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Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face
International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
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Technical Note
Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face Y.M. Cheng a,n, J.A. Wang b, G.X. Xie c, W.B. Wei a a
Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Kong School of Civil and Environmental Engineering, University of Science and Technology, Beijing, PR China c Department of Resource Exploration and Management Engineering, Anhui University of Science and Technology, PR China b
a r t i c l e in f o Article history: Received 11 September 2009 Received in revised form 19 February 2010 Accepted 16 August 2010 Available online 9 September 2010
1. Introduction In underground coal mining, an increase in coal recovery rate can be achieved by a decrease in the size of the supporting coal pillars [1]. The design of the coal pillar is important for both the success and the effective operation of coal extraction. Pillar design studies have been carried out by many engineers and researchers, and many strength formulae and design methods have been proposed, and these are reviewed in [2–4]. Pillar formation in bord and pillar coal mining is required for (1) panel isolation, (2) protection of roadways, shafts and surface features, and (3) protection of mine workings from flooding and roof caves in the face area while depillaring [4]. Different methods are used for the design of the pillar for different situations or functions. A goaf is an extracted coal panel during coal mining. Here, we consider the pillar along the goaf, which is a chain pillar used for the protection of the roadway and the extraction panel and exists only along one side. Sheorey [4] has suggested three methods for chain pillar analysis and design: (i) choose a pillar strength formula, determine the average load (depending on one-sided or two-sided goaf, caving or stowing) and the width of pillar with a suitable safety factor; (ii) choose the width of pillar such that the gate road beyond is not much affected by the longwall or depillaring face movement; and (iii) perform numerical stress analysis with different chain pillar sizes and various design parameters, and the pillar size is defined by applying a suitable failure criterion to the seam. When the first two methods are selected, Wilson’s formula [5] and Sheorey’s formula [6] can be used, and these are derived from analysis of the abutment
n
Corresponding author. Tel.: +852 27666042; fax: + 852 23346389. E-mail address: [email protected] (Y.M. Cheng).
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pressure across the width of the pillar. When the third method is selected, it is not necessary to choose the strength or load formulae as an estimation of the pillar load and the strength is not required, but it is important to model the realistic constitutive model using suitable caved goaf properties with the adoption of a suitable failure criterion for the in situ seam. Numerical stress analysis of this problem has been done in [7–12]. Although the use of design formulae is more popular in chain pillar design and analysis, the use of numerical modeling is necessary for some complex and special situations, particularly when the excavation sequence is important for overall stability. Moreover, the use of design formulae relies on the assumption that the abutment pressure is not affected by the roadway position (Fig. 1a). From numerical simulation, Wei and Cheng [13] have shown that the distance of the roadway excavation from the goaf has some influence on the abutment stress distribution in the roof strata (Fig. 1b), which cannot be accounted for by simple design formulae. On the basis of a two-dimensional numerical simulation, Wei and Cheng [13] pointed out that an intermediate pillar width is not good for roadway stability, and this conclusion is similar to that reached by Whittaker and Singh [14], who recommended that a pillar 10–30 m wide is optimal for a gate road, because this range will give the highest gate road closure. Wilson [5] pointed out that a roadway placed in the highly stressed area of the ribside will suffer damage. Here, the pillar in a top-coal mining panel with an inclined thick coal seam is analyzed by numerical modeling. The 3D distribution of stress and failure zone is analyzed in detail, and the effects of the coal pillar shape are discussed. It is revealed that both the height and the width of the coal pillar are responsible for the stability of the roadway system. Finally, the results obtained from the present study have been used for a real project in Xieqiao with satisfactory outcome.
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abutment pressure
abutment pressure
roadway goaf
1373
roadway goaf
pillar
pillar
Fig. 1. The distribution of abutment pressure over the pillar and roadway along the goaf: (a) Abutment pressure not affected by roadway excavation and (b) Abutment pressure affected by roadway excavation.
1141(3)goaf coal pillar 4.6m
uphill roadway (ventilation tunnel) strike direction
1151(3) face
4.6m
231.8m
working face
downhill roadway (tramroad tunnel) Fig. 2. Plan of 1151(3) face.
Table 1 Mechanical properties of rock mass used in numerical modeling. Rock name
Density (kg/m3)
Elastic modulus (MPa)
Poisson ratio
Cohesion (Mpa)
Friction angle (deg)
Tensile strength (Mpa)
Roof strata Siltstone Mudstone Sandy mudstone Fine sandstone Sandstone
2460 2461 2510 2873 2487
1.95 104 0.875 104 0.5425 104 3.34 104 1.35 104
0.2 0.26 0.147 0.235 0.123
2.75 1.2 2.16 3.2 2.06
38 30 36 42 40
1.84 0.605 0.75 1.29 1.13
Coal 13#coal
1380
0.53 104
0.32
1.25
32
0.15
Floor strata Mudstone Siltstone Sandstone Sandy mudstone
2483 2460 2580 2530
1.77 104 1.95 104 2.5 104 1.085 104
0.204 0.2 0.159 0.147
1.2 3.75 2.5 2.45
32 38 42 40
0.58 1.84 3.6 2.01
The present analysis is based on the mining conditions of the 1151(3) mining panel in the Xieqiao colliery, Huainan City, PR China. Mining panel 1151(3) is a top-coal caving mining face, and on the north of this face is a mining panel 1141(3), where the coal has been extracted (Fig. 2). The elevation of the ground surface is from +20.4 to +25.8 m and the elevation of the working face is from –588 to –662 m. The mining face is 1674 m along the strike and 231.8 m along the dip. The average thickness of the coal stratum is 5.4 m and the dip angle is 131. The main roof is siltstone or fine sandstone with an average thickness of 6.2 m. The immediate roof is mudstone or sandy mudstone with an average thickness of 3.26 m. The immediate floor is mudstone and the thickness is about 1.5 m. The main floor is siltstone 2.8 m thick. The properties of the rock, which are based on laboratory tests and are consistent with the design parameters commonly adopted
for design in Xieqiao, are given in Table 1. In this analysis, six different coal pillar widths are considered: 3, 5, 7, 10, 15, and 20 m.
2. Numerical simulation model Six different 3D models were developed by FLAC3D for the project at Huainan City, and the pillar widths are 3, 5, 7, 10, 15 and 20 m. All the numerical models use a length of 500 m along the strike, a width of 600 m along the dip and a height of 214.17 m (Fig. 3). In these models, there are 87,898–91,036 zones and 103,531–107,134 grid-points. At the top of the models, a vertical load (p¼ gH) is applied to simulate the overburden weight. The elasto-plastic Mohr–Coulomb model with the
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
214.17m
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600m
500m
Fig. 3. Plot of the three-dimensional model mesh.
Table 2 Variation of mechanical properties of coal with plastic shear strain. Plastic shear strain Cohesion (MPa) Friction angle (deg)
0 1.25 32
0.0015 0.8 28
0.005 0.6 26
0.01 0.03 24
non-associated flow rule is used for the roof and floor strata. The strain softening Mohr–Coulomb model is used for the coal. The variation of cohesion and friction angle properties with plastic shear strain is shown in Table 2. The roadways in these models are 4.6 m wide and 2.7 m high. On the basis of statistical field measurement at Xieqiao, the at-rest pressure coefficients along the x and y directions are taken as 0.8 and 1.2, respectively. In this simulation, the caved area was filled by a very soft elastic material (the elastic modulus is very small) in order to consider the (very limited) supporting capability of the caved rock. The horizontal displacement is fixed at the lateral boundary while the vertical displacement is fixed at the bottom boundary. The modeling process is as follows: (1) calculating the initial state induced by the gravity; (2) modeling the excavation of panel 1141(3) step by step; (3) modeling the excavation of the uphill roadway and downhill roadway; and (4) modeling the excavation of panel 1151(3) step by step. During the process of modeling excavation panels 1141(3) and 1151(3), both the coal seam and immediate roof are excavated to simulate the caving of the roof during coal extraction. Then the cave area is filled with very soft elastic material. The ultimate elastic modulus of this material is 190 MPa and the Poisson ratio is 0.25. Fig. 4 shows the 3D view of the vertical stress distribution in the coal seam at panel 1151(3). In front of the mining face, there is a peak stress region at the upper corner of the panel that is superimposed by two parts: the stress induced by the adjacent extraction of the continuous longwall mining panel 1141(3) and the abutment pressure induced by the advancement of the mining face 1151(3). The pillar between mining faces 1141(3) and 1151(3) is the focus of this investigation.
3. Distribution of vertical stress along the strike with respect to pillar width The vertical stress for coal pillars of different widths along the strike is shown in Fig. 5. When the pillar width is 3 m, the vertical
stress in the pillar is relatively low and the stress in front of the working face is even lower than that at the back of the working face. For pillars of other widths, the peak stress is in front of the working face. When the pillar width is 5–20 m wide, the stress is relatively low near the working face and increases quickly in front of the working face. The vertical stress reaches a peak value at a distance between 5 and 20 m in front of the working face, and then decreases with increase in distance away from the working face. At the back of the working face, the stress increases slowly with increase in distance from the working face, and this is probably because the goaf is gradually filled by the caved rocks. For a pillar width within 5–15 m, the peak stress in the pillar increases gradually with the increase in pillar width. The peak stress is relatively constant for pillars 15–20 m wide. The distance from the peak stress to the working face decreases slowly with increase in width of the coal pillar. The vertical stress in the solid coal seam of the 1151(3) face along the strike with respect to different pillar widths is shown in Fig. 6. The stress is relatively low near the working face and increases quickly in front of the working face. The vertical stress reaches a peak value at a distance of 16–19 m in front of the working face. In general, the peak value of the vertical stress decreases gradually with increase in pillar width, except when the pillar width is 3 m or lesser. From these two figures, it can be seen that with increase in pillar width, the peak vertical stress in the coal pillar increases gradually, while the peak value in the solid coal seam of mining face 1151(3) decreases gradually. This indicates that with increase in pillar width, the load-bearing capacity is gradually improved, so the mining pressure is transferred gradually from the coal seam to the coal pillar.
4. Distribution of vertical stress along the dip with respect to pillar width The vertical stress in different cross-sections along the dip with respect to pillar width is shown in Fig. 7. There is less constraint to the 1151(3) coal seam, since the coal seam at the back of the working face has been extracted. For a section less than 11.25 m from the working face, the vertical stress is very low in the coal seam, while it is relatively high in the pillar, since there are more constraints at the back of the coal pillar. At sections further than 22.25 m away from the working face, the stress increases in the coal seam with a peak value as the constraint
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Fig. 4. The vertical stress distribution in near vicinity of the mining face 1151(3).
120
vertical stress (MPa)
100 80
3m 5m 7m 10 m 15 m 20 m
60 40 20 0 -100
-80
-60
-40
-20
0
20
40
60
80
100
distance (m) Fig. 5. The vertical stress for different pillar widths along the strike.
140
vertical stress (MPa)
120 3m 5 m 7m 10 m 15 m 20 m
100 80 60 40 20 0 -100 -80
-60
-40
-20 0 20 distance (m)
40
60
80
100
Fig. 6. The vertical stress in solid coal seam of 1151(3) face along the strike with respect to different pillar widths.
along the strike is the greatest at this location. The peak value of the vertical stress in the coal pillar is less than that in the coal seam when the width of the pillar is less than 10 m, while the peak value in the pillar is larger than that in the seam when the pillar width is very wide (410 m). This shows that when the pillar is thin, the mining pressure is carried mainly by the solid coal seam and the pressure will transfer gradually to the pillar as the width of the pillar increases. In general, with increase in pillar width, the peak value of the vertical stress decreases gradually in the coal seam, while the vertical stress increases gradually in the pillar. However, the peak stress is relatively constant for pillar between 15 and 20 m. In the coal pillar, the distance from the peak stress to the roadway side increases as the width of the pillar increases. In the solid coal seam, when the pillar is 3 m wide, the distance from the peak stress to the roadway side is always about 11 m. When the width of the pillar varies between 5 and 20 m, this distance is always about 7 m, except that when the width of the pillar is large the peak stress is far away from the working face.
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vertical stress (MPa)
1376
3m 5m 7m 10 m 15 m 20 m
80 70 60 50 40 30
1141(3) goaf
20 10 pillar
-50
-40
-30
-20
tunnel
1151(3) coal seam
0
-10
0
10
20
30
40
10
20
30
40
30
40
vertical stress (MPa)
distance (m)
3m 5m 7m 10 m 15 m 20 m
120 100 80 60 40 20 0
-50
-40
-30
-20
-10
0
distance (m) 120 3m 5m 7m 10 m
vertical stress (MPa)
100 80
15 m 20 m
60 40 20 0
-50
-40
-30
-20
-10
0
10
20
distance (m) Fig. 7. The vertical stress in different cross-sections along the dip with respect to different pillar width: (a) 105 m before working face, (b) 31.25 m before working face and (c) 3.75 m before working face.
When the width of the coal pillar is 3–15 m, the distribution of the stress in the pillar has a ‘‘single peak’’ shape. When the width of the pillar is 20 m, the distribution of stress in the pillar varies as follows. For a section 3.75 m in front of the working face, the curve has a single peak shape, while for a section 11.25 m in front of the working face, the curve changes to a ‘‘double peak’’ shape, but the width between the two maximum points is small (about 5 m). When the section is not less than 22.25 m in front of the working face, the curve changes to a relatively steady
double peak shape, and the width between the two maximum stress points is relatively large ( 7.5 m).
5. Vertical stress distribution in the plane of the coal seam As shown in Fig. 8, there is a peak stress region in the coal seam at a distance of 15–20 m in front of the mining face along the strike, and at a distance of 7–11 m from the edge of the coal
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gradually. With the increase in pillar width, a peak stress region appears gradually in the pillar similar to that in the coal seam. When the width of the pillar increases from 15 to 20 m, however, the peak value is essentially constant. The center of the peak stress region moves further away from the pillar edge with increase in pillar width. The distance between the center of the peak stress region and the mining face is greater in the coal seam than that in the pillar. This is probably because of the constraint effect due to the pillar along the strike, which causes the peak stress to appear relatively nearer to the mining face. The general results can be summarized as follows: (1) The coal seam and coal pillar can be divided into three regions along the strike (Fig. 9). In front of the mining face along the strike, the vertical stresses in both the coal seam and the coal pillar increase firstly, and then decrease with the increase in pillar width. At some distance in front of the mining face, the vertical stress reaches the peak value. The peak stress in the coal seam along the strike is located at the boundary between regions I and II, and the peak stress in the pillar along the strike is located at the boundary between regions IV and V. Region III is the de-stressed zone in the coal seam. Since the goaf is immediately behind this region, the stress in the coal seam near the goaf has been released. With increasing distance from the goaf, the abutment pressure is gradually released to the coal seam and the stress exceeds the original stress by several-fold. Therefore, region II is the plasticstrengthened region. Region V can be viewed as a plastic-strengthened region in the coal pillar. This region is constrained at the mining face by the pillar along the strike. If the width of the pillar is relatively small (but not less than 7 m in this study), the stress in this region cannot be freely released during mining. In contrast, the stress is slightly higher than the original value. However, when the pillar is very thin, the stress in this region can be released. When the width of the pillar is 5 m, this region can be divided into a de-stressed zone and a plastic-strengthened zone, which is represented by region VI at the back of the mining face along the strike. In regions I and IV, when the distance from the mining face increases, the stress decreases gradually and finally approaches its original value. (2) In Fig. 9, L1 and L2 are the distances between the mining face and the peak stress in the coal pillar and in the coal seam, respectively. Both L1 and L2 decrease gradually with increase in pillar width. This indicates that the wider the pillar, the closer the peak stress to the mining face. For the present project, L1 varies from 20 to 5 m, and L2 varies from 19 to
coal pillar
IV
V
PA
L3
VI
L1 L2
roadway L4 I
PB
II
III
working face
dip
coal seam strike
Fig. 8. The distribution of vertical stress in horizontal plane with respect to different pillar widths.
rib along the dip. If the width of the coal pillar increases, the distance between the mining face and the center of the peak stress region decreases slowly, while the peak value decreases
Fig. 9. The sketch map of vertical stress distribution in coal strata.
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16 m, with increasing pillar width. L1 is more affected than L2 by the width of the pillar. (3) The two broken lines in Fig. 9 show the position of the peak stress along the dip in the coal pillar and in the coal seam, respectively, where L3 and L4 are the distances from the peak stress to the edge of the coal pillar and to the edge of the coal seam, respectively. With increase in width of the pillar, L3 increases but L4 does not change much. (4) The two points PA and PB in Fig. 9 are the centers of peak stress region in the coal pillar and in the coal seam, respectively. With increase in width of the pillar, the peak stress in the coal pillar increases gradually, and when the width changes from 15 to 20 m, the peak stress does not change much. In contrast, the peak stress in the coal seam decreases gradually with the increase in pillar width. When the pillar width is less than 10 m, the peak stress in the pillar is less than that in the coal seam. When the width of the pillar is greater than 15 m, the peak stress in the pillar is greater than that in the coal seam. For a pillar width within 10–15 m, the largest peak stress of the roadway system transfers from the coal seam to the coal pillar.
20 m, the failure zone does not run through the pillar, and there is an elastic core 5–8 m wide in the center of the pillar. In coal seam 1151(3), when the pillar width is 20 m, there is little failure zone along the roadway. When the width of the pillar is 5–15 m, there is always a failure zone 5 m wide in the coal seam along the roadway. When the width of the pillar is 3 m, the load-bearing capacity of the pillar is weak, and the failure zone along the roadway becomes much wider.
6. Distribution of failure zone with respect to pillar width
8. Stability analysis of roadway system with respect to pillar width
The distribution of the failure zone in the coal stratum is shown in Fig. 10. When the width of the pillar is less than 15 m, the failure zone runs through the whole pillar. When the width is
1141(3) goaf roadway
7. Distribution of displacement with respect to pillar width The contour of the vertical displacement and the displacement vector around the roadway are shown in Fig. 11. When the pillar is 5–20 m wide, the largest vertical displacement around the roadway is located at its roof. When the pillar is 3 m wide, the largest vertical displacement is located at the side of the pillar, and the vertical displacement value is much larger than that for the other pillar widths. From the displacement vector, it can be seen that the horizontal displacement for a pillar 3 m wide is very large. So, when the width of the pillar is very small, the pillar will be crushed and it will be very difficult to keep the roadway stable.
The stability of the roadway system can be assessed from the distribution of the stress and failure zone as follows. When the
coal pillar
1151(3) face
pillar width = 3m
pillar width = 5m
pillar width = 7m
pillar width = 10m
pillar width = 15m
pillar width = 20m
Fig. 10. The distribution of failure zone in coal stratum with respect to different pillar widths.
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
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is very small, such as 3 m, the largest vertical displacement is located at the side of the pillar and the horizontal displacement is very large; the pillar is crushed and cannot be maintained in a stable condition. When the width of the pillar is 5–10 m, the failure zone runs through the whole pillar and the stress distribution has a single peak shape, but the largest vertical displacement is located at the roadway roof and the pillar is not crushed. Moreover, the peak value of the vertical stress is located at the coal seam and not at the coal pillar. Although a coal pillar has suffered peak pressure during the top caving mining and is in the yield state, the peak stress is transferred to the coal seam and the lower vertical load over the pillar is carried by the residual strength of the coal seam after failure. Therefore, for the case where the width of the coal pillar is 5–10 m, the pressure over the coal pillar is not large, and if the integrity can be maintained by some supporting measures, the pillar can be maintained in a stable condition by relying on the residual strength. When the width of the pillar is 15 m, it is in the yield state, and there is no elastic state region in the center. At the same time, the peak stress is located above the coal pillar and cannot be transferred to the coal seam (Fig. 12). A medium-sized coal pillar is hence not beneficial for the stability of the roadway. According to the Mohr–Coulomb principle, the rock compressive strength can be stated as
s1 ¼ s3 tan2 y þ sc
Fig. 11. The displacement in surrounding rocks with respect to different pillar widths (150 m before working face).
width of the pillar is very large, such as 20 m, the vertical stress has a double peak distribution in the pillar. There are plastic zones in both sides of the pillar and there is an elastic zone in the center of the pillar. In this situation, the pillar has sufficient load-bearing capacity to maintain pillar stability. When the width of the pillar
ð1Þ
where s1 is the compressive strength, s3 is the confining pressure, y is the shearing angle (y ¼(p/4)+ (f/2)), f is the friction angle, sc is the uniaxial compressive strength (s3 ¼0) (sc ¼2c cos f/(1 sin f)), and c is the cohesion. From Eq. (1), it can be seen that the compressive strength increases with the increase in lateral pressure (horizontal pressure). As shown in Fig. 13, with increase in pillar width, both the horizontal and vertical peak stress increase, except for a pillar width of 15–20 m, for which there is little change in the vertical peak stress. The value of sc for coal is small, so if we ignore the influence of sc, Eq. (1) becomes s1/s3 ¼tan2 y. This shows that the ratio of the vertical stress to horizontal stress has a great influence on the failure of the rock and the roadway stability. The larger the ratio, the more likely the rock will fail. The ratio of the vertical peak stress to horizontal peak stress with respect to different widths of pillar is shown in Fig. 14. The ratio has a peak value at a width of 15 m, so this width is not adequate for roadway stability. Although the ratio is also relatively large when the width of the pillar is small, e.g. 5–7 m, the peak mining pressure is transferred to the coal seam and the stress in the coal pillar is not large. So, the pillar will be in a stable state and appropriate supports can maintain the integrity of the pillar. When the width of the pillar is less than 10 m, the peak horizontal stress in the pillar increases rapidly. When the width of the pillar is greater than 10 m, the peak horizontal stress increases very slowly (Fig. 13), so we can say that the pillar is strengthened when the width is more than 10 m, as the pillar has a relatively strong lateral constraint by the large horizontal stress in this situation (Fig. 14). The pillar is weakened when the width is less than 10 m, as the lateral constraint is weak due to the small horizontal stress. When the pillar is weakened, the relatively large stress release will induce a relatively large deformation. In this situation, a relatively wider pillar (such as 7 m) will give a larger displacement around the roadway, which is demonstrated by a site observation as shown in Fig. 15. From the above discussion, the stability area of the roadway system can be divided into four regions, which are shown in Fig. 16. The stability of the roadway system in regions I and III is worse than that in regions II and IV. Region II can be further
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None shear-n shear-p
vertical stresscurve in roof strata
shear-n shear-p tension-p shear-n tension-n shear-p ter shear-p shear-p tension-p
pillar width = 5m-10m pillar width = 20m
pillar width = 15m
goaf goaf
goaf
tunnel
coal pillar
coal pillar
coal pillar
peak value of stress (MPa)
Fig. 12. The vertical stress and failure zone distribution with respect to different pillar widths.
120 vertical stress
100
horizontal stress
80 60 40 20 0 0
5
10 15 pillar width (m)
20
25
ratio of vertical peak stress to horizontal peak stress
Fig. 13. The peak value of vertical and horizontal stress with respect to different pillar widths.
2.5
condition, and its stability will be compromised. When the width of the pillar is reduced further (region II), the roadway is in a good stress environment and it can be maintained at a stable condition with proper supports. There is a relatively large deformation region (IIb) caused by pillar weakening. When the width of the pillar is reduced further to a very small value (region I), the pillar is crushed and the roadway system is not stable. So, the reasonable width of the pillar should be in region IIa in Fig. 16, as this width can take advantage of the enhanced coal recovery rate while maintaining a stable condition. Fig. 16 shows a plot of the roadway deformation observed on-site by Wei and Cheng [13] in similar colliery (detailed measurement results are not available). The width of the pillar ranges from 12 to 14 m in this project, and the roadway is seriously deformed, which is caused by a large pressure imposed on it. Though detailed measurements are not available from this project due to the rapid coal extraction, but the results illustrated in Figs. 13–16 are probably appropriate.
2
9. Effect of the shape of coal pillar 1.5 1 0.5 0 0
5
10
15
20
25
pillar width (m) Fig. 14. The ratio of vertical peak stress to horizontal peak stress with respect to different pillar widths.
divided into three sub-regions. In regions IIa and IIc, the deformation of the roadway is relatively small, while the deformation is relatively large in region IIb. In Fig. 16, the width of the pillar at point A is the critical width at which the location of the peak stress in the coal stratum will have a sudden change, i.e. the peak stress jumps from the coal pillar to the coal seam. When the pillar width is large enough (region IV), the roadway system is in a stable state. When the width of the pillar approaches the critical value (region III), then the roadway will be in a poor stress
As early as in 1907, Daniels and Moore [15] discovered that the larger the coal specimen, the less will be its strength. Furthermore, the strength of the sample is found to decrease with the height of the specimen. Bunting [16] called these two phenomena as size effect and shape effect. It has been determined by numerous tests that a coal pillar has a critical size. When the size of the coal specimen exceeds the critical size, the strength does not decrease with further increase in size. Bieniawski and Van Heerden [2] reported that the critical size is about 1.5 m in a coal seam in South Africa. Pariseau et al. [17] reported that the critical size is about 0.9 m in a coal seam in western USA. Hustrulid [3] suggested that 0.9 m can be taken as the general critical size in coal mining. In this present study, the pillar size is definitely larger than the critical size, so the size effect is neglected here. The shape effect has been discussed by many researchers but there is no consensus about this effect. Many researchers believe that the coal strength is affected more by the shape than by the size. When the shape effect (the ratio of width to height) is considered, the commonly used coal pillar strength can be divided into two categories, namely, linear expression and exponent
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
3.35
3.4
3.70
3.70
1.4
1.8
1.7
1.2
Before deformation
1.6
2.9
Width = 12m
After deformation
3.08
2.9
3.70
3.70 Width = 14m
Width = 13m Note :
1.8 2.96 3.70
Width = 7.5m
2.2
2.1
1.6
2.9
3.38
Width = 7m
2.9
2.96
2.7
2.9
2.1
2.0
1.88
2.9
3.30
1381
The selected crosses are where the deformation is serious; The unit is meter. Fig. 15. Schematic plot of a roadway condition as observed at a site.
pillar width from small to large I worse stability
II better stability II a small deformation
II b
III worse stability II c
IV better stability
A
large small deformation deformation
Fig. 16. The division of stability area of roadway system with respect to different pillar widths.
expressions [18], which are given as
ss ¼ K1 ½Aþ BðW=hÞ
ð2Þ
ss ¼ K2 W a =hb
ð3Þ
where ss is coal strength (MPa), K1 and K2 are the limit cube strength on-site (MPa), and A and B are dimensionless and A+B¼1, a and b are dimensionless parameters in the range 0.33–0.67. Eq. (3) indicates that the coal strength increases with the increase in the ratio of width to height. When this ratio is more than 10, the coal pillar is unlikely to fail. Wu et al. [19] examined the shape effect of the coal pillar by finite element analysis and arrived at the conclusion that the coal strength increases with the ratio of width to height. Beyond a ratio of 8, the pillar strength will be stable and will not increase with any further increase in the ratio. In the present work, the roadway height (pillar height) is 2.7 m and the pillar width is 20 m. The ratio of width to height is hence 7.4, which is slightly less than 8. On the basis of the work by Wu et al. [19], the pillar has nearly attained its maximum strength. The above analysis shows that the roadway system is in a stable state when the width of the pillar is 20 m.
The six different models used here have the same roadway height of 2.7 m. In order to investigate the influence of roadway height, another model was developed in which the roadway height is 5.4 m and the pillar width is 20 m. It can be seen that the whole pillar is at the yield state and there is no elastic region in the pillar center (Fig. 17). The distribution of the vertical stress is compared with that of the previous three models (Fig. 18). It can be seen that the vertical stress distribution (Fig. 18d) is similar to the previous model with a 10 m wide pillar (Fig. 18a). The difference between the peak stress in the coal pillar and in the coal seam is greater when the roadway height is 2.7 m. However, beyond some distance away from the working face, the peak stress in the coal seam is larger than that in the coal pillar in both models. Even if the ratio of width to height is the same, the roadway with the greater height is less stable, which is expected. For coal extraction, both the width and the height (roadway height) of the coal pillar are the variables that will determine the stability of the roadway system. In general, the ratio of width to height is a major variable of the roadway system, but the actual sizes of the pillars may affect the stability of the system. In practice, the roadway height is often controlled by the mining
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Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
Fig. 17. Failure zone in coal pillar (pillar width¼ 20 m, roadway height ¼ 5.4 m).
8.75 m
100
11.25 m 21.25 m
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.25 m 105 m
80
31.25 m 46.25 m
60
105 m
120 100 szz (MPa)
szz(MPa)
120 3.75 m
80 60
40
40
20
20 0
0 -50
-40
-30
-20
-10
0
10
20
30
40 -50
-40
-30
-20
-10
0
10
20
30
40
-10 0 L (m)
10
20
30
40
L (m)
100 80 60
szz (MPa)
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.2 5m 105 m
120
szz (MPa)
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.25 m 105 m
L (m)
40 20 -50
-40
-30
-20
0 -10 0 L (m)
10
20
30
40 -50
-40
-30
-20
100 90 80 70 60 50 40 30 20 10 0
Fig. 18. The vertical stress at different distances from the working face: (a) pillar width¼ 10 m, roadway height ¼2.7 m, (b) pillar width ¼15 m, roadway height¼ 2.7 m, (c) pillar width¼ 20 m, roadway height ¼ 2.7 m and (d) pillar width¼ 20 m, roadway height ¼ 5.4 m.
conditions, so the width of the pillar will be the main variable that can be controlled by the engineers.
10. Discussion and conclusions From the present study, it was found that in front of the working face, there is a range that is seriously affected by top coal mining. In this range, stress increases rapidly to a peak value. In the coal pillar, the peak stress was located at about 5–20 m in front of the working face and in the coal seam the peak stress is at about 16–20 m in front of the working face. Beyond the seriously affected range, with increase in pillar width, the stress in the coal seam is always decreasing, while the stress in the coal pillar is increasing. When the width of the pillar
is about 15 m, the stress in the coal seam reaches the peak value and then remains basically unchanged with further increase in width of the pillar. When the width is less than 10 m, the peak stress is located in the coal seam. When the pillar width is greater than 15 m, the peak stress is located in the pillar. This indicates that with the increase in the width of the pillar, the mining pressure is initially imposed on the coal seam and is then transferred gradually to the coal pillar. At the same time, when the width of the pillar is less than 15 m, the failure zone runs through the whole pillar. When the width of the pillar is 20 m, there is an elastic region in the pillar center. Therefore, when the width of the pillar is 15 m, it is in an unfavorable stress environment and the whole pillar will break. In this situation, the stability of the roadway is worse and the roadway will experience major distortion. When the width of the pillar is
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
small (not more than 3 m), the pillar would be crushed and the roadway stability will also be worse. When the pillar is wide enough, such as 20 m, the pillar has the ability to resist the mining pressure, so the roadway stability will be improved. When the width of the pillar is between 5 and 10 m, though the pillar will suffer great mining pressure during the top caving mining, the peak stress will be transferred quickly to the coal seam and the ultimate pressure imposed on the pillar is not large. The roadway will still be in good condition in this case, and the integrity of the pillar can be maintained by proper supports. It should be emphasized that once the whole pillar is broken, excessive deformation cannot be avoided. When the width of the pillar is 10 m, the pillar is strengthened with a relatively high horizontal stress, so the deformation is relatively small. When the width of the pillar is not more than 7 m, the pillar is weakened. In this case, the wider the pillar, the greater will be the deformation. For a pillar width of 7 m, the roadway deformation will be slightly greater than is acceptable. Therefore, for the design of a reasonable pillar width for a mining panel along the goaf, there are three points to bear in mind. Firstly, the width of the pillar should not be close to the critical width (at this width, the peak stress is transferred from the coal seam to the coal pillar), since the roadway system will be in a bad stress environment and the width of the pillar is not large enough to resist the peak stress. Secondly, the width of the pillar should not be too small, since in this situation the load-bearing capacity of the pillar will be small and the pillar will be crushed. Thirdly, when the width of the pillar is small (less than the critical width but not very small), the roadway will have a relatively large deformation because of the small lateral constraint. The first two issues will cause serious problems for the roadway system. The last issue will not be too troublesome, since the deformation will be released only when the peak stress is imposed on it. When the pillar is in a relatively low stress condition, the deformation will not be large and integrity can be maintained with proper supports. The results from the present study apply to the geotechnical conditions given in Table 1 and Fig. 3 and for a specified roadway height. Nevertheless, the general behavior of stability and stress distribution of the roadway system with respect to pillar width and size will be applicable to fully mechanized top caving mining problems. Since the progress of coal extraction is extremely fast, no detailed measurement apart from visual examination is
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available during the extraction. On the basis of the present study, a pillar width of 6 m was used in this project and coal extraction was carried out with satisfaction. References [1] Donovan JG, Karfakis MG. Design of backfilled thin-seam coal pillars using earth pressure theory. Geotech Geol Eng 2004;22:627–42. [2] Bieniawski ZT, Van Heerden WL. The significance of in situ tests on large rock specimens. Int J Rock Mech Min Sci Geomech Abstr 1975;12:101–13. [3] Hustrulid WA. A review of coal pillar strength formulas. Rock Mech 1976;8: 115–45. [4] Sheorey PR. Design of coal pillar arrays and chain pillars. In: Hudson JA, editor. Comprehensive rock engineering, vol. 2. Oxford: Pergamon; 1993. p. 631–70. [5] Wilson AH. Research into the determination of pillar size, Part 1. The Mining Engineer, vol. 131. The Institution of Mining Engineers; 1972. p. 409–17. [6] Sheorey PR, Singh TN, Singh B. Considerations for the stability of longwall chain pillars and adjacent roadway. In: Farmer IW, editor. Proceedings of the symposium on strata mechanics, Newcastle-upon-Tyne. Amsterdam: Elsevier; 1982. p. 129–33. [7] Fadeev AB, Abdyldayev EK. Elastoplastic analysis of stresses in coal pillars by finite element method. Rock Mech 1979;11:243–51. [8] Hsiung SM, Peng SS. Chain pillar design for US longwall panels. J Min Sci Technol (China) 1985;2:279–305. [9] Thin IGT, Pine RJ, Trueman R. Numerical modeling as an aid to the determination of the stress distribution in the goaf due to longwall coal mining. Int J Rock Mech Min Sci Geomech Abstr 1993;30(7):1403–9. [10] Mukherjee C, Sheorey PR, Sharma KG. Numerical simulation of caved goaf behaviour in longwall workings. Int J Rock Mech Min Sci Geomech Abstr 1994;31(1):35–45. [11] Murali Mohan G, Sheorey PR, Kushwaha A. Numerical estimation of pillar strength in coal mines. Int J Rock Mech Min Sci 2001;38(8):1185–92. [12] Jaiswal A, Sharma SK, Shrivastva BK. Numerical modeling study of asymmetry in the induced stresses over coal mine pillars with advancement of the goaf line. Int J Rock Mech Min Sci 2004;41:859–64. [13] Wei WB, Cheng YM. Stress evolution in irregular pillar protected and bolting supported roadway retained along top-coal caving mining panel. Rock Mech Rock Eng 2009. in preparation. [14] Whittaker BN, Singh RN. Design and stability of pillars in longwall mining. Min Eng (London) 1979;138:59–70. [15] Daniels J, Moore LD. The ultimate crushing strength of coal. Eng Min J 1907;10:263–8. [16] Bunting D. Chamber pillar in deep anthracite mines. Trans AIME 1991;42: 236–45. [17] Pariseau WG, Hustrulid WA, Swanson SR, Van Sambeek LL. Coal pillar strength study. University of Utah, Mining Department, Report to US Bur Mines, Contract H0242059, 1977. [18] Xie HP, Duan FB, Zhou HW, Zhao GX. Recent developments of theory and analysis methods of strip pillar stability. China Min Mag 1998;7(5):37–41. in Chinese. [19] Wu LX, Wang JZ, Guo ZZ. Fundamentals of designing and monitoring for coal pillar. Beijing: China University of Mining and Technology Press; 2000. in Chinese.
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International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
Technical Note
Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face Y.M. Cheng a,n, J.A. Wang b, G.X. Xie c, W.B. Wei a a
Department of Civil and Structural Engineering, Hong Kong Polytechnic University, Hong Kong School of Civil and Environmental Engineering, University of Science and Technology, Beijing, PR China c Department of Resource Exploration and Management Engineering, Anhui University of Science and Technology, PR China b
a r t i c l e in f o Article history: Received 11 September 2009 Received in revised form 19 February 2010 Accepted 16 August 2010 Available online 9 September 2010
1. Introduction In underground coal mining, an increase in coal recovery rate can be achieved by a decrease in the size of the supporting coal pillars [1]. The design of the coal pillar is important for both the success and the effective operation of coal extraction. Pillar design studies have been carried out by many engineers and researchers, and many strength formulae and design methods have been proposed, and these are reviewed in [2–4]. Pillar formation in bord and pillar coal mining is required for (1) panel isolation, (2) protection of roadways, shafts and surface features, and (3) protection of mine workings from flooding and roof caves in the face area while depillaring [4]. Different methods are used for the design of the pillar for different situations or functions. A goaf is an extracted coal panel during coal mining. Here, we consider the pillar along the goaf, which is a chain pillar used for the protection of the roadway and the extraction panel and exists only along one side. Sheorey [4] has suggested three methods for chain pillar analysis and design: (i) choose a pillar strength formula, determine the average load (depending on one-sided or two-sided goaf, caving or stowing) and the width of pillar with a suitable safety factor; (ii) choose the width of pillar such that the gate road beyond is not much affected by the longwall or depillaring face movement; and (iii) perform numerical stress analysis with different chain pillar sizes and various design parameters, and the pillar size is defined by applying a suitable failure criterion to the seam. When the first two methods are selected, Wilson’s formula [5] and Sheorey’s formula [6] can be used, and these are derived from analysis of the abutment
n
Corresponding author. Tel.: +852 27666042; fax: + 852 23346389. E-mail address: [email protected] (Y.M. Cheng).
1365-1609/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmms.2010.08.008
pressure across the width of the pillar. When the third method is selected, it is not necessary to choose the strength or load formulae as an estimation of the pillar load and the strength is not required, but it is important to model the realistic constitutive model using suitable caved goaf properties with the adoption of a suitable failure criterion for the in situ seam. Numerical stress analysis of this problem has been done in [7–12]. Although the use of design formulae is more popular in chain pillar design and analysis, the use of numerical modeling is necessary for some complex and special situations, particularly when the excavation sequence is important for overall stability. Moreover, the use of design formulae relies on the assumption that the abutment pressure is not affected by the roadway position (Fig. 1a). From numerical simulation, Wei and Cheng [13] have shown that the distance of the roadway excavation from the goaf has some influence on the abutment stress distribution in the roof strata (Fig. 1b), which cannot be accounted for by simple design formulae. On the basis of a two-dimensional numerical simulation, Wei and Cheng [13] pointed out that an intermediate pillar width is not good for roadway stability, and this conclusion is similar to that reached by Whittaker and Singh [14], who recommended that a pillar 10–30 m wide is optimal for a gate road, because this range will give the highest gate road closure. Wilson [5] pointed out that a roadway placed in the highly stressed area of the ribside will suffer damage. Here, the pillar in a top-coal mining panel with an inclined thick coal seam is analyzed by numerical modeling. The 3D distribution of stress and failure zone is analyzed in detail, and the effects of the coal pillar shape are discussed. It is revealed that both the height and the width of the coal pillar are responsible for the stability of the roadway system. Finally, the results obtained from the present study have been used for a real project in Xieqiao with satisfactory outcome.
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
abutment pressure
abutment pressure
roadway goaf
1373
roadway goaf
pillar
pillar
Fig. 1. The distribution of abutment pressure over the pillar and roadway along the goaf: (a) Abutment pressure not affected by roadway excavation and (b) Abutment pressure affected by roadway excavation.
1141(3)goaf coal pillar 4.6m
uphill roadway (ventilation tunnel) strike direction
1151(3) face
4.6m
231.8m
working face
downhill roadway (tramroad tunnel) Fig. 2. Plan of 1151(3) face.
Table 1 Mechanical properties of rock mass used in numerical modeling. Rock name
Density (kg/m3)
Elastic modulus (MPa)
Poisson ratio
Cohesion (Mpa)
Friction angle (deg)
Tensile strength (Mpa)
Roof strata Siltstone Mudstone Sandy mudstone Fine sandstone Sandstone
2460 2461 2510 2873 2487
1.95 104 0.875 104 0.5425 104 3.34 104 1.35 104
0.2 0.26 0.147 0.235 0.123
2.75 1.2 2.16 3.2 2.06
38 30 36 42 40
1.84 0.605 0.75 1.29 1.13
Coal 13#coal
1380
0.53 104
0.32
1.25
32
0.15
Floor strata Mudstone Siltstone Sandstone Sandy mudstone
2483 2460 2580 2530
1.77 104 1.95 104 2.5 104 1.085 104
0.204 0.2 0.159 0.147
1.2 3.75 2.5 2.45
32 38 42 40
0.58 1.84 3.6 2.01
The present analysis is based on the mining conditions of the 1151(3) mining panel in the Xieqiao colliery, Huainan City, PR China. Mining panel 1151(3) is a top-coal caving mining face, and on the north of this face is a mining panel 1141(3), where the coal has been extracted (Fig. 2). The elevation of the ground surface is from +20.4 to +25.8 m and the elevation of the working face is from –588 to –662 m. The mining face is 1674 m along the strike and 231.8 m along the dip. The average thickness of the coal stratum is 5.4 m and the dip angle is 131. The main roof is siltstone or fine sandstone with an average thickness of 6.2 m. The immediate roof is mudstone or sandy mudstone with an average thickness of 3.26 m. The immediate floor is mudstone and the thickness is about 1.5 m. The main floor is siltstone 2.8 m thick. The properties of the rock, which are based on laboratory tests and are consistent with the design parameters commonly adopted
for design in Xieqiao, are given in Table 1. In this analysis, six different coal pillar widths are considered: 3, 5, 7, 10, 15, and 20 m.
2. Numerical simulation model Six different 3D models were developed by FLAC3D for the project at Huainan City, and the pillar widths are 3, 5, 7, 10, 15 and 20 m. All the numerical models use a length of 500 m along the strike, a width of 600 m along the dip and a height of 214.17 m (Fig. 3). In these models, there are 87,898–91,036 zones and 103,531–107,134 grid-points. At the top of the models, a vertical load (p¼ gH) is applied to simulate the overburden weight. The elasto-plastic Mohr–Coulomb model with the
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
214.17m
1374
600m
500m
Fig. 3. Plot of the three-dimensional model mesh.
Table 2 Variation of mechanical properties of coal with plastic shear strain. Plastic shear strain Cohesion (MPa) Friction angle (deg)
0 1.25 32
0.0015 0.8 28
0.005 0.6 26
0.01 0.03 24
non-associated flow rule is used for the roof and floor strata. The strain softening Mohr–Coulomb model is used for the coal. The variation of cohesion and friction angle properties with plastic shear strain is shown in Table 2. The roadways in these models are 4.6 m wide and 2.7 m high. On the basis of statistical field measurement at Xieqiao, the at-rest pressure coefficients along the x and y directions are taken as 0.8 and 1.2, respectively. In this simulation, the caved area was filled by a very soft elastic material (the elastic modulus is very small) in order to consider the (very limited) supporting capability of the caved rock. The horizontal displacement is fixed at the lateral boundary while the vertical displacement is fixed at the bottom boundary. The modeling process is as follows: (1) calculating the initial state induced by the gravity; (2) modeling the excavation of panel 1141(3) step by step; (3) modeling the excavation of the uphill roadway and downhill roadway; and (4) modeling the excavation of panel 1151(3) step by step. During the process of modeling excavation panels 1141(3) and 1151(3), both the coal seam and immediate roof are excavated to simulate the caving of the roof during coal extraction. Then the cave area is filled with very soft elastic material. The ultimate elastic modulus of this material is 190 MPa and the Poisson ratio is 0.25. Fig. 4 shows the 3D view of the vertical stress distribution in the coal seam at panel 1151(3). In front of the mining face, there is a peak stress region at the upper corner of the panel that is superimposed by two parts: the stress induced by the adjacent extraction of the continuous longwall mining panel 1141(3) and the abutment pressure induced by the advancement of the mining face 1151(3). The pillar between mining faces 1141(3) and 1151(3) is the focus of this investigation.
3. Distribution of vertical stress along the strike with respect to pillar width The vertical stress for coal pillars of different widths along the strike is shown in Fig. 5. When the pillar width is 3 m, the vertical
stress in the pillar is relatively low and the stress in front of the working face is even lower than that at the back of the working face. For pillars of other widths, the peak stress is in front of the working face. When the pillar width is 5–20 m wide, the stress is relatively low near the working face and increases quickly in front of the working face. The vertical stress reaches a peak value at a distance between 5 and 20 m in front of the working face, and then decreases with increase in distance away from the working face. At the back of the working face, the stress increases slowly with increase in distance from the working face, and this is probably because the goaf is gradually filled by the caved rocks. For a pillar width within 5–15 m, the peak stress in the pillar increases gradually with the increase in pillar width. The peak stress is relatively constant for pillars 15–20 m wide. The distance from the peak stress to the working face decreases slowly with increase in width of the coal pillar. The vertical stress in the solid coal seam of the 1151(3) face along the strike with respect to different pillar widths is shown in Fig. 6. The stress is relatively low near the working face and increases quickly in front of the working face. The vertical stress reaches a peak value at a distance of 16–19 m in front of the working face. In general, the peak value of the vertical stress decreases gradually with increase in pillar width, except when the pillar width is 3 m or lesser. From these two figures, it can be seen that with increase in pillar width, the peak vertical stress in the coal pillar increases gradually, while the peak value in the solid coal seam of mining face 1151(3) decreases gradually. This indicates that with increase in pillar width, the load-bearing capacity is gradually improved, so the mining pressure is transferred gradually from the coal seam to the coal pillar.
4. Distribution of vertical stress along the dip with respect to pillar width The vertical stress in different cross-sections along the dip with respect to pillar width is shown in Fig. 7. There is less constraint to the 1151(3) coal seam, since the coal seam at the back of the working face has been extracted. For a section less than 11.25 m from the working face, the vertical stress is very low in the coal seam, while it is relatively high in the pillar, since there are more constraints at the back of the coal pillar. At sections further than 22.25 m away from the working face, the stress increases in the coal seam with a peak value as the constraint
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Fig. 4. The vertical stress distribution in near vicinity of the mining face 1151(3).
120
vertical stress (MPa)
100 80
3m 5m 7m 10 m 15 m 20 m
60 40 20 0 -100
-80
-60
-40
-20
0
20
40
60
80
100
distance (m) Fig. 5. The vertical stress for different pillar widths along the strike.
140
vertical stress (MPa)
120 3m 5 m 7m 10 m 15 m 20 m
100 80 60 40 20 0 -100 -80
-60
-40
-20 0 20 distance (m)
40
60
80
100
Fig. 6. The vertical stress in solid coal seam of 1151(3) face along the strike with respect to different pillar widths.
along the strike is the greatest at this location. The peak value of the vertical stress in the coal pillar is less than that in the coal seam when the width of the pillar is less than 10 m, while the peak value in the pillar is larger than that in the seam when the pillar width is very wide (410 m). This shows that when the pillar is thin, the mining pressure is carried mainly by the solid coal seam and the pressure will transfer gradually to the pillar as the width of the pillar increases. In general, with increase in pillar width, the peak value of the vertical stress decreases gradually in the coal seam, while the vertical stress increases gradually in the pillar. However, the peak stress is relatively constant for pillar between 15 and 20 m. In the coal pillar, the distance from the peak stress to the roadway side increases as the width of the pillar increases. In the solid coal seam, when the pillar is 3 m wide, the distance from the peak stress to the roadway side is always about 11 m. When the width of the pillar varies between 5 and 20 m, this distance is always about 7 m, except that when the width of the pillar is large the peak stress is far away from the working face.
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
vertical stress (MPa)
1376
3m 5m 7m 10 m 15 m 20 m
80 70 60 50 40 30
1141(3) goaf
20 10 pillar
-50
-40
-30
-20
tunnel
1151(3) coal seam
0
-10
0
10
20
30
40
10
20
30
40
30
40
vertical stress (MPa)
distance (m)
3m 5m 7m 10 m 15 m 20 m
120 100 80 60 40 20 0
-50
-40
-30
-20
-10
0
distance (m) 120 3m 5m 7m 10 m
vertical stress (MPa)
100 80
15 m 20 m
60 40 20 0
-50
-40
-30
-20
-10
0
10
20
distance (m) Fig. 7. The vertical stress in different cross-sections along the dip with respect to different pillar width: (a) 105 m before working face, (b) 31.25 m before working face and (c) 3.75 m before working face.
When the width of the coal pillar is 3–15 m, the distribution of the stress in the pillar has a ‘‘single peak’’ shape. When the width of the pillar is 20 m, the distribution of stress in the pillar varies as follows. For a section 3.75 m in front of the working face, the curve has a single peak shape, while for a section 11.25 m in front of the working face, the curve changes to a ‘‘double peak’’ shape, but the width between the two maximum points is small (about 5 m). When the section is not less than 22.25 m in front of the working face, the curve changes to a relatively steady
double peak shape, and the width between the two maximum stress points is relatively large ( 7.5 m).
5. Vertical stress distribution in the plane of the coal seam As shown in Fig. 8, there is a peak stress region in the coal seam at a distance of 15–20 m in front of the mining face along the strike, and at a distance of 7–11 m from the edge of the coal
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gradually. With the increase in pillar width, a peak stress region appears gradually in the pillar similar to that in the coal seam. When the width of the pillar increases from 15 to 20 m, however, the peak value is essentially constant. The center of the peak stress region moves further away from the pillar edge with increase in pillar width. The distance between the center of the peak stress region and the mining face is greater in the coal seam than that in the pillar. This is probably because of the constraint effect due to the pillar along the strike, which causes the peak stress to appear relatively nearer to the mining face. The general results can be summarized as follows: (1) The coal seam and coal pillar can be divided into three regions along the strike (Fig. 9). In front of the mining face along the strike, the vertical stresses in both the coal seam and the coal pillar increase firstly, and then decrease with the increase in pillar width. At some distance in front of the mining face, the vertical stress reaches the peak value. The peak stress in the coal seam along the strike is located at the boundary between regions I and II, and the peak stress in the pillar along the strike is located at the boundary between regions IV and V. Region III is the de-stressed zone in the coal seam. Since the goaf is immediately behind this region, the stress in the coal seam near the goaf has been released. With increasing distance from the goaf, the abutment pressure is gradually released to the coal seam and the stress exceeds the original stress by several-fold. Therefore, region II is the plasticstrengthened region. Region V can be viewed as a plastic-strengthened region in the coal pillar. This region is constrained at the mining face by the pillar along the strike. If the width of the pillar is relatively small (but not less than 7 m in this study), the stress in this region cannot be freely released during mining. In contrast, the stress is slightly higher than the original value. However, when the pillar is very thin, the stress in this region can be released. When the width of the pillar is 5 m, this region can be divided into a de-stressed zone and a plastic-strengthened zone, which is represented by region VI at the back of the mining face along the strike. In regions I and IV, when the distance from the mining face increases, the stress decreases gradually and finally approaches its original value. (2) In Fig. 9, L1 and L2 are the distances between the mining face and the peak stress in the coal pillar and in the coal seam, respectively. Both L1 and L2 decrease gradually with increase in pillar width. This indicates that the wider the pillar, the closer the peak stress to the mining face. For the present project, L1 varies from 20 to 5 m, and L2 varies from 19 to
coal pillar
IV
V
PA
L3
VI
L1 L2
roadway L4 I
PB
II
III
working face
dip
coal seam strike
Fig. 8. The distribution of vertical stress in horizontal plane with respect to different pillar widths.
rib along the dip. If the width of the coal pillar increases, the distance between the mining face and the center of the peak stress region decreases slowly, while the peak value decreases
Fig. 9. The sketch map of vertical stress distribution in coal strata.
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16 m, with increasing pillar width. L1 is more affected than L2 by the width of the pillar. (3) The two broken lines in Fig. 9 show the position of the peak stress along the dip in the coal pillar and in the coal seam, respectively, where L3 and L4 are the distances from the peak stress to the edge of the coal pillar and to the edge of the coal seam, respectively. With increase in width of the pillar, L3 increases but L4 does not change much. (4) The two points PA and PB in Fig. 9 are the centers of peak stress region in the coal pillar and in the coal seam, respectively. With increase in width of the pillar, the peak stress in the coal pillar increases gradually, and when the width changes from 15 to 20 m, the peak stress does not change much. In contrast, the peak stress in the coal seam decreases gradually with the increase in pillar width. When the pillar width is less than 10 m, the peak stress in the pillar is less than that in the coal seam. When the width of the pillar is greater than 15 m, the peak stress in the pillar is greater than that in the coal seam. For a pillar width within 10–15 m, the largest peak stress of the roadway system transfers from the coal seam to the coal pillar.
20 m, the failure zone does not run through the pillar, and there is an elastic core 5–8 m wide in the center of the pillar. In coal seam 1151(3), when the pillar width is 20 m, there is little failure zone along the roadway. When the width of the pillar is 5–15 m, there is always a failure zone 5 m wide in the coal seam along the roadway. When the width of the pillar is 3 m, the load-bearing capacity of the pillar is weak, and the failure zone along the roadway becomes much wider.
6. Distribution of failure zone with respect to pillar width
8. Stability analysis of roadway system with respect to pillar width
The distribution of the failure zone in the coal stratum is shown in Fig. 10. When the width of the pillar is less than 15 m, the failure zone runs through the whole pillar. When the width is
1141(3) goaf roadway
7. Distribution of displacement with respect to pillar width The contour of the vertical displacement and the displacement vector around the roadway are shown in Fig. 11. When the pillar is 5–20 m wide, the largest vertical displacement around the roadway is located at its roof. When the pillar is 3 m wide, the largest vertical displacement is located at the side of the pillar, and the vertical displacement value is much larger than that for the other pillar widths. From the displacement vector, it can be seen that the horizontal displacement for a pillar 3 m wide is very large. So, when the width of the pillar is very small, the pillar will be crushed and it will be very difficult to keep the roadway stable.
The stability of the roadway system can be assessed from the distribution of the stress and failure zone as follows. When the
coal pillar
1151(3) face
pillar width = 3m
pillar width = 5m
pillar width = 7m
pillar width = 10m
pillar width = 15m
pillar width = 20m
Fig. 10. The distribution of failure zone in coal stratum with respect to different pillar widths.
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
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is very small, such as 3 m, the largest vertical displacement is located at the side of the pillar and the horizontal displacement is very large; the pillar is crushed and cannot be maintained in a stable condition. When the width of the pillar is 5–10 m, the failure zone runs through the whole pillar and the stress distribution has a single peak shape, but the largest vertical displacement is located at the roadway roof and the pillar is not crushed. Moreover, the peak value of the vertical stress is located at the coal seam and not at the coal pillar. Although a coal pillar has suffered peak pressure during the top caving mining and is in the yield state, the peak stress is transferred to the coal seam and the lower vertical load over the pillar is carried by the residual strength of the coal seam after failure. Therefore, for the case where the width of the coal pillar is 5–10 m, the pressure over the coal pillar is not large, and if the integrity can be maintained by some supporting measures, the pillar can be maintained in a stable condition by relying on the residual strength. When the width of the pillar is 15 m, it is in the yield state, and there is no elastic state region in the center. At the same time, the peak stress is located above the coal pillar and cannot be transferred to the coal seam (Fig. 12). A medium-sized coal pillar is hence not beneficial for the stability of the roadway. According to the Mohr–Coulomb principle, the rock compressive strength can be stated as
s1 ¼ s3 tan2 y þ sc
Fig. 11. The displacement in surrounding rocks with respect to different pillar widths (150 m before working face).
width of the pillar is very large, such as 20 m, the vertical stress has a double peak distribution in the pillar. There are plastic zones in both sides of the pillar and there is an elastic zone in the center of the pillar. In this situation, the pillar has sufficient load-bearing capacity to maintain pillar stability. When the width of the pillar
ð1Þ
where s1 is the compressive strength, s3 is the confining pressure, y is the shearing angle (y ¼(p/4)+ (f/2)), f is the friction angle, sc is the uniaxial compressive strength (s3 ¼0) (sc ¼2c cos f/(1 sin f)), and c is the cohesion. From Eq. (1), it can be seen that the compressive strength increases with the increase in lateral pressure (horizontal pressure). As shown in Fig. 13, with increase in pillar width, both the horizontal and vertical peak stress increase, except for a pillar width of 15–20 m, for which there is little change in the vertical peak stress. The value of sc for coal is small, so if we ignore the influence of sc, Eq. (1) becomes s1/s3 ¼tan2 y. This shows that the ratio of the vertical stress to horizontal stress has a great influence on the failure of the rock and the roadway stability. The larger the ratio, the more likely the rock will fail. The ratio of the vertical peak stress to horizontal peak stress with respect to different widths of pillar is shown in Fig. 14. The ratio has a peak value at a width of 15 m, so this width is not adequate for roadway stability. Although the ratio is also relatively large when the width of the pillar is small, e.g. 5–7 m, the peak mining pressure is transferred to the coal seam and the stress in the coal pillar is not large. So, the pillar will be in a stable state and appropriate supports can maintain the integrity of the pillar. When the width of the pillar is less than 10 m, the peak horizontal stress in the pillar increases rapidly. When the width of the pillar is greater than 10 m, the peak horizontal stress increases very slowly (Fig. 13), so we can say that the pillar is strengthened when the width is more than 10 m, as the pillar has a relatively strong lateral constraint by the large horizontal stress in this situation (Fig. 14). The pillar is weakened when the width is less than 10 m, as the lateral constraint is weak due to the small horizontal stress. When the pillar is weakened, the relatively large stress release will induce a relatively large deformation. In this situation, a relatively wider pillar (such as 7 m) will give a larger displacement around the roadway, which is demonstrated by a site observation as shown in Fig. 15. From the above discussion, the stability area of the roadway system can be divided into four regions, which are shown in Fig. 16. The stability of the roadway system in regions I and III is worse than that in regions II and IV. Region II can be further
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None shear-n shear-p
vertical stresscurve in roof strata
shear-n shear-p tension-p shear-n tension-n shear-p ter shear-p shear-p tension-p
pillar width = 5m-10m pillar width = 20m
pillar width = 15m
goaf goaf
goaf
tunnel
coal pillar
coal pillar
coal pillar
peak value of stress (MPa)
Fig. 12. The vertical stress and failure zone distribution with respect to different pillar widths.
120 vertical stress
100
horizontal stress
80 60 40 20 0 0
5
10 15 pillar width (m)
20
25
ratio of vertical peak stress to horizontal peak stress
Fig. 13. The peak value of vertical and horizontal stress with respect to different pillar widths.
2.5
condition, and its stability will be compromised. When the width of the pillar is reduced further (region II), the roadway is in a good stress environment and it can be maintained at a stable condition with proper supports. There is a relatively large deformation region (IIb) caused by pillar weakening. When the width of the pillar is reduced further to a very small value (region I), the pillar is crushed and the roadway system is not stable. So, the reasonable width of the pillar should be in region IIa in Fig. 16, as this width can take advantage of the enhanced coal recovery rate while maintaining a stable condition. Fig. 16 shows a plot of the roadway deformation observed on-site by Wei and Cheng [13] in similar colliery (detailed measurement results are not available). The width of the pillar ranges from 12 to 14 m in this project, and the roadway is seriously deformed, which is caused by a large pressure imposed on it. Though detailed measurements are not available from this project due to the rapid coal extraction, but the results illustrated in Figs. 13–16 are probably appropriate.
2
9. Effect of the shape of coal pillar 1.5 1 0.5 0 0
5
10
15
20
25
pillar width (m) Fig. 14. The ratio of vertical peak stress to horizontal peak stress with respect to different pillar widths.
divided into three sub-regions. In regions IIa and IIc, the deformation of the roadway is relatively small, while the deformation is relatively large in region IIb. In Fig. 16, the width of the pillar at point A is the critical width at which the location of the peak stress in the coal stratum will have a sudden change, i.e. the peak stress jumps from the coal pillar to the coal seam. When the pillar width is large enough (region IV), the roadway system is in a stable state. When the width of the pillar approaches the critical value (region III), then the roadway will be in a poor stress
As early as in 1907, Daniels and Moore [15] discovered that the larger the coal specimen, the less will be its strength. Furthermore, the strength of the sample is found to decrease with the height of the specimen. Bunting [16] called these two phenomena as size effect and shape effect. It has been determined by numerous tests that a coal pillar has a critical size. When the size of the coal specimen exceeds the critical size, the strength does not decrease with further increase in size. Bieniawski and Van Heerden [2] reported that the critical size is about 1.5 m in a coal seam in South Africa. Pariseau et al. [17] reported that the critical size is about 0.9 m in a coal seam in western USA. Hustrulid [3] suggested that 0.9 m can be taken as the general critical size in coal mining. In this present study, the pillar size is definitely larger than the critical size, so the size effect is neglected here. The shape effect has been discussed by many researchers but there is no consensus about this effect. Many researchers believe that the coal strength is affected more by the shape than by the size. When the shape effect (the ratio of width to height) is considered, the commonly used coal pillar strength can be divided into two categories, namely, linear expression and exponent
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
3.35
3.4
3.70
3.70
1.4
1.8
1.7
1.2
Before deformation
1.6
2.9
Width = 12m
After deformation
3.08
2.9
3.70
3.70 Width = 14m
Width = 13m Note :
1.8 2.96 3.70
Width = 7.5m
2.2
2.1
1.6
2.9
3.38
Width = 7m
2.9
2.96
2.7
2.9
2.1
2.0
1.88
2.9
3.30
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The selected crosses are where the deformation is serious; The unit is meter. Fig. 15. Schematic plot of a roadway condition as observed at a site.
pillar width from small to large I worse stability
II better stability II a small deformation
II b
III worse stability II c
IV better stability
A
large small deformation deformation
Fig. 16. The division of stability area of roadway system with respect to different pillar widths.
expressions [18], which are given as
ss ¼ K1 ½Aþ BðW=hÞ
ð2Þ
ss ¼ K2 W a =hb
ð3Þ
where ss is coal strength (MPa), K1 and K2 are the limit cube strength on-site (MPa), and A and B are dimensionless and A+B¼1, a and b are dimensionless parameters in the range 0.33–0.67. Eq. (3) indicates that the coal strength increases with the increase in the ratio of width to height. When this ratio is more than 10, the coal pillar is unlikely to fail. Wu et al. [19] examined the shape effect of the coal pillar by finite element analysis and arrived at the conclusion that the coal strength increases with the ratio of width to height. Beyond a ratio of 8, the pillar strength will be stable and will not increase with any further increase in the ratio. In the present work, the roadway height (pillar height) is 2.7 m and the pillar width is 20 m. The ratio of width to height is hence 7.4, which is slightly less than 8. On the basis of the work by Wu et al. [19], the pillar has nearly attained its maximum strength. The above analysis shows that the roadway system is in a stable state when the width of the pillar is 20 m.
The six different models used here have the same roadway height of 2.7 m. In order to investigate the influence of roadway height, another model was developed in which the roadway height is 5.4 m and the pillar width is 20 m. It can be seen that the whole pillar is at the yield state and there is no elastic region in the pillar center (Fig. 17). The distribution of the vertical stress is compared with that of the previous three models (Fig. 18). It can be seen that the vertical stress distribution (Fig. 18d) is similar to the previous model with a 10 m wide pillar (Fig. 18a). The difference between the peak stress in the coal pillar and in the coal seam is greater when the roadway height is 2.7 m. However, beyond some distance away from the working face, the peak stress in the coal seam is larger than that in the coal pillar in both models. Even if the ratio of width to height is the same, the roadway with the greater height is less stable, which is expected. For coal extraction, both the width and the height (roadway height) of the coal pillar are the variables that will determine the stability of the roadway system. In general, the ratio of width to height is a major variable of the roadway system, but the actual sizes of the pillars may affect the stability of the system. In practice, the roadway height is often controlled by the mining
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Fig. 17. Failure zone in coal pillar (pillar width¼ 20 m, roadway height ¼ 5.4 m).
8.75 m
100
11.25 m 21.25 m
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.25 m 105 m
80
31.25 m 46.25 m
60
105 m
120 100 szz (MPa)
szz(MPa)
120 3.75 m
80 60
40
40
20
20 0
0 -50
-40
-30
-20
-10
0
10
20
30
40 -50
-40
-30
-20
-10
0
10
20
30
40
-10 0 L (m)
10
20
30
40
L (m)
100 80 60
szz (MPa)
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.2 5m 105 m
120
szz (MPa)
3.75 m 8.75 m 11.25 m 21.25 m 31.25 m 46.25 m 105 m
L (m)
40 20 -50
-40
-30
-20
0 -10 0 L (m)
10
20
30
40 -50
-40
-30
-20
100 90 80 70 60 50 40 30 20 10 0
Fig. 18. The vertical stress at different distances from the working face: (a) pillar width¼ 10 m, roadway height ¼2.7 m, (b) pillar width ¼15 m, roadway height¼ 2.7 m, (c) pillar width¼ 20 m, roadway height ¼ 2.7 m and (d) pillar width¼ 20 m, roadway height ¼ 5.4 m.
conditions, so the width of the pillar will be the main variable that can be controlled by the engineers.
10. Discussion and conclusions From the present study, it was found that in front of the working face, there is a range that is seriously affected by top coal mining. In this range, stress increases rapidly to a peak value. In the coal pillar, the peak stress was located at about 5–20 m in front of the working face and in the coal seam the peak stress is at about 16–20 m in front of the working face. Beyond the seriously affected range, with increase in pillar width, the stress in the coal seam is always decreasing, while the stress in the coal pillar is increasing. When the width of the pillar
is about 15 m, the stress in the coal seam reaches the peak value and then remains basically unchanged with further increase in width of the pillar. When the width is less than 10 m, the peak stress is located in the coal seam. When the pillar width is greater than 15 m, the peak stress is located in the pillar. This indicates that with the increase in the width of the pillar, the mining pressure is initially imposed on the coal seam and is then transferred gradually to the coal pillar. At the same time, when the width of the pillar is less than 15 m, the failure zone runs through the whole pillar. When the width of the pillar is 20 m, there is an elastic region in the pillar center. Therefore, when the width of the pillar is 15 m, it is in an unfavorable stress environment and the whole pillar will break. In this situation, the stability of the roadway is worse and the roadway will experience major distortion. When the width of the pillar is
Y.M. Cheng et al. / International Journal of Rock Mechanics & Mining Sciences 47 (2010) 1372–1383
small (not more than 3 m), the pillar would be crushed and the roadway stability will also be worse. When the pillar is wide enough, such as 20 m, the pillar has the ability to resist the mining pressure, so the roadway stability will be improved. When the width of the pillar is between 5 and 10 m, though the pillar will suffer great mining pressure during the top caving mining, the peak stress will be transferred quickly to the coal seam and the ultimate pressure imposed on the pillar is not large. The roadway will still be in good condition in this case, and the integrity of the pillar can be maintained by proper supports. It should be emphasized that once the whole pillar is broken, excessive deformation cannot be avoided. When the width of the pillar is 10 m, the pillar is strengthened with a relatively high horizontal stress, so the deformation is relatively small. When the width of the pillar is not more than 7 m, the pillar is weakened. In this case, the wider the pillar, the greater will be the deformation. For a pillar width of 7 m, the roadway deformation will be slightly greater than is acceptable. Therefore, for the design of a reasonable pillar width for a mining panel along the goaf, there are three points to bear in mind. Firstly, the width of the pillar should not be close to the critical width (at this width, the peak stress is transferred from the coal seam to the coal pillar), since the roadway system will be in a bad stress environment and the width of the pillar is not large enough to resist the peak stress. Secondly, the width of the pillar should not be too small, since in this situation the load-bearing capacity of the pillar will be small and the pillar will be crushed. Thirdly, when the width of the pillar is small (less than the critical width but not very small), the roadway will have a relatively large deformation because of the small lateral constraint. The first two issues will cause serious problems for the roadway system. The last issue will not be too troublesome, since the deformation will be released only when the peak stress is imposed on it. When the pillar is in a relatively low stress condition, the deformation will not be large and integrity can be maintained with proper supports. The results from the present study apply to the geotechnical conditions given in Table 1 and Fig. 3 and for a specified roadway height. Nevertheless, the general behavior of stability and stress distribution of the roadway system with respect to pillar width and size will be applicable to fully mechanized top caving mining problems. Since the progress of coal extraction is extremely fast, no detailed measurement apart from visual examination is
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available during the extraction. On the basis of the present study, a pillar width of 6 m was used in this project and coal extraction was carried out with satisfaction. References [1] Donovan JG, Karfakis MG. Design of backfilled thin-seam coal pillars using earth pressure theory. Geotech Geol Eng 2004;22:627–42. [2] Bieniawski ZT, Van Heerden WL. The significance of in situ tests on large rock specimens. Int J Rock Mech Min Sci Geomech Abstr 1975;12:101–13. [3] Hustrulid WA. A review of coal pillar strength formulas. Rock Mech 1976;8: 115–45. [4] Sheorey PR. Design of coal pillar arrays and chain pillars. In: Hudson JA, editor. Comprehensive rock engineering, vol. 2. Oxford: Pergamon; 1993. p. 631–70. [5] Wilson AH. Research into the determination of pillar size, Part 1. The Mining Engineer, vol. 131. The Institution of Mining Engineers; 1972. p. 409–17. [6] Sheorey PR, Singh TN, Singh B. Considerations for the stability of longwall chain pillars and adjacent roadway. In: Farmer IW, editor. Proceedings of the symposium on strata mechanics, Newcastle-upon-Tyne. Amsterdam: Elsevier; 1982. p. 129–33. [7] Fadeev AB, Abdyldayev EK. Elastoplastic analysis of stresses in coal pillars by finite element method. Rock Mech 1979;11:243–51. [8] Hsiung SM, Peng SS. Chain pillar design for US longwall panels. J Min Sci Technol (China) 1985;2:279–305. [9] Thin IGT, Pine RJ, Trueman R. Numerical modeling as an aid to the determination of the stress distribution in the goaf due to longwall coal mining. Int J Rock Mech Min Sci Geomech Abstr 1993;30(7):1403–9. [10] Mukherjee C, Sheorey PR, Sharma KG. Numerical simulation of caved goaf behaviour in longwall workings. Int J Rock Mech Min Sci Geomech Abstr 1994;31(1):35–45. [11] Murali Mohan G, Sheorey PR, Kushwaha A. Numerical estimation of pillar strength in coal mines. Int J Rock Mech Min Sci 2001;38(8):1185–92. [12] Jaiswal A, Sharma SK, Shrivastva BK. Numerical modeling study of asymmetry in the induced stresses over coal mine pillars with advancement of the goaf line. Int J Rock Mech Min Sci 2004;41:859–64. [13] Wei WB, Cheng YM. Stress evolution in irregular pillar protected and bolting supported roadway retained along top-coal caving mining panel. Rock Mech Rock Eng 2009. in preparation. [14] Whittaker BN, Singh RN. Design and stability of pillars in longwall mining. Min Eng (London) 1979;138:59–70. [15] Daniels J, Moore LD. The ultimate crushing strength of coal. Eng Min J 1907;10:263–8. [16] Bunting D. Chamber pillar in deep anthracite mines. Trans AIME 1991;42: 236–45. [17] Pariseau WG, Hustrulid WA, Swanson SR, Van Sambeek LL. Coal pillar strength study. University of Utah, Mining Department, Report to US Bur Mines, Contract H0242059, 1977. [18] Xie HP, Duan FB, Zhou HW, Zhao GX. Recent developments of theory and analysis methods of strip pillar stability. China Min Mag 1998;7(5):37–41. in Chinese. [19] Wu LX, Wang JZ, Guo ZZ. Fundamentals of designing and monitoring for coal pillar. Beijing: China University of Mining and Technology Press; 2000. in Chinese.