Numerical Simulation of Supporting Technology for Soft Seam Tailgate in Hongling Coal Mine Wang Long Postgraduate student School of Energy Science and Engineering, Henan Polytechnic University, JiaoZuo 454000 China e-mail:
[email protected]
Guo Baohua* Associate Professor, School of Energy Science and Engineering, Henan Polytechnic university, Jiaozuo 454000 China *Corresponding Author, e-mail:
[email protected]
ABSTRACT Surrounding rock and coal of tailgate in 15141 isolated longwall face is soft and fractured in Hongling coal mine, thus the tailgate supporting technology was studied. The reasonable location of the tailgate and the width of coal pillar are analyzed theoretically firstly. Then, the reasonable width of coal pillar, bolt and cable parameters, and the effect of the advanced abutment pressure on tailgate stability are studied by using geotechnical software FLAC3D. The results show that the reasonable width of the coal pillar is about 14~16 m. Bolt & cable combined supporting technology can reduce the deformation of the tailgate section effectively, and the reinforced tailgate can bear the influence of advanced abutment pressure. These conclusions have a guiding significance for practice engineering of roadway supporting.
KEYWORDS:
mining engineering; coal pillar width; bolt & cable combined supporting;
numerical simulation
INTRODUCTION To keep stability of mine roadway is quite difficult or costly in the excavation and mining period in some coal mines, and about 100 km roadway need to be repaired and maintained in China per year [1]. Especially, supporting technology of soft rock roadway in deep mine is becoming one of the main challenges which restrict coal exploitation seriously [2-4]. The stability of roadway in soft rock or coal seam is hard to be maintained due to long term deformation. Specially, secondary deformation due to the effect of advanced abutment pressure may occur even though the roadway itself is stable previously[5]. In addition, coal pillars in underground coal mines play a key role to provide support to the superincumbent strata. Traditionally, the width of coal pillars are designed based on the principle that the bearing capacity of the coal pillar must be greater than the load imposed on itself [6]. The bigger the coal pillar width is, the less the stress concentration coefficient is in coal pillar, and the easier to support the roadway along gob [7]. However, conventional coal pillars with the width of 20~30 m will lead to massive coal loss. Thus, it is very important to determine the optimal width of coal pillar with bolt & cable combining support technology. - 5667 -
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ENGINEERING CONDITION 15141 isolated face is located to the upper part of 15161 gob, and the lower part of 15121 gob. The tailgate of 15141 isolated face is excavated in 2-1 coal seam with average thickness of 6.05 m, average dip angle of 20.5º and buried depth of 637 m. The surrounding rock and coal is soft and fractured, thus, the displacement of tailgate section in adjacent face is obvious when under support of U type steel shed. In order to reduce the deformation of tailgate section of 15141 face, it is urgent to restudy the reasonable tailgate location and supporting pattern. The roof and floor features are shown in Table 1.
Table 1: The roof and floor features of 15141 isolated face Name
Rock type
Main roof
Fine-grain sandstone
Average thickness/m 11.62
Immediate roof
Sandy mudstone
5.32
Coal seam
2-1 coal seam
6.05
Immediate floor
Mudstone
5.64
Main floor
Siltstone
4.47
Lithology description Hard, fractured, not easy falling Loose structure, easy falling, high carbon content Soft, black powder, vitreous luster, conchoidal fracture Stratification development, bottom swells reacting with water Hard
THEORETICAL ANALYSIS OF COAL PILLAR WIDTH Tailgate Location Selection zone
As shown in Figure 1, if the tailgate is located in position 1 which is namely stress relaxed [8-9] , the tailgate would be easy to keep stable with conventional supporting pattern. But the
1 2
3
4
Figure 1: Diagram of the gob-side roadway location
excavating and supporting process. If the tailgate is located in position 2 which is namely gobside roadway driving with small coal pillar width, where the stress of roof has released and the recovery rate of coal is high as well. However if the immediate roof and the immediate floor are too cracked probably, it is difficult to install support and keep stable of the roadway. If the
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roadway is located in position 3 which is namely stress concentrated zone, it is difficult to maintain the roadway for it is undergoing higher abutment pressure. If the tailgate is located in position 4, the original rock stress zone, there is more loss of coal, while roadway is stable with lower abutment pressure and intact surrounding rock. But if the surrounding rock is soft and fractured, taking position 4 as the roadway location is the optimal choice.
Calculation of coal pillar width As shown in Figure 2, the width of coal pillar could be divided into three parts. When adopting bolt & anchor combined supporting technology, we can calculate the reasonable width of coal pillar with formula (1) [10],
X1
X3
X2
B
Figure 2: Sketch map of coal pillar width calculation
B
X1 X 2
X3
(1)
where B is the coal pillar width; X1 is the fractured zone width of upper gob, which is given as the following formula (2) [10],
X1
M 2 tan
c0 tan 0 Px
k H ln 0
c0 tan
(2)
0
where M is the thickness of coal seam; is the lateral pressure coefficient, = (1 ); is Poisson’s ratio; 0 is the internal friction angle of coal seam; k is the stress concentration factor; is the average bulk density of overlying strata; H is the buried depth; C0 is the cohesion of coal seam; Px is the supporting force, the value of which is zero near the gob side. In formula (1), X2 is the effective length of the bolt near the coal pillar side, X3 is the margin width of coal pillar, and X3=0.1~0.3(X1+X2), therefore, formula (1) can be change into formula (3),
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(3)
The formula (3) is suitable to determine the width of coal pillar for the roadway with bolt supporting pattern. According to the geological condition of 15141 face, we get M=6.05m, =0.49, =0.33, 0=22°, C0=0.84MPa, k=2.5, =25KN/m3, H=640m, Px=0. Putting them into formula (2), the fractured zone width X1 is equal to 11.03m. The bolt effective length (X2) here is 1.8m. When X1 and X2 are fed into formula (3), we can obtain that the theoretical range of coal pillar width is from 14.11m to 16.68 m. Nevertheless, when determining the coal pillar width by above analytical calculation, the mechanical model is rather complicated, and some factors may be neglected. So that further study by numerical simulation method is needed to confirm above results.
SIMULATION AND ANALYSIS Model Building In order to analyze the stress and deformation distribution of the tailgate, the geotechnical software of FLAC3D is used to investigate the reasonable coal pillar width and supporting pattern. The simulation model is established according to the geological and mining condition of Hongling coal mine without considering the influence of structural plane, soft interlayer, and groundwater, and all relevant strata are regarded as isotropic and homogeneous in the model. As shown in Figure 3, the width (X) of the model is 70 m, the thickness (Y) is 10 m, and the height (Z) is 75 m, respectively. The dip angle of rock stratum is 20.5°. The top layer and the bottom layer in the model are the loading cushion. This model adopts the single direction constraint boundary (zero horizontal displacement) to limit two vertical boundaries and the two directions constrain boundary (zero horizontal and vertical displacement) to fixed bottom, and the vertical stress which is determined by the overburden weight is imposed on the top stress boundary. The tailgate of 15141 face is excavated after the calculation convergence for excavation and caving of the upper gob. The height of the model itself is 75 m, so the uniform distributed load of 15 MPa is imposed on the top boundary. The Strain-Softening model is chosen in numerical simulation, and the physical and mechanical parameters are listed in Table 2, in which these mechanical parameter values are reduced from the test values of rock specimens based on formula (4) by Hani S. Mitri [11].
Rf
Eint Erm
0.5 1 cos(
RMR ) 100
(4)
where Rf is the reduction coefficient of elastic modulus, RMR is the rock mass rating, Eint is elastic modulus of rock sample tested in lab, Erm is elastic modulus of engineering rock mass. The values of other mechanical parameters are reduced using the same method.
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Gob
Coal Pillar Tailgate
Figure 3: Numerical simulation model Table 2: Physical and mechanical parameters of surrounding rock Rock type
Density /kg·m-3
Bulk modulus /GPa
Shear modulus /GPa
Cohesion /MPa
/°
Tension /MPa
Siltstone
2720
6.52
4.24
5.04
34
3.42
Medium-grain sandstone
2665
5.13
3.41
3.62
36
2.62
Mudstone
2612
3.4
2.05
3.31
26
0.95
2-1 coal seam
1416
1.62
0.96
0.84
22
0.47
Sandy mudstone
2589
3.82
1.98
2.89
24
0.87
Fine-grain sandstone
2724
5.32
3.31
4.3
32
3.2
Sandstone and mudstone interbed
2580
3.53
1.82
3.13
28
1.32
Simulation Analysis of Coal Pillar Width According to the analytical calculation for pillar width (in section 3), three models with different pillar width are established in which the width is 10 m, 15 m, and 20 m respectively. As shown in Figure 4 and Figure 5, the reasonable pillar width can be determined by comparing the deformation and vertical stress distribution of the tailgate surrounding rock during the excavation. Figure 4 shows that the vertical displacements of the tailgate section decrease gradually with the increase of coal pillar width. The maximum horizontal convergences between two ribs are 35.76 cm, 26.42 cm, and 22.65 cm respectively; the vertical displacement of the roof is 26.04 cm, 13.84 cm, and 9.86 cm respectively; and the heave value of the floor is 15.23 cm, 10.42 cm, and 8.78 cm, respectively. Especially, when the width of the coal pillar increases from 10 m to 15 m,
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the total deformation of the tailgate is reduced more obviously than that when the coal pillar width increases from 15 m to 20 m. 40
Convergence between two ribs Displacement of roof Heave of floor
35 30 25 20 15 10 5 10
15 Coal pillar width/m
20
Figure 4: The tailgate deformation under different coal pillar widths
(a) Coal pillar width of 10m
(b) Coal pillar width of 15m
(c) Coal pillar width of 20m
Figure 5: The vertical stress diagrams of surrounding rock under different coal pillar widths From Figure 5, we can see that the phenomenon of vertical stress concentration arises in all the three models and the distance between position of the maximum vertical stress point and tailgate increases gradually with the increase of coal pillar width. Furthermore, the peak vertical stresses and the coefficients of stress concentration decrease gradually. Specifically, the peak stresses are 34.6 MPa, 31.3 MPa, and 30.2 MPa, respectively and the coefficient of vertical stress concentration are 2.31, 2.09, and 2.01, respectively. The vertical stress in the coal pillar is lower when the distance between the vertical stress point and tailgate or gob is smaller; otherwise, the vertical stress in coal pillar is relatively higher. It is mainly because there is a large fractured zone in low-stressed rock mass, and the stress can be reduced for release. To sum up, choosing the coal pillar width of 15 m or more is good to keep stability of the tailgate, but it is not necessary to enlarge the coal pillar width blindly. Therefore, the coal pillar width of 15 m is selected, which is consistent with the theoretical analysis result.
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Bolt Parameters Analysis Bolt parameters are the key factors that influence the effect of bolt & cable combined supporting technology [12], so the main supporting parameters are studied by numerical simulation. The relationships between the deformation of the tailgate section and bolt length, bolt spacing, and bolt pretension are shown in Figure 6.
50 45 40 35 30 25 20 15 10 5
40
30
35
25
30 25
20
20
15
15 10 2
2.2 2.4 Bolt length/m
2.6
Convergence between two ribs Displacement of roof Heave of floor
Convergence between two ribs Displacement of roof Heave of floor
Convergence between two ribs Displacement of roof Heave of floor
10 600
700 800 Bolt spacing /mm
900
30
50
70
90
Bolt pretension/kN
Figure 6: The relationships between tailgate section deformation and bolt parameters From Figure 6, we can see that the displacement of the tailgate section decreases with the blot length and the bolt pretension, and increase with the bolt spacing. That is to say, bolt & anchor combined supporting pattern can enhance the stability of the tailgate. However, when the bolt length and bolt pretension increase, or the bolt spacing decreases to some extend, the deformation of the tailgate section will almost no longer change [13]. Therefore, the bolt diameter of 20 mm, the bolt length of 2.2 m, the bolt pretension of 50 kN, the roof-bolt spacing of 700 mm, the rib-bolt spacing of 800 mm are selected to save supporting costs and reduce labor intensity.
The Design of Cables Layout In the process of mining, the fractured zone range of tailgate may be near or beyond the length of bolt, so that the cables are added to further enhance the stability of bolt supporting system. Cables are conventionally arranged in roof in both sides of the roof vertical centerline in engineering practice. However, if cables are arranged in roof between two adjacent bolting sections, the spacing between bolt and cable is too small. If the high density of supporting material can not significantly reduce the deformation of tailgate section, it will lead to waste of the supporting material at the same time. If the bolts in both sides of the roof vertical centerline are replaced by cables, and letting cables and bolts be arranged in a same tailgate section can make the integration of bolt and cable supporting and enlarge the bolt pretension zone of roof. The two supporting patterns of tailgate section are shown in Figure 7, in which model represents cables arranged in both sides of the bolting section in the axial direction, and Model represents cables replacing of bolts in both sides of the roof vertical centerline in the bolting section. The maximum displacement distribution, plastic zone, and vertical stress nephogram under two supporting patterns are shown in Figure 8, Figure 9 and Figure 10, respectively.
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3700
3700
1600
2100
800 700
700
150
1050
2200
2200
(a) Roadway section with supporting pattern I
(b) Roadway section with supporting pattern
anchor
anchor
anchor beam
anchor beam
bolt
bolt
w
w
steel strip
1600
(c) Top view of supporting pattern I
150
steel strip
2100
(d) Top view of supporting pattern
Figure 7: Two designs of the tailgate supporting pattern (The unit of the number is mm) As Shown in Figure 8, the effect of bolt & cable combined supporting pattern is much better than that of bolt supporting pattern without cables. Additionally, the section deformation of the tailgate with supporting pattern I is smaller than that with pattern during both excavation and mining process (In this model, two times of overlying stratum weight are applied to the top boundary of the model to simulate the influence of mining process.), but the difference of deformation between two supporting patterns is not obvious. The plastic zone widths of two tailgate ribs in three models are similar as shown in Figure 9, however, the plastic zones in roof and bottom are different obviously, and thus the supporting pattern I is slightly better than the supporting pattern in the control of roof and floor deformation. Figure 10 shows that the range of vertical stress concentration with supporting pattern I is smaller than that with supporting pattern , and the vertical stress concentration factor is 2.17 and 2.23 in two supporting patterns respectively. Overall, the vertical stress distributions nephograms of two supporting patterns are similar, and tailgate with supporting pattern can also withstand the larger advanced abutment pressure. Therefore, it is more efficient to select the supporting pattern for the tailgate.
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Pat tern during excavation Pat tern during mining Non-cables during excavation
45
Pat tern during excavation Pat tern during mining Non-cables during mining
40 35 30 25 20 15 10 5 1
2
3
Observation items
(Notes: 1- Convergence of both sides; 2-Vetical displacement of roof; 3-Heave of floor)
Figure 8: The displacement of tailgate section with three supporting patterns
(a) Pattern I
(b) Pattern
Figure 9: The plastic zone distribution of tailgate with two supporting patterns during mining
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(b) Pattern
Figure 10: The vertical stress nephogram of tailgate with two patterns during mining
CONCLUSIONS 1. When located in soft and fractured coal seam, the roadway should be arranged at the position of original rock stress; the theoretical range of coal pillar width is from 14.11m to 16.68 m, and it is consistent with the numerical simulation result. 2. The stability of tailgate increase gradually in a certain extent with the increase of the bolt length, bolt pretension or the decrease of the bolt spacing, Therefore, the bolt diameter of 20 mm, the bolt length of 2.2 m, the bolt pretension of 50 kN, the roof-bolt spacing of 700 mm, the ribbolt spacing of 800 mm are selected to save supporting costs and reduce labor intensity. 3. When adopting bolt & cable combined supporting pattern, cables can further reduce the deformation of roadway section by suspending the anchorage volume into the higher roof. If the bolts in both sides of the roof vertical centerline are replaced by cables, compared with setting the cables in roof between two adjacent bolting sections, the deformation and the fractured zone of tailgate section, and the stress concentration factor is a little bigger. However, the tailgate can also bear the larger advanced abutment pressure. Therefore, it is more efficient to select this supporting pattern for the tailgate.
ACKNOWLEDGEMENT This work was carried out under the National Natural Science Foundation of China (No. 51109076). This support is gratefully acknowledged.
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