanics theory. The relationships between the fracture length of the roof and the working resistance of the supports were discovered, and the correlations between the load on the overlying strata and the ratio of the crack’s length to the thickness of the roof were obtained. Using a working face of Jindi Coal Mine, Xing county Shanxi province as an example, the relationships between the fracture length of the roof and the working resistance of the supports were analysed in detail. The results give a design basis in hydraulic top coal caving supports, which could provide useful references in the practical application. On-site experiment proves that the periodic weighting step interval of the caving face is 15–16 m, which is basically consistent with the theoretical analysis results, and indicates that the mechanized caving hydraulic support is capable of meeting the support requirements in the mining of a super-thick but shallowly buried coal seam. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction The wall rock of a working face in coal mines moves under the gravitational effects of the overlying strata after active coal mining, which exerts stress on the hydraulic support systems, and that is the root cause of mine pressure [1]. Signs of pressure are specific appearance of the activities of the surrounding wall rocks, and to study these signs, it must firstly study the roof activity through monitoring and assumptions to the roof structure and the pattern of activities [2]. Studies in China have concluded the beneficial results on strata movement and pressure control in long-wall mining. Qian established the masonry beam theory, in which he established the condition of stability between the key block masonry beam slide and rotary deformation, namely the S-R condition [3,4]. Shi proposed two structure forms in terms of the characteristics of roof caving in shallow coal seams, namely the ‘‘short block structure of voussoir beam’’ and the ‘‘step rock beam’’, and he used the theory of rigid body equilibrium to analyze the stability of the key blocks in the main roof [5]. Current roof structure models generally assume the rock beam as homogeneous continuum medium, and analyze its stability and fracture all based on the basic theories and methods in theoretical mechanics and material mechanics of materials [6,7]. In fact, due ⇑ Corresponding author. Tel.: +86 13905207498. E-mail address: [email protected] (Y. Huang).
to long-term geological tectonic movement, different sizes of crannies and joints, and even faults inevitably exist in the rock, destroying the integrity of rock beam and playing a key role in the fracture of main roof [8,9]. In the western mining areas of Shendong, Pingshuo and Xingxian, the coal seams are not buried deep, and the roof bedrock is thin, but the earth surface is thick aeolian sand or loess. Especially in the weathered upper rock, fractures and joints are well developed, affecting the integrity of the rock mass to a certain extent. The pressure caused mining with hydraulic shears can lead to cut-down of the roof bedrock along the face [10,11]. Based on analyzing the characteristics of fully mechanized top coal caving mining of super-thick shallow seams, a fracture mechanics model for the main roof was established in this study. The relationship between fracture length of the main roof and the reasonable working resistance of top coal caving supports was deduced. This study takes the working face of the Xingxian Jindi Coal Mine 1113 in Shanxi as an example, and presents a design of hydraulic supports for thick coal seams. The pattern of mine pressure behavior was measured and analyzed. 2. Break characteristics of the main roof with fully mechanized top coal caving mining in shallow coal seam For shallow coal seams, available studies generally focus on the surrounding rock and the ways to control the pressure, but the
http://dx.doi.org/10.1016/j.ijmst.2014.03.011 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
2
J. Zhang et al. / International Journal of Mining Science and Technology xxx (2014) xxx–xxx
break characteristics and strata behavior have rarely been studied. During the fully mechanized top-coal caving of a super-thick shallow coal seam, as the mining height increases sharply, the caving of the immediate roof cannot fill the goaf. When the mining height reaches a certain value, the cutting after support appears to the rock beam of the main roof, and leads to roof rock cut down along the face, as shown in Fig. 1 [12]. Meanwhile, when the main roof breaks, it cannot form a bonded beam structure because there is little extrusion force. Therefore the main roof can be regarded as clamped or cantilever beam, breaking at the clamped end.
3. Fracture mechanics model of main roof and its solution 3.1. Establishment of the fracture mechanics model The shallow coal seams in Shendong, Pingshuo, Xingxian and other mining areas in western China are shallowly buried with thin roof bedrock, and the surface layers are thick aeolian sands or loess. The bedrock is especially affected by weathering, and the fracture and joint development affect the integrity of rock mass to a certain extent. In the mining process, fractures and joints develop easily in part due to the pressure of the hydraulic shearer, and in turn increase the overall instability of the rock. A main roof fracture mechanics model was established for the fully mechanized top-caving mining of super-thick shallow coal seams [13]. However, due to the overlying layer of thin bedrock and sand above the coal seam, an ‘‘arch’’ damage of the main roof can easily occur during the first weighting and periodic weighting. If this develops, the load on the key blocks of the roof is not the entire weight of the overburden [14]. In addition, the load on the support was treated as a triangularly distributed load in the model, instead of a uniformly distributed load [15]. Therefore, in shallow coal seam mining, with the main roof’s first weighting or periodic weightings, the main roof can be considered as a clamped beam or cantilever beam structure in order to calculate the interval of weightings. However, since the breaks are at the ends of the clamped beam, the main roof above the goaf can be regarded as a kind of beam structure with cracks [16,17]. Cracks of the main roof beam are composite compression-shear cracks under complex loading, not simply I open type, II slide type, III tear type. Fig. 2 shows the main roof fracture mechanics model. Here, q is the load of overlying strata; T is the extrusion pressure; p is the support force; L is the periodic weighting interval; l is the overhead zenith distance; a is the crack length; and h is the main roof thickness [18,19]. 3.2. Solution of the fracture mechanics model As shown in Fig. 2, stress intensity factor of the main roof can be treated as a composition of horizontal stress, overburden stress and bending under composite loadings.
q a h
T
L l
p
Fig. 2. Fracture mechanics model of main roof.
When the first or periodic weighting of the main roof occurs in super-thick shallow coal seam, the overburden stress q can be obtained by using the equilibrium arch theory [20]. It can be calculated by Eq.(1):
q¼
cL2
ð1Þ
2d tan u
where c is the average bulk density; d the coefficient of lateral stress; and u the angle of internal friction. T has extruding effects on cracks and causes Type I stress intensity factor, as follows:
pffiffiffiffiffiffi K I ¼ ðT=hÞF T ða=hÞ pa
ð2Þ
where 2
F T ða=hÞ ¼ 1:12 0:231ða=hÞ þ 10:55ða=hÞ 21:72ða=hÞ
3
4
þ 30:39ða=hÞ
q and p have shearing effects on cracks and cause Type II stress intensity factor, as follows:
pffiffiffiffiffiffi K II ¼ ð2qL plÞF q ða=hÞ= pa
ð3Þ
where
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 3 F q ða=hÞ ¼ ½1:30 0:65ða=hÞ þ 0:37ða=hÞ þ 0:28ða=hÞ = 1 a=h Bending moments are caused by overburden stress. Its stress intensity factor can be calculated by Eq. (4):
K M ¼ 3qL2 F M ða=hÞ
pffiffiffiffiffiffi 2 pa=h
ð4Þ
where 2
3
F M ða=hÞ ¼ 1:122 1:40ða=hÞ þ 7:33ða=hÞ 13:08ða=hÞ þ 14:0ða=hÞ
4
The stress intensity factor of a cracked tip of the main roof is equal to the addition of the above three simple loadings, as follows:
(P P
pffiffiffiffiffiffi 2 pffiffiffiffiffiffi K I ¼ 3qL2 F M ða=hÞ pa=h TF T ða=hÞ pa=h pffiffiffiffiffiffi K II ¼ ð2qL plÞF q ða=hÞ= pa
ð5Þ
Compression-shear fracture criterion of rock and concrete is in the following form [21]:
k
X
X KI þ K II ¼ K c
ð6Þ
where k is the coefficient of compression–shear ratio; and Kc the fracture toughness. By substituting Eqs. (1) and (5) into Eq. (6), the criterion can be further obtained as follows:
Fig. 1. Break characteristics of roof during fully mechanized top-caving mining in super-thick shallow coal seam.
h 4 pffiffiffiffiffiffi 2 pffiffiffiffiffiffi i q 1 l2 cL k 3 d tan u þ 2 F M ða=hÞ pa=h TF r ða=hÞ pa=h þ 3 pffiffiffiffiffiffi cL d tan u q1 l F q ða=hÞ= pa ¼ K c
ð7Þ
Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011
3
J. Zhang et al. / International Journal of Mining Science and Technology xxx (2014) xxx–xxx Table 1 Technical parameters of hydraulic support.
17.6 17.4 17.2
Number
Item
Value or description
1 2 3 4 5 6 7 8 9 10 11
Type Height (mm) Width (mm) Centerline spacing (mm) Setting load (kN) Working risistance (kN) Supporting intensity (MPa) Intensity ratio between base and roof (MPa) Movement distance (mm) Operation style Pump station pressure (kg/cm2)
ZF10000/23/35 2300/3500 1430/1600 1500 7754 10000 1.21–1.23 3.2 900/800 Operate on itself 315
L (m)
17.0 16.8 16.6 16.4 16.2 16.0 15.8 0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
p (MPa)
Fig. 3. Relation between periodic fracture length and support intensity.
According to field tests, the periodic weighting interval of the caving face is 15–16 m, which is basically consistent with the theoretical analysis result. The valve of the support at the fully mechanized caving face did not open during weightings, indicating that the mechanized caving hydraulic support is capable of meeting the support requirements in the mining of a super-thick but shallowly buried coal seam.
32 28 24
a/h increases to a certain number, the periodic fracture length becomes stable. The main reason is that: with the increases of a/h, it is difficult for the fracture to develop when it is under complex load, and the main roof will be easier to cut down, resulting in the decrease of the periodic fracture length.
L (m)
20 16 12 8 4 2
4
6
8
10
12
14
16
18
20
22
a/h (%)
Fig. 4. Relation between periodic fracture length and ratio of crack length to main roof thickness.
where Kc is the fracture toughness; p the working resistance of hydraulic support; q the overburden stress; and a/h the ratio of crack length to thickness. q and a/h have important impacts on fracture length of the main roof. This observation provides theoretical guidance for determining the working resistance of hydraulic supports. 4. Engineering example and application 4.1. Engineering example A fully mechanized caving face of Jindi Coal Mine makes use of fully mechanized top coal caving mining. The average seam thickness is 11.9 m with an average buried-depth of 175 m. The average thickness of the immediate roof is 1.4 m, including sandy mudstone, mudstone, siltstone and sandstone. The main roof is largely made up of siltstone and fine sandstone, and has an average thickness of 5 m. The fracture toughness and compression-shear ratio of the main roof are 1.0 MN/m3/2 and 1.0, respectively. The average bulk density of the rock above the main roof is 23 kN/m3 with an internal friction angle of 22°. The width of the workface, i.e., the support length, is 5.5 m and the horizontal extrusion pressure is approximately 0. With these values substituted into Eq. (7), the relation between the support strength p and the periodic fracture length L (relation curve shown in Fig. 3) can be obtained. In Fig. 4, the curve illustrates the relationship between the periodic fracture length L and a/h (main roof fracture length to thickness ratio) obtained with Eq. (7). From Fig. 3, we can see that: with the improvement of support strength, the periodic fracture length of the main roof decreases gradually, but the decrease of size increases. The main reason is that: with the improvement of support strength, the shear stress that the support imposed on the main roof increases, which makes it easier for top fall, causing the periodic fracture length to decrease gradually. With the increase of a/h, the periodic fracture length decreases gradually and the size shows a smaller decrease. When the value of
4.2. Engineering application Based on mining pressure measurement of an adjacent working face, the average periodic fracture length is identified as 16 m and a/h as 4–8% for the studied working face. According to coal seam conditions and the results of the above theoretical studies, it should be ensured that the support strength will not be less than 1.0 MPa, and therefore the zf10000/23/35 top-caving fully mechanized support is chosen to hold the roof, whereas ZFG10000/25/ 37H top-caving fully mechanized support is chosen to hold the top and bottom of the head board as a transitional support. Table 1 shows the main technical parameters of zf10000/23/35 support.
5. Conclusions (1) Based on the break characteristics of main roof in fully mechanized top coal caving mining of super-thick shallow coal seams, a fracture mechanics model of the main roof was built, and the criterion of the crack propagation and break in the main roof was derived using the fracture mechanics theory. (2) The relationship between the fracture length of the main roof, the working resistance of the top coal caving fully mechanized support, loads of overlying strata, and the ratio of the crack’s length to the thickness of the main roof was found. (3) According to the results, it was identified for the sample, a working face of Jindi Coal Mine, Xing county, Shanxi province, that the working resistance of the top coal caving fully mechanized support should not be less than 1.0 MPa, and the type of the hydraulic support is ZF10000/23/35. Good results have been obtained in practical applications.
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Please cite this article in press as: Zhang J et al. Fracture mechanics model of fully mechanized top coal caving of shallow coal seams and its application. Int J Min Sci Technol (2014), http://dx.doi.org/10.1016/j.ijmst.2014.03.011