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Discrimination conditions and process of water-resistant key strata
M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0224–0229
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Discrimination conditions and process of water-resistant key strata WANG Lianguo*, MIAO Xiexing, WU Yu, SUN Jian, YANG Hongbo State Key Laboratory of Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221008, China Abstract: Water-preservation mining is one of the most important parts of the ‘Green Mining’ technology system, which can realize the effective regulation of groundwater resources by controlling strata movement, changing passive prevention and governance of water disasters to active conservation and utilization of groundwater resources and thus obtaining coal and water simultaneously in mining. The concept of water-resistant key strata further enriches the content of the key stratum theory and provides a theoretical basis for water-preservation mining. In order to realize the idea of water-resistant key strata as a guideline in the design of water-preservation mining and engineering applications, the conditions for discrimination in the process of water-resistant key strata, we have presented a mechanical model, as well as its corresponding computer program, based on a large number of theoretical analyses and field measurements, as well as on a comprehensive consideration of the position, structural stability and seepage stability of key strata. Practical engineering applications indicate that this discrimination method and its corresponding computer program on water-resistant key strata are accurate and reliable and can satisfy the actual design needs of water-preservation mining and thus have instructional importance for water-preservation mining in mining areas lacking water. Keywords: water-resistant key stratum; water-preservation mining; structural stability; seepage stability
1
Introduction
The key stratum theory of strata control proposed by Qian et al. has been widely applied in the identification of suitable stratigraphic horizons of bed separation grouting, in the design of drilling hole arrangements of ground gas drainage, in the control of overlying strata and surface subsidence and elsewhere[1]. The ‘key strata’ in this key stratum theory of strata control are referred to as structural key strata, bearing the main effect of rock mass movements during mining, which controls the structural shape of ruptured rock masses. Given ground pressure control problems in water-preservation mining, Miao et al. have, in the last few years, presented the concept of water-resistant key strata[2-5]. A water-resistant key stratum in water-preservation mining is difficult to define but can be described as follows: provided that the upper aquifer of a coal seam is above the structural key stratum, or the lower aquifer of a coal seam is below the structural key stratum and if the structural key stratum cannot break under mining, then the Received 10 October 2009; accepted 11 December 2009 *Corresponding author. Tel: 86 13115225568 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60188-5
structural key stratum has a water-resistant function and is called the water-resistant key stratum. If the structural key stratum can break under mining, but the broken fissures can be filled with weak rock strata and a seepage water-inrush channel cannot be formed, then a compound water-resistant key stratum is formed by combining the structural key stratum with a weak rock stratum. From this description, one can see that the water-resistant key stratum can consist of only one single rock stratum, or by compounding several layers of weak rock strata with hard rock strata. The rock strata of the water-resistant key stratum must include hard rock strata which can bear the strain of rock mass movement during mining. That is to say, a water-resistant key stratum must be formed by structural key stratum or by compounding weak rock strata with hard rock strata that can bear a certain amount of strain of rock mass movement during mining. The concept of a water-resistant key stratum further enriches the content of the key stratum theory and provides a theoretical basis for water-preservation mining. However, distinguishing water-resistant key strata largely depends on field experience and is still short of a scientific basis. In order to realize the idea of water-resistant key strata as a guideline in the design of water-preservation mining and engineering
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
applications, in this study, we present the conditions for discrimination and process of water-resistant key strata and their corresponding computer programs based on a large number of theoretical analyses and field measurements. In this method of discrimination, the position, structural stability and seepage stability of key strata are considered comprehensively. We also provide for a corresponding force analysis model. Our practical applications suggest that the discrimination method and its corresponding computer programs of water-resistant key strata are accurate and reliable, which should satisfy the actual design needs of water-preservation mining and are thus of instructional importance for water-preservation mining in mining areas lacking water.
2
225 n
q1 =
E1 h13 ¦ γ i hi
(1)
i =1
n
¦E h
3 i i
i =1
where Ei, Ȗi and hi are the elastic model, the body force, and the thickness of each rock strata (i=1, 2, …, n, s, m).
Conditions for discrimination
According to our description of a water-resistant key stratum, we see that the decision of whether there is a water-resistant key stratum in the overlying strata should be considered from two sides. First, we need to decide whether there is a rock stratum that can control the movement of the overlying strata above the coal seam (roof) or below the coal seam (floor). Rock strata can consist either of a single rock stratum called the structural key stratum or is a compound stratum which can control the movement of the overlying strata in a particular combination of several layers of weak and hard rock strata. These rock strata have the capacity of water-resistance when their structure remain stable and do not break. Secondly, the risk of water-inrush can be judged by the abrupt changes in the characteristics of broken rock seepage as the rock strata break when mining. If no water-inrush from the broken rock strata occurs, we can conclude that these rock strata are water-resistant key strata. Therefore, the discrimination of water-resistant key strata can be classified into three steps: 1) Discrimination based on position The basis for the formation of water-resistant key strata is that there are structural key strata in the overlying strata. We must first identify the position of structural key strata and then identify whether these structural key strata can form water-resistant key strata[1]. The position of a structural key stratum can be identified by combining the drilling data and the mined geological conditions with the key stratum theory. Provided that the rock stratum S1 is the lowest key stratum as shown in Fig. 1, which controls n layers of rock strata (S2, …, Sn, Sm are refer to rock strata above S1, and 1, 2, …, n and m are the layer number, where n
Fig. 1
Load calculation model of overlying strata
If a specific rock stratum is a key stratum, it should simultaneous satisfy the conditions of discrimination based on stiffness (deformation) and strength, which can be presented as follows: n +1
n
n
s
¦ E h ¦γ h < ¦ E h ¦γ h i =1
3 i i
i i
i =1
3 i i
i =1
(l1 < ln +1 , n < s < m)
i i
i =1
(2)
where li indicates the first interval of roof breaking of the ith rock stratum. Given the condition of a fixed-fixed beam, the first interval of roof breaking of the 1st rock stratum can be expressed as follows:
l1 = h1
σc 5q1
(3)
where ıc is the limit of the compressive bearing capacity. 2) Discrimination based on structural stability The structural stability of a key stratum in the overlying strata is very important for controlling water-inrush under mining conditions. If a key stratum does not break during mining, water-inrush will not occur. Taking a compound water-resistant key stratum of the overlying strata as an example, its lower rock stratum must be a hard rock stratum no matter whether it is made up of several layers of rock strata. In order to carry out a strength analysis of a representative compound water-resistant key stratum, we studied a compound water-resistant key stratum made up of four rock layers, similar to the structural procedure followed by Miao et al[4]. Its mechanical model is shown in Figs. 2 and 3, where l is the advancing distance of the mining workface, 4h is the height of the cross section and its width is unit length.
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be calculated as follows:
T
n
5RFNVWUDWXP
ξ=
5RFNVWUDWXP 5RFNVWUDWXP 5RFNVWUDWXP +DUGURFN VWUDWXP
:HDNURFN VWUDWXP
Mechanical model of compound water-resistant key strata made up of four layers of rock strata
Fig. 3
Mechanical model of compound water-resistant key strata given the condition of a fixed-fixed beam
K
Fig. 2
According to the mechanical model of a compound water-resistant key stratum, the normal stress and shear stress of each rock stratum in a composite rock beam can be calculated by using Eqs.(4) and (5), respectively. Otherwise, the interval of roof breaking of the composite rock beam can be analyzed by the Mohr-Coulomb failure criterion, where the first interval of roof breaking l1 can be calculated by using Eq.(6). Hence, we can distinguish the structural stability characteristics of key strata in the overlying strata[4]. Ei y 1 − μ i2 σi = 4 Ei I ¦ 2 i i =1 1 − μ i M
No.2
( i = 1, 2, 3, 4 )
(4)
En i 1 − μ n2 1 S n* ⋅ ⋅ Q τi = ¦ 4 E b n =1 i I ¦ 2 i i =1 1 − μ i
(5)
σ 1 − Kσ 3 = Rt
(6)
3) Discrimination based on seepage stability According to the seepage theory of a mined rock mass, we can theoretically decide whether a broken structural key stratum in the overlying strata still has the ability of water-resistance, or still is a water-resistant key stratum[6-11]. We have used the n rock layers (including the key stratum) between the key stratum and the main aquifer to analyze its permeability and the seepage catastrophe coefficient ȟ of the roof (floor) to determine the seepage stability of the key stratum in the overlying strata[8-9], which can
4 ρ ( p0 − pn )¦ i =1
1 n β j hj ¦ cai j =1 caj
§ n μ hi · ¨¦ ¸ © i =1 ki ¹
2
(7)
where ȡ is the mass density of water; p0–pn the difference in pressure between roof (floor) and aquifer; ȕ a non-Darcy flow factor; μ the dynamic viscosity; i hj the thickness of the jth rock layer; ca the acceleration coefficient and ki the permeability. When ȟ<1, the composite rock strata between the key stratum and its overlying strata still has the ability of water-resistance, which can form a compound water-resistant key stratum, where water-inrush accidents do not occur. When ȟ1, the composite rock strata between the key stratum and its overlying strata does not have the ability to resist water and therefore cannot form a compound water-resistant key stratum and water-inrush accidents may occur. According to this three-step discrimination method, we can solve the problems of whether there is a water-resistant key stratum in the overlying strata and what kind of rock stratum can form a water-resistant key stratum.
3
Discrimination process
Given our analysis of the determination and conditions of water-resistant key strata, it can be seen that the determination of water-resistant key strata is a comparatively complex system. Our proposed method of a three-step determination is also a comparatively complex calculation in practice and may be difficult in practical engineering applications. For convenience therefore, we designed a special program for the determination of water-resistant key strata and a corresponding computer program for the determination process of water-resistant key strata as shown in Fig. 4.
Fig. 4
Discrimination process of water-resistant key strata
Input for the computer program is geometric, physical and mechanical parameters, seepage properties of each rock stratum in the overlying strata, hydro
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
geological conditions and other properties. The output parameters of the computer program are structural stability and seepage stability of water-resistant key strata. Elsewhere a corresponding interpretation and feasible countermeasures are given. The determination program of the water-resistant key strata includes five parts: a structural calculation module of the key stratum, an analytical seepage characteristics module, a database module, a fuzzy reasoning module and an interpretation module. The structural calculation module largely calculates structural stability of the key stratum, including a forced state and the first interval of roof breaking of the key stratum. The seepage characteristic module mainly calculates the seepage stability of the broken key stratum. The database module preserves the physical and mechanical characteristic parameters and seepage properties of each rock stratum and a part of the expert knowledge that can be used by fuzzy reasoning. The fuzzy reasoning module determines the deleted parameters based on reasoning given a condition of scarcity of data. The interpretation module mainly interprets the obtained results and presents expert opinions and suggestions, in accordance with many types of conditions.
4
Example analysis
The strike length of the 12610 fully-mechanized workface in the Daliuta coal mine of the Shendong mining area is 5293.4 m and the length of its inclinaTable 1
227
tion 239.8 m, so the area of fully-mechanized workface is 1269357 m2. The coal seam pitch is around 1°~5° and its average thickness 5.19 m. The geological reserves are about 8498474 t and the recoverable reserves approach 7903581 t. Given the capacity of the mine, this can be mined over a period of 10 months. The 12610 fully-mechanized workface has 141 sets of JOY 8670 installed for support and is equipped with a set of JOY 7LS shearers. The 12610 workface belongs to the spring area of Halagou and Sanbulago, and there is a Quaternary loose bed and Yanan formation bedrock in the overlying stratum of the 2–2-coal seam. Loose soils are from 10~100 m thick, with an average of 67 m. There is a sandy gravel layer at the bottom of this loose bed, 0~15 m thick, with an average of 6.3 m (the sandy gravel layer in the Baijiaqu ditch is 2.4~9.6 m thick). The thickness of the Yanan formation bedrock in the overlying stratum is 27.5~65.0 m, with an average thickness of 48 m (the Yanan formation bedrock in the Baijiaqu ditch is 27.5 m thick). On the first caving segment of the cut-hole, the Yanan formation bedrock is about 60 m thick, the loose bed 50 m, the weathered bedrock 22 m and the sandy gravel layer between 0~5 m. The first caving segment of the cut-hole belongs to the area of loose beds with lots of water. The physical and mechanical parameters of the roof rock strata of the 12610 workface are shown in Table 1.
Physical and mechanical parameters of roof rock strata of the 12610 workface
Lithology
Depth (m)
Thickness (m)
Bulk density (kN/m3)
Compressive strength (MPa)
Elastic modulus (GPa)
Siltstone (First-aquifer)
19
19
10
10
8
Clay
79
60
11
15
12
Gritstone
84
5
24
35
30
7
25
45
35
Fine-sandstone Medium-sandstone
(Second91 aquifer) 96
5
24
45
30
Coal quality mudstone
100
4
21
20
20
Siltstone
103
3
25
40
35
Argillaceous siltstone
110
7
22
20
20
Main roof (Third-aquifer)
128
18
25
45
35
Immediate roof
130
2
21
5
20
2–2-coal
135
5
14
10
15
Floor
150
15
25
22.3
35
According to our three-step discrimination method of the water-resistant key strata, we must first identify the position of the structural key stratum in the overlying strata. On the basis of the physical and mechanical parameters of the roof rock strata of the 12610 workface, we determined that the main roof, 18 m thick, is the structural key stratum of the mining overlying strata based on our theoretical calculation, using Eq.(2). The deformation and movement of the main roof leads to the movement of all overlying strata. So the main roof can form a single wa-
ter-resistant key stratum until it breaks. The main roof will cause the movement of all overlying strata when it breaks. Secondly, we must determine the structural stability of structural key strata in the overlying strata. We calculated the bearing load q1 and the first breaking interval l1 of the key strata based on the physical and mechanical parameters of the rock strata, given in Table 1. The calculated results can be expressed as follows:
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Mining Science and Technology
q1 = 1.44 MPa l1 = h1
σc 5q1
= 18 ×
45 = 45.0 m 5 ×1.44
We know from the calculated results, that the key strata must break when the 12610 workface is pushed. However, from Table 1, we can see that there is a soft clayey rock between the key stratum and the aquifer (between the first and the second aquifer). We should also determine from the seepage calculation whether the compound water-resistant key stratum, a combination of the key stratum and the soft clayey rock, has Table 2
No.2
a water-resistant function. So, in a third step, we must identify the permeability performance of the compound key strata that combines the key stratum and the soft clayey rock. There are 10 layers of rock strata above the 12610 workface. The mass density of water is ρ = 1000 kg/m3 and its dynamic viscosity
μ = 0.98 × 10−3 Pa ⋅ s . The average pressure difference between the 12610 workface and its upper aquifer, i.e., p0–pn is 0.8 MPa. Based on the rock strata strain fields, calculated by RFPA simulation, we can obtain the seepage properties of each layer when the workface advances 50 m, as shown in Table 2.
Rock strata seepage properties of the 12610 workface (the workface advances 50 m)
No.
Lithology
Thickness (m)
Strain
Permeability (m–2)
Non-Darcy flow factor
Acceleration coefficient
1
Siltstone
19
0.005
3.50×10–16
2.48×1016
1.62×1013
2
Clay
60
0.005
2.50×10–18
3.64×1016
8.65×1014
6.62×10
–17
4.69×10
17
9.67×1012
–19
5.40×10
20
8.49×1015
3
Gritstone
5
0.01
4
Fine-sandstone
7
0.014
2.78×10
5
Medium-sandstone
5
0.016
7.45×10–18
7.29×1017
3.82×1013
0.017
8.68×10
–19
7.68×10
20
2.69×1014
–18
7.89×10
17
6.49×1014
6
Coal quality mudstone
4
7
Siltstone
3
0.02
3.54×10
8
Argillaceous siltstone
7
0.022
3.32×10–19
4.55×1020
1.48×1016
–18
20
5.63×1014
2.90×1018
2.65×1016
9
Main roof
18
0.028
4.61×10
10
Immediate roof
2
0.031
1.26×10–20
According to the seepage properties of each layer, as shown in Table 2 and Eq.(7), we can calculate the seepage catastrophe coefficient ȟ as follows: 10
1
i =1
i a
¦c
= 2.00 × 10−13 , 10
μ hi
i =1
i a i
¦c k
10
β i hi
i =1
cai
¦
= 2.15 × 107
= 9.04 × 10 (N ⋅ s / m3 )
4 ρ ( p0 − pn ) = 4 × 1000 × 0.8 × 106 = 3.2 × 109 (N 2 ⋅ s 2 /m 6 )
ξ=
3.2 × 109 × 2.00 × 10−13 × 2.15 × 107
( 9.04 × 10 )
2
= 1.68
From this result, we see that the seepage catastrophe coefficient ȟ>1 when the workface advances 50 m and the overlying strata cannot form a compound water-resistant key stratum, so that accidents of water-inrush will occur. Therefore, we must take measures to prevent water-inrush and to ensure safe mining at the workface.
5
Vol.20
Conclusions
1) Whether there is a water-resistant key stratum in the overlying strata should be considered from two sides. In first instance, we should consider whether
2.84×10
there is a rock stratum controlling the movement of the overlying strata above the coal seam (roof) or below coal seam (floor). This rock stratum can be either a single rock stratum called the structural key stratum or a compound stratum which can control the movement of the overlying in a special combination of several weak rock layers and a hard rock stratum. These rock strata have the ability to resist water when their structure remains stable and does not break. Secondly, the risk of water-inrush can be judged by the abrupt changes in the characteristics of the broken rock seepage, when the rock stratum breaks during mining. If there is no water-inrush from the broken rock stratum, we can conclude that these rock strata are water-resistant key strata. 2) We have presented a three-step discrimination method of water-resistant key strata. We first identified the position of the structural key stratum in the overlying strata. Secondly, we identified the structural stability of this structural key stratum in the overlying strata. Finally, we identified the seepage stability of structural key stratum, also in the overlying strata. We outlined the corresponding discrimination conditions and the calculation formula for the water-resistant key stratum. 3) Based on the three-step discrimination method of the water-resistant key stratum, we have presented the discrimination process of the water-resistant key stratum and its corresponding computer program. 4) Practical applications indicate that this dis-
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
crimination method and its corresponding computer program of water-resistant key strata are accurate and reliable, which can satisfy the actual design needs of water-preservation mining and are therefore of instructional importance for water-preservation mining in areas short of water.
Acknowledgements This study was supported by the National Natural Science Foundation of China (No.50874103), the National Basic Research Program of China (Nos.2006CB202210 and 2007CB209408) and the Natural Science Foundation of Jiangsu Province (No.KB2008135), as well as by the Qinglan Project of Jiangsu Province.
References [1] [2] [3]
Qian M G, Miao X X, Xu J L, Mao X B. Theory of Key Strata in Strata Control. Xuzhou: China University of Mining & Technology Press, 2003. (In Chinese) Qian M G, Miao X X, Xu J L. Resources and environment harmonics (green) mining. Journal of China Coal Society, 2007, 32(1): 1-7. (In Chinese) Qian M G, Miao X X, Xu J L. Resources and environment harmonics (green) mining and its technological system. Journal of Mining & Safety Engineering, 2006, 23(1): 1-5. (In Chinese)
[4]
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Miao X X, Chen R H, Bai H B. Fundamental concepts and mechanic analysis of water-proof key strata in water-keeping mining. Journal of China Coal Society, 2007, 32(6): 561-564. (In Chinese) [5] Miao X X, Pu H, Bai H B. Principle of water-resisting key strata and its application in water-preserved mining. Journal of China University of Mining & Technology, 2008, 37(1): 1-4. (In Chinese) [6] Pu H, Miao X X, Yao B H, Tian M J. Structural motion of water-resisting key strata lying on overburden. Journal of China University of Mining & Technology, 2008, 18(3): 353-357. [7] Feng M M, Mao X B, Bai H B, Miao X X. Analysis of water insulating effect of compound water-resisting key strata in deep mining. Journal of China University of Mining & Technology, 2007, 17(1): 1-5. [8] Miao X X, Liu W Q, Chen Z Q. Seepage Theory of Mined Rock Mass. Beijing: Science Press, 2004. (In Chinese) [9] Chen Z Q, Miao X X, Liu W Q. Analysis on stability of parametric system of seepage flow in wall rock affected by mining. Journal of Center South University, 2004, 35(1): 129-132. (In Chinese) [10] Kong H L, Miao X X, Wang L Z, Zhang Y, Chen Z Q. Analysis of the harmfulness of water-inrush from coal seam floor based on seepage instability theory. Journal of China University of Mining & Technology, 2007, 17(4): 453-458. [11] Wang L G, Miao X X. Cusp catastrophe model of relations among permeability, stress and strain of rocks. Chinese Journal of Rock Mechanics & Engineering, 2005, 24(23): 4210-4214. (In Chinese)
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Discrimination conditions and process of water-resistant key strata WANG Lianguo*, MIAO Xiexing, WU Yu, SUN Jian, YANG Hongbo State Key Laboratory of Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221008, China Abstract: Water-preservation mining is one of the most important parts of the ‘Green Mining’ technology system, which can realize the effective regulation of groundwater resources by controlling strata movement, changing passive prevention and governance of water disasters to active conservation and utilization of groundwater resources and thus obtaining coal and water simultaneously in mining. The concept of water-resistant key strata further enriches the content of the key stratum theory and provides a theoretical basis for water-preservation mining. In order to realize the idea of water-resistant key strata as a guideline in the design of water-preservation mining and engineering applications, the conditions for discrimination in the process of water-resistant key strata, we have presented a mechanical model, as well as its corresponding computer program, based on a large number of theoretical analyses and field measurements, as well as on a comprehensive consideration of the position, structural stability and seepage stability of key strata. Practical engineering applications indicate that this discrimination method and its corresponding computer program on water-resistant key strata are accurate and reliable and can satisfy the actual design needs of water-preservation mining and thus have instructional importance for water-preservation mining in mining areas lacking water. Keywords: water-resistant key stratum; water-preservation mining; structural stability; seepage stability
1
Introduction
The key stratum theory of strata control proposed by Qian et al. has been widely applied in the identification of suitable stratigraphic horizons of bed separation grouting, in the design of drilling hole arrangements of ground gas drainage, in the control of overlying strata and surface subsidence and elsewhere[1]. The ‘key strata’ in this key stratum theory of strata control are referred to as structural key strata, bearing the main effect of rock mass movements during mining, which controls the structural shape of ruptured rock masses. Given ground pressure control problems in water-preservation mining, Miao et al. have, in the last few years, presented the concept of water-resistant key strata[2-5]. A water-resistant key stratum in water-preservation mining is difficult to define but can be described as follows: provided that the upper aquifer of a coal seam is above the structural key stratum, or the lower aquifer of a coal seam is below the structural key stratum and if the structural key stratum cannot break under mining, then the Received 10 October 2009; accepted 11 December 2009 *Corresponding author. Tel: 86 13115225568 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60188-5
structural key stratum has a water-resistant function and is called the water-resistant key stratum. If the structural key stratum can break under mining, but the broken fissures can be filled with weak rock strata and a seepage water-inrush channel cannot be formed, then a compound water-resistant key stratum is formed by combining the structural key stratum with a weak rock stratum. From this description, one can see that the water-resistant key stratum can consist of only one single rock stratum, or by compounding several layers of weak rock strata with hard rock strata. The rock strata of the water-resistant key stratum must include hard rock strata which can bear the strain of rock mass movement during mining. That is to say, a water-resistant key stratum must be formed by structural key stratum or by compounding weak rock strata with hard rock strata that can bear a certain amount of strain of rock mass movement during mining. The concept of a water-resistant key stratum further enriches the content of the key stratum theory and provides a theoretical basis for water-preservation mining. However, distinguishing water-resistant key strata largely depends on field experience and is still short of a scientific basis. In order to realize the idea of water-resistant key strata as a guideline in the design of water-preservation mining and engineering
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
applications, in this study, we present the conditions for discrimination and process of water-resistant key strata and their corresponding computer programs based on a large number of theoretical analyses and field measurements. In this method of discrimination, the position, structural stability and seepage stability of key strata are considered comprehensively. We also provide for a corresponding force analysis model. Our practical applications suggest that the discrimination method and its corresponding computer programs of water-resistant key strata are accurate and reliable, which should satisfy the actual design needs of water-preservation mining and are thus of instructional importance for water-preservation mining in mining areas lacking water.
2
225 n
q1 =
E1 h13 ¦ γ i hi
(1)
i =1
n
¦E h
3 i i
i =1
where Ei, Ȗi and hi are the elastic model, the body force, and the thickness of each rock strata (i=1, 2, …, n, s, m).
Conditions for discrimination
According to our description of a water-resistant key stratum, we see that the decision of whether there is a water-resistant key stratum in the overlying strata should be considered from two sides. First, we need to decide whether there is a rock stratum that can control the movement of the overlying strata above the coal seam (roof) or below the coal seam (floor). Rock strata can consist either of a single rock stratum called the structural key stratum or is a compound stratum which can control the movement of the overlying strata in a particular combination of several layers of weak and hard rock strata. These rock strata have the capacity of water-resistance when their structure remain stable and do not break. Secondly, the risk of water-inrush can be judged by the abrupt changes in the characteristics of broken rock seepage as the rock strata break when mining. If no water-inrush from the broken rock strata occurs, we can conclude that these rock strata are water-resistant key strata. Therefore, the discrimination of water-resistant key strata can be classified into three steps: 1) Discrimination based on position The basis for the formation of water-resistant key strata is that there are structural key strata in the overlying strata. We must first identify the position of structural key strata and then identify whether these structural key strata can form water-resistant key strata[1]. The position of a structural key stratum can be identified by combining the drilling data and the mined geological conditions with the key stratum theory. Provided that the rock stratum S1 is the lowest key stratum as shown in Fig. 1, which controls n layers of rock strata (S2, …, Sn, Sm are refer to rock strata above S1, and 1, 2, …, n and m are the layer number, where n
Fig. 1
Load calculation model of overlying strata
If a specific rock stratum is a key stratum, it should simultaneous satisfy the conditions of discrimination based on stiffness (deformation) and strength, which can be presented as follows: n +1
n
n
s
¦ E h ¦γ h < ¦ E h ¦γ h i =1
3 i i
i i
i =1
3 i i
i =1
(l1 < ln +1 , n < s < m)
i i
i =1
(2)
where li indicates the first interval of roof breaking of the ith rock stratum. Given the condition of a fixed-fixed beam, the first interval of roof breaking of the 1st rock stratum can be expressed as follows:
l1 = h1
σc 5q1
(3)
where ıc is the limit of the compressive bearing capacity. 2) Discrimination based on structural stability The structural stability of a key stratum in the overlying strata is very important for controlling water-inrush under mining conditions. If a key stratum does not break during mining, water-inrush will not occur. Taking a compound water-resistant key stratum of the overlying strata as an example, its lower rock stratum must be a hard rock stratum no matter whether it is made up of several layers of rock strata. In order to carry out a strength analysis of a representative compound water-resistant key stratum, we studied a compound water-resistant key stratum made up of four rock layers, similar to the structural procedure followed by Miao et al[4]. Its mechanical model is shown in Figs. 2 and 3, where l is the advancing distance of the mining workface, 4h is the height of the cross section and its width is unit length.
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be calculated as follows:
T
n
5RFNVWUDWXP
ξ=
5RFNVWUDWXP 5RFNVWUDWXP 5RFNVWUDWXP +DUGURFN VWUDWXP
:HDNURFN VWUDWXP
Mechanical model of compound water-resistant key strata made up of four layers of rock strata
Fig. 3
Mechanical model of compound water-resistant key strata given the condition of a fixed-fixed beam
K
Fig. 2
According to the mechanical model of a compound water-resistant key stratum, the normal stress and shear stress of each rock stratum in a composite rock beam can be calculated by using Eqs.(4) and (5), respectively. Otherwise, the interval of roof breaking of the composite rock beam can be analyzed by the Mohr-Coulomb failure criterion, where the first interval of roof breaking l1 can be calculated by using Eq.(6). Hence, we can distinguish the structural stability characteristics of key strata in the overlying strata[4]. Ei y 1 − μ i2 σi = 4 Ei I ¦ 2 i i =1 1 − μ i M
No.2
( i = 1, 2, 3, 4 )
(4)
En i 1 − μ n2 1 S n* ⋅ ⋅ Q τi = ¦ 4 E b n =1 i I ¦ 2 i i =1 1 − μ i
(5)
σ 1 − Kσ 3 = Rt
(6)
3) Discrimination based on seepage stability According to the seepage theory of a mined rock mass, we can theoretically decide whether a broken structural key stratum in the overlying strata still has the ability of water-resistance, or still is a water-resistant key stratum[6-11]. We have used the n rock layers (including the key stratum) between the key stratum and the main aquifer to analyze its permeability and the seepage catastrophe coefficient ȟ of the roof (floor) to determine the seepage stability of the key stratum in the overlying strata[8-9], which can
4 ρ ( p0 − pn )¦ i =1
1 n β j hj ¦ cai j =1 caj
§ n μ hi · ¨¦ ¸ © i =1 ki ¹
2
(7)
where ȡ is the mass density of water; p0–pn the difference in pressure between roof (floor) and aquifer; ȕ a non-Darcy flow factor; μ the dynamic viscosity; i hj the thickness of the jth rock layer; ca the acceleration coefficient and ki the permeability. When ȟ<1, the composite rock strata between the key stratum and its overlying strata still has the ability of water-resistance, which can form a compound water-resistant key stratum, where water-inrush accidents do not occur. When ȟ1, the composite rock strata between the key stratum and its overlying strata does not have the ability to resist water and therefore cannot form a compound water-resistant key stratum and water-inrush accidents may occur. According to this three-step discrimination method, we can solve the problems of whether there is a water-resistant key stratum in the overlying strata and what kind of rock stratum can form a water-resistant key stratum.
3
Discrimination process
Given our analysis of the determination and conditions of water-resistant key strata, it can be seen that the determination of water-resistant key strata is a comparatively complex system. Our proposed method of a three-step determination is also a comparatively complex calculation in practice and may be difficult in practical engineering applications. For convenience therefore, we designed a special program for the determination of water-resistant key strata and a corresponding computer program for the determination process of water-resistant key strata as shown in Fig. 4.
Fig. 4
Discrimination process of water-resistant key strata
Input for the computer program is geometric, physical and mechanical parameters, seepage properties of each rock stratum in the overlying strata, hydro
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
geological conditions and other properties. The output parameters of the computer program are structural stability and seepage stability of water-resistant key strata. Elsewhere a corresponding interpretation and feasible countermeasures are given. The determination program of the water-resistant key strata includes five parts: a structural calculation module of the key stratum, an analytical seepage characteristics module, a database module, a fuzzy reasoning module and an interpretation module. The structural calculation module largely calculates structural stability of the key stratum, including a forced state and the first interval of roof breaking of the key stratum. The seepage characteristic module mainly calculates the seepage stability of the broken key stratum. The database module preserves the physical and mechanical characteristic parameters and seepage properties of each rock stratum and a part of the expert knowledge that can be used by fuzzy reasoning. The fuzzy reasoning module determines the deleted parameters based on reasoning given a condition of scarcity of data. The interpretation module mainly interprets the obtained results and presents expert opinions and suggestions, in accordance with many types of conditions.
4
Example analysis
The strike length of the 12610 fully-mechanized workface in the Daliuta coal mine of the Shendong mining area is 5293.4 m and the length of its inclinaTable 1
227
tion 239.8 m, so the area of fully-mechanized workface is 1269357 m2. The coal seam pitch is around 1°~5° and its average thickness 5.19 m. The geological reserves are about 8498474 t and the recoverable reserves approach 7903581 t. Given the capacity of the mine, this can be mined over a period of 10 months. The 12610 fully-mechanized workface has 141 sets of JOY 8670 installed for support and is equipped with a set of JOY 7LS shearers. The 12610 workface belongs to the spring area of Halagou and Sanbulago, and there is a Quaternary loose bed and Yanan formation bedrock in the overlying stratum of the 2–2-coal seam. Loose soils are from 10~100 m thick, with an average of 67 m. There is a sandy gravel layer at the bottom of this loose bed, 0~15 m thick, with an average of 6.3 m (the sandy gravel layer in the Baijiaqu ditch is 2.4~9.6 m thick). The thickness of the Yanan formation bedrock in the overlying stratum is 27.5~65.0 m, with an average thickness of 48 m (the Yanan formation bedrock in the Baijiaqu ditch is 27.5 m thick). On the first caving segment of the cut-hole, the Yanan formation bedrock is about 60 m thick, the loose bed 50 m, the weathered bedrock 22 m and the sandy gravel layer between 0~5 m. The first caving segment of the cut-hole belongs to the area of loose beds with lots of water. The physical and mechanical parameters of the roof rock strata of the 12610 workface are shown in Table 1.
Physical and mechanical parameters of roof rock strata of the 12610 workface
Lithology
Depth (m)
Thickness (m)
Bulk density (kN/m3)
Compressive strength (MPa)
Elastic modulus (GPa)
Siltstone (First-aquifer)
19
19
10
10
8
Clay
79
60
11
15
12
Gritstone
84
5
24
35
30
7
25
45
35
Fine-sandstone Medium-sandstone
(Second91 aquifer) 96
5
24
45
30
Coal quality mudstone
100
4
21
20
20
Siltstone
103
3
25
40
35
Argillaceous siltstone
110
7
22
20
20
Main roof (Third-aquifer)
128
18
25
45
35
Immediate roof
130
2
21
5
20
2–2-coal
135
5
14
10
15
Floor
150
15
25
22.3
35
According to our three-step discrimination method of the water-resistant key strata, we must first identify the position of the structural key stratum in the overlying strata. On the basis of the physical and mechanical parameters of the roof rock strata of the 12610 workface, we determined that the main roof, 18 m thick, is the structural key stratum of the mining overlying strata based on our theoretical calculation, using Eq.(2). The deformation and movement of the main roof leads to the movement of all overlying strata. So the main roof can form a single wa-
ter-resistant key stratum until it breaks. The main roof will cause the movement of all overlying strata when it breaks. Secondly, we must determine the structural stability of structural key strata in the overlying strata. We calculated the bearing load q1 and the first breaking interval l1 of the key strata based on the physical and mechanical parameters of the rock strata, given in Table 1. The calculated results can be expressed as follows:
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Mining Science and Technology
q1 = 1.44 MPa l1 = h1
σc 5q1
= 18 ×
45 = 45.0 m 5 ×1.44
We know from the calculated results, that the key strata must break when the 12610 workface is pushed. However, from Table 1, we can see that there is a soft clayey rock between the key stratum and the aquifer (between the first and the second aquifer). We should also determine from the seepage calculation whether the compound water-resistant key stratum, a combination of the key stratum and the soft clayey rock, has Table 2
No.2
a water-resistant function. So, in a third step, we must identify the permeability performance of the compound key strata that combines the key stratum and the soft clayey rock. There are 10 layers of rock strata above the 12610 workface. The mass density of water is ρ = 1000 kg/m3 and its dynamic viscosity
μ = 0.98 × 10−3 Pa ⋅ s . The average pressure difference between the 12610 workface and its upper aquifer, i.e., p0–pn is 0.8 MPa. Based on the rock strata strain fields, calculated by RFPA simulation, we can obtain the seepage properties of each layer when the workface advances 50 m, as shown in Table 2.
Rock strata seepage properties of the 12610 workface (the workface advances 50 m)
No.
Lithology
Thickness (m)
Strain
Permeability (m–2)
Non-Darcy flow factor
Acceleration coefficient
1
Siltstone
19
0.005
3.50×10–16
2.48×1016
1.62×1013
2
Clay
60
0.005
2.50×10–18
3.64×1016
8.65×1014
6.62×10
–17
4.69×10
17
9.67×1012
–19
5.40×10
20
8.49×1015
3
Gritstone
5
0.01
4
Fine-sandstone
7
0.014
2.78×10
5
Medium-sandstone
5
0.016
7.45×10–18
7.29×1017
3.82×1013
0.017
8.68×10
–19
7.68×10
20
2.69×1014
–18
7.89×10
17
6.49×1014
6
Coal quality mudstone
4
7
Siltstone
3
0.02
3.54×10
8
Argillaceous siltstone
7
0.022
3.32×10–19
4.55×1020
1.48×1016
–18
20
5.63×1014
2.90×1018
2.65×1016
9
Main roof
18
0.028
4.61×10
10
Immediate roof
2
0.031
1.26×10–20
According to the seepage properties of each layer, as shown in Table 2 and Eq.(7), we can calculate the seepage catastrophe coefficient ȟ as follows: 10
1
i =1
i a
¦c
= 2.00 × 10−13 , 10
μ hi
i =1
i a i
¦c k
10
β i hi
i =1
cai
¦
= 2.15 × 107
= 9.04 × 10 (N ⋅ s / m3 )
4 ρ ( p0 − pn ) = 4 × 1000 × 0.8 × 106 = 3.2 × 109 (N 2 ⋅ s 2 /m 6 )
ξ=
3.2 × 109 × 2.00 × 10−13 × 2.15 × 107
( 9.04 × 10 )
2
= 1.68
From this result, we see that the seepage catastrophe coefficient ȟ>1 when the workface advances 50 m and the overlying strata cannot form a compound water-resistant key stratum, so that accidents of water-inrush will occur. Therefore, we must take measures to prevent water-inrush and to ensure safe mining at the workface.
5
Vol.20
Conclusions
1) Whether there is a water-resistant key stratum in the overlying strata should be considered from two sides. In first instance, we should consider whether
2.84×10
there is a rock stratum controlling the movement of the overlying strata above the coal seam (roof) or below coal seam (floor). This rock stratum can be either a single rock stratum called the structural key stratum or a compound stratum which can control the movement of the overlying in a special combination of several weak rock layers and a hard rock stratum. These rock strata have the ability to resist water when their structure remains stable and does not break. Secondly, the risk of water-inrush can be judged by the abrupt changes in the characteristics of the broken rock seepage, when the rock stratum breaks during mining. If there is no water-inrush from the broken rock stratum, we can conclude that these rock strata are water-resistant key strata. 2) We have presented a three-step discrimination method of water-resistant key strata. We first identified the position of the structural key stratum in the overlying strata. Secondly, we identified the structural stability of this structural key stratum in the overlying strata. Finally, we identified the seepage stability of structural key stratum, also in the overlying strata. We outlined the corresponding discrimination conditions and the calculation formula for the water-resistant key stratum. 3) Based on the three-step discrimination method of the water-resistant key stratum, we have presented the discrimination process of the water-resistant key stratum and its corresponding computer program. 4) Practical applications indicate that this dis-
WANG Lianguo et al
Discrimination conditions and process of water-resistant key strata
crimination method and its corresponding computer program of water-resistant key strata are accurate and reliable, which can satisfy the actual design needs of water-preservation mining and are therefore of instructional importance for water-preservation mining in areas short of water.
Acknowledgements This study was supported by the National Natural Science Foundation of China (No.50874103), the National Basic Research Program of China (Nos.2006CB202210 and 2007CB209408) and the Natural Science Foundation of Jiangsu Province (No.KB2008135), as well as by the Qinglan Project of Jiangsu Province.
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Miao X X, Chen R H, Bai H B. Fundamental concepts and mechanic analysis of water-proof key strata in water-keeping mining. Journal of China Coal Society, 2007, 32(6): 561-564. (In Chinese) [5] Miao X X, Pu H, Bai H B. Principle of water-resisting key strata and its application in water-preserved mining. Journal of China University of Mining & Technology, 2008, 37(1): 1-4. (In Chinese) [6] Pu H, Miao X X, Yao B H, Tian M J. Structural motion of water-resisting key strata lying on overburden. Journal of China University of Mining & Technology, 2008, 18(3): 353-357. [7] Feng M M, Mao X B, Bai H B, Miao X X. Analysis of water insulating effect of compound water-resisting key strata in deep mining. Journal of China University of Mining & Technology, 2007, 17(1): 1-5. [8] Miao X X, Liu W Q, Chen Z Q. Seepage Theory of Mined Rock Mass. Beijing: Science Press, 2004. (In Chinese) [9] Chen Z Q, Miao X X, Liu W Q. Analysis on stability of parametric system of seepage flow in wall rock affected by mining. Journal of Center South University, 2004, 35(1): 129-132. (In Chinese) [10] Kong H L, Miao X X, Wang L Z, Zhang Y, Chen Z Q. Analysis of the harmfulness of water-inrush from coal seam floor based on seepage instability theory. Journal of China University of Mining & Technology, 2007, 17(4): 453-458. [11] Wang L G, Miao X X. Cusp catastrophe model of relations among permeability, stress and strain of rocks. Chinese Journal of Rock Mechanics & Engineering, 2005, 24(23): 4210-4214. (In Chinese)