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Zonal extraction technology and numerical simulation analysis in open pit coal mine
International Journal of Mining Science and Technology 22 (2012) 487–491
Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Zonal extraction technology and numerical simulation analysis in open pit coal mine Chen Yanlong a,b,⇑, Cai Qingxiang a, Shang Tao a, Peng Hongge a, Zhou Wei a, Chen Shuzhao a a b
School of Mines, China University of Mining & Technology, Xuzhou 221116, China Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 19 November 2011 Received in revised form 15 December 2011 Accepted 17 January 2012 Available online 4 July 2012 Keywords: Extraction technology Numerical simulation Zonal mining Residual coal End-walls
a b s t r a c t In order to enhance coal recovery ratio of open pit coal mines, a new extraction method called zonal mining system for residual coal around the end-walls is presented. The mining system can improve economic benefits by exploiting haulage and ventilation roadways from the exposed position of coal seams by utilizing the existing transportation systems. Moreover, the main mining parameters have also been discussed. The outcome shows that the load on coal seam roof is about 0.307 MPa and the drop step of the coal seam roof about 20.3 m when the thickness of cover and average volume weight are about 120 m and 0.023 MN/m3 respectively. With the increase of mining height and width, the coal recovery ratio can be improved. However, when recovery ratio is more than 0.85, the average stress on the coal pillar will increase tempestuously, so the recovery ratio should also be controlled to make the coal seam roof safe. Based on the numerical simulation results, it is concluded that the ratio of coal pillar width to height should be more than 1.0 to make sure the coal pillars are steady, and there are only minor displacements on the end-walls. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Because of slope angle, mining boundary, changes of coal seam thickness, and so on, a large number of coal will be left under the end-walls in the open pit coal mines [1–3]. In traditional mining systems, the coal under the end-walls will be buried by the inner dumping site. The coal resources are normally discarded and wasted in vain. So how to recycle the residual coal under the endwalls with safe and efficient methods is a valuable and challenging research. To recover coal remnants around the end-walls, underground mining system is normally adopted by excavating some adits into end-walls [4–6]. Due to long wall underground mining system, the fully caving method is used to manage the coal roof, and it will cause a greater ground subsidence [7,8]. Hence, zonal mining is usually adopted in some regions where only minor ground placement is permissible [9,10]. In zonal mining system, the coal field is divided into some regular strips, one strip is mined and the next strip is reserved. The reserved strips can support the load of the overburdened strata, as a result only minor and uniform movements happen on the ground [11–15]. Thus, some unrecoverable reserves can be extracted on the premise of the controllable ground subsidence. Practice has proved that zonal mining is an effective method to control the overlying strata and ground subsidence, and is usually used for extracting coal seams under buildings and ⇑ Corresponding author. Tel.: +81 80 43163456. E-mail address: [email protected] (Y. Chen).
railway lines [16–21]. According to the past investigations about zonal mining technology, we try to recover the residual coal under end-walls of open pit mines. Thus, the key issue of this research is to ensure the extraction system and mining parameters. 2. Zonal extraction system for residual coal around end-walls There was a lot of coal which could not be mined because of end-walls covering at the studied open pit coal mine. And it is a good solution to recycle the coal remnants by zonal mining with existing production systems. In the extraction system, the interval at which the main haulage and ventilation roadways are excavated from the exposed position of coal seam in end-walls, and the exploitation zone during two haulage roadways in the mining area is shown in Fig. 1. Each mining area is alternatively established in a similar way. After the haulage and ventilation roadways are formed, exploitation roadways will be excavated. After the mining roadways are excavated, working faces will be laid out towards the left and right of the main ventilation roadway and a retreated mining manner is carried out. 3. Zonal mining parameters [6,21] 3.1. Load on the coal seam roof In the zonal mining system, the roof is born the load includes not only the deadweight, but also the overburden load of upper
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.01.008
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Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
Table 1 Mechanics properties of the rock layers. Lithology
Thickness (m)
Loess Weathered sandstone Sandstone Mudstone Siltite Sandstone Coal
Volume weight (MN/m3)
30 14 30 24 12 12 8
0.0196 0.0230 0.0238 0.0249 0.0232 0.0238 0.0144
Elastic modulus (MPa)
b l ¼ pffiffiffi 2l m
150 2000 4200 2800 4600 5500 1000
rock. According to the combination beam theory, the nth load of rock layer on the rock (assumed the first layer) can be calculated with the following formula [22]. 3
ðQ n Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 þ þ cn hn Þ 3 E 1 h1
þ
3 E 2 h2
þ þ
ð1Þ
3 E n hn
where ðQ n Þ1 is the nth load of rock layer on the first layer, MPa; E the elastic modulus of rock, MPa; h the thickness of rock, m; and c the volume weight of rock, MN/m3. According to the Table 1, the load on the coal seam roof is calculated as follows: The first dead weight of the rock layer q1 is: q1 ¼ c1 h1 ¼ 0:0238 12 ¼ 0:2856 MPa The second load of layer on the first rock layer ðq2 Þ1 is: 3
ðq2 Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 Þ 3
3
E1 h1 þ E2 h2
the rock mass, MPa; and q the upper rock load on the coal seam roof, MPa. Then Eq. (3) can be obtained as:
¼ 0:3071 MPa
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 4 4 b b 4lm
ð3Þ
where l is the drop step of coal seam roof, m; and b the length of the working face, m. According to the design of mining system and geological data, the values of all variables in Eq. (3) and the calculation result of the drop step are shown in Table 2. 3.3. Mining width The mining width is related to the mining depth. Practice shows that lumpy displacements will not occur on the surface if the mined width meets the following condition:
H H 6 Bm 6 10 4
ð4Þ
where Bm is the mining width, m; H the mining depth, m. From Table 1, the coal seam is about 120 m from the ground. According to Eq. (4), the mining width of coal seam is: 12:0 m 6 Bm 6 30 m. Meanwhile, from the mentioned result, the drop step of coal seam roof is about 20.3 m. If the mining width is less than 20.3 m, it is good for controlling the roof. Hence, the reasonable value of the mining width is: 12:0 m 6 Bm 6 20 m
The third load of layer on the first rock layer ðq3 Þ1 is: 3.4. Coal pillar width
3
ðq3 Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 þ c3 h3 Þ 3
3
3
E1 h1 þ E2 h2 þ E3 h3
¼ 0:1929 MPa
If found that ðq3 Þ1 is less than ðq2 Þ1 , then we should consider the first and second loads of rock layers on the first layer, because the third layer is thick, and has no effect on the first one. Therefore, the load of coal seam roof is about 0.307 MPa. 3.2. Drop step of the coal seam roof In zonal mining system, the roof of coal seam is supported by coal pillars, so the immediate roof and the main roof can be considered as a fixed board while the drop step of the roof in the mining room can be derived from the stress analysis of a board model [22]. If we assume
lm ¼
h 1 l2
sffiffiffiffiffiffiffi 2St q
ð2Þ
In this paper, the coal pillar width was analyzed by invoking the Obert-Dwvall formula [22]:
Bc R ¼ Rc 0:778 þ 0:222 hm
ð5Þ
where R is the strength of the coal pillar, MPa; Rc the uniaxial compressive strength of the coal block, MPa; Bc the width of the coal pillar, m; and hm the height of the coal pillar, m. The average stress on the coal pillar can be derived from the Tributary area theory:
R ¼ ðBm þ Bc ÞkcH=Bc
ð6Þ
Table 2 Calculation result of the drop step.
where lm is the drop step criterion; h the thickness of the overburden rock, m; l the Poisson’s ratio of coal; St the tensile strength of
Variables
b (m)
h (m)
l
St (MPa)
q (MPa)
Value Drop step of coal seam roof (m)
100 20.3
12
0.30
0.40
0.307
Mining area 2
Mining area 1
Exposed position of coal seam Main haulage roadway
Main ventilation roadway
Main haulage roadway
Main ventilation roadway
Fig. 1. Mining area of zonal mining system.
Main haulage roadway
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Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
Recovery ratio
0.64
Recovery ratio
k=1 k=2
0.74
k=3
0.54 0.44 0.34 0.24
3
4
5
6
7
8
0.84
0.84
0.74
0.74
Recovery ratio
0.84
0.64 0.54 0.44 0.34
0.64 0.54 0.44 0.34 0.24
0.24 3
Mining height (m)
4
5
6
7
3
8
4
Mining height (m)
(a) Mining width is 10 m
(b) Mining width is 15 m
5 6 7 Mining height (m)
8
(c) Mining width is 20 m
0.65 0.62
k=1 k=2 k=3
8 9 10 11 12 13 14 15 16 17 18 19 20 Mining width (m)
(a) Mining height is 4 m
Recovery ratio
0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30
Recovery ratio
Recovery ratio
Fig. 2. Relation between mining height and recovery ratio.
0.59 0.56 0.53 0.50 0.47 0.44
8 9 10 11 12 13 14 15 16 17 18 19 20 Mining width (m)
0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 8
10
(b) Mining height is 6 m
12 14 16 Mining width (m)
18
20
(c) Mining height is 8 m
Fig. 3. Relation between mining width and recovery ratio.
where Bm is the mining width, m; k the stress concentration factor; c the average density of the overburdened rock, MN/m3; and H the thickness of the overburdened rock, m. From Eqs. (5) and (6), the minimum width of the coal pillar can be concluded as follows:
Bc ¼
17 13
R kγ h
ðFkcH 0:778Rc Þhm 0:444Rc qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ð0:778Rc FkcHÞ2 hm þ 0:888Rc hm Bm FkcH þ 0:444Rc
21
9 5 1 0
ð7Þ
where F is the safety factor.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ρ
Fig. 4. Relation between recovery ratio and average stress on the coal pillar.
3.5. Coal recovery ratio The recovery ratio q in the zonal system can be expressed as follows:
B m hm q¼ ðBm þ Bc Þhc
ð8Þ
where Bm is the mining width, m; Bc the width of the coal pillar, m; hm the mining height, m; and hc the thickness of the coal seam. Form Eqs. (7) and (8), the recovery ratio can be calculated when the mining height and width change. The relation between mining height and recovery ratio is shown in Fig. 2. From Fig. 2, it is indicated that the recovery ratio will increase as the mining height becomes thicker under the different values of stress concentration factor. If mining technology allows, the whole coal seam should be extracted, thus highest recovery ratio. The relation between the mining width and recovery ratio is shown in Fig. 3. It is concluded that the recovery ratio increases with increase of the mining width under different values of stress concentration factor. So we should increase the mining width as much as possible. According to these results, to improve the recovery ratio, we should increase the values of mining height and width as much as possible under the suitable conditions.
Table 3 Main mining parameters of zonal mining at studied mine.
c (N/m3)
H (m)
Rc (MPa)
hm (m)
F
Bm (m)
k
Bc (m)
q
21,850
120
22.1
8
1.1
20
1.0 2.0 3.0
4.0 7.0 11.0
0.85 0.73 0.64
Meanwhile, according to Eqs. (6) and (8), the relation between recovery ratio and the average stress on the coal pillar can be derived as Eq. (9), the relation between recovery ratio and the average stress on the coal pillar is shown as Fig. 4.
R 1 ¼ kch 1 q
ð9Þ
From the Fig. 4, when recovery ratio is more than 0.85, the average stress on the coal pillar increases very much, so we have to control the recovery ratio to make the coal seam roof safe. Considering these conclusions, as shown in Table 3, the optimal mining width should be 20 m and the mining height 8 m. According to Eq. (7), the width of the coal pillar is from 4 to 11 m, and the recovery ratio is less than 0.85.
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Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
(b) The Mesh was a graded and element type with three nodded triangles; (c) The type of elasticity was isotropic; (d) Failure criterion was Mohr-Coulomb principle; (e) The initial state of elements was field stress and body force; (f) Boundary conditions were that of ground surface; the left, right and bottom side slopes were free planes; and the vertical direction and the horizontal direction were fixed.
4. Numerical simulation In order to examine the stability of coal pillar under the mentioned mining parameters, numerical simulation was carried out with two-dimensional finite element method (FEM) software. More attention was paid to the end-wall slope stability since it will be affected when the coal under the end-walls is extracted. 4.1. Initial analysis conditions The overall view of the analysis model is shown in Fig. 5. The mechanical properties of the rock layers are shown in Table 1, and the initial analytical conditions were as follows: (a) The types of analysis were a two-dimensional elastic analysis and plane strain;
Fig. 5. Overall view of the analysis model.
4.2. Results and discussion Fig. 6 shows the strength factor of coal pillars under the different coal pillar widths (4, 7, 11 m), and Fig. 7 shows strength factor of the first four coal pillars with mining stage. From these two Figures, when the coal pillar width is 4 m (Fig. 6a) and the mining strip mined, most of the coal pillar strength factors are less than 1.0 and almost all the immediate roofs of the goaf are caved. When the coal pillar width is 7 m (Fig. 6b) and the mining strip mined, most of the coal pillar strength factors are about 1.0 and some of the immediate roofs are caved. Finally, when the coal pillar width is 11 m (Fig. 6c) and the mining strip mined, most of the coal pillar strength factors are more than 1.2 and most of the immediate roofs of the goaf are stable. From these results, we can conclude that to make the pillar steady in zonal mining system, the ratio of coal pillar width to height should be more than 1.0. Meanwhile, we noticed that the slope keeps stable under these conditions, so the mining width is reasonable, and only slope stability had minor effects. From these results, when the mining width is 20 m and
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
(a) k = 1, coal pillar width Bc = 4 m
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
(b) k = 2, coal pillar width Bc = 7 m
(c) k = 3, coal pillar width Bc = 11 m
2.0 1.8 1.6 1.4 1.2 1.0 0.8
Bc = 4 m Bc = 7 m Bc = 11 m
1
2
3
4 5 6 Mining stage (a) First coal pillar
Strength factor
2.0 1.8 1.6 1.4 1.2 1.0 0.8
7
8
Strength factor
Strength factor
Strength factor
Fig. 6. Strength factor of coal pillars.
1
2
3
4 5 6 Mining stage
(c) Third coal pillar
7
8
2.0 1.8 1.6 1.4 1.2 1.0 0.8
2.0 1.8 1.6 1.4 1.2 1.0 0.8
1
2
3
1
2
3
4 5 6 Mining stage (b) Second coal pillar
4
5
6
Mining stage (d) Fourth coal pillar
Fig. 7. Average strength factor of the first four coal pillars with the mining strip mined.
7
8
7
8
Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
mining height 8 m, the coal pillar width should be about 11 m. Therefore, most of the coal pillar strength factor is more than 1.2 and the recovery ratio is about 0.64. 5. Conclusions Based on the results mentioned in the discussion, the following conclusions were obtained: (1) The extraction of residual coal from the end-walls with zonal mining technology, a method which can utilize existing mining technology and transportation systems for open pit mines, the cost of transportation can be reduced. Moreover, since the coal around end-walls is exposed and there is no extra stripping engineering quantity, then the economic benefit of the open pit mine may be improved. (2) The main mining parameters of residual coal extraction system have already been presented in this paper. By analytical calculation, the load on coal seam roof is about 0.307 MPa and the drop step of the coal seam roof is about 20 m when the thickness of cover and average volume weight are about 120 m and 0.023 MN/m3 respectively. (3) With the increase of mining height and width, the coal recovery ratio can be improved. However, when recovery ratio is more than 0.85, the average stress on the coal pillar will increase tempestuously, so the recovery ratio should also be controlled to make the coal seam roof safe. (4) From the numerical simulation results, it is concluded that the ratio of the coal pillar width to height should be more than 1.0 to make sure the coal pillars are steady, and there are only minor displacements on the end-walls.
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Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Zonal extraction technology and numerical simulation analysis in open pit coal mine Chen Yanlong a,b,⇑, Cai Qingxiang a, Shang Tao a, Peng Hongge a, Zhou Wei a, Chen Shuzhao a a b
School of Mines, China University of Mining & Technology, Xuzhou 221116, China Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan
a r t i c l e
i n f o
Article history: Received 19 November 2011 Received in revised form 15 December 2011 Accepted 17 January 2012 Available online 4 July 2012 Keywords: Extraction technology Numerical simulation Zonal mining Residual coal End-walls
a b s t r a c t In order to enhance coal recovery ratio of open pit coal mines, a new extraction method called zonal mining system for residual coal around the end-walls is presented. The mining system can improve economic benefits by exploiting haulage and ventilation roadways from the exposed position of coal seams by utilizing the existing transportation systems. Moreover, the main mining parameters have also been discussed. The outcome shows that the load on coal seam roof is about 0.307 MPa and the drop step of the coal seam roof about 20.3 m when the thickness of cover and average volume weight are about 120 m and 0.023 MN/m3 respectively. With the increase of mining height and width, the coal recovery ratio can be improved. However, when recovery ratio is more than 0.85, the average stress on the coal pillar will increase tempestuously, so the recovery ratio should also be controlled to make the coal seam roof safe. Based on the numerical simulation results, it is concluded that the ratio of coal pillar width to height should be more than 1.0 to make sure the coal pillars are steady, and there are only minor displacements on the end-walls. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Because of slope angle, mining boundary, changes of coal seam thickness, and so on, a large number of coal will be left under the end-walls in the open pit coal mines [1–3]. In traditional mining systems, the coal under the end-walls will be buried by the inner dumping site. The coal resources are normally discarded and wasted in vain. So how to recycle the residual coal under the endwalls with safe and efficient methods is a valuable and challenging research. To recover coal remnants around the end-walls, underground mining system is normally adopted by excavating some adits into end-walls [4–6]. Due to long wall underground mining system, the fully caving method is used to manage the coal roof, and it will cause a greater ground subsidence [7,8]. Hence, zonal mining is usually adopted in some regions where only minor ground placement is permissible [9,10]. In zonal mining system, the coal field is divided into some regular strips, one strip is mined and the next strip is reserved. The reserved strips can support the load of the overburdened strata, as a result only minor and uniform movements happen on the ground [11–15]. Thus, some unrecoverable reserves can be extracted on the premise of the controllable ground subsidence. Practice has proved that zonal mining is an effective method to control the overlying strata and ground subsidence, and is usually used for extracting coal seams under buildings and ⇑ Corresponding author. Tel.: +81 80 43163456. E-mail address: [email protected] (Y. Chen).
railway lines [16–21]. According to the past investigations about zonal mining technology, we try to recover the residual coal under end-walls of open pit mines. Thus, the key issue of this research is to ensure the extraction system and mining parameters. 2. Zonal extraction system for residual coal around end-walls There was a lot of coal which could not be mined because of end-walls covering at the studied open pit coal mine. And it is a good solution to recycle the coal remnants by zonal mining with existing production systems. In the extraction system, the interval at which the main haulage and ventilation roadways are excavated from the exposed position of coal seam in end-walls, and the exploitation zone during two haulage roadways in the mining area is shown in Fig. 1. Each mining area is alternatively established in a similar way. After the haulage and ventilation roadways are formed, exploitation roadways will be excavated. After the mining roadways are excavated, working faces will be laid out towards the left and right of the main ventilation roadway and a retreated mining manner is carried out. 3. Zonal mining parameters [6,21] 3.1. Load on the coal seam roof In the zonal mining system, the roof is born the load includes not only the deadweight, but also the overburden load of upper
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.01.008
488
Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
Table 1 Mechanics properties of the rock layers. Lithology
Thickness (m)
Loess Weathered sandstone Sandstone Mudstone Siltite Sandstone Coal
Volume weight (MN/m3)
30 14 30 24 12 12 8
0.0196 0.0230 0.0238 0.0249 0.0232 0.0238 0.0144
Elastic modulus (MPa)
b l ¼ pffiffiffi 2l m
150 2000 4200 2800 4600 5500 1000
rock. According to the combination beam theory, the nth load of rock layer on the rock (assumed the first layer) can be calculated with the following formula [22]. 3
ðQ n Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 þ þ cn hn Þ 3 E 1 h1
þ
3 E 2 h2
þ þ
ð1Þ
3 E n hn
where ðQ n Þ1 is the nth load of rock layer on the first layer, MPa; E the elastic modulus of rock, MPa; h the thickness of rock, m; and c the volume weight of rock, MN/m3. According to the Table 1, the load on the coal seam roof is calculated as follows: The first dead weight of the rock layer q1 is: q1 ¼ c1 h1 ¼ 0:0238 12 ¼ 0:2856 MPa The second load of layer on the first rock layer ðq2 Þ1 is: 3
ðq2 Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 Þ 3
3
E1 h1 þ E2 h2
the rock mass, MPa; and q the upper rock load on the coal seam roof, MPa. Then Eq. (3) can be obtained as:
¼ 0:3071 MPa
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 4 4 b b 4lm
ð3Þ
where l is the drop step of coal seam roof, m; and b the length of the working face, m. According to the design of mining system and geological data, the values of all variables in Eq. (3) and the calculation result of the drop step are shown in Table 2. 3.3. Mining width The mining width is related to the mining depth. Practice shows that lumpy displacements will not occur on the surface if the mined width meets the following condition:
H H 6 Bm 6 10 4
ð4Þ
where Bm is the mining width, m; H the mining depth, m. From Table 1, the coal seam is about 120 m from the ground. According to Eq. (4), the mining width of coal seam is: 12:0 m 6 Bm 6 30 m. Meanwhile, from the mentioned result, the drop step of coal seam roof is about 20.3 m. If the mining width is less than 20.3 m, it is good for controlling the roof. Hence, the reasonable value of the mining width is: 12:0 m 6 Bm 6 20 m
The third load of layer on the first rock layer ðq3 Þ1 is: 3.4. Coal pillar width
3
ðq3 Þ1 ¼
E1 h1 ðc1 h1 þ c2 h2 þ c3 h3 Þ 3
3
3
E1 h1 þ E2 h2 þ E3 h3
¼ 0:1929 MPa
If found that ðq3 Þ1 is less than ðq2 Þ1 , then we should consider the first and second loads of rock layers on the first layer, because the third layer is thick, and has no effect on the first one. Therefore, the load of coal seam roof is about 0.307 MPa. 3.2. Drop step of the coal seam roof In zonal mining system, the roof of coal seam is supported by coal pillars, so the immediate roof and the main roof can be considered as a fixed board while the drop step of the roof in the mining room can be derived from the stress analysis of a board model [22]. If we assume
lm ¼
h 1 l2
sffiffiffiffiffiffiffi 2St q
ð2Þ
In this paper, the coal pillar width was analyzed by invoking the Obert-Dwvall formula [22]:
Bc R ¼ Rc 0:778 þ 0:222 hm
ð5Þ
where R is the strength of the coal pillar, MPa; Rc the uniaxial compressive strength of the coal block, MPa; Bc the width of the coal pillar, m; and hm the height of the coal pillar, m. The average stress on the coal pillar can be derived from the Tributary area theory:
R ¼ ðBm þ Bc ÞkcH=Bc
ð6Þ
Table 2 Calculation result of the drop step.
where lm is the drop step criterion; h the thickness of the overburden rock, m; l the Poisson’s ratio of coal; St the tensile strength of
Variables
b (m)
h (m)
l
St (MPa)
q (MPa)
Value Drop step of coal seam roof (m)
100 20.3
12
0.30
0.40
0.307
Mining area 2
Mining area 1
Exposed position of coal seam Main haulage roadway
Main ventilation roadway
Main haulage roadway
Main ventilation roadway
Fig. 1. Mining area of zonal mining system.
Main haulage roadway
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Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
Recovery ratio
0.64
Recovery ratio
k=1 k=2
0.74
k=3
0.54 0.44 0.34 0.24
3
4
5
6
7
8
0.84
0.84
0.74
0.74
Recovery ratio
0.84
0.64 0.54 0.44 0.34
0.64 0.54 0.44 0.34 0.24
0.24 3
Mining height (m)
4
5
6
7
3
8
4
Mining height (m)
(a) Mining width is 10 m
(b) Mining width is 15 m
5 6 7 Mining height (m)
8
(c) Mining width is 20 m
0.65 0.62
k=1 k=2 k=3
8 9 10 11 12 13 14 15 16 17 18 19 20 Mining width (m)
(a) Mining height is 4 m
Recovery ratio
0.44 0.42 0.40 0.38 0.36 0.34 0.32 0.30
Recovery ratio
Recovery ratio
Fig. 2. Relation between mining height and recovery ratio.
0.59 0.56 0.53 0.50 0.47 0.44
8 9 10 11 12 13 14 15 16 17 18 19 20 Mining width (m)
0.85 0.80 0.75 0.70 0.65 0.60 0.55 0.50 8
10
(b) Mining height is 6 m
12 14 16 Mining width (m)
18
20
(c) Mining height is 8 m
Fig. 3. Relation between mining width and recovery ratio.
where Bm is the mining width, m; k the stress concentration factor; c the average density of the overburdened rock, MN/m3; and H the thickness of the overburdened rock, m. From Eqs. (5) and (6), the minimum width of the coal pillar can be concluded as follows:
Bc ¼
17 13
R kγ h
ðFkcH 0:778Rc Þhm 0:444Rc qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ð0:778Rc FkcHÞ2 hm þ 0:888Rc hm Bm FkcH þ 0:444Rc
21
9 5 1 0
ð7Þ
where F is the safety factor.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ρ
Fig. 4. Relation between recovery ratio and average stress on the coal pillar.
3.5. Coal recovery ratio The recovery ratio q in the zonal system can be expressed as follows:
B m hm q¼ ðBm þ Bc Þhc
ð8Þ
where Bm is the mining width, m; Bc the width of the coal pillar, m; hm the mining height, m; and hc the thickness of the coal seam. Form Eqs. (7) and (8), the recovery ratio can be calculated when the mining height and width change. The relation between mining height and recovery ratio is shown in Fig. 2. From Fig. 2, it is indicated that the recovery ratio will increase as the mining height becomes thicker under the different values of stress concentration factor. If mining technology allows, the whole coal seam should be extracted, thus highest recovery ratio. The relation between the mining width and recovery ratio is shown in Fig. 3. It is concluded that the recovery ratio increases with increase of the mining width under different values of stress concentration factor. So we should increase the mining width as much as possible. According to these results, to improve the recovery ratio, we should increase the values of mining height and width as much as possible under the suitable conditions.
Table 3 Main mining parameters of zonal mining at studied mine.
c (N/m3)
H (m)
Rc (MPa)
hm (m)
F
Bm (m)
k
Bc (m)
q
21,850
120
22.1
8
1.1
20
1.0 2.0 3.0
4.0 7.0 11.0
0.85 0.73 0.64
Meanwhile, according to Eqs. (6) and (8), the relation between recovery ratio and the average stress on the coal pillar can be derived as Eq. (9), the relation between recovery ratio and the average stress on the coal pillar is shown as Fig. 4.
R 1 ¼ kch 1 q
ð9Þ
From the Fig. 4, when recovery ratio is more than 0.85, the average stress on the coal pillar increases very much, so we have to control the recovery ratio to make the coal seam roof safe. Considering these conclusions, as shown in Table 3, the optimal mining width should be 20 m and the mining height 8 m. According to Eq. (7), the width of the coal pillar is from 4 to 11 m, and the recovery ratio is less than 0.85.
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(b) The Mesh was a graded and element type with three nodded triangles; (c) The type of elasticity was isotropic; (d) Failure criterion was Mohr-Coulomb principle; (e) The initial state of elements was field stress and body force; (f) Boundary conditions were that of ground surface; the left, right and bottom side slopes were free planes; and the vertical direction and the horizontal direction were fixed.
4. Numerical simulation In order to examine the stability of coal pillar under the mentioned mining parameters, numerical simulation was carried out with two-dimensional finite element method (FEM) software. More attention was paid to the end-wall slope stability since it will be affected when the coal under the end-walls is extracted. 4.1. Initial analysis conditions The overall view of the analysis model is shown in Fig. 5. The mechanical properties of the rock layers are shown in Table 1, and the initial analytical conditions were as follows: (a) The types of analysis were a two-dimensional elastic analysis and plane strain;
Fig. 5. Overall view of the analysis model.
4.2. Results and discussion Fig. 6 shows the strength factor of coal pillars under the different coal pillar widths (4, 7, 11 m), and Fig. 7 shows strength factor of the first four coal pillars with mining stage. From these two Figures, when the coal pillar width is 4 m (Fig. 6a) and the mining strip mined, most of the coal pillar strength factors are less than 1.0 and almost all the immediate roofs of the goaf are caved. When the coal pillar width is 7 m (Fig. 6b) and the mining strip mined, most of the coal pillar strength factors are about 1.0 and some of the immediate roofs are caved. Finally, when the coal pillar width is 11 m (Fig. 6c) and the mining strip mined, most of the coal pillar strength factors are more than 1.2 and most of the immediate roofs of the goaf are stable. From these results, we can conclude that to make the pillar steady in zonal mining system, the ratio of coal pillar width to height should be more than 1.0. Meanwhile, we noticed that the slope keeps stable under these conditions, so the mining width is reasonable, and only slope stability had minor effects. From these results, when the mining width is 20 m and
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
(a) k = 1, coal pillar width Bc = 4 m
0 0.63 1.26 1.89 2.53 3.16 3.79 4.42 5.05 5.68
(b) k = 2, coal pillar width Bc = 7 m
(c) k = 3, coal pillar width Bc = 11 m
2.0 1.8 1.6 1.4 1.2 1.0 0.8
Bc = 4 m Bc = 7 m Bc = 11 m
1
2
3
4 5 6 Mining stage (a) First coal pillar
Strength factor
2.0 1.8 1.6 1.4 1.2 1.0 0.8
7
8
Strength factor
Strength factor
Strength factor
Fig. 6. Strength factor of coal pillars.
1
2
3
4 5 6 Mining stage
(c) Third coal pillar
7
8
2.0 1.8 1.6 1.4 1.2 1.0 0.8
2.0 1.8 1.6 1.4 1.2 1.0 0.8
1
2
3
1
2
3
4 5 6 Mining stage (b) Second coal pillar
4
5
6
Mining stage (d) Fourth coal pillar
Fig. 7. Average strength factor of the first four coal pillars with the mining strip mined.
7
8
7
8
Y. Chen et al. / International Journal of Mining Science and Technology 22 (2012) 487–491
mining height 8 m, the coal pillar width should be about 11 m. Therefore, most of the coal pillar strength factor is more than 1.2 and the recovery ratio is about 0.64. 5. Conclusions Based on the results mentioned in the discussion, the following conclusions were obtained: (1) The extraction of residual coal from the end-walls with zonal mining technology, a method which can utilize existing mining technology and transportation systems for open pit mines, the cost of transportation can be reduced. Moreover, since the coal around end-walls is exposed and there is no extra stripping engineering quantity, then the economic benefit of the open pit mine may be improved. (2) The main mining parameters of residual coal extraction system have already been presented in this paper. By analytical calculation, the load on coal seam roof is about 0.307 MPa and the drop step of the coal seam roof is about 20 m when the thickness of cover and average volume weight are about 120 m and 0.023 MN/m3 respectively. (3) With the increase of mining height and width, the coal recovery ratio can be improved. However, when recovery ratio is more than 0.85, the average stress on the coal pillar will increase tempestuously, so the recovery ratio should also be controlled to make the coal seam roof safe. (4) From the numerical simulation results, it is concluded that the ratio of the coal pillar width to height should be more than 1.0 to make sure the coal pillars are steady, and there are only minor displacements on the end-walls.
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