- Email: [email protected]
Numerical simulation of overburden and surface movements for Wongawilli strip pillar mining
International Journal of Mining Science and Technology 26 (2016) 71–76
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Numerical simulation of overburden and surface movements for Wongawilli strip pillar mining Guo Wenbing a,b,⇑, Xu Feiya a a b
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China Synergism Innovative Center of Coal Safety Production in Henan Province, Jiaozuo 454000, China
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 10 August 2015 Accepted 30 September 2015 Available online 22 December 2015 Keywords: Strip pillar mining Wongawilli Surrounding rock Mining under structures Numerical simulation
a b s t r a c t The Wongawilli strip pillar mining technique, which combines the strip pillar mining layout and Wongawilli mining technology, is a new high efficient mining technology for mining under surface structures. The Wongawilli strip pillar mining technique was studied in this paper using theoretical analysis and numerical simulation. As an example, the geological and mining conditions of a coal mine were used to design the Wongawilli strip pillar plans, including the support parameters of the entries and the mining technology. In order to control the surrounding rocks and manage the roof effectively during coal mining, the stress fields, displacement fields and plastic zones were studied by numerical simulation. The stress fields, displacement fields, and plastic zones generated by Wongawilli strip pillar mining were obtained. And the surface movement and deformation were also determined after mining was completed and its effects on surface structures were analyzed and evaluated. The results demonstrate that it is feasible to mine under surface structures with the Wongawilli strip pillar mining technique. This mining method can protect the surface structures from damages. Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction There is a great amount of coal (about 140 billion tons) left unmined under the surface structures, water bodies, railways (referred to as the ‘‘three-body”). Coal mining under ‘‘threebody” especially that under surface structures has become a major problem in the mining area [1]. The key problem of mining under surface structures is to control overburden strata and surface movements [2]. At present, the major surface subsidence control methods are strip pillar mining, backfilling mining method, room and pillar mining, grouting of bed separations, harmonic mining, and so on [3]. Wongawilli mining method is a high-efficient shortwall mining method developed based on the room and pillar mining, and named after the first successful trial in Australia ‘‘Wongawilli” coal seam [4,5]. The room and pillar mining equipment such as continuous miner is employed in this method. Its major advantage is flexible face layout which can be used to mine the small wedge coal blocks and those that are difficult to be mined with retreat longwall mining method. It requires less capital investment, short lead time for production, flexible equipment
⇑ Corresponding author. Tel.: +86 13503915892. E-mail address: [email protected] (W. Guo).
operation and face move and high productivity [6]. It has been applied in some China’s coal mines [7–9]. The Wongawilli strip coal mining technology is a new coal mining method for mining under surface structures. It combines the strip mining longwall layout and Wongawilli high efficient mining technology. This technology can fully utilize the advantages of both the strip pillar and Wongawilli mining methods, and overcomes the disadvantages such as frequent face move, and low mining efficiency in strip pillar mining, and poor ventilation condition and poor long-term stability problems of coal pillars. In this paper, the Wongawilli strip mining method was designed for Wangtaipu Mine and its feasibility was studied using numerical modeling and surface subsidence prediction.
2. Geological and mining conditions At present, most of the No. 15 coal seam reserves totaling 377.07 million tons in Wangtaipu mine are located under surface structures. In order to mine the coal under the surface structures, the Wongawilli strip mining method was employed. An appropriate panel was selected for trial and the surface movement monitoring lines were set up. The trial area is located in panel XV2214 (east) (Fig. 1). In this area, the floor elevation of coal seam is from +634 to +646 m, the surface elevation ranges from +810 to
http://dx.doi.org/10.1016/j.ijmst.2015.11.013 2095-2686/Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
72
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76 XV2214 (east) tailgate 4.6 m 15.8 m 4.6 m
3
m
45°
Branch entry
2
64 4
500
3938
Branch entry
Gob
00 5355
00 5354
XV2306 panel
00 5356
64 2
m
3.
64 0
63 8
N
63 6
63 4
64 6
XV2214 east panel
m o m 8 .8 Ro
2.83 m 9.33 m
XV2307 panel
Fig. 1. Panel location where the Wongawilli strip pillar mining was employed.
+817.5 m, and the average depth of coal seam is about 174 m. The average panel length is 282 m and average panel width is 73.5 m. The seam dips 1–3°, and is 1.8–2.5 m thick with an average of 2.15 m. The roof and floor rock characteristics are shown in Table 1.
20 m
25 m
20 m
XV2214 (east) headgate
Fig. 2. Layout of Wongawilli strip pillar mining.
3. Wongawilli strip mining layout Cable prestress 100 kN Cable prestress 100 kN
The XV2214 (east) panel in Wangtaipu mine is a longwall face. The mining plan employs both the strip pillar and Wongawilli mining methods in order to achieve high efficient coal mining under surface structures. Firstly, the design of strip pillar mining was determined based on the geological and mining conditions. The strip width is 25 m and the pillar width is 20 m. Secondly, the mining technology is determined. And the branch entry and rooms were driven with the continuous miner. In strip mining, two branch entries were used. In each branch entry, rooms were driven on the inside, i.e., double branch entry and single side cutting. The branch entry is 2.5 m high by 4.6 m wide. The room is 3.3 m wide and 8.8 m deep, with an inclination angle of 45° (Fig. 2).
Bolt φ20 mm×2000 mm 300 mm 300 mm Bolt 900 mm φ18 mm×1800 mm 1800 mm 900 mm
Bolt φ20 mm×2000 mm
500 mm
Bolt φ18 mm×1800 mm 1000 mm 1000 mm 1000 mm 300 mm
500 mm
100 mm
Center line
2500 mm
3.1. Mining plan and entry layout
500 mm 4400 mm 4600 mm
Fig. 3. Entry support of the Wongawilli method (rooms are on the right side).
3.2. Entry support design The branch entry is rectangular. They were supported by a combination of roof bolts and cable bolts. The rooms were not supported (Fig. 3). The roof bolts were 20 mm in diameter by 2 m long. The anchoring portion consisted of two different types of grouting agents; one was CK2335 slow acting agent and the other was CK2360, an ultrafast grouting agent. The diameter of the hole was 30 mm. The bolt row spacing was 1 m. Each row had 4 bolts. The cable row spacing near the solid coal side was 3 m, while near the mining side was 1 m. The distance from the cable to the solid coal side was 300 mm. The cable, made of high strength prestressed steel strand, was 15.24 mm in diameter and 6.7 m long. A K2335 quick anchoring agent and two Z2360 medium resin anchorage agents were used. The bolts used in the branch entries were 18 mm in diameter and 1.8 m long. Two anchoring agents a CK2335 and a CK2360 for drill hole diameter 28 mm were used.
3.3. Face equipment According to the coal seam geological conditions and the equipment specifications, an intermittent face transportation mode was used, because it is more flexible and appropriate. The coal mining process is ‘‘continuous miner-shuttle car-crusher-belt conveyor”. The main equipment is shown in Table 2. 3.4. Development of branch entry and retreat mining This mining process includes branch entry development and room mining. 3.4.1. Branch entry development The two branch entries are driven at the same time. It includes 5 steps: coal cutting, coal loading, coal transportation, coal cleaning
Table 1 Roof and floor of the coal seam. Roof and floor
Rock
Thickness (m)
Main roof Immediate roof Immediate floor
Limestone
9.820
Mudstone
9.670
Main floor
Limestone
20.79
Characteristics of rock Gray, blocky, thin chert layer in the upper part, well-developed fractures at the bottom, containing a large number of Reichelina, a small amount of fossils, the bottom is clayey limestone Light gray, top is light black blocky. The center part contains a small amount of clay. The lower part is imbedded with lumpy pyrite. At the bottom, there is a 0.15 m ‘‘Shanxi type iron” Gray, slightly whitish, massive, micro fractures are well-developed, a lot of calcite veins, the top is imbedded with lumpy pyrite
73
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76 Table 2 Equipments for Wongawilli mining. No.
Equipment
Specification and model
Quantity
Remark
1 2 3 4 5
Continuous miner Shuttle car Four-arms roof bolter Anti-explosive diesel scraper Continuous haulage
EML340-1600/3500 SC10 ZMF20-4 WJ-4FBA(B) LY2000/980-10
1 1 1 1 1
Development, mining Transportation Support Supplementary operation Transferring, breaking
and machines transferring. The continuous miner advances 6 m per cycle considering the geological conditions of Wangtaipu Mine. In preparation for the continuous miner’s cut, the operator steers the miner to the left side rib of the entry, center the cut using the laser sensor. The operator then raises the cutting drum, cutting forward and moves down to the floor. After cutting the bottom coal, trimming the entry floor flat. The 1st cut is completed and the miner starts to back out, which is ‘‘sumping”. It then starts the 2nd cut immediately next to the 1st cut to widen the opening to the designed width. This is known as the ‘‘winning block”. During coal cutting, the coal loading procedure includes three steps— the coal falls into the collection head, where the loading arms work continuously to feed the coal onto the chain conveyor at the center of the continuous miner. The conveyor transports the coal to the shuttle car at the rear of the continuous miner where it dumps on the shuttle car. The shutter car then transports the coal to the crusher located at the belt tail end. 3.4.2. Mining in the room After the development of the branch entry, room mining to recover coal pillars can be carried out by the double entry single wing mode. The continuous miner cuts coal from the end of the branch entry at 45° to the branch entry. The room cut is 8.8 m deep by 3.3 m wide by 2.5 m high, leaving a 2 m pillar between two adjacent cuts. The ‘‘three eight-hour shift system” operation mode was used; 2 shifts for production and 1 shift for maintenance. The production shift consisted of 12 people, while the maintenance shift was 9 people. The daily average production was 994 ton. Productivity was 30.12 t/day in entry development while the daily average production was 1275 ton, and productivity was 38.64 t/day in room mining. After mining, the branch entry was timely sealed on both sides. The gob was left to cave in.
4. Numerical simulation of Wongawilli strip pillar mining Based on the mining design and geological conditions, the surrounding rock stress distribution, entry displacement and plastic zone distribution were analyzed by numerical simulation. The results provide a guideline for future mining design and entry supports [10,11]. 4.1. Rock mechanics properties The rock mechanics properties for the simulation are shown Table 3. The roof strata are in ascending order limestone, mudstone, limestone, and fine sandstone, while the floor strata are thick bauxitic mudstone and limestone. In the numerical simulation, only thicker strata were considered, the thinner strata were combined with the similar thicker strata. 4.2. Entry support parameters The two branch entries were supported by roof bolts and cable bolts [12]. The roof bolts and cable bolts were simulated by coupling the cable structure and the surrounding rock in the numerical simulation. The support parameters are shown in Table 4. 4.3. Boundary condition of the model Owing to the boundary effects of the model on the entry, the model size is 65 m 40 m 96 m. It is a plane strain model. ANSYS software was used to construct the mesh and then imported to the FLAC3D for computation. The gravity stress is imposed to the upper boundary of the model. The displacements of X and Y directions are limited to the horizontal boundary of the model and the Z
Table 3 Rock mechanics properties of the surrounding rocks. Rock
Bulk modulus (GPa) Shear modulus (GPa) Tensile strength (MPa) Cohesion (MPa) Internal friction angle (°) Density (kg/m3) Thickness (m)
Fine sandstone Limestone Mudstone Limestone Coal Bauxitic mudstone Limestone
4.40 4.89 2.14 4.89 0.85 2.18 6.17
2.64 3.22 1.47 3.22 0.48 1.52 3.53
1.26 1.08 0.65 1.28 0.32 0.59 1.69
0.45 0.25 0.35 0.45 0.18 0.26 0.30
25 24 23 25 21 20 26
2717 2664 2589 2717 1447 2633 2717
3.870 4.300 2.150 8.330 2.700 7.710 15.000
Table 4 Parameters of the roof bolts and cable bolts. Diameter of bolt (cable) Length of bolt (cable) (mm) (mm) Bolt
20.00 18.00 Cable 15.24
2000 1800 8000
Pretension (kN)
Elasticity modulus of bolt (cable) (GPa)
Tensile strength of bolt (cable) (kN)
Bulk modulus (GPa)
Shear modulus (10 5)
60.0 60.0 100.0
200 163 195
210.5 175.0 260.0
14.3 13.3 12.7
2.88 2.65 2.56
74
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
direction is fixed. The model and inner mesh are as shown in Figs. 4 and 5 respectively. 7 3 2 1 6 8 9 4 5
Fig. 4. Numerical simulation model.
7 16 15 14 13 12 11 19 20 21 22 23 24 8
Fig. 5. Inner mesh of the model.
4.4. Analysis of simulation results 4.4.1. Analysis of stress field Two models were simulated, that is, one branch entry and two branch entries mining. The maximum major principal stress in the surrounding coal and rock mass after mining is shown in Fig. 6. Fig. 6a shows that after mining at left branch entry and the room is not supported, the maximum major principal stress moves to the wider coal pillar at the model center and the large irregular coal pillar at the center remains stable; Fig. 6b presents that after mining at right branch entry, the small irregular coal pillar at the center may be unstable.
4.4.2. Analysis of displacement field The horizontal displacement after one branch entry and two branch entries mining is shown in Fig. 7. In Fig. 7a, after mining at left branch entry, the displacement in the two ribs of branch entry is about 10 mm. The displacement of the irregular coal pillar on the left side is about 60.95 mm; as shown in Fig. 7b, after room mining at right side, the horizontal displacements of surrounding rock and coal further increased. The displacement of the opposite corners of the irregular large coal pillar at the center is essentially symmetric, but the left side one is slightly larger. Because the left is mined earlier, the mining effects mainly act on the left. The stress and displacement in the surrounding rock are not symmetrical owing to different mining time.
-5.4953e+006 to -5.0000e+006 -5.0000e+006 to -4.0000e+006 -4.0000e+006 to -3.0000e+006 -3.0000e+006 to -2.0000e+006 -2.0000e+006 to -1.0000e+006 -1.0000e+006 to 0.0000e+000 0.0000e+000 to 1.9265e+004 (a) After mining at left branch entry
-5.2753e+006 to -5.0000e+006 -5.0000e+006 to -4.0000e+006 -4.0000e+006 to -3.0000e+006 -3.0000e+006 to -2.0000e+006 -2.0000e+006 to -1.0000e+006 -1.0000e+006 to 0.0000e+000 0.0000e+000 to 1.4416e+005 (b) After mining at both branch entry
Fig. 6. Maximum main stress in the surrounding rocks.
-6.0945e-005 to -6.0000e-005 -6.0000e-005 to -5.0000e-005 -5.0000e-005 to -4.0000e-005 -4.0000e-005 to -3.0000e-005 -3.0000e-005 to -2.0000e-005 -2.0000e-005 to -1.0000e-005 -1.0000e-005 to 0.0000e+000 0.0000e+000 to 1.0000e-005 1.0000e-005 to 2.0000e-005 2.0000e-005 to 2.1153e-005 (a) After mining at left branch entry
-7.7451e-005 to -7.0000e-005 -7.0000e-005 to -6.0000e-005 -6.0000e-005 to -5.0000e-005 -5.0000e-005 to -4.0000e-005 -4.0000e-005 to -3.0000e-005 -3.0000e-005 to -2.0000e-005 -2.0000e-005 to -1.0000e-005 -1.0000e-005 to 0.0000e+000 0.0000e+000 to 1.0000e-005 1.0000e-005 to 2.0000e-005 2.0000e-005 to 2.4778e-005 (b) After mining at both branch entry
Fig. 7. Horizontal displacement after mining.
75
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p Tension-n shear-p tension-p
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-p Tension-n shear-p Tension-n shear-p tension-p (a) After mining at left branch entry
(b) After mining at both branch entry
Fig. 8. Plastic zones after mining.
Table 5 Prediction parameters of the surface subsidence. Plan
Average thickness (m)
Subsidence factor q
Offset of inflection point s
Tangent of major influence angle tan b
Propagation angle h (°)
Displacement factor b
Longwall mining Strip mining
1.7 1.7
0.77 0.07
0.05H 0
2.3 1.8
88.5 88.5
0.30 0.23
250
50
10
50
100
30 45
200
35 40
Coal pillar
55
Coal pillar
50
50 45 40 35 30 25 20 15
150
10
25
Coal pillar
Coal pillar
100
35 40
Coal pillar
30 25
15 20
Coal pillar
10
150
0
15
20
200
20 15 25 10
250
300
350
400
450
Fig. 9. Surface subsidence contour.
4.4.3. Analysis of plastic zone The stress will redistribute after entry developments and reaches a new equilibrium under the supporting structure and surrounding rocks. The plastic zone will form in the surrounding rocks. Both theory and practice have proven that the plastic zone size in the surrounding rocks is an important factor in entry stability. The distribution of plastic zone after one branch entry and two branch entry mining is shown in Fig. 8. As shown in Fig. 8a, after room mining at left branch entry, the plastic zone is small and the entry support is adequate; in Fig. 8b, after mining at right branch entry the plastic cells of the center pillar increases. The stress concentrates on the center area of coal pillar. However, some parts at the center of the pillar l are still intact. This is good for safety management during mining. Comparative analysis of the stress field, displacement field and plastic zone in the surrounding rocks shows that the roof bolts and cable bolts support structures in the branch entry changes the state of stress in the surrounding rock to some extent. The residual strength of the plastic surrounding rock is improved.
5. Analysis and prediction of surface movement and deformation 5.1. Prediction parameters of surface movement First, based on the special characteristics of the overburden strata and the geological and mining conditions of the area, the surface movement prediction parameters are determined when longwall mining is used, followed by the determination of the parameters for Wongawilli strip pillar mining based on the strip mining width, pillar width and recovery rate. The prediction parameters of surface movement and deformation of the Wongawilli strip mining are as shown in Table 5 [13]. 5.2. Prediction results and analysis Based on the prediction parameters for the Wongawilli strip pillar mining method, surface movement and deformation are predicted, from which the contour maps of surface subsidence,
76
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
Table 6 Maximum movement and deformation of the surface. Maximum subsidence (mm)
Maximum slope (mm/m)
Maximum displacement (mm)
58.1
0.6
18.2
horizontal displacement, surface slope, and surface horizontal strain are derived and plotted. The surface subsidence contour is shown in Fig. 9, and the units in Fig. 9 is mm. The maximal surface movement and deformation after mining are shown in Table 6. From Table 6 and the damage standard of masonry structures in China, the surface movement and deformation, after the Wongawilli strip pillar mining, are smaller than the damage grade I of masonry structures. So this method is feasible for mining under structures. 6. Conclusions Based on the geological and mining conditions, the Wongawilli strip pillar mining plans were designed. And the supporting parameters of the branch entries and the mining technology were determined. Based on the numerical simulation analysis, the stress fields, displacement fields, and plastic zones generated by Wongawilli strip pillar mining were studied. The surface movement and deformation after mining were predicted. The damage degree of surface structures are within grade I specified in Chinese masonry structures. The Wongawilli strip pillar mining technique, which combines the strip pillar mining layout and Wongawilli mining technology, is a new high efficient mining technology for mining under surface structures. The results demonstrate that it is feasible to mine coal reserves under surface structures with this method. Acknowledgments This project is sponsored by the National Natural Science Foundation of China (No. 51374092). The authors acknowledge Dr. Syd
Maximum horizontal strain (mm/m) 0.5
+0.2
S. Peng’s instructive suggestions and valuable comments on the writing of this paper. References [1] Guo WB, Chai HB. Coal mining damages and protection. Beijing: China Coal Industry Press; 2008. p. 50–87. [2] Qian MG, Miao XX, Xu JL. Green mining of coal resources harmonizing with environment. J China Coal Soc 2007;32(1):1–7. [3] Tesarik DR, Seymour JB, Yanske TR. Long-term stability of a backfilled roomand-pillar test section at the Buick mine, Missouri, USA. Int J Rock Mech Min Sci 2007;47(7):1182–96. [4] Liu KG, Xu JH, Miao XX. Short wall mining technology and its applications. Beijing: China Coal Industry Press; 2007. p. 60–97. [5] Peng SS. Longwall mining. America: Printed in the United States of America; 2006. p. 401–29. [6] Zhou AP. Roof control technology for Wongawilli coal mining method. Coal Sci Technol 2006;34(7):46–9. [7] Lu ZF, Wang A, Ma MS. Application of Wongawilli mining technology in Daliuta coal mine. Coal Sci Technol 2000;28(12):1–4. [8] Liang DH, Zhang Z. Application of EML340 continuous mining cutter in Wongawilli mining method. Coal Min Technol 2010;15(6):65–7. [9] Li HD. Application of Wongawilli mining method with hydraulic powered support for roof control to Daliuta mine. Coal Sci Technol 2008;36(8):15–7. [10] Su JP, Li RQ, Li DY. Study on shallow seam strata behavior numerical simulation of Wongawilli mining method. Coal Eng 2000;10:60–2. [11] Singh AK, Singh R, Maiti J. Assessment of mining induced stress development over coal pillars during depillaring. Int J Rock Mech Min Sci 2011;48 (5):805–18. [12] Kushwaha A, Singh SK, Tewari S. Empirical approach for designing of support system in mechanized coal pillar mining. Int J Rock Mech Min Sci 2010;47 (7):1063–78. [13] Guo WB, Deng KZ, Zou YF. Study on surface movement parameters of strip pillar mining. J China Coal Soc 2005;30(3):596–600.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Numerical simulation of overburden and surface movements for Wongawilli strip pillar mining Guo Wenbing a,b,⇑, Xu Feiya a a b
School of Energy Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China Synergism Innovative Center of Coal Safety Production in Henan Province, Jiaozuo 454000, China
a r t i c l e
i n f o
Article history: Received 9 June 2015 Received in revised form 10 August 2015 Accepted 30 September 2015 Available online 22 December 2015 Keywords: Strip pillar mining Wongawilli Surrounding rock Mining under structures Numerical simulation
a b s t r a c t The Wongawilli strip pillar mining technique, which combines the strip pillar mining layout and Wongawilli mining technology, is a new high efficient mining technology for mining under surface structures. The Wongawilli strip pillar mining technique was studied in this paper using theoretical analysis and numerical simulation. As an example, the geological and mining conditions of a coal mine were used to design the Wongawilli strip pillar plans, including the support parameters of the entries and the mining technology. In order to control the surrounding rocks and manage the roof effectively during coal mining, the stress fields, displacement fields and plastic zones were studied by numerical simulation. The stress fields, displacement fields, and plastic zones generated by Wongawilli strip pillar mining were obtained. And the surface movement and deformation were also determined after mining was completed and its effects on surface structures were analyzed and evaluated. The results demonstrate that it is feasible to mine under surface structures with the Wongawilli strip pillar mining technique. This mining method can protect the surface structures from damages. Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction There is a great amount of coal (about 140 billion tons) left unmined under the surface structures, water bodies, railways (referred to as the ‘‘three-body”). Coal mining under ‘‘threebody” especially that under surface structures has become a major problem in the mining area [1]. The key problem of mining under surface structures is to control overburden strata and surface movements [2]. At present, the major surface subsidence control methods are strip pillar mining, backfilling mining method, room and pillar mining, grouting of bed separations, harmonic mining, and so on [3]. Wongawilli mining method is a high-efficient shortwall mining method developed based on the room and pillar mining, and named after the first successful trial in Australia ‘‘Wongawilli” coal seam [4,5]. The room and pillar mining equipment such as continuous miner is employed in this method. Its major advantage is flexible face layout which can be used to mine the small wedge coal blocks and those that are difficult to be mined with retreat longwall mining method. It requires less capital investment, short lead time for production, flexible equipment
⇑ Corresponding author. Tel.: +86 13503915892. E-mail address: [email protected] (W. Guo).
operation and face move and high productivity [6]. It has been applied in some China’s coal mines [7–9]. The Wongawilli strip coal mining technology is a new coal mining method for mining under surface structures. It combines the strip mining longwall layout and Wongawilli high efficient mining technology. This technology can fully utilize the advantages of both the strip pillar and Wongawilli mining methods, and overcomes the disadvantages such as frequent face move, and low mining efficiency in strip pillar mining, and poor ventilation condition and poor long-term stability problems of coal pillars. In this paper, the Wongawilli strip mining method was designed for Wangtaipu Mine and its feasibility was studied using numerical modeling and surface subsidence prediction.
2. Geological and mining conditions At present, most of the No. 15 coal seam reserves totaling 377.07 million tons in Wangtaipu mine are located under surface structures. In order to mine the coal under the surface structures, the Wongawilli strip mining method was employed. An appropriate panel was selected for trial and the surface movement monitoring lines were set up. The trial area is located in panel XV2214 (east) (Fig. 1). In this area, the floor elevation of coal seam is from +634 to +646 m, the surface elevation ranges from +810 to
http://dx.doi.org/10.1016/j.ijmst.2015.11.013 2095-2686/Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
72
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76 XV2214 (east) tailgate 4.6 m 15.8 m 4.6 m
3
m
45°
Branch entry
2
64 4
500
3938
Branch entry
Gob
00 5355
00 5354
XV2306 panel
00 5356
64 2
m
3.
64 0
63 8
N
63 6
63 4
64 6
XV2214 east panel
m o m 8 .8 Ro
2.83 m 9.33 m
XV2307 panel
Fig. 1. Panel location where the Wongawilli strip pillar mining was employed.
+817.5 m, and the average depth of coal seam is about 174 m. The average panel length is 282 m and average panel width is 73.5 m. The seam dips 1–3°, and is 1.8–2.5 m thick with an average of 2.15 m. The roof and floor rock characteristics are shown in Table 1.
20 m
25 m
20 m
XV2214 (east) headgate
Fig. 2. Layout of Wongawilli strip pillar mining.
3. Wongawilli strip mining layout Cable prestress 100 kN Cable prestress 100 kN
The XV2214 (east) panel in Wangtaipu mine is a longwall face. The mining plan employs both the strip pillar and Wongawilli mining methods in order to achieve high efficient coal mining under surface structures. Firstly, the design of strip pillar mining was determined based on the geological and mining conditions. The strip width is 25 m and the pillar width is 20 m. Secondly, the mining technology is determined. And the branch entry and rooms were driven with the continuous miner. In strip mining, two branch entries were used. In each branch entry, rooms were driven on the inside, i.e., double branch entry and single side cutting. The branch entry is 2.5 m high by 4.6 m wide. The room is 3.3 m wide and 8.8 m deep, with an inclination angle of 45° (Fig. 2).
Bolt φ20 mm×2000 mm 300 mm 300 mm Bolt 900 mm φ18 mm×1800 mm 1800 mm 900 mm
Bolt φ20 mm×2000 mm
500 mm
Bolt φ18 mm×1800 mm 1000 mm 1000 mm 1000 mm 300 mm
500 mm
100 mm
Center line
2500 mm
3.1. Mining plan and entry layout
500 mm 4400 mm 4600 mm
Fig. 3. Entry support of the Wongawilli method (rooms are on the right side).
3.2. Entry support design The branch entry is rectangular. They were supported by a combination of roof bolts and cable bolts. The rooms were not supported (Fig. 3). The roof bolts were 20 mm in diameter by 2 m long. The anchoring portion consisted of two different types of grouting agents; one was CK2335 slow acting agent and the other was CK2360, an ultrafast grouting agent. The diameter of the hole was 30 mm. The bolt row spacing was 1 m. Each row had 4 bolts. The cable row spacing near the solid coal side was 3 m, while near the mining side was 1 m. The distance from the cable to the solid coal side was 300 mm. The cable, made of high strength prestressed steel strand, was 15.24 mm in diameter and 6.7 m long. A K2335 quick anchoring agent and two Z2360 medium resin anchorage agents were used. The bolts used in the branch entries were 18 mm in diameter and 1.8 m long. Two anchoring agents a CK2335 and a CK2360 for drill hole diameter 28 mm were used.
3.3. Face equipment According to the coal seam geological conditions and the equipment specifications, an intermittent face transportation mode was used, because it is more flexible and appropriate. The coal mining process is ‘‘continuous miner-shuttle car-crusher-belt conveyor”. The main equipment is shown in Table 2. 3.4. Development of branch entry and retreat mining This mining process includes branch entry development and room mining. 3.4.1. Branch entry development The two branch entries are driven at the same time. It includes 5 steps: coal cutting, coal loading, coal transportation, coal cleaning
Table 1 Roof and floor of the coal seam. Roof and floor
Rock
Thickness (m)
Main roof Immediate roof Immediate floor
Limestone
9.820
Mudstone
9.670
Main floor
Limestone
20.79
Characteristics of rock Gray, blocky, thin chert layer in the upper part, well-developed fractures at the bottom, containing a large number of Reichelina, a small amount of fossils, the bottom is clayey limestone Light gray, top is light black blocky. The center part contains a small amount of clay. The lower part is imbedded with lumpy pyrite. At the bottom, there is a 0.15 m ‘‘Shanxi type iron” Gray, slightly whitish, massive, micro fractures are well-developed, a lot of calcite veins, the top is imbedded with lumpy pyrite
73
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76 Table 2 Equipments for Wongawilli mining. No.
Equipment
Specification and model
Quantity
Remark
1 2 3 4 5
Continuous miner Shuttle car Four-arms roof bolter Anti-explosive diesel scraper Continuous haulage
EML340-1600/3500 SC10 ZMF20-4 WJ-4FBA(B) LY2000/980-10
1 1 1 1 1
Development, mining Transportation Support Supplementary operation Transferring, breaking
and machines transferring. The continuous miner advances 6 m per cycle considering the geological conditions of Wangtaipu Mine. In preparation for the continuous miner’s cut, the operator steers the miner to the left side rib of the entry, center the cut using the laser sensor. The operator then raises the cutting drum, cutting forward and moves down to the floor. After cutting the bottom coal, trimming the entry floor flat. The 1st cut is completed and the miner starts to back out, which is ‘‘sumping”. It then starts the 2nd cut immediately next to the 1st cut to widen the opening to the designed width. This is known as the ‘‘winning block”. During coal cutting, the coal loading procedure includes three steps— the coal falls into the collection head, where the loading arms work continuously to feed the coal onto the chain conveyor at the center of the continuous miner. The conveyor transports the coal to the shuttle car at the rear of the continuous miner where it dumps on the shuttle car. The shutter car then transports the coal to the crusher located at the belt tail end. 3.4.2. Mining in the room After the development of the branch entry, room mining to recover coal pillars can be carried out by the double entry single wing mode. The continuous miner cuts coal from the end of the branch entry at 45° to the branch entry. The room cut is 8.8 m deep by 3.3 m wide by 2.5 m high, leaving a 2 m pillar between two adjacent cuts. The ‘‘three eight-hour shift system” operation mode was used; 2 shifts for production and 1 shift for maintenance. The production shift consisted of 12 people, while the maintenance shift was 9 people. The daily average production was 994 ton. Productivity was 30.12 t/day in entry development while the daily average production was 1275 ton, and productivity was 38.64 t/day in room mining. After mining, the branch entry was timely sealed on both sides. The gob was left to cave in.
4. Numerical simulation of Wongawilli strip pillar mining Based on the mining design and geological conditions, the surrounding rock stress distribution, entry displacement and plastic zone distribution were analyzed by numerical simulation. The results provide a guideline for future mining design and entry supports [10,11]. 4.1. Rock mechanics properties The rock mechanics properties for the simulation are shown Table 3. The roof strata are in ascending order limestone, mudstone, limestone, and fine sandstone, while the floor strata are thick bauxitic mudstone and limestone. In the numerical simulation, only thicker strata were considered, the thinner strata were combined with the similar thicker strata. 4.2. Entry support parameters The two branch entries were supported by roof bolts and cable bolts [12]. The roof bolts and cable bolts were simulated by coupling the cable structure and the surrounding rock in the numerical simulation. The support parameters are shown in Table 4. 4.3. Boundary condition of the model Owing to the boundary effects of the model on the entry, the model size is 65 m 40 m 96 m. It is a plane strain model. ANSYS software was used to construct the mesh and then imported to the FLAC3D for computation. The gravity stress is imposed to the upper boundary of the model. The displacements of X and Y directions are limited to the horizontal boundary of the model and the Z
Table 3 Rock mechanics properties of the surrounding rocks. Rock
Bulk modulus (GPa) Shear modulus (GPa) Tensile strength (MPa) Cohesion (MPa) Internal friction angle (°) Density (kg/m3) Thickness (m)
Fine sandstone Limestone Mudstone Limestone Coal Bauxitic mudstone Limestone
4.40 4.89 2.14 4.89 0.85 2.18 6.17
2.64 3.22 1.47 3.22 0.48 1.52 3.53
1.26 1.08 0.65 1.28 0.32 0.59 1.69
0.45 0.25 0.35 0.45 0.18 0.26 0.30
25 24 23 25 21 20 26
2717 2664 2589 2717 1447 2633 2717
3.870 4.300 2.150 8.330 2.700 7.710 15.000
Table 4 Parameters of the roof bolts and cable bolts. Diameter of bolt (cable) Length of bolt (cable) (mm) (mm) Bolt
20.00 18.00 Cable 15.24
2000 1800 8000
Pretension (kN)
Elasticity modulus of bolt (cable) (GPa)
Tensile strength of bolt (cable) (kN)
Bulk modulus (GPa)
Shear modulus (10 5)
60.0 60.0 100.0
200 163 195
210.5 175.0 260.0
14.3 13.3 12.7
2.88 2.65 2.56
74
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
direction is fixed. The model and inner mesh are as shown in Figs. 4 and 5 respectively. 7 3 2 1 6 8 9 4 5
Fig. 4. Numerical simulation model.
7 16 15 14 13 12 11 19 20 21 22 23 24 8
Fig. 5. Inner mesh of the model.
4.4. Analysis of simulation results 4.4.1. Analysis of stress field Two models were simulated, that is, one branch entry and two branch entries mining. The maximum major principal stress in the surrounding coal and rock mass after mining is shown in Fig. 6. Fig. 6a shows that after mining at left branch entry and the room is not supported, the maximum major principal stress moves to the wider coal pillar at the model center and the large irregular coal pillar at the center remains stable; Fig. 6b presents that after mining at right branch entry, the small irregular coal pillar at the center may be unstable.
4.4.2. Analysis of displacement field The horizontal displacement after one branch entry and two branch entries mining is shown in Fig. 7. In Fig. 7a, after mining at left branch entry, the displacement in the two ribs of branch entry is about 10 mm. The displacement of the irregular coal pillar on the left side is about 60.95 mm; as shown in Fig. 7b, after room mining at right side, the horizontal displacements of surrounding rock and coal further increased. The displacement of the opposite corners of the irregular large coal pillar at the center is essentially symmetric, but the left side one is slightly larger. Because the left is mined earlier, the mining effects mainly act on the left. The stress and displacement in the surrounding rock are not symmetrical owing to different mining time.
-5.4953e+006 to -5.0000e+006 -5.0000e+006 to -4.0000e+006 -4.0000e+006 to -3.0000e+006 -3.0000e+006 to -2.0000e+006 -2.0000e+006 to -1.0000e+006 -1.0000e+006 to 0.0000e+000 0.0000e+000 to 1.9265e+004 (a) After mining at left branch entry
-5.2753e+006 to -5.0000e+006 -5.0000e+006 to -4.0000e+006 -4.0000e+006 to -3.0000e+006 -3.0000e+006 to -2.0000e+006 -2.0000e+006 to -1.0000e+006 -1.0000e+006 to 0.0000e+000 0.0000e+000 to 1.4416e+005 (b) After mining at both branch entry
Fig. 6. Maximum main stress in the surrounding rocks.
-6.0945e-005 to -6.0000e-005 -6.0000e-005 to -5.0000e-005 -5.0000e-005 to -4.0000e-005 -4.0000e-005 to -3.0000e-005 -3.0000e-005 to -2.0000e-005 -2.0000e-005 to -1.0000e-005 -1.0000e-005 to 0.0000e+000 0.0000e+000 to 1.0000e-005 1.0000e-005 to 2.0000e-005 2.0000e-005 to 2.1153e-005 (a) After mining at left branch entry
-7.7451e-005 to -7.0000e-005 -7.0000e-005 to -6.0000e-005 -6.0000e-005 to -5.0000e-005 -5.0000e-005 to -4.0000e-005 -4.0000e-005 to -3.0000e-005 -3.0000e-005 to -2.0000e-005 -2.0000e-005 to -1.0000e-005 -1.0000e-005 to 0.0000e+000 0.0000e+000 to 1.0000e-005 1.0000e-005 to 2.0000e-005 2.0000e-005 to 2.4778e-005 (b) After mining at both branch entry
Fig. 7. Horizontal displacement after mining.
75
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p Tension-n shear-p tension-p
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-p Tension-n shear-p Tension-n shear-p tension-p (a) After mining at left branch entry
(b) After mining at both branch entry
Fig. 8. Plastic zones after mining.
Table 5 Prediction parameters of the surface subsidence. Plan
Average thickness (m)
Subsidence factor q
Offset of inflection point s
Tangent of major influence angle tan b
Propagation angle h (°)
Displacement factor b
Longwall mining Strip mining
1.7 1.7
0.77 0.07
0.05H 0
2.3 1.8
88.5 88.5
0.30 0.23
250
50
10
50
100
30 45
200
35 40
Coal pillar
55
Coal pillar
50
50 45 40 35 30 25 20 15
150
10
25
Coal pillar
Coal pillar
100
35 40
Coal pillar
30 25
15 20
Coal pillar
10
150
0
15
20
200
20 15 25 10
250
300
350
400
450
Fig. 9. Surface subsidence contour.
4.4.3. Analysis of plastic zone The stress will redistribute after entry developments and reaches a new equilibrium under the supporting structure and surrounding rocks. The plastic zone will form in the surrounding rocks. Both theory and practice have proven that the plastic zone size in the surrounding rocks is an important factor in entry stability. The distribution of plastic zone after one branch entry and two branch entry mining is shown in Fig. 8. As shown in Fig. 8a, after room mining at left branch entry, the plastic zone is small and the entry support is adequate; in Fig. 8b, after mining at right branch entry the plastic cells of the center pillar increases. The stress concentrates on the center area of coal pillar. However, some parts at the center of the pillar l are still intact. This is good for safety management during mining. Comparative analysis of the stress field, displacement field and plastic zone in the surrounding rocks shows that the roof bolts and cable bolts support structures in the branch entry changes the state of stress in the surrounding rock to some extent. The residual strength of the plastic surrounding rock is improved.
5. Analysis and prediction of surface movement and deformation 5.1. Prediction parameters of surface movement First, based on the special characteristics of the overburden strata and the geological and mining conditions of the area, the surface movement prediction parameters are determined when longwall mining is used, followed by the determination of the parameters for Wongawilli strip pillar mining based on the strip mining width, pillar width and recovery rate. The prediction parameters of surface movement and deformation of the Wongawilli strip mining are as shown in Table 5 [13]. 5.2. Prediction results and analysis Based on the prediction parameters for the Wongawilli strip pillar mining method, surface movement and deformation are predicted, from which the contour maps of surface subsidence,
76
W. Guo, F. Xu / International Journal of Mining Science and Technology 26 (2016) 71–76
Table 6 Maximum movement and deformation of the surface. Maximum subsidence (mm)
Maximum slope (mm/m)
Maximum displacement (mm)
58.1
0.6
18.2
horizontal displacement, surface slope, and surface horizontal strain are derived and plotted. The surface subsidence contour is shown in Fig. 9, and the units in Fig. 9 is mm. The maximal surface movement and deformation after mining are shown in Table 6. From Table 6 and the damage standard of masonry structures in China, the surface movement and deformation, after the Wongawilli strip pillar mining, are smaller than the damage grade I of masonry structures. So this method is feasible for mining under structures. 6. Conclusions Based on the geological and mining conditions, the Wongawilli strip pillar mining plans were designed. And the supporting parameters of the branch entries and the mining technology were determined. Based on the numerical simulation analysis, the stress fields, displacement fields, and plastic zones generated by Wongawilli strip pillar mining were studied. The surface movement and deformation after mining were predicted. The damage degree of surface structures are within grade I specified in Chinese masonry structures. The Wongawilli strip pillar mining technique, which combines the strip pillar mining layout and Wongawilli mining technology, is a new high efficient mining technology for mining under surface structures. The results demonstrate that it is feasible to mine coal reserves under surface structures with this method. Acknowledgments This project is sponsored by the National Natural Science Foundation of China (No. 51374092). The authors acknowledge Dr. Syd
Maximum horizontal strain (mm/m) 0.5
+0.2
S. Peng’s instructive suggestions and valuable comments on the writing of this paper. References [1] Guo WB, Chai HB. Coal mining damages and protection. Beijing: China Coal Industry Press; 2008. p. 50–87. [2] Qian MG, Miao XX, Xu JL. Green mining of coal resources harmonizing with environment. J China Coal Soc 2007;32(1):1–7. [3] Tesarik DR, Seymour JB, Yanske TR. Long-term stability of a backfilled roomand-pillar test section at the Buick mine, Missouri, USA. Int J Rock Mech Min Sci 2007;47(7):1182–96. [4] Liu KG, Xu JH, Miao XX. Short wall mining technology and its applications. Beijing: China Coal Industry Press; 2007. p. 60–97. [5] Peng SS. Longwall mining. America: Printed in the United States of America; 2006. p. 401–29. [6] Zhou AP. Roof control technology for Wongawilli coal mining method. Coal Sci Technol 2006;34(7):46–9. [7] Lu ZF, Wang A, Ma MS. Application of Wongawilli mining technology in Daliuta coal mine. Coal Sci Technol 2000;28(12):1–4. [8] Liang DH, Zhang Z. Application of EML340 continuous mining cutter in Wongawilli mining method. Coal Min Technol 2010;15(6):65–7. [9] Li HD. Application of Wongawilli mining method with hydraulic powered support for roof control to Daliuta mine. Coal Sci Technol 2008;36(8):15–7. [10] Su JP, Li RQ, Li DY. Study on shallow seam strata behavior numerical simulation of Wongawilli mining method. Coal Eng 2000;10:60–2. [11] Singh AK, Singh R, Maiti J. Assessment of mining induced stress development over coal pillars during depillaring. Int J Rock Mech Min Sci 2011;48 (5):805–18. [12] Kushwaha A, Singh SK, Tewari S. Empirical approach for designing of support system in mechanized coal pillar mining. Int J Rock Mech Min Sci 2010;47 (7):1063–78. [13] Guo WB, Deng KZ, Zou YF. Study on surface movement parameters of strip pillar mining. J China Coal Soc 2005;30(3):596–600.