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Arch structure effect of the coal gangue flow of the fully mechanized caving in special thick coal seam and its impact on the loss of top coal
International Journal of Mining Science and Technology 26 (2016) 593–599
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Arch structure effect of the coal gangue flow of the fully mechanized caving in special thick coal seam and its impact on the loss of top coal Zhang Ningbo, Liu Changyou ⇑ State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, Xuzhou 221116, China School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
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Article history: Received 25 October 2015 Received in revised form 19 December 2015 Accepted 28 January 2016 Available online 26 May 2016 Keywords: Extra thick coal seam Coal gangue flow Top coal loss Dynamic random arch effect
a b s t r a c t Based on the characteristics of the top coal thickness of the fully mechanized caving in special thick coal seam, the long distance of coal gangue caving, as well as the different sizes of the coal gangue broken fragment dimension and spatial variation of drop flow, this paper uses laboratory dispersion simulation experiment and theoretical analysis to study the arch structure effect and its influence rule on the top coal loss in the process of coal gangue flow. Research shows that in the process of coal gangue flow, arch structure can be formed in three types: the lower arch structure, middle arch structure, and upper arch structure. Moreover, the arch structure has the characteristics of dynamic random arch, the formation probability of dynamic random arch with different layers is not the same, dynamic random arch caused the reduction of the top coal fluency; analyzing the dynamic random arch formation mechanism, influencing factors, and the conditions of instability; the formation probability of the lower arch structure is the highest, the whole coal arch and the coal gangue arch structure has the greatest impact on top coal loss. Therefore, to prevent or reduce the formation of lower whole coal arch structure, the lower coal gangue arch structure and the middle whole coal arch structure is the key to reduce the top coal loss. The research conclusion provides theoretical basis for the further improvement of the top coal recovery rate of the fully mechanized caving in extra thick coal seam. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Coal is the main energy source in China [1] and thick coal seam reserves account for about 45% of the total reserves. Fully mechanized caving technology is an effective way for mining thick and extra thick coal seam [2–6]. The improvement of the top coal drawing ratio is one of the key difficulties in the fully mechanized caving mining technology [7,8]. The technology of top coal loss in the fully mechanized caving face accounts for 1/2–2/3 of the total loss, and it shows a trend of increase. Therefore, a solution to the technology loss problem is crucial to improvements in the recovery rate of the fully mechanized caving face. The technology loss is mainly caused by the unreasonable coal caving technology mode and parameter. The top coal is the basis for the determination of the coal caving technology parameter and mode. Top coal caving law is the result of many factors. These factors include top coal thickness, the broken fragment size of the immediate roof, the collapsing characteristics of the immediate roof and the support type.
⇑ Corresponding author. Tel.: +86 13585478385.
The formation and instability of the arch structure in the coal gangue caving flow field is a unique phenomenon [9] which can affect the recovery of top coal and coal quality. With the increase of the thickness of top coal, the range of the top coal and gangue caving flow increases, both the broken fragment dimension of coal gangue and caving flow space changes. This enlarges the probability of the mixing process of top coal and gangue as well as the top coal arching [10], especially under the condition of fully mechanized caving in the special thick coal seam (14–20 m). In order to further improve the recovery rate of top coal and reasonably determine the caving technology parameter, it is necessary to understand the formation characteristics of the arch structure in the coal caving flow process and its influence on coal gangue flow pattern, fluency, and top coal recovery under this condition. Therefore, this paper uses a granular physical simulation experiment to study the arching characteristics of coal gangue in the process of fully mechanized caving in extra thick coal seam and top coal caving and its influence on top coal loss, in order to provide a basis for the reasonable determination of the coal caving technology parameter and the formulation of the technical measures to improve the recovery rate of top coal.
E-mail address: [email protected] (C. Liu). http://dx.doi.org/10.1016/j.ijmst.2016.05.010 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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2. Granular simulation experiment project According to the similarity theory, under the geological conditions of the No. 8105 coal mining face in the Tashan mine, a granular simulation experiment method was used to study the top coal fragment distribution of the fully mechanized caving in extra thick coal seam with large mining height. The granular simulation experiment method was also used to study the structure effect of coal gangue flow field and its influence on top coal loss law. The average thickness of the coal seam of the Tashan mine No. 8105 working face is 14.50 m, the mining height is 4.20 m, caving height is 10.30 m, mining and caving ratio is about 1:2.45. The immediate roof is the alternate occurrence layer consisting of magmatic rocks, mudstone and silicified coal with the average height 4.49 m. The basic roof consists of siltstone, fine sandstone, and muddy-conglomeratic sandstone with an average height of 22.93 m. Average dip angle of the coal seam is 4°, the coal mining cycle progress is 0.80 m, and the top coal drawing interval is 0.80 m per cut [11,12]. Theoretical analysis and field tests [13] show that the order of top coal along the thickness direction of the broken fragment dimension is: the lower top coal fragment dimension < middle top coal fragment dimension < upper top coal fragment dimension. According to the results of the field-observed broken coal gangue fragment dimension, the top coal is divided into three layers: upper top coal, middle top coal and lower top coal. The immediate roof is divided into lower immediate roof, middle immediate roof, and upper immediate roof [14]. An experiment was conducted to analyze the caving of top coal flow and the arching law. Different colors and sizes of stones were used to replace all layers of coal, with the addition of clay to simulate smaller particles of coal and rock powder. The testing program is shown in Table 1. The coal gangue caving mixed experiment model table adopted in the experiment mainly includes a control system, a simple selfmoving hydraulic support, a mobile system, and framework. It is 300 cm long, 15 cm wide, and 220 cm tall. The advancing support and coal caving can be automatically carried out by setting the relevant parameters, as shown in Fig. 1. The model bracket height is 40 cm and the field machine mining height is 4.20 cm. Therefore, the geometric similarity ratio of the model and the prototype is C = 40/420 = 1:10.5. The thickness of top coal is 10.30 m and the corresponding thickness of top coal in the model is 0.98 m. At the boundary, the simulated coal pillar is 6.50 cm and the openoff cut is 6.00 m.
Fig. 1. Combined experiment of fully mechanized coal gangue caving.
divided into three types: the lower arch structure, the middle arch structure, and the upper arch structure. The specific distribution area is shown in Fig. 2. (1) Lower arch structure: the arch structure is located near the drawing opening, it has the smaller span and high stability, and it is easy to be formed. This is because the space near the drawing opening is the smallest and the large coal mass or coal rock mass is easy to be squeezed by each other to form hinged arch. The lower arch structure contains coal arch and coal gangue arch. The arching material of the coal arch is coal, as shown in Fig. 3a. The arching material of the coal gangue arch is a mixture of coal and gangue, as shown in Fig. 3b. (2) Middle arch structure: the middle arch structure is located between the caving shield and the top beam. The arch structure can be divided into coal gangue arch structure and pure coal arch structure (Fig. 4). (3) Upper arch structure: the arch structure is located at the top coal above the support top beam. It is less formed, it has a large span, can form a coal arch, and a coal gangue arch. The stability of the arch structure is lower, the front arch angle is generally located on the top beam of support, the rear arch angle is located on the gangue in goaf, and, therefore, the impact of the swing of support tail beam is small (Fig. 5).
4. Dynamic random arch effect in the process of top coal caving 3. Arch characteristics and types of top coal caving in extra thick coal seam Through the observation on the coal gangue flow law of the fully mechanized caving in extra thick coal seam during the experiment process, we can see that arching is an important factor that affects the top coal caving in the process of top coal flowing. According to the arching area [15,16], in the caving process of the working surface, the arch structure formed by top coal is
4.1. Dynamic random arch characteristics in the process of top coal caving In the top coal caving process, the flow field fracture surface contracts with the decline of the height. The shrinkage of the fracture surface provides horizontal extrusion pressure between coal gangue blocks which provides the mechanical condition for the block extrusion into arch. Through the experimental analysis of arching characteristics and types in the top coal caving process, we can conclude that the formation of the arch structure has a
Table 1 Similar simulation scheme (mm). Layer position
Upper immediate roof Middle immediate roof Lower immediate roof Upper top coal Middle top coal Lower top coal
Parameter
Arching area of upper arch structure
Thickness
Fragment dimension
600 400 300 326 326 326
90 75 70 55 45 27
Top beam Arching area of middle arch structure Caving shield Arching area of lower arch structure Angle of repose Tail beam
Fig. 2. Three different types of arch structures.
N. Zhang, C. Liu / International Journal of Mining Science and Technology 26 (2016) 593–599
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caving, the coal gangue arch structure that is formed by the boundary conditions such as the fragmentation difference and the support caving shield is called the dynamic random arch effect. The formation mechanism, influence factors, instability conditions of the dynamic random arch, and its influence on top coal flow field and the recovery rate are analyzed as follows.
4.2. Formation mechanism and influence factors of dynamic random arch (a) Lower coal arch
(b) Lower coal gangue arch
Fig. 3. Lower arch structure.
By the characteristics and types of arch structures formed during the process of top coal flow, the formation of the arch structure should have some conditions: (1) Resulting in uneven displacement or relative displacement. (2) There exists a structure support point or constraint which can act as skewback. As the top coal caving is a dynamic process, and the coal gangue boundary is a changed curve which is from top to bottom, there are two kinds of situations between the coal gangue boundary along the side of (drawing opening) goaf and the top coal movement trace along the side of support:
(a) Middle pure coal arch
(b) Middle coal gangue arch
Fig. 4. Middle arch structure.
(a) Upper pure coal arch
(b) Upper coal gangue arch
Fig. 5. Upper arch structure.
dynamic random feature, specifically manifested in the following aspects: (1) Arching position: In the flow field, there is a random possibility of arching in different top coal positions. With the coal gangue flow in the flow field, the shape and position of the arch may be changed to a certain extent. (2) Arching probability: Although the formation of the arch is random, the formation probability is different at different positions. The arching probability increases with the shrinkage of the fracture surface in the caving flow field. (3) Arching material: arching blocks contain both coal and gangue. In summary, the formation of the arch in the fully mechanized top coal caving process has a certain randomness. From the formation process of the arch structure, the arch structure is formed by extrusion between coal gangue of broken top-coal and gangue in the dynamic process of caving flow. Its formation time, position, and shape are random. Therefore, in the dynamic process of coal
(1) The end face of the angle of repose and the top beam will serve as a skew back support for the flowing coal gangue. (2) Because the coal gangue broken fragmentation is not the same size, leading to different coal gangue liquidity in different positions, when most of the lower coal gangue liquidity is less than the upper coal gangue, the lower coal gangue will likely serve as the skew back support for the upper coal gangue. Before the start of coal caving technology of fully mechanized working face there is a relatively static state between coal gangue blocks, as shown in Fig. 6. The caving coal gangue blocks behind the support, in the state of stress equilibrium, are restricted to an inclined body by the support. Before the start of coal caving technology, for any stress circle of coal gangue blocks behind the support, the external force diagram is shown in Fig. 7. When opening the drawing opening, the coal gangue block at the bottom lose support, breaking the original state of stress equilibrium. At this moment, the coal gangue near the drawing opening will first flow to the working face, changing the original state of the stress equilibrium at point M as the vertical stress r2 begins to decrease. The vertical stress r2 will become less than the horizontal stress r1 if the flow is smooth. The vertical stress r2 will be 0 with the continuation of top coal flow process, in a certain range, when granular coal gangue blocks form a continuum under the action of friction. If the ends of the continuum are supported by the support of the shield beam and the repose end face behind the support, the arch structure will be formed. At this point, the
Fig. 6. Sketch map of the arching of fully mechanized top coal caving space.
N. Zhang, C. Liu / International Journal of Mining Science and Technology 26 (2016) 593–599
σ2
596
σ1
m f1
N2 G
N1
f2
Assuming that the acting combined stress on surface AB at the front skew back is N1, N1 is decomposed into horizontal normal stress N10 and vertical shear stress N11, and the acting combined stress on surface CD at the back skew back is N2. Assuming the arch body is symmetrical, N2 is decomposed into horizontal normal stress N20 and the vertical shear stress N21, then:
G ¼ L Dh q g
Fig. 7. Sketch map of the stressing of fully mechanized top coal caving space.
resultant stress direction at the two skew backs will tilt toward the vault, as shown in Fig. 8, and the component force in the horizontal direction of the resultant stress exerts pressure on the granular block which formed to arch. The friction formed by this pressure will be greater than the gravity of the coal gangue above the arch, as shown in Fig. 8, and the coal gangue block beneath the arch will smooth out. However, the flow of coal gangue block above the arch will be restricted. Although the formation of the dynamic random arch effect is random, the formation of arch structure has certain conditions. The factors influencing the formation of the dynamic random arch effect are as follows: (1) Coal gangue fragmentation size. Because the coal gangue contact and squeeze each other in the flow process, it is easy to form a relatively stable hinged structure only when the top coal is larger. Therefore, coal gangue fragmentation and fragmentation distribution has great influence on the formation of a stable arch structure. (2) The span of arch structure. Because the span of the arch structure determines the number of coal gangue blocks in the arch structure, usually the arch structure is relatively stable overall. Once a certain block in the arch structure loses stability, however, it will lead to structural damage and instability of the entire arch structure. Therefore, a greater span and higher position of the arch lowers the stability of arch structure. Conversely, a lower position of the arch, closer to the drawing opening, results in a higher stability of the arch structure. (3) Binding force along horizontal direction. Because the arch structure is usually a curved arch structure, to ensure the stability of arch structure, in the horizontal direction, a constraint is an important factor to maintain the stability of arch structure. Usually, a horizontal force constraint can provide constraints for the skew back of the arch structure. At the top of the arch structure, a horizontal force constraint can also increase the extrusion of block in the arch structure, improving the stability of the whole arch structure. 4.3. Instability condition of dynamic random arch The arch structure formed during the top coal caving process of fully mechanized caving is simplified as a mechanical geometry model, as shown in Fig. 9.
ð1Þ
In the equation: q is the density of coal gangue in the arch body. The balanced equation for the arch body is:
G ¼ N10 Dh þ N 20 Dh
ð2Þ
Combining Eqs. (1) and (2):
N10 þ N20 ¼ L q g
ð3Þ
Eq. (3) is called the mechanical geometric equilibrium conditions for the existence of ‘‘granular arch”. If the arch is symmetric, then
N10 ¼ N20 ¼
1 Lqg 2
ð4Þ
The dynamic random arch formed by coal gangue in the top coal caving process affects the flow law of top coal. This causes the top coal and gangue to be mixed in advance of when the gangue is seen at the drawing opening and the window is closed. Thus, part of the top coal is lost. Therefore, it is necessary to avoid the emergence of dynamic random arch. If, however, the formation of the arch cannot be avoided, it is necessary to take measures to make the arch unstable. Because the dynamic random arch belongs to the granular arch, there are two ways to cause the destruction of the granular arch. The first way is making the skewback without a stable support point. The second way involves component factors of moving a granular arch, making it lose continuity, leading to the instability of the arch caused by the loss of continuous carrier of horizontal compressive stress. A random dynamic arch has strong compression capability, but the tensile and jacking force resistance ability is very poor. Therefore, the dynamic random arch can be made unstable by way of breaking the skew back, pulling the arch, and pushing up the arch. The specific measures are as follows: (1) In the case of the random arch, with skew back on the support shield beam at a relatively high position, the arching probability is lower and the span of the arch is relatively large, which is generally more unstable. But, once the arch is formed at a lower position, it will be a more stable arch, and the arch instability can then be caused by traversing support at a small range from up to down or front to back, or continuously swinging the support tail beam in the caving process. This makes the continuously flowing coal gangue transfer displacement to the tail beam, and causes the instability of a piece of rock of the arch structure, so as to make the arch unstable due to the reason that compressive stress of the arch cannot be transmitted.
L
f1
N1
N2 G
N0
Δh
m
σ 2′ σ 1′ ψ
N1
f2
Fig. 8. Arching stress distribution diagram of coal gangue behind the support.
G
B A
N1
N 20
N2
N21
D C
Fig. 9. Mechanical geometry model of arch structure (Dh – Height of arch body, L – Span of arch body, G – Gravity).
N. Zhang, C. Liu / International Journal of Mining Science and Technology 26 (2016) 593–599
(2) For the random arch with skew back on the tail beam of the support shield beam, the front skew back can be damaged by swaying the support tail beam, so as to cause the instability of arch and ensure smooth running of top coal.
5. Analysis of the influence of dynamic random arch structure on the flow and loss of top coal The formation probability of the dynamic random arch is higher in the caving process of the extra thick coal seam. In general, the dynamic random arch causes the reduction of the top coal flowability, reducing the risk of top coal caving, as shown in Fig. 10. We can see from Fig. 10, the top coal above arch structure cannot be caved to the drawing opening. Usually, when dynamic random arch is formed, the coal gangue flow above the arch structure stagnates. The arch structure is also gradually formed in the process of top coal flow, so the effect of dynamic random arch on top coal flow is also a gradual process of change and development. Closing the window and shifting the support during the top coal caving process will cause the coal gangue boundary move backward. How much the coal gangue boundary moves backward relates to the amount of top coal above the arch structure. If the amount is more, then the coal gangue boundary notably moves backward, as shown in Fig. 11. We can see from Fig. 11, the lower coal gangue boundary cannot be caved before the support shift, so the amount of released top coal is less. However, after the support shift, with the top coal homeopathy rolled, arch structure will be also destroyed, and with the flow of top coal, because the top coal is not fully caved and discharged, the coal gangue boundary is near the goaf. This will inevitably make the top coal flow into the coal gangue repose angle near the lower region of the coal gangue boundary after the support shift and the amount of top coal in the lower parts which cannot be caved increases, leading to the permanent loss of top coal. On the other hand, the coal gangue boundary significantly moves backwards due to the support shift, so the amount of top coal in the next caving process is more. This happens because the mixed gangue will easily appear in the flowing process of top coal. The degree of mixed gangue increases with the increasing flow time of top coal. Thus, the more the amount of the top coal caved, the longer the caved time is, and the top coal loss caused by mixed gangue will be more. So, carrying out the shift operation in the case that the top coal above the dynamic random arch is not fully caved, the impact on the loss of top coal is more serious, including the loss caused by two aspects: the mixed gangue caused by top coal flow and top coal flow into the area which cannot be caved. For the coal which can be caved, the top coal loss is mainly caused by the reason that the mixed gangue and top coal arch
Top coal is not released at the top
Fig. 10. Effect of dynamic random arch on top coal flow.
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make it so the top coal cannot be released in the coal working face, usually closing the window when seeing gangue and stopping caving. Therefore, there are two cases for seeing gangue in the process of coal caving. The first case is the gangue near repose angle in goaf is easy to flow to the drawing opening with top coal in caving process. At this point, the top coal is not fully caved, so the top coal loss is very serious. This situation is unlikely to occur, but once the top coal loss appears, it is quite serious. The second case is when top coal and gangue are mixed in the flowing process. Because the density of gangue is larger, the coal gangue easily flows into the upper top coal in the flowing process, leading to the result that some upper top coal is not caved yet when seeing gangue, thus causing losses. Usually, the loss of top coal in this case is not very serious, and the loss of top coal increases with the increase of top coal thickness, as shown in Fig. 11. Furthermore, if the top coal above the arch structure has been caved more, the coal gangue boundary is smooth and it has little effect on the loss of top coal. The influence of different arch structures on top coal flow is different. The formation of the upper arch structure will hinder the flowability of coal or gangue in the upper part of the arch structure. If the upper part of arch structure is all gangue, the formation of arch will help to slow down the time of seeing gangue, improving the recovery rate of top coal. The formation of middle arch structure will disturb the top coal flow state at the upper arch structure. At the same time, it is easy for the gangue of goaf to enter into the caving space of the lower part of arch structure, causing seeing gangue in advance at the drawing opening and arching near the drawing opening. This will prevent the top coal caving. At this point, if taking the arch breaking measures, caving time will be increased. If stopping caving, it will positively affect the recovery of top coal. Carrying out the statistics of the arching number, arching probability of different arch structures in 20 cycles of normal caving, and recovery rate of top coal under the conditions of different arch structures in the test process, the results are shown in Table 2. The recovery rate of top coal is the ratio result of caved top coal quantity and theoretical caving quantity before the first formation of the arch structure. With the statistical process, the influence of a certain kind of arch structure on the recovery rate can be predicted, before the appearance of the arch. If other arch structures are formed, the arch breaking measures can be adopted to guarantee the accuracy of the statistics of the recovery rate. From Table 2, we can see that in the process of caving, arching probability at different horizons are different; the higher the height of the arch, the smaller the probability of the arch. The arching probability of upper arch structure is the formation probability of upper top coal which is 3.12%, middle arch structure accounted for 16.67%, and the lower arch structure accounted for 80.21%. The lower arch structure, then, has the highest probability. In the same layer of arch structure, pure coal arch effect on recovery rate is greater than the coal gangue arch. When the pure coal arch of lower arch structure is first formed, the top coal recovery rate is only 24%, so the treatment of the pure coal arch structure of lower top coal is an effective way to improve the top coal recovery. As the recovery rate of coal gangue arch structure is already higher, if the amount of gangue in coal gangue arch structure is large, once the arch structure is destroyed, there will be a lot of gangue to reach the drawing opening, causing the higher percentage of gangue content. The gangue content will also be higher in the subsequent caving. Therefore, if the amount of gangue is more, it is not suitable to deal with the arch structure. For the middle arch structure, the two types of arch have great influence on the recovery of top coal. Therefore, it is necessary to carry out the broken arch operation. For the upper arch structure, top coal recovery rate has reached more than 60%, its arching probability is lower, and it is difficult
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Coal gangue boundary
Coal gangue repose angle before the support shift
Coal gangue repose angle after the support shift
Support after support shift
Fig. 11. Flow law of top coal before and after the support shift.
Table 2 Arching number and probability of different arch structures. Arch structure type
Arching number
Arching probability (%)
Arching ratio (%)
Recovery rate of top coal (%)
Upper arch structure
Pure coal arch Coal gangue arch
1 2
5 10
1.04 2.08
60.30 78.80
Middle arch structure
Pure coal arch Coal gangue arch
9 7
45 35
9.38 7.29
51.34 63.23
Lower arch structure
Pure coal arch Coal gangue arch
56 21
280 105
58.33 21.88
24.14 82.23
to break the arch through support action. Therefore, the impact of the arch at this position can be ignored. 6. Conclusions (1) According to the position characteristics of the arching of top coal caving in the working face, the arch structure is divided into upper arch structure, middle arch structure, and lower arch structure. Arch structures of three layers include two types of pure coal and coal gangue mixture. (2) Through the analysis of the top coal caving arching law of the fully mechanized coal caving in the extra thick coal seam, putting forward the concept of dynamic random arch, influencing factors of dynamic random arch include coal gangue fragmental size, span of arch structure, and constraining force of horizontal direction. Random dynamic arch has strong compression capability, but the tensile and jacking force resistance ability is very poor. So, the dynamic random arch can be made unstable in the way of breaking the skew back, pulling and pushing up the arch. (3) Dynamic random arch causes the decrease of top coal flowability. Closing the window and moving the support when it is arching in the process of top coal caving, will cause the coal gangue boundary to move backward. After shift moving, the top coal near the lower region of coal gangue boundary flowed into goaf behind the shift, causing permanent loss of top coal. The formation probability of dynamic random arch is different at different positions. The formation proba-
bility of the lower dynamic arch is the highest. The whole coal arch structure has the biggest impact on top coal loss. The lower arch structure, especially the lower pure coal arch structure, is the key point of control, as it can significantly improve the top coal recovery rate.
Acknowledgments Financial support for this work, provided by the Independent Research Subject of State Key Laboratory of Coal Resources and Mine Safety of China University of Mining and Technology (No. SKLCRSM12X03), the Scientific Research and Innovation Project for College Graduates in Jiangsu (No. CXZZ13_0947), Top-Notch Academic Programs of Jiangsu Higher Education Institutions, and the Priority Academic Development Program of Jiangsu Higher Education Institutions, is gratefully acknowledged. References [1] Xie YS, Zhao YS. Numerical simulation on the law of top-coal caving process under vibration condition. J Min Saf Eng 2008;25(2):188–91. [2] Wang JC, Li ZG, Chen YJ, Zheng HF. Experimental study of loose medium flow field on the long wall top-coal caving. J China Coal Soc 2004;29(3):260–3. [3] Zhang YD, Zhang FT, Ji M, Gao LS, Jin ZY, Cheng L, Wu LP. Research on the reasonable coal caving technological parameters of extra-thick coal seam. J Min Saf Eng 2012;29(6):808–14. [4] Ma LQ, Qiu XX, Dong T. Huge thick conglomerate movement induced by full thick long wall mining huge thick coal seam. Int J Min Sci Technol 2012;22 (3):399–404.
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Arch structure effect of the coal gangue flow of the fully mechanized caving in special thick coal seam and its impact on the loss of top coal Zhang Ningbo, Liu Changyou ⇑ State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, Xuzhou 221116, China School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of Education of China, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 25 October 2015 Received in revised form 19 December 2015 Accepted 28 January 2016 Available online 26 May 2016 Keywords: Extra thick coal seam Coal gangue flow Top coal loss Dynamic random arch effect
a b s t r a c t Based on the characteristics of the top coal thickness of the fully mechanized caving in special thick coal seam, the long distance of coal gangue caving, as well as the different sizes of the coal gangue broken fragment dimension and spatial variation of drop flow, this paper uses laboratory dispersion simulation experiment and theoretical analysis to study the arch structure effect and its influence rule on the top coal loss in the process of coal gangue flow. Research shows that in the process of coal gangue flow, arch structure can be formed in three types: the lower arch structure, middle arch structure, and upper arch structure. Moreover, the arch structure has the characteristics of dynamic random arch, the formation probability of dynamic random arch with different layers is not the same, dynamic random arch caused the reduction of the top coal fluency; analyzing the dynamic random arch formation mechanism, influencing factors, and the conditions of instability; the formation probability of the lower arch structure is the highest, the whole coal arch and the coal gangue arch structure has the greatest impact on top coal loss. Therefore, to prevent or reduce the formation of lower whole coal arch structure, the lower coal gangue arch structure and the middle whole coal arch structure is the key to reduce the top coal loss. The research conclusion provides theoretical basis for the further improvement of the top coal recovery rate of the fully mechanized caving in extra thick coal seam. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Coal is the main energy source in China [1] and thick coal seam reserves account for about 45% of the total reserves. Fully mechanized caving technology is an effective way for mining thick and extra thick coal seam [2–6]. The improvement of the top coal drawing ratio is one of the key difficulties in the fully mechanized caving mining technology [7,8]. The technology of top coal loss in the fully mechanized caving face accounts for 1/2–2/3 of the total loss, and it shows a trend of increase. Therefore, a solution to the technology loss problem is crucial to improvements in the recovery rate of the fully mechanized caving face. The technology loss is mainly caused by the unreasonable coal caving technology mode and parameter. The top coal is the basis for the determination of the coal caving technology parameter and mode. Top coal caving law is the result of many factors. These factors include top coal thickness, the broken fragment size of the immediate roof, the collapsing characteristics of the immediate roof and the support type.
⇑ Corresponding author. Tel.: +86 13585478385.
The formation and instability of the arch structure in the coal gangue caving flow field is a unique phenomenon [9] which can affect the recovery of top coal and coal quality. With the increase of the thickness of top coal, the range of the top coal and gangue caving flow increases, both the broken fragment dimension of coal gangue and caving flow space changes. This enlarges the probability of the mixing process of top coal and gangue as well as the top coal arching [10], especially under the condition of fully mechanized caving in the special thick coal seam (14–20 m). In order to further improve the recovery rate of top coal and reasonably determine the caving technology parameter, it is necessary to understand the formation characteristics of the arch structure in the coal caving flow process and its influence on coal gangue flow pattern, fluency, and top coal recovery under this condition. Therefore, this paper uses a granular physical simulation experiment to study the arching characteristics of coal gangue in the process of fully mechanized caving in extra thick coal seam and top coal caving and its influence on top coal loss, in order to provide a basis for the reasonable determination of the coal caving technology parameter and the formulation of the technical measures to improve the recovery rate of top coal.
E-mail address: [email protected] (C. Liu). http://dx.doi.org/10.1016/j.ijmst.2016.05.010 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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2. Granular simulation experiment project According to the similarity theory, under the geological conditions of the No. 8105 coal mining face in the Tashan mine, a granular simulation experiment method was used to study the top coal fragment distribution of the fully mechanized caving in extra thick coal seam with large mining height. The granular simulation experiment method was also used to study the structure effect of coal gangue flow field and its influence on top coal loss law. The average thickness of the coal seam of the Tashan mine No. 8105 working face is 14.50 m, the mining height is 4.20 m, caving height is 10.30 m, mining and caving ratio is about 1:2.45. The immediate roof is the alternate occurrence layer consisting of magmatic rocks, mudstone and silicified coal with the average height 4.49 m. The basic roof consists of siltstone, fine sandstone, and muddy-conglomeratic sandstone with an average height of 22.93 m. Average dip angle of the coal seam is 4°, the coal mining cycle progress is 0.80 m, and the top coal drawing interval is 0.80 m per cut [11,12]. Theoretical analysis and field tests [13] show that the order of top coal along the thickness direction of the broken fragment dimension is: the lower top coal fragment dimension < middle top coal fragment dimension < upper top coal fragment dimension. According to the results of the field-observed broken coal gangue fragment dimension, the top coal is divided into three layers: upper top coal, middle top coal and lower top coal. The immediate roof is divided into lower immediate roof, middle immediate roof, and upper immediate roof [14]. An experiment was conducted to analyze the caving of top coal flow and the arching law. Different colors and sizes of stones were used to replace all layers of coal, with the addition of clay to simulate smaller particles of coal and rock powder. The testing program is shown in Table 1. The coal gangue caving mixed experiment model table adopted in the experiment mainly includes a control system, a simple selfmoving hydraulic support, a mobile system, and framework. It is 300 cm long, 15 cm wide, and 220 cm tall. The advancing support and coal caving can be automatically carried out by setting the relevant parameters, as shown in Fig. 1. The model bracket height is 40 cm and the field machine mining height is 4.20 cm. Therefore, the geometric similarity ratio of the model and the prototype is C = 40/420 = 1:10.5. The thickness of top coal is 10.30 m and the corresponding thickness of top coal in the model is 0.98 m. At the boundary, the simulated coal pillar is 6.50 cm and the openoff cut is 6.00 m.
Fig. 1. Combined experiment of fully mechanized coal gangue caving.
divided into three types: the lower arch structure, the middle arch structure, and the upper arch structure. The specific distribution area is shown in Fig. 2. (1) Lower arch structure: the arch structure is located near the drawing opening, it has the smaller span and high stability, and it is easy to be formed. This is because the space near the drawing opening is the smallest and the large coal mass or coal rock mass is easy to be squeezed by each other to form hinged arch. The lower arch structure contains coal arch and coal gangue arch. The arching material of the coal arch is coal, as shown in Fig. 3a. The arching material of the coal gangue arch is a mixture of coal and gangue, as shown in Fig. 3b. (2) Middle arch structure: the middle arch structure is located between the caving shield and the top beam. The arch structure can be divided into coal gangue arch structure and pure coal arch structure (Fig. 4). (3) Upper arch structure: the arch structure is located at the top coal above the support top beam. It is less formed, it has a large span, can form a coal arch, and a coal gangue arch. The stability of the arch structure is lower, the front arch angle is generally located on the top beam of support, the rear arch angle is located on the gangue in goaf, and, therefore, the impact of the swing of support tail beam is small (Fig. 5).
4. Dynamic random arch effect in the process of top coal caving 3. Arch characteristics and types of top coal caving in extra thick coal seam Through the observation on the coal gangue flow law of the fully mechanized caving in extra thick coal seam during the experiment process, we can see that arching is an important factor that affects the top coal caving in the process of top coal flowing. According to the arching area [15,16], in the caving process of the working surface, the arch structure formed by top coal is
4.1. Dynamic random arch characteristics in the process of top coal caving In the top coal caving process, the flow field fracture surface contracts with the decline of the height. The shrinkage of the fracture surface provides horizontal extrusion pressure between coal gangue blocks which provides the mechanical condition for the block extrusion into arch. Through the experimental analysis of arching characteristics and types in the top coal caving process, we can conclude that the formation of the arch structure has a
Table 1 Similar simulation scheme (mm). Layer position
Upper immediate roof Middle immediate roof Lower immediate roof Upper top coal Middle top coal Lower top coal
Parameter
Arching area of upper arch structure
Thickness
Fragment dimension
600 400 300 326 326 326
90 75 70 55 45 27
Top beam Arching area of middle arch structure Caving shield Arching area of lower arch structure Angle of repose Tail beam
Fig. 2. Three different types of arch structures.
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caving, the coal gangue arch structure that is formed by the boundary conditions such as the fragmentation difference and the support caving shield is called the dynamic random arch effect. The formation mechanism, influence factors, instability conditions of the dynamic random arch, and its influence on top coal flow field and the recovery rate are analyzed as follows.
4.2. Formation mechanism and influence factors of dynamic random arch (a) Lower coal arch
(b) Lower coal gangue arch
Fig. 3. Lower arch structure.
By the characteristics and types of arch structures formed during the process of top coal flow, the formation of the arch structure should have some conditions: (1) Resulting in uneven displacement or relative displacement. (2) There exists a structure support point or constraint which can act as skewback. As the top coal caving is a dynamic process, and the coal gangue boundary is a changed curve which is from top to bottom, there are two kinds of situations between the coal gangue boundary along the side of (drawing opening) goaf and the top coal movement trace along the side of support:
(a) Middle pure coal arch
(b) Middle coal gangue arch
Fig. 4. Middle arch structure.
(a) Upper pure coal arch
(b) Upper coal gangue arch
Fig. 5. Upper arch structure.
dynamic random feature, specifically manifested in the following aspects: (1) Arching position: In the flow field, there is a random possibility of arching in different top coal positions. With the coal gangue flow in the flow field, the shape and position of the arch may be changed to a certain extent. (2) Arching probability: Although the formation of the arch is random, the formation probability is different at different positions. The arching probability increases with the shrinkage of the fracture surface in the caving flow field. (3) Arching material: arching blocks contain both coal and gangue. In summary, the formation of the arch in the fully mechanized top coal caving process has a certain randomness. From the formation process of the arch structure, the arch structure is formed by extrusion between coal gangue of broken top-coal and gangue in the dynamic process of caving flow. Its formation time, position, and shape are random. Therefore, in the dynamic process of coal
(1) The end face of the angle of repose and the top beam will serve as a skew back support for the flowing coal gangue. (2) Because the coal gangue broken fragmentation is not the same size, leading to different coal gangue liquidity in different positions, when most of the lower coal gangue liquidity is less than the upper coal gangue, the lower coal gangue will likely serve as the skew back support for the upper coal gangue. Before the start of coal caving technology of fully mechanized working face there is a relatively static state between coal gangue blocks, as shown in Fig. 6. The caving coal gangue blocks behind the support, in the state of stress equilibrium, are restricted to an inclined body by the support. Before the start of coal caving technology, for any stress circle of coal gangue blocks behind the support, the external force diagram is shown in Fig. 7. When opening the drawing opening, the coal gangue block at the bottom lose support, breaking the original state of stress equilibrium. At this moment, the coal gangue near the drawing opening will first flow to the working face, changing the original state of the stress equilibrium at point M as the vertical stress r2 begins to decrease. The vertical stress r2 will become less than the horizontal stress r1 if the flow is smooth. The vertical stress r2 will be 0 with the continuation of top coal flow process, in a certain range, when granular coal gangue blocks form a continuum under the action of friction. If the ends of the continuum are supported by the support of the shield beam and the repose end face behind the support, the arch structure will be formed. At this point, the
Fig. 6. Sketch map of the arching of fully mechanized top coal caving space.
N. Zhang, C. Liu / International Journal of Mining Science and Technology 26 (2016) 593–599
σ2
596
σ1
m f1
N2 G
N1
f2
Assuming that the acting combined stress on surface AB at the front skew back is N1, N1 is decomposed into horizontal normal stress N10 and vertical shear stress N11, and the acting combined stress on surface CD at the back skew back is N2. Assuming the arch body is symmetrical, N2 is decomposed into horizontal normal stress N20 and the vertical shear stress N21, then:
G ¼ L Dh q g
Fig. 7. Sketch map of the stressing of fully mechanized top coal caving space.
resultant stress direction at the two skew backs will tilt toward the vault, as shown in Fig. 8, and the component force in the horizontal direction of the resultant stress exerts pressure on the granular block which formed to arch. The friction formed by this pressure will be greater than the gravity of the coal gangue above the arch, as shown in Fig. 8, and the coal gangue block beneath the arch will smooth out. However, the flow of coal gangue block above the arch will be restricted. Although the formation of the dynamic random arch effect is random, the formation of arch structure has certain conditions. The factors influencing the formation of the dynamic random arch effect are as follows: (1) Coal gangue fragmentation size. Because the coal gangue contact and squeeze each other in the flow process, it is easy to form a relatively stable hinged structure only when the top coal is larger. Therefore, coal gangue fragmentation and fragmentation distribution has great influence on the formation of a stable arch structure. (2) The span of arch structure. Because the span of the arch structure determines the number of coal gangue blocks in the arch structure, usually the arch structure is relatively stable overall. Once a certain block in the arch structure loses stability, however, it will lead to structural damage and instability of the entire arch structure. Therefore, a greater span and higher position of the arch lowers the stability of arch structure. Conversely, a lower position of the arch, closer to the drawing opening, results in a higher stability of the arch structure. (3) Binding force along horizontal direction. Because the arch structure is usually a curved arch structure, to ensure the stability of arch structure, in the horizontal direction, a constraint is an important factor to maintain the stability of arch structure. Usually, a horizontal force constraint can provide constraints for the skew back of the arch structure. At the top of the arch structure, a horizontal force constraint can also increase the extrusion of block in the arch structure, improving the stability of the whole arch structure. 4.3. Instability condition of dynamic random arch The arch structure formed during the top coal caving process of fully mechanized caving is simplified as a mechanical geometry model, as shown in Fig. 9.
ð1Þ
In the equation: q is the density of coal gangue in the arch body. The balanced equation for the arch body is:
G ¼ N10 Dh þ N 20 Dh
ð2Þ
Combining Eqs. (1) and (2):
N10 þ N20 ¼ L q g
ð3Þ
Eq. (3) is called the mechanical geometric equilibrium conditions for the existence of ‘‘granular arch”. If the arch is symmetric, then
N10 ¼ N20 ¼
1 Lqg 2
ð4Þ
The dynamic random arch formed by coal gangue in the top coal caving process affects the flow law of top coal. This causes the top coal and gangue to be mixed in advance of when the gangue is seen at the drawing opening and the window is closed. Thus, part of the top coal is lost. Therefore, it is necessary to avoid the emergence of dynamic random arch. If, however, the formation of the arch cannot be avoided, it is necessary to take measures to make the arch unstable. Because the dynamic random arch belongs to the granular arch, there are two ways to cause the destruction of the granular arch. The first way is making the skewback without a stable support point. The second way involves component factors of moving a granular arch, making it lose continuity, leading to the instability of the arch caused by the loss of continuous carrier of horizontal compressive stress. A random dynamic arch has strong compression capability, but the tensile and jacking force resistance ability is very poor. Therefore, the dynamic random arch can be made unstable by way of breaking the skew back, pulling the arch, and pushing up the arch. The specific measures are as follows: (1) In the case of the random arch, with skew back on the support shield beam at a relatively high position, the arching probability is lower and the span of the arch is relatively large, which is generally more unstable. But, once the arch is formed at a lower position, it will be a more stable arch, and the arch instability can then be caused by traversing support at a small range from up to down or front to back, or continuously swinging the support tail beam in the caving process. This makes the continuously flowing coal gangue transfer displacement to the tail beam, and causes the instability of a piece of rock of the arch structure, so as to make the arch unstable due to the reason that compressive stress of the arch cannot be transmitted.
L
f1
N1
N2 G
N0
Δh
m
σ 2′ σ 1′ ψ
N1
f2
Fig. 8. Arching stress distribution diagram of coal gangue behind the support.
G
B A
N1
N 20
N2
N21
D C
Fig. 9. Mechanical geometry model of arch structure (Dh – Height of arch body, L – Span of arch body, G – Gravity).
N. Zhang, C. Liu / International Journal of Mining Science and Technology 26 (2016) 593–599
(2) For the random arch with skew back on the tail beam of the support shield beam, the front skew back can be damaged by swaying the support tail beam, so as to cause the instability of arch and ensure smooth running of top coal.
5. Analysis of the influence of dynamic random arch structure on the flow and loss of top coal The formation probability of the dynamic random arch is higher in the caving process of the extra thick coal seam. In general, the dynamic random arch causes the reduction of the top coal flowability, reducing the risk of top coal caving, as shown in Fig. 10. We can see from Fig. 10, the top coal above arch structure cannot be caved to the drawing opening. Usually, when dynamic random arch is formed, the coal gangue flow above the arch structure stagnates. The arch structure is also gradually formed in the process of top coal flow, so the effect of dynamic random arch on top coal flow is also a gradual process of change and development. Closing the window and shifting the support during the top coal caving process will cause the coal gangue boundary move backward. How much the coal gangue boundary moves backward relates to the amount of top coal above the arch structure. If the amount is more, then the coal gangue boundary notably moves backward, as shown in Fig. 11. We can see from Fig. 11, the lower coal gangue boundary cannot be caved before the support shift, so the amount of released top coal is less. However, after the support shift, with the top coal homeopathy rolled, arch structure will be also destroyed, and with the flow of top coal, because the top coal is not fully caved and discharged, the coal gangue boundary is near the goaf. This will inevitably make the top coal flow into the coal gangue repose angle near the lower region of the coal gangue boundary after the support shift and the amount of top coal in the lower parts which cannot be caved increases, leading to the permanent loss of top coal. On the other hand, the coal gangue boundary significantly moves backwards due to the support shift, so the amount of top coal in the next caving process is more. This happens because the mixed gangue will easily appear in the flowing process of top coal. The degree of mixed gangue increases with the increasing flow time of top coal. Thus, the more the amount of the top coal caved, the longer the caved time is, and the top coal loss caused by mixed gangue will be more. So, carrying out the shift operation in the case that the top coal above the dynamic random arch is not fully caved, the impact on the loss of top coal is more serious, including the loss caused by two aspects: the mixed gangue caused by top coal flow and top coal flow into the area which cannot be caved. For the coal which can be caved, the top coal loss is mainly caused by the reason that the mixed gangue and top coal arch
Top coal is not released at the top
Fig. 10. Effect of dynamic random arch on top coal flow.
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make it so the top coal cannot be released in the coal working face, usually closing the window when seeing gangue and stopping caving. Therefore, there are two cases for seeing gangue in the process of coal caving. The first case is the gangue near repose angle in goaf is easy to flow to the drawing opening with top coal in caving process. At this point, the top coal is not fully caved, so the top coal loss is very serious. This situation is unlikely to occur, but once the top coal loss appears, it is quite serious. The second case is when top coal and gangue are mixed in the flowing process. Because the density of gangue is larger, the coal gangue easily flows into the upper top coal in the flowing process, leading to the result that some upper top coal is not caved yet when seeing gangue, thus causing losses. Usually, the loss of top coal in this case is not very serious, and the loss of top coal increases with the increase of top coal thickness, as shown in Fig. 11. Furthermore, if the top coal above the arch structure has been caved more, the coal gangue boundary is smooth and it has little effect on the loss of top coal. The influence of different arch structures on top coal flow is different. The formation of the upper arch structure will hinder the flowability of coal or gangue in the upper part of the arch structure. If the upper part of arch structure is all gangue, the formation of arch will help to slow down the time of seeing gangue, improving the recovery rate of top coal. The formation of middle arch structure will disturb the top coal flow state at the upper arch structure. At the same time, it is easy for the gangue of goaf to enter into the caving space of the lower part of arch structure, causing seeing gangue in advance at the drawing opening and arching near the drawing opening. This will prevent the top coal caving. At this point, if taking the arch breaking measures, caving time will be increased. If stopping caving, it will positively affect the recovery of top coal. Carrying out the statistics of the arching number, arching probability of different arch structures in 20 cycles of normal caving, and recovery rate of top coal under the conditions of different arch structures in the test process, the results are shown in Table 2. The recovery rate of top coal is the ratio result of caved top coal quantity and theoretical caving quantity before the first formation of the arch structure. With the statistical process, the influence of a certain kind of arch structure on the recovery rate can be predicted, before the appearance of the arch. If other arch structures are formed, the arch breaking measures can be adopted to guarantee the accuracy of the statistics of the recovery rate. From Table 2, we can see that in the process of caving, arching probability at different horizons are different; the higher the height of the arch, the smaller the probability of the arch. The arching probability of upper arch structure is the formation probability of upper top coal which is 3.12%, middle arch structure accounted for 16.67%, and the lower arch structure accounted for 80.21%. The lower arch structure, then, has the highest probability. In the same layer of arch structure, pure coal arch effect on recovery rate is greater than the coal gangue arch. When the pure coal arch of lower arch structure is first formed, the top coal recovery rate is only 24%, so the treatment of the pure coal arch structure of lower top coal is an effective way to improve the top coal recovery. As the recovery rate of coal gangue arch structure is already higher, if the amount of gangue in coal gangue arch structure is large, once the arch structure is destroyed, there will be a lot of gangue to reach the drawing opening, causing the higher percentage of gangue content. The gangue content will also be higher in the subsequent caving. Therefore, if the amount of gangue is more, it is not suitable to deal with the arch structure. For the middle arch structure, the two types of arch have great influence on the recovery of top coal. Therefore, it is necessary to carry out the broken arch operation. For the upper arch structure, top coal recovery rate has reached more than 60%, its arching probability is lower, and it is difficult
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Coal gangue boundary
Coal gangue repose angle before the support shift
Coal gangue repose angle after the support shift
Support after support shift
Fig. 11. Flow law of top coal before and after the support shift.
Table 2 Arching number and probability of different arch structures. Arch structure type
Arching number
Arching probability (%)
Arching ratio (%)
Recovery rate of top coal (%)
Upper arch structure
Pure coal arch Coal gangue arch
1 2
5 10
1.04 2.08
60.30 78.80
Middle arch structure
Pure coal arch Coal gangue arch
9 7
45 35
9.38 7.29
51.34 63.23
Lower arch structure
Pure coal arch Coal gangue arch
56 21
280 105
58.33 21.88
24.14 82.23
to break the arch through support action. Therefore, the impact of the arch at this position can be ignored. 6. Conclusions (1) According to the position characteristics of the arching of top coal caving in the working face, the arch structure is divided into upper arch structure, middle arch structure, and lower arch structure. Arch structures of three layers include two types of pure coal and coal gangue mixture. (2) Through the analysis of the top coal caving arching law of the fully mechanized coal caving in the extra thick coal seam, putting forward the concept of dynamic random arch, influencing factors of dynamic random arch include coal gangue fragmental size, span of arch structure, and constraining force of horizontal direction. Random dynamic arch has strong compression capability, but the tensile and jacking force resistance ability is very poor. So, the dynamic random arch can be made unstable in the way of breaking the skew back, pulling and pushing up the arch. (3) Dynamic random arch causes the decrease of top coal flowability. Closing the window and moving the support when it is arching in the process of top coal caving, will cause the coal gangue boundary to move backward. After shift moving, the top coal near the lower region of coal gangue boundary flowed into goaf behind the shift, causing permanent loss of top coal. The formation probability of dynamic random arch is different at different positions. The formation proba-
bility of the lower dynamic arch is the highest. The whole coal arch structure has the biggest impact on top coal loss. The lower arch structure, especially the lower pure coal arch structure, is the key point of control, as it can significantly improve the top coal recovery rate.
Acknowledgments Financial support for this work, provided by the Independent Research Subject of State Key Laboratory of Coal Resources and Mine Safety of China University of Mining and Technology (No. SKLCRSM12X03), the Scientific Research and Innovation Project for College Graduates in Jiangsu (No. CXZZ13_0947), Top-Notch Academic Programs of Jiangsu Higher Education Institutions, and the Priority Academic Development Program of Jiangsu Higher Education Institutions, is gratefully acknowledged. References [1] Xie YS, Zhao YS. Numerical simulation on the law of top-coal caving process under vibration condition. J Min Saf Eng 2008;25(2):188–91. [2] Wang JC, Li ZG, Chen YJ, Zheng HF. Experimental study of loose medium flow field on the long wall top-coal caving. J China Coal Soc 2004;29(3):260–3. [3] Zhang YD, Zhang FT, Ji M, Gao LS, Jin ZY, Cheng L, Wu LP. Research on the reasonable coal caving technological parameters of extra-thick coal seam. J Min Saf Eng 2012;29(6):808–14. [4] Ma LQ, Qiu XX, Dong T. Huge thick conglomerate movement induced by full thick long wall mining huge thick coal seam. Int J Min Sci Technol 2012;22 (3):399–404.
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