Numerical simulation on top-coal movement law under caving mining ...

7i'ansit Development in Rock Mechanics - Cai, Yang & Wang (Eds) © 2015 Taylor & Francis Group, London, ISBN 978-1-138-02730-5

Numerical simulation on top-coal movement law under caving mining technique in steeply dipping seam influenced by caving sequence J.W. Zhang, Z.Y Song & H.L. Geng School of Resource and Safety Engineering, China University of Mining and Technology (Beijing), Beijing, China

ABSTRACT: In order to understand the top-coal movement law influenced by caving sequence in steeply dipping seam, taking NO. 1201 working face of Shanxi Dayuan coal Ltd. as engineering background, established the PFC 20 numerical model and simulated caving process under three different caving sequences: caving from top to bottom, caving from bottom to top and mixed caving sequence. The characteristic of recovery ratio, top-coal draw body and the boundary of coal and rock under the condition of steeply dipping seam are obtained from the simulation. The results show that the recovery ratio is the lowest when caving from top to bottom, while it is the highest when caving from bottom to top; the topcoal draw body has the tendency of developing towards the top of working face; the boundary of coal and rock shows significant asymmetry during the developing process; under the condition of steep coal seam, the top-coal at the top-of working face left blank seriously, which has a significant impact on the stability of the air-return way, so special measures are suggested to reinforce the top-coal above the airreturn way. (Huang eta!. 2010). Based on the theory of loose medium flow, simulated caving process of the loose top-coal and the fractured immediate roof and made comprehensive analysis about the recovery ratio, draw body and the character of boundary of coal and rock. The result can provide the theoretical foundation to optimize the caving sequence and increase the top-coal recovery ratio.

1 INTRODUCTION With the exhaustion of the eastern mining area's coal resources, the focus of coal mining in our country gradually shifted to the west part of China, and the occurrence conditions of coal seams in west part of China are complex, the steeply dipping and steep seam are widely distributed in western China (Wu 2001). Therefore, the research which focuses on the top-coal movement law of steeply dipping seam caving mining technique has important significance to ensure the security and efficiency in the caving mining and to improve the resource recovery ratio. At present, many scholars have conducted researches on the law of top-coal movement under the steeply dipping seam condition and the failure mechanism of the top-coal (Wang et al. 2006, Wang eta!. 2013). However, the former researches on the top-coal movement law under steeply dipping seam conditions always place extra emphasis on the analysis of top-coal recovery ratio, while rare researches have been conducted on the aspect of combination with top-coal caving pattern and evolution about boundary of coal and rock (Wang et a!. 2005, Yang et a!. 2010). It is especially rare for scholars to conduct the research combining the top-coal caving pattern under the steeply dipping seam conditions in different caving sequences, topcoal movement law and the relations of top-coal recovery ratio between different caving sequences

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2 ENGINEERING SITUATION The main coal seam of DAYUAN coal mine in Shanxi province is NO.2 coal seam, which has an average thickness of 6.5 m, the angle of coal seam is between 40°-55°, and the average angle is 47°. The immediate roof is light gray/gray mudstone or sandy mudstone with a thickness of 0.1 m-1.5 m. The main roof is gray siltstone with a thickness of 8 m, the immediate floor is siltstone with a thickness of 2.0 m-2.6 m. The main roof is composed of gray medium-grained sandstone with a thickness of 12 m, and some parts of it are composed of grit stone and the cementation degree is not uniform. Specifically, most parts are rigid, some loose structures exist in a few area. The strike and incline length of NO. 1201 working face are 680 m and 60 m respectively, the mining height is 2.5 m and the support type is ZFY4800/l7/28, whose width is 1.5 m and the

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height is 1.7-2.8 m, the average caving height is 4 m while the depth of coal-bed is 242.6 m-195.6 m. 3 NUMERICAL MODEL AND SIMULATION SCHEME 3.1

Model size and boundary conditions

The steeply dipping seam caving mining numerical calculation model was built through particle flow code 2D (PFC 20), Figure 1 illustrates the initial state of the numerical model. The model size: length x height::: 60 m x 14.5 m. . . The coal are simulated by the black partlcles which locate at the under-part of the Figure 1 while the fractured immediate roof are simulated by the gray particles which locate at the top of the Figure 1. . The initial conditions are as follows: the particles initial velocity are 0 and are experienced the gravity only, g::: 9.81 rn/s2, both the velocity and the acceleration velocity of the wall are 0. The boundary conditions are as follows: the boundary of the particles is limited by wall, whose velocity and acceleration velocity are both fixed to 0. 3.2 Mechanical parameters of model Based on results of the rock mechanics experimental and take the size effect into consideration, the rock mechanics parameter adopted in the numerical simulation are shown in Table 1. 3.3 Simulation scheme design Similar to the field condition, 36 caving supports are set in the model. Specifically, there are I transitive support at the upper end and 2 transitive supports at the lower end; there are 5 caving supports were prohibited caving at the upper and lower end respectively, in order to protect the crossheadings. Consequently 23 caving supports can be utilized to

Table 1. Mechanical parameters of model. Density Radius Material (kglm 3) (m) Gangue 2500 Coal 1500

Normal Shear stiffness stiffness Friction (N/m) (N/m) coefficient

0.3-0.5 4 X lOS 4 X 10' 0.4 0.1-0.15 2 X lOS 2 X 108 0.4

caving in the simulation which can reflect the real condition in the coal mine. Based on the PFC 20 model, three caving sequences were taken into consideration: caving from top to bottom, caving from bottom to top and mixed caving sequence. The research has been conducted to get a clear perspective of the impact on the law of top-coal caving in di~erent caving sequence so that it can help to detenrune the most reasonable caving sequence. Three different schemes are as follows: Scheme 1: Caving from top to bottom-The coal caving started from the N0. 31 caving support at the top and continued to caving from the top to bottom. The process of coal caving stops at the NO. 9 caving support at the bottom, 23 caving supports involved during the process of caving. Scheme 2: Caving from bottom to top-The coal caving started from the NO. 9 caving support at the bottom and continued to caving from the bottom to top. The process of coal caving stops at the NO. 31 caving support at the top, 23 caving supports involved during the process of cavin~. Scheme 3: Mixed caving sequence-The coal cavmg started from the NO. 31 caving support at the top and continue to caving from the top to bottom. It stops at the NO. 26 caving support and the process of caving to bottom ceased. Then The coal caving started from NO. 9 caving support at the bottom and continued to caving from the bottom to top. It stops at the NO. 25 support. 23 caving supports involved during the process of caving.

4 RESULT ANALYSIS 4.1

Analysis of the recovery ratio

The morphology of residual coal in the goaf in three simulation schemes are shown in Figure 2. Analyze of the number of residual and initial coal particles of the three schemes, Formula 1 can be used to calculate the recovery ratio in three schemes, calculation results are shown in Table 2. (1) Figure I. Initial state of the PFC20 model.

彩＝（・一孙・ｏｏ％ｍａｎｉｓａｖｅｒａｇｅｍ，ｔｈｅ１．７－２．８ｈｅｉｇｈｔ３Ｎｍ．ｍ－１９５．６２４２．６ｃｏａｌ・ｂｅｄｉｄｅｐｔｈ２Ｄ（ＰＦＣ２Ｄ）．Ｆｉｇｕｒｅｃｏｄｅｆｌｏｗｐａｒｔｉｃｌｅｔｈｒｏｕｇｈｂｕｉｌｔｗａｓｃａｌｃｕｌａｔｉｏｎｎｕｍｅｒｉ・ｍｉｎｉｎｇｓｅａｍｄｉｐｐｉｎｇｓｔｅｅｐｌｙ１１１ｅｃ伽ｄｉｔｉｏｎｓｓｉｚｅＭｏｄｅｆ３．１ＳＣＨＥＭＥＳＩＭＵＬＡＴＩＯＮＡＮＤＭＯＤＥＬｌｗｕｎｄｅｒ－ｐａｒｔｌｏｃａｔｅｂｌａｃｋｓｉｍｕｌａｔｅｄＴｈｃ１４．５ｍ．ｌｅｎｇｔｈ×ｈｅｉｇｌｌｔ＝６０ｍｘｓｉｚｅ：ｍｏｄｅｌｍｏｄｅｌ．Ｔｈｅｎｕｍｅｒｉｃａｌｓｔａｔｅｔｉａｌ０ａｉｎｉ．ｃｌｅｓｐａｒｔｉ．ｆｏ・ｌｌｏｗｓ：ｔｈｅｃｏｎｄｉｔｉｏｎｓＴｈｅ１．Ｆｉｇｕｌｅｔ１１ｅｌｏｃａｔｅｇｒａｙｂｙｓｉｍｕｌａｔｅｄｒｏｏｆｉｍｍｅｄｉａｔｅｆｒａｃｔｕｒｅｄｍｏｄｅｌ．Ｓｐｅｃｉｆｉｃａｌｌｙ，ｔｈｅｒｅｓｅｔｃｏｎｄｉｔｉｏｎ，３６ｆｉｅｌｄＳｉｍｉｌａｒｄｅｓｉｇｎｓｃｈｅｍｅＳｉｍｕｌａｔｉｏｎ３．３１．Ｔｆ出ｌｅｃａｌｎｕｍｅｒｉ－ａｄｏｐｔｅｄｐａｒａｍｅｔｅｒｃｏｎｓｉｄｅｒａｔｉｏｎ，ｔｈｅｅｆｆｅｃｔｓｉｚｅｔａｋｅｔａｌｅｘｐｅｒｉｍｅｎ－ｍｅｃｈａｎｉｃｓｒｏｃｋｏｆｍｏｄｅｌＭｅｃｈａｎｉｃａｌｐａｒａｍｅｔｅｒｓ３．２ｆｉｘｅｄｂｏｔｈｖｅｌｏｃｉｔｙｗａｌｌ．ｗｈｏｓｅｂｖｌｉｍｉｔｅｄｉｓｂｏｕｎｄａｒｙｆｏｌｌｏｗｓ：ｔｈｅｃｏｎｄｉｔｉｏｎｓｂｏｕｎｄａｒｙ０．ｗａｌｌａｃｃｅｌｅｒａｔｉｏｎｖｅｌｏｃｉｔｙＩｎ／ｓ２．ｂｏｔｈｏｎｌｙ，ｇ＝９．８ｌｇｒａｖｉｔｙ２ｔｅｘｐｅｒｉｅｎｃｅｄｕｐｐｅｒ５ｔｉｖｅａｌｅｅｎｄ；ｔｈｅｒｅｐｏｒｔｓａｓｕｐ－ｇｅｔｃｏｎｄｕｃｔｅｄｂｅｅｎｈａｓｒｅｓｅａｒｃｈｓｅｑｕｅｎｃｅ．Ｔｈｅｍｉｘｅｄｂｏｔｔｏｍ．ｃａｖｉｎｇｉｎｇｃｏｎｓｉｄｅｒａｔｉｏｎ：ｃａｖ－ｉｎｔｏｔａｋｅｎｓｅｑｕｅｎｃｅｓｍｏｄｅｌ．ｔｈｒｅｅＰＦＣ２ＤＢａｓｅｄｍｉｎｅ．ｃｏｎｄｉｔｉｏｎｒｅａｌｒｅｆｌｅｃｔｗｈｉｃｈｓｉｍｕｌａｔｉｏｎｃｏｅｆｆｉｃｉｅｎｔ（Ｎ／ｍ）（Ｎ／ｍ）ＦｒｉｃｔｉｏｎｓｔｍｅｓｓｓｔｉｆｆｎｅｓｓＳｈｅａｒＮｏｒｍａｌＭａｔｅｒｉａｌ（ｋｇ／ｍ３）（ｍ）ＲａｄｉｕｓＤｅｎｓｉｔｙｍｏｄｅｌｐａｒａｍｅｔｅｒｓ１．ＭｅｃｈａｎｉｃａｌＴａｂｌｅｍｏｄｅｌ．ＰＦＣｚｏｔｈｅｏｆｓｔａｔｅ１．ＩｎｉｔｉａｌＦｉｇｕｒｅｕｔｉｌｉｚｅｄ２３Ｃｏｎｓｅｑｕｅｎｔｌｙｃｒｏｓｓｈｅａｄｉｎｇｓ．ｐｒｏｔｅｃｔｏｒｄｅｒｒｅｓｐｅｃｔｉｖｅｌｙ，ｉｎｅｎｄｌｏｗｅｒｕｐｐｅｒｐｒｏｈｉｂｉｔｅｄｗｅｒｅｓｕｐｐｏｒｔｓｂｏｔｔｏｍ—＿１’ｈｅ１：Ｃａｖｉｎｇ３１４ｆｏｆｌｏｗｓ：ａｓｄｉｆｆｅｒｅｎｔｓｅｑｕｅｎｃｅ．Ｔｈｒｅｅｒｅａｓｏｎａｂｌｅｍｏｓｔｄｅｔｅｍａｉｎｅｈｅｌｐｃａｎｉｔｔｈａｔｓｏｓｅｑｕｅｎｃｅｄｉｔｌｅｒｅｎｔｔｏｐ．ｃｏａｌｌａｗｏｎｉｍｐａｃｔｐｅｒｓｐｅｃｔｉｖｅｔｏｐ．Ｔｈｅｔｏ伊ｊ功ｅ２：Ｃａｖｉｎｇｂｏｔｔｏｍ．２３ｂｏｔｔｏｍ．ＴｈｅｔｏｃｃｏｎｔｉｎｕｅｄＮＯ．３ｃｏａｌｓｅｑｕｅｎｃｅ—孔ｅ３：ＭｉｘｅｄＳｃｈｅｍｅｐｒｏｃｅｓｓｓｕｐｐｏｒｔｓｔｏｐ，２３４Ｒｓｕｐｐｏｎｃａｖｉｎｇ．ｐｒｏｃｅｓｓｄｕｒｉｎｇｉｎｖｏｌｖｅｄｓｕｐｐｏｒｔｓＳＵｐｐＯｒｔ．２３ＮＯ．２５ｍｅｔｏｐ．ＩｔｂｏｔｔｏｍｃｏｎｔｉｎｕｅｄａｔＮＯ．９ｓｔａｒｔｅｄｃａｖｉｎｇｃｏａｌｃｅａｓｅｄ．ＴｈｅｎｂｏｔｔｏｍｅＳＳｐｒｏｃ－ＮＯ．２６ｓｔｏｐｓｂｏｔｔｏｍ．Ｉｔｔｏｐｆｒｏｍｃａｖｉｎｇｃｏｎｔｉｎｕｅｔｏｐａｔ１ｃｓｕｐｐｏｒｔｓｃｈｅｒｕｅｓ，ＦｏｒｍｕｌａｐａｒｔｉｃｌｅｓｉｎｉｔｉａｌａｎｄｎｕｍｂｅｒＡｎａｌｙｚｅＦｉｇｕｒｅｓｈｏｗｎａｒｅｓｃｈｅｍｅｓｓｉｍｕｌａｔｉｏｎｇｏａｆｃｏａｌｒｅｓｉｄｕａｌｏｆｍｏｒｐｈｏｌｏｇｙＴｈｅｒａｔｉｏｒｅｃｏｖｅｒｙＤ厂ｔｈｅＡｎａｌｙｓｉｓ４．１ＡＮＡＤ【，ｓＩＳ２．Ｔａｂｌｅｓｈｏｗｎａｒｅｒｅｓｕｌｔｓｓｃｈｅｍｅｓ．ｃａｌｃｕｌａｔｉｏｎｔｈｒｅｅｉｎｒａｔｉｏｒｅｃｏｖｅｒｙｔｈｅｃａｌｃｕｌａｔｅｔｏｕｓｅｄｂｅＥＳＵＬＴｌｌｕｓｔｒａｔｅｓｈｉｌｅｎｄｒａｎｓｉ．ｒａｎｓｉｔｉｖｅｌｅａｒａｖｉｎｇＵＭＥＲＩＣＡＬａｖｉｎｇ

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the top-coal can be utilized in the caving from bottom to top under the condition of steeply dipping seam. It was much more easier for the caving of the top cola in the upper end and contributed to a higher recovery ratio. 4. Since the specific gravity of the gangue is bigger that the coal, the gangue from the goaf of the upper end can easily went into the middle drawing opening, consequently the recovery ratio is low.

(b) cheme 2

4.2 Analysis of the draw body

(c) Scheme 3 Figure 2. The morphology of residual coal in the goaf under different caving sequences.

Table 2. Recovery ratio of different scheme.

Caving sequence

Particle number of initial coal individual

Particle number of residual coal individual

Recovery ratio(%)

Scheme 1 Scheme 2 Scheme 3

6236 6236 6236

2821 2330 2485

54.76 62.63 60.15

Analyze the draw body Inversion and compare the various pattern of draw body in different caving sequences, the results of inversion are shown in the Figure 3. Figure 3 illustrates that when caving from top to bottom, There was no difference in draw body volume on either side of the drawing opening center line, The morphology of draw body were symmetrical to the drawing opening center line; No matter caving from bottom to top or mixed caving sequence, it was revealed significant asymmetry. What's more, it is the unique characteristic of caving mining technique under steeply dipping seam that the top-coal draw body has the tendency of developing towards the top of working face. In order to quantitatively describe this asymmetry, draw body asymmetry coefficient value W is defined as follows: (2)

where W =draw body asymmetry coefficient value; V, = draw body volume on the right side of the where w = recovery ratio; Nc= particle number of residual coal; and Nd = particle number of initial coal. Combined Figure 2 and Table 2, we knew that: I. Among the three schemes, the maximum recovery ratio appeared in the caving from top to bottom while the minimum recovery ratio appeared in the caving from bottom to top, the medium recovery ratio appeared in the mixed caving sequence. 2. All the recovery ratio remained low in the three caving sequences, this result can be attributed to the short length of 1201 mining face (60 m), moreover, 10 caving supports at the end of face were prohibited to caving,consequently the recovery ratio was mainly influenced by the end loss and lead to such low recovery ratio. 3. The gangue in the goaf was located under the top-coal which have not been caved when caving from bottom to top, it has gentle influence on the caving of top-coal which located in middle part and upper end. The gravity flow ability of

怠六一纛ｎｕｍｂｅｒｍｅｄｉｕｍｍｉｎｉｍｕｍｍａｘｉｍｕｍ ∥：兰ａｓｙｍｍｅｔｒｙ．、№ａｔ＇ｓｌｉｎｅ，Ｔｈｅｂｏｔｔｏｍ．ＴｈｅｒｅａＦＩｎｄＦｉｇｕｒｅＣｏｍｂｉｎｅｄｃｏａｌ．ｉｎｉｔｉａｌＮｄ＝ｐａｒｔｉｃｌｅｃｏａｌ；ａｎｄｒｅｓｉｄｕａｌｎｕｍｂｅｒｒａｔｉｏ；札＝ｐａｒｔｉｃｌｅ１０＝ｒｅｃｏｖｅｒｙｗｈｅｒｅｓｃｈｅｍｅ．２．Ｒｅｃｏｖｅｒｙｓｅｑｕｅｎｃｅｓ，ｄｉｆｆｅｒｅｎｔｇｏａｆｉｎｃｏａｌｒｅｓｉｄｕａｌｍｏｒｐｈｏｌｏｇｙ１２．Ｔｈｅｌｅｎｇｔｈｓｈｏｒｔａｔｔｒｉｂｕｔｅｄｂｅｒｅｓｕｌｔｓｅｑｕｅｎｃｅｓ，ｔｈｉｓｌＯＷｒｅｍａｉｎｅｄｒａｔｉｏ２．Ａ１１ｓｅｑｕｅｎｃｅ・ｍｉｘｅｄｔｏｐ，ｔｈｅｒｅｃｏｖｅｒｙｗｈｉｌｅｔｏｍｂｏｔ－ａｐｐｅａｒｅｄｅｒｙｒｅｃｏｖ・ｓｃｈｅｍｅｓ，ｔｈｅｔｈｒｅｅ１．Ａｍｏｎｇｔｈａｔ：ｋｎｅｗ２，ｗｅｌｍＴａｂｌｅａｈｔｒｉｂｕｔｅｄｃｏｎ－ｕｐｐｅｒｃｏｌａｆｏｒｅａｓｉｅｒｍｏｒｅｍｕｃｈｓｅａｍ．Ｉｔｃｏｎｄｉｔｉｏｎｕｔｉｌｉｚｅｄｂｅｔｏｐ－ｃｏａｌ３１５ａｂｉｆｉｔｙｆｌｏｗｅｎｄ．Ｔｈｅｕｐｐｅｒｐａｒｔｗｈｉｃｈｔｏｐ．ｃｏａｌｉｎｆｌｕｅｎｃｅｇｅｎｔｌｅｔｏｐ．ｉｔｃａｖｅｄｂｅｅｎｎｏｔｈａｖｅｗｈｉｃｈｔｏｐ．ｃｏａｌｌｏｃａｔｅｄ３．Ｔｈｅｌｏｗｓｕｃｈｌｅａｄｌｏｓｓｂｙｉｎｆｉｕｅｎｃｅｄｍａｉｎｌｙｒｅｃｏｖｅｒｙｃａｖｉｎｇ，ｃｏｎｓｅｑｕｅｎｔｌｙｐｒｏｈｉｂｉｔｅｄｆａｃｅａｔｓｕｐｐｏｒｔｓｍｏｒｅｏｖｅｒ，１０ｍ），３ｉｆａｃｅ（６０３．Ｆｉｇｕｒｅｓｈｏｗｎａｒｅｉｎｖｅｒｓｉｏｎｒｅｓｕｌｔｓｓｅｑｕｅｎｃｅｓ，ｔｈｅｄｉｆｆｅｒｅｎｔｐａｔｔｅｒｎｖａｒｉｏｕｓｃｏｍｐａｒｅａｎｄＩｎｖｅｒｓｉｏｎＡｎａｌｙｚｅｂｏａｙｏｆＡｎａｌｙｓｉｓ４．２ｌｏｗ．ｉＳｒａｔｉｏｏｐｅｎｉｎｇ，ｃｏｎｓｅｑｕｅｎｔｌｙｍｉｄｄｌｅｉｎｔｏｗｅｎｔｅａｓｉｌｙｃａｎｅｎｄｕｐｐｅｒｇｏａｆｇａｎｇｕｅｃｏａｌ．ｔｈｅｂｉｇｇｅｒｉｓｇａｎｇｕｅｔｈｅｇｒａｖｉｔｙｓｐｅｃｉｆｉｃ４．Ｓｉｎｃｅｒａｔｉｏ．（ｒｅｃｏｖｅｒｙ巧ｆｏｌｌｏｗｓ：ａｓｄｅｆｉｎｅｄＷｉｓｖａｌｕｅａｓｙｍｍｅｔｒｙ，ｄｒａｗｔｈｉｓｄｅｓｃｒｉｂｅｑｕａｎｔｉｔａｔｉｖｅｌｙｏｒｄｅｒｆａｃｅ．Ｉｎｗｏｒｋｉｎｇｔｏｗａｒｄｓｄｅｖｅｌｏｐｉｎｇｔｅｎｄｅｎｃｙｈａｓｂｏｄｙｔｏｐ－ｃｏａｌｔｈａｔｓｅａｍｄｉｐｐｉｎｇｓｔｅｅｐｌｙｕｎｄｅｒｎｉｑｕｅｔｅｃｈ－ｍｉｎｉｎｇｃａｖｉｎｇｃｈａｒａｃｔｅｒｉｓｔｉｃｕｎｉｑｕｅｉｓｍｏｒｅ．ｓｉｇｎｉｆｉｃａｎｔｒｅｖｅａｌｅｄｉｔｓｅｑｕｅｎｃｅ，ｍｉｘｅｄｏｒｔｏｐｔｏｂｏｔｔｏｍｍａｔｔｅｒｆｉｎｅ；Ｎｏｃｅｎｔｅｒｃａｌｓｙｍｍｅｔｒｉ．ｗｅｒｅｃｅｎｔｅｒｏｐｅｎｉｎｇｄｒａｗｉｎｇｅｉｔｈｅｒｕｍｅｖ０１．ｂｏｄｖｄｒａｗｉｎｄｉｆｆｅｒｅｎｃｅ１１０ｗａｓｔｏｔｏｐｆｒｏｍｃａｖｉｎｇｗｈｅｎｔｈａｔ２Ｆ（ｂ）Ｓｃｈｅｍｅａ）Ｓｃｈｅｍｅｏｆｓｉｄｅｒｉｇｈｔｔｈｅｏｎｖｏｌｕｍｅ矿＝ｄｒａｗｖａｌｕｅ；ｃｏｅｆｆｉｃｉｅｎｔａｓｙｍｍｅｔｒｙｂｏｄｙｗｈｅｒｅ缈＝ｄｒａｗｓｅｑｕｅｎｃｅｓ－ｃａｖｉｎｇｅｎｔｄｉｆｆｅｒ－ｕｎｄｅｒｂｏｄｙｄｒａｗｏｆｍｏｒｐｈｏｌｏｇｙ３．Ｔｈｅｉｇｕｒｅ０ｈ）Ｓｃｈｅｍｅｉｎｉｎｇｉｇｈｅｒｌｌｕｓｔｍｔｅｓ２）ｉｇｕｒｅ

(a) cheme I

(b) Scheme 2

(c) Scheme 3 Figure 3. The morphology of draw body under different caving sequences.

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flow from NO. 31 support, and this led to a higher value of W.

caving center line, m3; V1 for draw body volume on the left side of the caving center line, m3; V, and V1 has the following relationship:

4.3 Analysis of the boundary of coal and rock

(2)

where V =the volume of draw body. Calculate V, V,, V1 and Draw body asymmetry coefficient value Wunder three kinds of ca~ ing sequence respectively, Results are shown m Table 3. Combined Figure 3 and Table 3, we knew that:

I. Among the three schemes, compare the value of W, the maximum appeared in the caving from bottom to top while the minimum appeared in the caving from top to bottom, the medium appeared in the mixed caving sequence; 2. The top-coal which located in the middle-upper part of working face would move to bottom along support beam under the force of gravity during the process of caving from bottom to top, and in particular, the condition of steep coal seam would make the flow state more obvious. Especially, when caving from bottom to top, there would be more upper coal released through the lower part of drawing opening, Therefore draw body asymmetry coefficient value Wreached maximum, as high as 2.817; 3. During the process of mixed caving sequence, because of the first stage of the caving in upper end has come to the end (The coal caving of NO. 31-26 support has been finished),Top-coal release quantity located in middle-upper part will reduce obviously when caving from NO. 9 support to the top. Therefore the Draw body volume on the right side of the caving center line will be smaller, namely Draw body asymmetry coefficient W achieve a smaller value of 2.817. 4. Due to the upper end of the border restrictions, Draw body volume on the right side of the caving center was less than caving from bottom to top and mixed caving sequence, smaller values of W was caused by the boundary effect of the model. Top-coal which located at the upper end of working force and upper air-return would

Along with the caving, Initial boundary of coal and rock would gradually destroy along the coal seam inclination direction. Due to the intrusion of gangue, complete boundary of coal and rock was divided into two parts: front of the boundary of coal and rock and back of the boundary of coal and rock. Three schemes of the coal and rock boundary characteristics are shown in Figure 4. Figure 4 illustrates that during the process of caving from top to bottom, the back boundary of coal and rock was located at the top of working face, while the front boundary of coal and rock moves down gradually along with the caving from top to bottom, the shape of the front and back boundary of coal and rock showed more obvious asymmetry in coal seam inclination direction. Due to the density of gangue is greater than coal particle, the upper triangular coal which was compacted by the influx of gangue has not been released in the top of working face, since the coal seam inclination angle is very big, the direct roof gangue and the unreleased upper triangular coal moved down gradually with the process of caving from top to bottom, the shape of the back boundary of coal and rock would change greatly during the process of movement, as shown in Figure 4b. Due to the specific gravity of gangue is greater than coal, and the condition of steep coal seam would also led to significant influence, coal gangue particles would enter into the drawing opening ahead of the coal particles during the process of caving from top to bottom, as Figure 4b showed, which led to some coal be left in goaf, as Figure 4a showed. So this caving mode is not suggested to use. Figure 5 illustrates that during the process of caving from bottom to top, the back boundary of coal and rock was located at the bottom of working

Table 3. The value of V, V,, V1 and W of different scheme. Caving sequence

V(m 3)

V,(m 3)

Vl(ml)

w

Scheme 1 Scheme 2 Scheme 3

5.380 8.051 8.037

3.289 5.942 5.921

2.091 2.109 2.116

1.573 2.817 2.798

ｒｏｃｋ．ｍｅ

Ｋ＋巧＝Ｖｌｉｎｅ，ｍ３；¨ａｎｄ■ ｌｉｎｅ，ｍ３；■ｆｏｒＫｌ４ｎ口咖妇矿胁Ｐ６Ｄ“ｎ如秒∥ｃＤ口ｆ铡ｄｒＤ伙（２）ｏｆ缈ｒｉ曲ｔｗｉｌｌｍｉｄｄｌｅ－ｕｐｐｅｒｆｉｎｉｓｈｅｄ）．１＿０Ｐ．ｃｏａｌｍｉｘｅｄｍａｘｉｍｕｍ．ａｓＷｒｅａｃｈｅｄＷｍｅｄｉｕｍｖｏｌｕｍｅｕｐｐｅｒｍｏｄｅｌ．ＴｏＰ－ｃｏａｌｃｏａｌｂｏｕｎｄａｒｙｇｏａｌｂｅｄｒａｗｉｎｇｃｏｎｄｉｔｉｏｎ，一ａｎｄ１ｂ）Ｃａ、’ｉｎｇＮＯ．３ｒｅｌａｔｉｏｎｓｈｉｐ：ｆｏｌｌｏｗｉｎｇｈｈａｓＣａｌｃｕｌａｔｅｙ＝ｔｈｅｗｈｅｒｅｕ４．３ｔｒｙａｓｙｍｍｅ－３ＤｒａｗＣｏｍｂｉｎｅｄ３．Ｔｒ出ｌｅａｒｅｒｅｓｐｅｃｔｉｖｅｌｙ，Ｒｅｓｕｌｔｓｓｅｑｕｅｎｃｅｃａｖ・ａｋｉｎｄｓｍｅｔｒｙａｓｙｍ．ｓｍａｌｌｅｒ，ｎａｍｅｌｙｗｉｌｌｌｉｎｅｃｅｎｔｅｒｖｏｌｕｍｅｔｏｐ．ＴｈｅｒｅｆｏｒｅＮＯ．９ｗｈｅｎｏｂｖｉｏｕｓｌｙｒｅｄｕｃｅ１ｔｅａｔｅｄｑｕａｎｔｉｔｙｒｅｌｅａｓｅ１～２６Ｎ０．３ｅｎｄ（Ｔｈｅｃｏｍｅｓｔａｇｅｆｉｒｓｔｂｅｃａｕｓｅｓｅｑｕｅｎｃｅ，３．Ｄｕｒｉｎｇ２．８１７：ｈｉｇｈｃｏｅｆｆｉｃｉｅｎｔｂｏｄｙｄｒａｗＴｈｅｒｅｆｏｒｅｏｐｅｎｉｎｇ．ｄｒａｗｉｎｇｌｏｗｅｒｔｈｒｏｕｇｈｒｅｌｅａｓｅｄｂｅｔｏｐ’ｔｈｅｒｅＯＵＳ．Ｅｓｐｅｃｉａｌｌｙ，ｗｈｅｎｏｂｖｉ－ｓｔａｔｅｆｌｏｗｍａｋｅｐａｒｔｉｃｕｌａｒ，ｔｈｅｔｏｐ．ａｎｄｔｒｏｍｃａｖｍｇＯｌｔｌｌｅｄｕｎｎｇｌｔｙｇｒａｖ－ｏｔｔｏｒｃｅｕｎｄｅｒｂｅａｍｓｕｐｐｏｒｔａｌｏｎｇｂｏｔｔｏｍｍｄｙｅｆａｃｅｗｏｒｋｉｎｇ０１ｐａｒｔｍｉｄｄｌｅ．ｕｐｐｅｒｔｏｐ．ｃｏａｌ２．Ｔｈｅｓｅｑｕｅｎｃｅ：ｃａｖａｎｇ１１１１ｘｅｄｔｎｅｉｎａｐｐｅａｒｅａｍｅｌｎｍｌｍｍｕｍｗｈｉｌｅｆｒｏｍｃａｗｎｇｉｎａｐｐｅａｒｅｄｍａＸｌｍｕｍ ∥．ｔｈｅｖａｌｕｅｓｃｈｅｍｅｓ，ｃｏｍｐａｒｅｔｈｒｅｅ１．Ａｍｏｎｇｋｎｅｗ３，ｗｅＴａｂｌｅｗａｓｖａｌｕｅｓｓｅｑｕｅｎｃｅ，ｓｍａｌｌｅｒｍｉｘｅｄｌｅｓｓｃｅｎｔｅｒｉｎｇｃａｖ．ｓｉｄｅｒｉｇｈｔｏｎｂｏｄｖＤｒａｗｒｅｓｔｒｉｃｔｉｏｎｓ，ｂｏｒｄｅｒ４．ＤｕｅＷｏ２．８１７．Ｋ一，Ｋｖａｌｕｅ３．ＴｈｅＴａｂｌｅｗｏｕｌｄａｉｒ－ｒｅｔｕｒｎｕｐｐｅｒｆｏｔｅｅｅｎｄｅｆｆｅｃｔ４ｃａｕｓｅｄ４．ａｒｅｃｈａｒａｃｔｅｒｉｓｔｉｃｓｓｃｈｃｒｅｅｓｒｅｃｋｐａｒｔｓ：ｆｒｏｎｔｔｗｏｄｉｖｉｄｅｄｇａｎｇｕｅ，ｃｏｍｐｌｅｔｅｉｎｔｒｕｓｉｏｎｄｉｒｅｃｔｉｏｎ．Ｄｕｅｓｅａｍｄｅｓｔｒｏｙｃａｖｉｎｇ，ＩｎｉｔｉａｌＡｌｏｎｇ３１６ｓｃｈｅｍｅ．５ｉｄｉｆｆｅｒｅｎｔＵＳｅ．ｓｕｇｇｅｓｔｅｄｎｏｔｉｓｍｏｄｅｔｈｉｓｓｈｏｗｅｄ．Ｓｏ４ａａｓｌｅｆｔｓｏｍｅｓｈｏｗｅｄ，４ｂｂｏｔｔｏｍ，ａｓｃａｖｉｎｇａｈｅａｄｏｐｅｎｉｎｇｉｎｔｏｅｎｔｅｒｐａｒｔｉｃｌｅｓｉｎｆｌｕｅｎｃｅ．ｃｏａｌｓｉｇｎｉｆｉｃａｎｔｌｅｄａｌｓｏｓｔｅｅｐｃｏａｌ，ａｎｄｇｒａｖｉｔｙｓｐｅｃｉｆｉｃＤＨｅ４ｂ．Ｆｉｇｕｒｅｓｈｏｗｎｍｏｖｅｍｅｎｔ，ａｓｇｒｅａｔｌｙｃｈａｎｇｅｗｏｕｌｄｔｈｅｂｏｔｔｏｍ．ｔｈｅｉｎｇｃａｖ－ｍｏｖｅｄｕｎｒｅｌｅａｓｅｄｒｏｏｆｄｉｒｅｃｔｂｉｇ．ｔｈｅｖｅｒｙａｎｇｌｅｆａｃｅ，ｓｉｎｃｅｒｅｌｅａｓｅｄｂｅｅｎｎｏｔｈａｓｉｎｆｌｕｘｂｙｃｏｍｐａｃｔｅｄｗｈｉｃｈｔｒｉａｎｇｕｌａｒｕｐｐｅｒｐａｒｔｉｃｌｅ，ｔｈｅＣＯａｌｔｈａｎｇｒｅａｔｅｒｉｓｇａｎｇｕｅｄｅｎｓｉｔｙＤｕｅｄｉｒｅｃｔｉｏｎ．ｉｎｃｌｉｎａｔｉｏｎｓｅａｍｉｎａｓｙｍｍｅｔｒｙｏｂｖｉｏｕｓｍｏｒｅｓｈｏｗｅｄｓｈａｐｅｗｉｔｈａｌｏｎｇｇｒａｄｕａｌｌｙｄｏｗｎｍｏｖｅｓｆｒｏｎｔｆａｃｅ，ｗｈｉｌｅｌｔｅａｔｅｄｂｏｔｔｏｍ，ｔｈｅｔｏｐｐｒｏｃｅｓｓ１（ＮＯ．３ａ）ＣａＶｉｎｇｗｏｒｋｉｎｇａｔｌｏｃａｔｅｄｗａｓｒｏｃｋａｎｄｃｏａｌｂｏｕｎｄａｒｙｂａｃｋｔｏｐ，ｔｈｅｔｏｂｏｔｔｏｍｆｒｏｍｃａｖｉｎｇｏｆｐｒｏｃｅｓｓｔｈｅｄｕｒｉｎｇｔｈａｔ１．ｓｃｈｅｍｅｉｎｒｏｃｋａｎｄｃｏａｌｂｏｕｎｄａｒｙｏｆ４．ＭｏｒｐｈｏｌｏｇｙＦｉｇｕｒｅＮＯ，２２ｔｏｆａｓｕｐｐｏｒｔ，ａｎｄｉｇｈｅｒｎｄｅｒｎｄｃｈｉｅｖｅｍａｌｌｅｒｌｌｕｓｔｒａｔｅｓ

(a) Caving to NO.J I

(b) Caving 10 N0.22

Figure 4. Morphology of boundary of coal and rock in scheme!.

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(a) Caving to

0 .9

(b) Caving to 0.22

(b) Caving to N0. 16

Figure 6. Morphology of boundary of coal and rock in scheme 3.

Figure 5. Morphology of boundary of coal and rock in scheme 2.

face and the morphology remained unchanged basically. While the front boundary of coal and rock moved up gradually along with the caving from bottom to top, following the caving from the bottom of the working face, beside the coal particles near by the drawing opening entering the drawing opening. Because the large inclined angle lead to the larger gravity's component force along the working face's tilt direction, the coal particles in the upper and middle levels of the working face show the tendency that flow to drawing opening. Therefore, the top of working face's top-coal became thinner, as shown in the ellipse's region of Figure 5. It was illustrated that the working face's top-coal leakage is very serious under the large inclined angle, which having a significant impact on the air-return roadway's stability. So measures should be taken to strengthen the coal in the top of the air-return roadway. After caving 5 supports from top to bottom at the top of working face, then turned to cave from bottom to top in the end of working face during the process of mixed caving sequence. While the space above the top of caving support has filled with gangue of immediate roof, so it wouldn't appear the thickness of to~oal thinning phenomenon during caving from bottom to top process, which has happened to scheme 2, also the boundary of coal and rock was basically unchanged at the top of working face. Meanwhile, as a result of the coal were near from the end of caving support, morphology of front and back boundary of coal and rock at the end of working face remained unchanged basically.

4.4 Analysis of the contact force field During the process of caving of the three schemes, the fractured coal and rock will move to the caving opening along the path of least resistance. During the process of movement, the contact force field will form in the internal part of the coal and rock. The Figure 6 illustrates the distribution of the contact force field under three different caving

／懑／馥／多 ∥７ＦｅｓｕｌｔＮＯ．Ｉ（ｂ）ＣａｖｉｎｇＮＯ．９ａ（ａ）Ｃａｖｉｎｇｈａｖｉｎｇａｎｇｌｅ．ｗｈｉｃｈｌａｒｇｅｓｅｒｉｏｕｓｖｅｒｙｉｓｌｅａｋａｇｅｔｏｐ—ｃｏａｌｉｌｌｕｓｔｒａｔｅｄ５．Ｉｔｒｅｇｌｏｒｌｅｌｌｉｐｓｅ＇ｓｓｈｏｗｎｔｈｉｎｎｅｒ，ａｓｂｅｃａｍｅｔｏｐ．ｃｏａｌＴｈｅｒｅｆｏｒｅ．ｔｈｅｏｐｅｎｉｎｇ．ｆｌｏｗｔｈａｔｔｅｎｄｅｎｃｙｓｈｏｗｌｅｖｅｌｓｍｉｄｄｌｅｕｐｐｅｒｐａｒｔｉｃｌｅｓｄｉｒｅｃｔｉｏｎ，ｔｈｅｔｉｌｔｆａｃｅ＇ｓｃｏｍｐｏｎｅｎｔｇｒａｖｉｔｙ＇ｓｌａｒｇｅｒｌｅａｄａｎｇｌｅｉｎｃｆｉｎｅｄｌａｒｇｅｏｐｅｎｉｎｇ．Ｂｅｃａｕｓｅｅｎｔｅｒｉｎｇｄｒａｗｉｎｇｂｙｔｉｃｌｅｓｐａｒ－ｆａｃｅ，ｂｅｓｉｄｅｔｏｐ，ｆｏｌｌｏｗｉｎｇａｌｏｎｇｇｒａｄｕａｌｌｙｕｐｍｏｖｅｄｂａｓｉｃａｌｌｙ．肋ｉｌｅｍｏｒｐｈｏｌｏｇｙ２．ｓｃｈｅｍｅｉｎｂｏｕｎｄａｒｙ５ｓ５．ＭｏｒｐｈｏｌｏｇｙＡｆｔｅｒｒｏａｄｗａｙ．ｓｔｒｅｎｇｔｈｅｎｔａｋｅｎｂｅｓｈｏｕｌｄｍｅａｓｕｒｅｓｓｔａｂｉｆｉｔｙ．Ｓｏｒｏａｄｗａｙ＇ｓａｉｒ－ｒｅｔｕｒｎｏｎｉｍｐａｃｔａｒｆａｃｅ．Ｍｅａｎｗｈｉｌｅ，ａｓｂａｓｉｃａｌｌｙｗａｓｒｏｃｋ２，ａｌｓｏｓｃｈｅｍｅｈａｐｐｅｎｅｄｐｒｏｃｅｓｓ，ｗｈｉｃｈｔｏｐｐｈｅｎｏｍｅｎｏｎｔ１１ｉｎｎｉｎｇｔｏＤ—＿ｃｏａｌｔｈｉｃｋｎｅｓｓａｐｐｅａｒｗｏｕｌｄｎ’ｔｉｔｒｏｏｆ，ｓｏｉｍｍｅｄｉａｔｅｗｉｔｈｆｉｌｌｅｄｈａｓｓｕｐｐｏｒｔｓｐａｃｅｓｅｑｕｅｎｃｅ．Ⅵｈｉｌｅｍｉｘｅｄｄｕｒｉｎｇｔｏｃａｖｅｔｕｒｎｅｄ６ｉｆａｃｅ，ｔｈｅｎｒｏｃｋ．Ｔｈｅｐａｒｔｉｎｔｅｒｎａｌｉｎｍｏｖｅｍｅｎｔ，ｔｈｅｐｒｏｃｅｓｓＤｕｒｉｎｇｒｅｓｉｓｔａｎｃｅ．ｌｅａｓｔｐａｔｈａｌｏｎｇｏｐｅｎｉｎｇｉｎｇｃａｖ— ｍｏｖｅｗｉｌｌｃｏａｌｆｒａｃｔｕｒｅｄｓｃｈｅｍｅｓ，ｐｒｏｃｅｓｓＤｕｒｉｎｇｃｏｎｔａｃｔｆｏｒｃｅｆｉｅ掰ｏｆＡｎａｌｙｓｉｓ４．４ｂａｓｉｃａｌｌｙ．ｕｎｃｈａｎｇｅｄｒｅｍａｉｎｅｄｆａｃｅｗｏｒｋｉｎｇａｔｒｏｃｋｂｏｕｎｄａｒｙｂａｃｋｆｒｏｎｔｍｏｒｐｈｏｌｏｇｙｓｕｐｐｏｒｔ，ｅｎｄｎｅａｒｗｅｒｅｃｏａｌ１３２０ａｒｃｈ（Ｂｉｐｒｅｓｓｕｒｅｉｎｔａｃｔｓｅｑｕｅｎｃｅ．ｔｕｒｔｔｌｅｒｍｏｒｅ．ｔｌａｅｃａｗｎｇｍｌｘｅｄａｎｄｏｔｄｉＵｏｎｓｃｏｎ＝ｉｎａｐｐｅａｒｅｄｃｈａｉｎｓｔｈａｎｓｐａｒｓｅｒａｒｅｏｏｔｔｏｍｔｏｐｔｒｏｍｃａｖａｎｇｏｔｃｏｎｄｌｔｌＯｎｃｈａｉｎｓｓｔｒｏｎｇｔｈａｔｋｎｏｗｃａｎ７．ｗｅｆｒｏｍｉＵｕｓｔｒａｔｅｄＡｓａｐｐｅａｒｅａ．ｇａｎｇｕｅｔｌｌｅｗ１１ｅｎｏｐｅｎｉｎｇｃａｖｉｎｇｔｌａｅａｒｏｕｎｄｔｒａｉｎｓｆｏｒｃｅｏｆｓｈｏｗｓｒｉｇｈｔａｔｌｉｅｓｆｉｇｕｒｅｃｏｎｄｉｔｉｏｎｓ，ｔｈｅｓｅｑｕｅｎｃｅｓｅｑｕｅｎｃｅｓ．ｄｉｆｆｅｒｅｎｔｕｎｄｅｒｆｉｅｌｄ７．ＴｈｅＦｉｇｕｒｅ３１７ｄｉｆｆｅｒｅｎｔｔｈｒｅｅｕｎｄｅｒｆｉｅｌｄｆｏｒｃｅｃｏｎｔａｃｔｄｉｓｔｒｉｂｕｔｉｏｎｔｏｐ，ｔｈｅｆｒｏｍｃａｖｉｎｇａｎｄｂｏｔｔｏｍｔｏｐｔｉ＇ｏｍｃａｖｉｎｇｏｆｃｏｎｄｉｔｉｏｎｓｔｈｅｔｏｌｎｇ：Ｃｏｍｐａｒｅｄｏｐｅｎ－ｃａｖｉｎｇｔｈｅａｂｏｖｅｆｏｒｍｎｏｔｄｉｄｉｇｕｒｅｕｐｐｏｒｔｓｉｇｎｉｆｉｃａｎｔ１ｌｌｕｓｔｒａｔｅｓ

(a) Scheme I

(b) Scheme 2

(c) Scheme 3

Figure 7. The contact force field under different caving sequences.

sequence conditions, the figure lies at the right shows the distribution of the force trains around the caving opening when the gangue appeared. As illustrated from Figure 7, we can know that the distribution of strong force chains under the condition of caving from top to bottom are sparser than the strong force chains appeared in the conditions of caving from bottom to top and mixed caving sequence, furthermore, the intact pressure arch (Bi 2013) did not form above the caving opening; Compared to the conditions of caving from top to bottom and caving from bottom to top, the

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strong force chains in the mixed caving sequence show the trend that much more strong force chains distributed along the coal seam's incline direction, these strong chains can be the internal factors of the pressure arch. The contact force under the pressure arch dipped dramatically, this phenomenon can be explained that it is the dynamic process of pressure arch's forming and fracturing contributed to the fluent caving of the top-coal. Consequently the recovery is increased. 5 CONCLUSIONS 1. Caving sequence has a great impact on the

recovery ratio. The maximum recovery ratio appears when caving from top to bottom while the minimum appears when caving from bottom to top, and the mixed caving sequence has a medium recovery ratio. 2. Under different caving sequences, the draw body appears varying degrees of tendency of developing towards the top of working face. Caving from bottom to top has the maximum value of draw body asymmetry coefficient W, which is 2.817. 3. The boundary of coal and rock shows significant asymmetry during the developing process. Under the condition of steep coal seam, the top-coal at the top-of working face left blank seriously, which has a significant impact on the stability of the air-return way, measures are suggested to be taken to reinforce the top-coal above the air-return way.

５Ｃ．ｉｎｃｒｅａｓｅｄ．ｔｏｐ－ＣＯａｌ．Ｃｏｎｓｅｑｕｅｎｔｌｙｆｌｕｅｎｔｃｏｎｔｒｉｂｕｔｅｄｆｒａｃｔｕｒｉｎｇｆｏｒｍｉｎｇａｒｃｈ＇ｓｐｒｏｃｅｓｓｄｙｎａｍｉｃｉｓｉｔｅｘｐｌａｉｎｅｄｂｅｃａｎｐｈｅｎｏｍｅｎｏｎｄｒａｍａｔｉｃａｌｌｙ，ｔｈｉｓｄｉｐｐｅｄｓｕｒｅｐｒｅｓ．ｕｎｄｅｒｆｏｉｃｅｃｏｎｔａｃｔａｒｃｈ．Ｔｈｅｐｒｅｓｓｕｒｅｆａｃｔｏｒｓｉｎｔｅｒｎａｌｃａｎｔｈｅｓｅｄｉｒｅｃｔｉｏｎ．ｉｎｃｌｉｎｅｓｅａｍ＇ｓＣＯａｌａｌｏｎｇｄｉｓｔｒｉｂｕｔｅｄｍＯｌｌｍｕｃｈｔｈａｔｔｒｅｎｄｓｈｏｗｍｉｘｅｄｉｎｆｏｒｏｅｇｓｔｒｏｎｇｓｅｑｕｅｎｃｅｍ１．Ｃａｖｉｎｇｓｅｑｕｅｎｃｅｔｏｐ，ａｎｄｔｏｍｂｏｔ－ｗｈｅｎａｐｐｅａｒｓｍｉｎｉｍｕｍｗｈｉｌｅｗｈｅｎａＤｐｅａｒｓｒａｔｉｏｒｅｃｏｖｅｒｙｓｒａｔｉｏ．Ｔｈｅｓｅｒｉｏｕｓｌｙ，ｗｈｉｃｈｂｌａｎｋ１ｅｆｔｆａｃｅｔｏｐ．ｏｆａｔｓｅａｍ，ｔｈｅｃｏａｌｓｔｅｅｐｃｏｎｄｉｄｏｎＵｎｄｅｒｐｒｏｃｅｓｓ．ｄｕｒｉｎｇｃａｎｔｓｉｇｎｉｆｉ－ｓｈｏｗｓｒｏｃｋｂｏｕｎｄａｒｙ３．Ｔｈｅ２．８１７．ｉＳｗｈｉｃｈｃｏｅｆｆｉｃｉｅｎｔ形ａｓｙｍｍｅｔｒｙｂｏｄｙｖａｌｕｅｍａｘｉｍｕｍｈａｓＣａｖｉｎｇｆａｃｅ．ｗｏｒｋｉｎｇｔｏｗａｒｄｓｄｅｖｅｌｏｐｉｎｇｔｅｎｄｅｎｃｙｄｅｇｒｅｅｓｖａｒｙｉｎｇａｐｐｅａｒｓｂｏｄｙｄｒａｗｓｅｑｕｅｎｃｅｓ，ｔｈｅｄｉｆｉｅ：ｒｅｎｔ２．Ｕｎｄｅｒｒａｔｉｏ．ｒｅｃｏｖｅｒｙａｒｆｏｒｍｗｉｌｌｏｐｅｎｉｎｇｄｒａｗｉｎｇｃｈａｉｎｓｆｏｒｃｅｓｅｑｕｅｎｃｅ，ｓｔｒｏｎｇｍｉｘｅｄａｎｄｂｏｔｔｏｍｆｒｏｍ４．Ｗｈｅｎ３１８ｗａｙ．ａｂｏｖｅｔｏｐ．ｃｏａｌｒｅｉｎｆｏｒｃｅｔｏｔａｋｅｎｂｅｔｏｓｕｇｇｅｓｔｅｄａｒｅｗａｙ，ｍｅａｓｕｒｅｓａｉｒ－ｒｅｔｕｒｎｓｔａｂｉｌｉｔｙｏｎ１ｉｍｐａｃｔ２８（９）：８０８—８Ｂｅｉｆｉｎｇｆｌｏｗａ１．２００６．ＰａｒｔｉｃｌｅＷａｎｇ’Ｓ．Ｒ．＆Ｗａｎｇ＇Ｊ．Ａ．ｅｔ３５（１１１：１８７８—１８８２．ＪＳｂｃ招ｔｙａｎｇｌｅ．ＪｏｕｒｎａｌｄｉｐｇｒｅａｔｗｉｔｈＣＯａｌｓｏｆｔｅｘｔｒａ－ＣａＶｉｎｇｔｏｐ－ｃｏａｌｎｏｌｏｇｙｔｅｃｈ－ｋｅｙａ１．２０１０．ＳｔｕｄｙｅｔＨｕａｎｇ。Ｚ．Ｚ．＆Ｐｅｎ．ＹＦ４０（３）：４３－４６．ＰｒｏｔｅｃｔｉｏｎＥｎｖｉｒｏｎｍｅｎｔａｌＳａｆｅｔｙＰＦＣ２Ｄ．Ｍｉｎｉｎｇｂａｓｅｄｓｅａｍ】ＯＷｓｈａｌ．ｂｅｈａｖｉｏｒｇｒｏｕｎｄ３．Ａｎａｌｙｓｉｓ２０１Ｂｉ．Ｚ．ＷＲＥＦＥＲＥＮＣＥＳＣＯａｌ．ｃａｖｉｎｇｔｈｅｂｅｎｅｆｉｔｓｓｔｒｕｃｔｕｒｅ，ｗｈｉｃｈａｒｃｈｓｔａｂｌｅ８一１１．３８（３）：Ｔｅｃｈｎｏｌｏｇｙｓｅａｍ．ＣｏａｌＳＯｆｔｔｈｒｅｅｆａｃｅｔｏｐｆｕｌｌｙｉｎｃｌｉｎｅｄｈｉｇｈｄｏｗｎｔｏｐｐｌｅｄｓｕｐｐｏｒｔｐｏｗｅｒｅｄｈｙｄｒａｕｌｉｃｍｅａｓｕｒｅｓｃｏｕｎｔｅｒ．ａ１．２０１０．ＣａｕｓｅＹｊｎ戥Ｒ．Ｓ．＆Ｚｈｕ，Ｘ．Ｌ．ｅｔ１．２９（５）：５２７—５３Ｃｈ跏口ＣｏａｌＪｏｕｒｎａｌｍｍｍｇ．ｓｅａｍｄｉｐｐｉｎｇｓｔｅ印ｌｙｉｎｆｌｏｏｒｒｏｏｔ－ｓｕｐｐｏｒｔｓｙｓｔｅｍｏｆＳｔａｂｉｌｉｔｙｍｏｄｅｌ２００１．ＤｙｎａｍｉｃＷｕ．Ｙｐ２７（１）：５－８．Ｂｅｏ＇ｉｎｇＴｅｃｈｎｏｌｏｇｙＳｃｉｅｎｃｅＵｎｉｖｅｒｓｉｔｙｓｅａｍ．Ｊｏｕｒｎａｌｔｈｉｃｋｓｔｅｅｐｉｎｃａｖｉｎｇｔｏｐ．ｃｏａｌｍｅｃｈａｎｉｚｅｄｄｕｒｉｎｇｎｉｓｍｍｅｃｈａ— ｆａｉｌｕｒｅａｎｄ１ａｗａｎａｌｙｓｉｓｅｌｅｍｅｎｔａ１．２００５．ＤｉｓｔｉｎｃｔＷａｎ品Ｓ．Ｒ．＆Ｗａｎｇ，Ｊ．Ａ．ｅｔ３８（１ｎ：１９０５—１９１１．ＳｏｃｉｅｔｙＣｏａｌＣｈｉｎａｏｆｔｅｃｎｉｑｕｅ．Ｊｏｕｒｎａｌｍｉｎｉｎｇｉｎｇｃａｖ・ｕｎｄｅｒｌａｗｍｏｖｅｍｅｎｔｃｏａｌｔｏｐｔｈｅｏｎｓｉｍｕｌａｔｉｏｎｎｕｍｅｒｉｃａｌａ１．２０１３．３．ＤＷａｎｇ＇Ｊ．Ｃ．＆Ｗ瓯Ｌ．Ｋ．ｅｔｒｅａｔＯＮＣＩｊＵＳＩＯＮＳｅｄｉｕｍｉｇｎｉｆｉｃａｎｔｅｌａｔｉｖｅｌｙ

4. When caving from top to bottom and the mixed caving sequence, strong force chains above the drawing opening will form a relatively stable arch structure, which benefits the caving of coal. REFERENCES Bi, Z.W. 2013. Analysis on ground behavior law of shallow coal seam based on PFC 20 • Mining Safety and Environmental Protection 40(3): 43-46. Huang, Z.Z. & Ren, Y.F. et al. 2010. Study on key technology of fully mechanized top-coal caving in extrathick soft coal seam with great dip angle. Journal of China Coal Society 35(11): 1878-1882. Wang, S.R. & Wang, J.A. et al. 2006. Particle flow analysis on mechanized top-coal caving in steep thick seam. Journal of University of Science and Technology Beijing 28(9): 808-811. Wang, J.C. & Wei, L.K. et al. 2013. 3-D numerical simulation on the top coal movement law under caving mining tecnique. Journal of China Coal Society 38(11 ): 1905-1911. Wang, S.R. & Wang, J.A. et al. 2005. Distinct element analysis on coal movement law and failure mechanism during mechanized top-coal caving in steep thick seam. Journal of University of Science and Technology Beijing 27(1 ): 5-8. Wu, Y.P. 2001. Dynamic model and stability of system roof-support floor in steeply dipping seam mining. Journal of China Coal Society 29(5): 527-531. Yang, R.S. & Zhu, X.L. et al. 2010. Cause and countermeasures of hydraulic powered support toppled down in high inclined fully mechanized top coal caving face in three soft seam. Coal Science and Technology 38(3): 8-11.

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