Abutment pressure distribution for longwall face mining through abandoned roadways

International Journal of Mining Science and Technology 29 (2019) 59–64

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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Abutment pressure distribution for longwall face mining through abandoned roadways Yang Li a,⇑, Mingxing Lei a, Haosen Wang a, Cheng Li b, Weiwei Li a, Yang Tao a, Jingyi Wang a a b

College of Resources and Safety Engineering, China University of Mining and Technology Beijing, Beijing 100083, China China National Machinery Imp & Exp. Corp, Beijing 100037, China

a r t i c l e

i n f o

Article history: Received 3 June 2018 Received in revised form 19 July 2018 Accepted 15 August 2018 Available online 28 December 2018 Keywords: Abandoned roadways Abutment pressure Theoretical calculation Numerical simulation

a b s t r a c t Abutment pressure distribution is different when a longwall panel is passing through the abandoned gate roads in a damaged coal seam. According to the geological condition of panel E13103 in Cuijiazhai Coal Mine in China, theoretical analysis and finite element numerical simulation were used to determine the front pressure distribution characteristics when the longwall face is 70, 50, 30, 20, 10, and 5 m from the abandoned roadways. The research results show that the influence range of abutment pressure is 40 to 45 m outby the face, and the peak value of front abutment pressure is related to the distance between the face and abandoned roadways. When the distance between the longwall face and abandoned roadways is reduced from 50 to 10 m, the front abutment pressure peak value kept increasing. When the distance is 10 m, it has reached the maximum. The peak value is located in 5 to 6 m outby the faceline. When the distance between the longwall face and abandoned roadways is reduced from10 to5 m, the front abutment pressure sharply decreases, the intact coal yields and is even in plastic state. The peak value transfers to the other side of the abandoned roadways. The research results provide a theoretical basis for determining the advance support distance of two roadways in the panel and the reinforcement for face stability when the longwall face is passing through the abandoned roadways. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Front abutment pressure is the result of in-situ stress was readjusted for a new state of equilibrium in the longwall mining process. Research on the distribution of abutment pressure has significant importance in confirming the width of the coal pillar and the distance of advance support in roadways [1–4]. Currently, there is a lot of research on abutment pressure characteristics of traditional longwall mining, thick coal seam longwall faces, isolated faces, and extra-wider face [5–8]. However, theoretical analysis and research on the distribution of longwall face passing the abandoned roadways are relatively less. Abandoned roadways are often left from the roadway mining or room and pillar mining. Due to the existence of these roadways, the in-situ stress has already been redistributed around the roadway. Thus, when the longwall face approaches and passes through the abandoned roadways, the abutment pressure distribution has different characteristics than those of traditional longwall mining [9,10]. Especially when the face approaches the abandoned roadway, the distribu⇑ Corresponding author. E-mail address: [email protected] (Y. Li).

tion of abutment pressure has significant importance to set-up the advanced support to prevent the coal or rock dynamic disasters and make the roadway passing process safe and efficient [11,12]. This paper analyses the panel E13103 in Cuijiazhai Coal Mine of Yuzhou Mining Co., Ltd in Kailuan Group. FLAC3D numerical simulation was used to determine the abutment pressure distribution in long wall face passing the abandoned roadways.

1.1. Geological condition E13103 face is located in No.3 northern area of Cuijiazhai coal mine. Its north is the goaf ofPanel E13105 and its south part is E13101 panel. The panel length is 1086.5 m, and panel width is 110 m. The elevation of coal seam is +830 m. The depth is 300 to 320 m, average thickness of coal seam is 4 m and the coal seam dip is 0° to 21°, with an average of 7°. According to geological exploration data and distribution of the abandoned roadways, there are several roadways parallel to the longwall face in panel E13103. The width of roadway varies from 2.5 to 4.2 m, and interval is from 60 to 70 m. The panel layout is shown in Fig. 1

https://doi.org/10.1016/j.ijmst.2018.11.018 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Fig. 3. Unit analysis in limit equilibrium zone. Fig. 1. Panel layout.

(2) Abutment pressure calculation in elastic region 2. Theoretical analysis of abutment pressure distribution

Similarly, the mechanical equilibrium of a unit in the elastic region is analyzed, as shown in Fig. 4.

2.1. Front abutment pressure calculation without roadways ahead

hðrx þ drx Þ  hrx þ 2ry fdx ¼ 0

After coal was extracted during the face advance, the stress in front of the face should be redistributed. With the face advance, the gob space is increasing from the setup room, and coal seam in front of face gradually forms limit equilibrium region and elastic region [8]. The mechanical model of front abutment pressure distribution is shown in Fig. 2. (1) Limit equilibrium region calculation of abutment pressure A unit in the limit equilibrium region is selected to conduct the mechanical analysis, as shown in Fig. 3. According to the forces balance in the limit equilibrium region, Eq. (1) is calculated below:

hðrx þ drx Þ  hrx  2ry fdx ¼ 0

ð1Þ

According to the limit equilibrium zone—Mohr-Coulomb strengthcriterion, Eq. (2) is as follows:

ry ¼ rc þ

1 þ sinu rx 1  sinu

ð2Þ

CombiningEqs. (2) to (1), Eq. (3) is as follows: 2fx 1þsinu

ry ¼ N0 e h ð1sinuÞ

ð3Þ

where N0 is the residual support strength of coal, and N 0 ¼ s0 cotu; rx is the stress along the coal seam direction; ry is the vertical stress on the coal seam; f is the friction coefficient between coal seam and roof; h is the mining heigh; rc is the uniaxial compressive strength of coal; and w is the internal friction angle ofcoal. Putting ry ¼ kcH into Eq. (3), the position of the abutment pressure peak value is

x0 ¼

h 1 þ sinu kcH lnð Þ 2f 1  sinu N0

ð4Þ

where k is the abutment pressure concentration coefficient; H is coal seam depth; c is the unit weight of overlying strata.

ð5Þ

In the plastic region, there is

rx ¼ kry ; drx ¼ dkry

ð6Þ

where k is the ratio of major principal horizontal stress to vertical stress. Combining Eqs. (6) with (5) 2f k

ry ¼ kcHe h ðxx0 Þ Inserting tance is

xt ¼ x0 þ

ð7Þ

ry ¼ cH to Eq. (7), the abutment pressure impact dis-

h lnk 2f k

ð8Þ

According to the above analysis, the front abutment pressure distribution is a positive exponential curve in limit equilibrium region, and the abutment pressure distribution curve in elastic region is in the shape of negative exponential curve. Abutment pressure peak value can be calculated in Eq. (3), and the width of limit equilibrium region (the distance between peak value point and panel face) can be calculated by Eq. (4). The full abutment pressure influence can be calculated by Eq. (8). In panel E13103, the coal thickness (h) is 4 m; depth (H) is 320 m; internal friction angle (w) is 23°; overburden density (c) is 25 kN/m3; friction between coal seam and roof friction (f) is 0.2; the ratio of major principal horizontal stress to vertical stress (k) is 0.3; and stress concentration coefficient (k) is 2.5. By combining Eqs. (4) and (8) to calculate the width of limit equilibrium, the distance betweenthe peak value point and face (x0) is 5.6 m and the distribution of abutment pressure is (xt) 38.5 m. It is calculated that when the distancebetween face and abandoned roadways is more than 39 m, the front abutment pressure does not affect the abandoned roadways. 2.2. Abutment pressure calculation analysis when the longwall face approaches the roadway When there is no roadway ahead, the influence distance of front abutment pressure is 38.5 m. The distanceoflimit equilibrium

Fig. 2. Front abutment pressure distribution without roadways ahead.

Fig. 4. Force analysis in elastic region.

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region is 5.6 m. It is known that when the distance from face to roadway is more than 38.5 m, the roadway is not affected by front abutment pressure. Due to thetime factorsandstress redistribution around the abandoned roadway, the original supporting was no longer effective in the abandoned roadways. As a result, the stress concentration coefficient would decrease and the limit equilibrium region around the roadway would increase.Fig. 5 shows the stressdistribution around the abandoned roadway. Puttingcoal seam parameters into Eqs. (4) and (8), the limit equilibrium region is x00 = 2.5 m, xt0 = 8.1 m. Thus, when the differentbetween face and roadway L > xt ± xt0 = 46.6 m, the longwall face and roadway do not affect each other.Theabutment pressure distribution is shown in Fig. 6.

When the distancebetween the face and roadway L < xt + xt0 = 46.6 m, the abutment pressure from the panel face will overlap the pressure from the roadway, as shown in Fig. 7. Front abutment pressure in the face and abutment pressure on the inbye side of the roadway will both increase. When the distance between the face and roadway L < x0 ± x00 = 8.1 m, the coal pillar between the face and abandoned roadway is completely in plastic state and becomes the yield pillar. Abutment pressure will be transferred outbye the roadway to the intact coal. The pressure peak outby the roadway will continue to increase with the yield of the coal pillar. The full distance of abutment pressure is also increasing. 3. Numerical simulation analysis 3.1. Model establishment

Fig. 5. Abutment pressure distribution on the sides of abandoned roadway.

According to the geological condition of panel E13103, a numerical simulation model with 200 m width and 300 m long and 100 m high is established. Thus, the size of model is 300 m  200 m  100 m. The model is divided into 92,000 blocks and 108,158 nodal points. The model uses the displacement boundary method at the side of model to limit its horizontal movement, applying horizontal stress with the changes of depth. The bottom surface of the model is limited by horizontal and vertical movement. The upper part of model applies equivalent stress with overlying layer. Mohr Coloumb model is used within the coal and rock mass. The three-dimensional mesh of the computational model is shown in Fig. 8. The physical mechanical parameters of coal and strata layer from the bottom are shown in Table 1. 3.2. Excavation process simulation

Fig. 6. Distance between face and roadway is more than 46.6 m.

Fig. 7. Distance between face and roadway is less than 46.6 m.

After excavating the abandoned roadway until balance in the model, the longwall face starts advancing in different distance. Due to the size of model and influence range of abutment pressure, the longwall face advanced 10 m at a time when the distance between simulated face and roadway is less than 100 m. During this time, the vertical stress should be monitored until the face is 20 m inby the abandoned roadway. Then the stress in each 5 m would be recorded until the face is intersected with the abandoned roadway. From Fig. 9, it is shown that the front abutment pressure distribution is in 30, 20, 10, and 5 m distance between the face and the roadways. 3.3. Result analysis numerical simulation The numerical simulation results are shown in 3D visualization in Fig. 10. The front abutment pressure distribution was monitored when face advances 70, 50, 30, 20, 10, and 5 m distance from the roadway as shown in Fig. 11. And the statistical analysis of the results are shown in Table 2. From Figs. 9 and 10 and Table 2, it is shown that the peak of front abutment pressure reaches maximum value 18.09 MPa, when

Fig. 8. Three-dimensional numerical model.

Table 1 Rock properties in numerical model. Lithology

Bulk modulus (GPa)

shear modulus (GPa)

Cohesive force (MPa)

Internal friction angle (°)

Extension strength (MPa)

Unit weight (kN/m3)

Sandy mudstone Packsand Siltstone Claypan #3 coal #1 coal Limestone

5.12 10.87 5.56 5.28 4.86 4.86 13.47

3.42 6.27 4.26 2.61 1.35 1.26 8.75

3.5 9.1 5.5 3 1.5 1.5 12

34 40 36 35 23 23 41

2.5 8.6 2.5 2.5 2 2 9

25.1 28.7 24.6 24.3 13.8 13.8 26.5

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Fig. 9. Excavation process simulation in different distance.

the face is 70 m distance to the abandoned roadway. Stress concentration coefficient is 2.26. The point of peak value is 5 m. The influence of front abutment pressure ranges from 40 to 45 m and the roadway is not affected in this distance. The stress on two sides of roadway does not change so much by the abutment pressure from the face. When the face is 50 m distance to the abandoned roadway, the front abutment pressure peak value in face is 18.99 MPa, peak value point is 5 to 6 m in front of the face, and stress concentration coefficient is 2.37. Compared with the abutment pressure distribution as the roadway is 70 m distance to the face, the abutment pressure increases in 50 m, and the peak value point moves 1 m forward, which shows that the range of abutment pressure starts affecting the roadway and the pressure is overlapped by both the face and roadway.

When the face is 30 m distance to the roadway, the front abutment pressure peak value is 20.33 MPa, peak value point is 5 to 6 m in front of face, and stress concentration coefficient is 2.54. Compared with the abutment pressure distribution as the roadway is 50 and 70 m to the face, the abutment pressure peak value continues to increase, which shows that front abutment pressure continues to overlap. When face is 20 m distance to roadway, front abutment pressure peak value is 23.25 MPa, peak value point is 4 m in front of face, stress concentration coefficient is 2.9. Compared with abutment pressure distribution curves when the roadway is 30, 50, 70 m distance to the face, the abutment pressure peak value is sharply increasing, which shows that pressures are overlapping significantly and makes the front abutment pressure sharply increase. The influence range of abutment pressure is 20 to 30 m in front of the face. When the face is 10 m distance to the abandoned roadway, the peak value of front abutment pressure reached 25.75 MPa, while the stress concentration coefficient increased to 3.22. At this time, abutment pressure peak value is maximum. Peak value point is still 5 m in front of the panel face, and 10 m pillar has reached its limited width. When face is 5 m distance to the roadway, abutment pressure between face and roadway is sharply decreasing to 15.91 MPa. Stress concentration coefficient is reduced from 3.22 to 1.98, and the abutment pressure peak value on the other side of the roadway rises to 22.38 MPa. The stress concentration coefficient is 2.79, and peak value point is 2 m from the roadway. It is 11 m to the face, which shows that the front abutment pressure is transferred from the longwall face to the other side of the roadway in this process. When the distance is from 10 m to 5 m, the pillar between roadway and the face is in the plastic state. The pillar strength is sharply decreasing as the face advanced, but it still has certain residual strength. Abutment pressure formed by the longwall face is transferred to the other side of the roadway.. Then the coal face is unstable, so the reinforcement measures should be done before panel face is 10 m distance to the roadway. The face must be protected more, especially 5 m from the roadway, to avoid large area damage due to unstable coal pillar.

Fig. 10. Abutment pressure distribution in different advance distance.

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Fig. 11. Front abutment pressure distribution in different advance distance.

Table 2 Front abutment pressure characteristics in the panel. Gap between face and roadway (m)

Abutment pressure peak value (MPa)

Stress concentration coefficient

Distance of peak value point between coal wall (m)

Roadway stress concentration factor

70 50 30 20 10 5

18.09 18.99 20.23 23.25 25.75 15.98

2.26 2.37 2.54 2.91 3.22 1.98

5 5-6 5-6 5-6 5 3

1.16 1.25 1.43 1.67 2.04 2.80

4. Strength analysis of face and roadway coal seam When face advances close to the roadway, the coal panel between the roadway and face are considered as a process of pillar retreat. According to a previous section of the paper, it is known that with the decrease of pillar width, abutment pressure in the pillar is increasing first and decreasing after. The pillar strength is decreasing with the size smaller. According to Bienawski, the coal pillar strength empirical formula is [13]:

  B R ¼ RC 0:64 þ 0:36 h

ð9Þ

where R is the coal pillar strength; RC is the compressive strength of coal pillar; B is the coal pillar width; and h is the coal seam thickness. The coal pillar strength with different size can be calculated. So the pillar strength, stress peak value inside the coal pillar and stress concentration coefficient are shown in Table 3 and Fig. 12. According to Fig. 12 and Table 3: (1) The pillar strength changes linearly in different pillar width, and the peak value of the abutment pressure inside the coal pillar and stress concentration factor show a up and down trend with the decrease of coal pillar size. (2) When the pillar width is 10 m, the peak value of stress inside the coal pillar and stress concentration coefficient reaches the

Table 3 Stress peak value and pillar strength in different pillar widths. Coal pillar width (m)

Stress coefficient inside coal pillar

Stress peak value inside coal pillar (MPa)

Coal pillar strength (MPa)

70 50 30 20 15 10 5

2.26 2.37 2.54 2.91 2.98 3.22 1.98

18.09 18.99 20.23 23.25 23.87 25.75 15.98

101.92 75.18 48.43 35.06 28.38 21.69 15.01

Fig.12. Comparation with the pillar strength and stress peak value in different pillar width.

maximum; when the pillar width is less than 10 m, the peak value of stress decreases sharply. (3) When the pillar width is less than 13 m, the peak value of stress in the coal pillar will be greater than the pillar strength. Then the pillar is unstable and starts to yeild. So the coal face and roof need extra supports and reinforcement within this distance to prevent serious accident happened. 5. Conclusions (1) The results of theoretical analysis show that the influence range of front abutment pressure is 38.5 m in panel E13103, and it is between 40 and 45 m in numerical simulation. When the distance between the face and the roadway is more than 50 m, there are no interaction happened for both face and abandoned roadways. The peak locates at 4 to 5 m in front of the face and the value is below 19 MPa. (2) When the distance is less than 50 m, abutment pressure overlaps from both longwall face and abandoned roadway. As the distance is 20 m, the peak value of front abutment pressure increases sharply. When the distance is 10 m, front abutment pressure reaches maximum value. (3) When the distance is less than 13 m, the pillar load is greater than its ultimate strength, then the coal pillar will be unstable and in plastic state. At this time, reinforcement should be taken to strengthen the coal face and roof in time.

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(4) The research results provide certain theoretical basis for longwall mining passing through abandoned roadways in damage coal seam and also supply significant reference for mining in similar geological conditions in other areas. Acknowledgments The research is supported by National Key R&D Program of China (No. 2017YFC060300204), Yue Qi Young Scholar Project, CUMTB and Yue Qi Distinguished Scholar Project (No. 800015Z1138), China University of Mining & Technology, Beijing. References [1] Li Y. Overburden movement in solid waste rock cemented backfill mining methods. J China Coal Soc 2011;36(s2):370–4. [2] Li Y, Zhu EG, Zhang KN, Qi BD. Mining methods and roof caving mechanism in longwall mining through the abandoned gateroads of small mines. J China Coal Soc 2017;42(s1):16–23. [3] Wang H, Jiang YD, Zhao YX, Zhu J, Liu S. Numerical investigation of the dynamic mechanical state of a coal pillar during longwall mining panel extraction. Rock Mech Rock Eng 2013;46(5):1211–21.

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