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Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining
International Journal of Mining Science and Technology xxx (2016) xxx–xxx
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining Jiang Bangyou, Wang Lianguo ⇑, Lu Yinlong, Sun Xiaokang, Jin Gan State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 27 November 2015 Received in revised form 14 January 2016 Accepted 22 February 2016 Available online xxxx Keywords: Residual coal after room and pillar mining Repeated mining Fully mechanized caving face Roof control Support resistance
a b s t r a c t To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining, the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis. The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained. The results indicate that when the repeated mining face passes the residual pillars, the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face. A relatively stable ‘‘Voussoir beam” structure is formed after the advance fracturing of the main roof. When the repeated mining face passes the old goaf, as the large rock block revolves and touches gangue, the rock block will break secondarily under overburden rock loads. An example calculation was performed involving an integrated mine in Shanxi province, results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412 kN and 10,743 kN when repeated mining face passed through residual pillar and goaf. On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction For many years, the major working of small coal mines in China involved the room and pillar mining method. This mining configuration involved great degrees of risk and uncertainty, with numerous abandoned coal pillars and a low recovery rate. Consequently, large coal resources were abandoned. A relatively unique mining method, the repeated mining of residual coal, has been proposed for the retrieval of residual coal deposits that were damaged and abandoned by this old-style mining method [1,2]. This repeated mining is performed in a residual coal deposit damaged by disorderly mining. However, the abnormal roof fracturing and movement in such repeated mining practices have resulted in serious threats to the safety of workers and equipment, such as those associated with mine pillars [3,4]. In repeated mining of residual coal, there have been frequent accidents by way of abnormal roof collapse such as mine-face roof falls and support failures, resulting in great economic losses and casualties [5]. To address the challenges of abnormal pressure characteristics of the face roof and its control, Zhang et al. [6] conducted theoret-
⇑ Corresponding author. Tel.: +86 516 83885205. E-mail address: [email protected] (L. Wang).
ical analysis of a main roof fracture step size based on Ref. [7]. Believing that the roof rock in the residual coal zone rock is in a state of coexisting ‘‘complete-not completely collapse-complete collapse”, Zhang et al. proposed four structures for the roof above a residual-coal repeated mining face and constructed corresponding mechanical models. Ref. [8] developed a ‘‘dispersion-block” structural model for the repeated mining face roof and studied the roof structure instability and roof-fall mechanism associated with a residual pillar based on dispersion and block theories. Building on a study of Ref. [8], Zhu et al. [9] developed a force model for a repeated mining face support system associated with a ‘‘dispersion-block” roof structure and derived a calculation equation for support loading. It is inevitable that repeated mining of residual coal will encounter empty areas resulting from the original mining. Based on key-block theory, Bai and Hou [10] and Zhang et al. [11] established a mechanical model for an unsupported roadway roof and proposed a high-water-content backfill material and a roof-control method when the mining face passes through empty roadways. Liu et al. [5] developed a fracture mechanical model for a repeated mining face that passes under an unsupported roadway roof to reveal the basic initial fracture mechanism; they also derived calculation equations for support loading to represent conditions before and after the repeated mining face passes through an empty roadway.
http://dx.doi.org/10.1016/j.ijmst.2016.05.017 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
However, current research on the damage characteristics of roof movements and control mechanism when a repeated mining face passes through old mining residual pillars and goafs is relatively scarce. Old mining residual pillars and goafs alternate in small coal mines, which significantly increases the roof fracture complexity and control difficulties in repeated mining. To address issues such as ground pressure abnormalities and roof control difficulties in a repeated mining fully mechanized caving face of residual coal after small scale room and pillar mining, a specific residual coal repeated mining project in an integrated mine in Shanxi province was studied. The repeated mining face roof movements and fracture characteristics were analyzed, and a mechanical model of the roof-support structure for a mining face that passes through old residual pillars and goafs was developed. The study results yielded repeated mining face roof-control mechanisms, a reasonable resistance calculation formula for the working resistance of support, and a basis for selecting supports and working resistance settings in a repeated mining face. 2. Geological overview of the project The integrated coal mine in Shanxi consists of five small coal mines and has a design production capacity of 0.60 Mt/a, primarily from mining of 3# coal seam. This coal layer is at an average depth of 370 m and has a thickness of 3.06–5.02 m, with an average of 4.23 m. The coal is sufficiently stable for mining. The intermediate roofs are composed mostly of siltstone and mudstone with a thickness of 15.0–18.1 m. The roof rocks are thinly to moderately thickly bedded, and this particular level contains abundant plant debris and relatively well-developed fissures. The floor is composed of black mudstone with some siltstone. Limited by coal mining technology and equipment, small coal mines have been using room and pillar mining for the 3# coal seam prior to integration, with a criss-crossing roadway layout. A site survey revealed that the mining of small coal mines has usually included manual construction of cut-in holes in the floor of 3# coal seam. These holes are 2 m wide, 2 m high, and approximately 20 m long, and the pillar spacing is approximately 15 m. Subsequently, rib expansion was performed on the cut-in holes a distance of 2– 3 m on either side. A residual coal pillar was formed between adjacent goafs after mining; these residual pillars are roughly 20 m long and 11 m wide, as shown in Fig. 1. The mine roadways are approximately 3 m wide and 2 m high. This type of old small coal mine room and pillar mining poses many difficulties for control of the repeated mining face roof. 3. Numerical simulation of the repeated mining face 3.1. Model construction The numerical calculation software product FALC3D was used to construct a numerical model. The model takes into account 6m
11 m
2m
4.23 m
3m
20 m
Old goaf
Old roadway
6m Residual coal pillar
Fig. 1. Small coal mine room and pillar mining layout (m).
the geologic conditions of repeated mining of the 3rd-layer residual coal in such a mine. The dimensions of the model are 222 m 158 m 69.5 m. The model includes displacement control boundaries. The side horizontal displacement is set to zero, and the vertical and horizontal displacements of the bottom are both set to zero. The top surface is allowed to move freely. A uniform vertical load of 7.5 MPa is exerted on the top surface to simulate the load from the weight of the overlaying strata. The model uses Mohr–Coulomb constitutive relations. Due to the original mining damage and the long-term effects of the overlying rock load, the residual coal layer is modeled as a lowelasticity-modulus medium with a Poisson’s ratio of nearly zero [12]. Specific parameters are shown in Table 1. 3.2. Simulated plan After constructing the initial model, a null command is used to model a series of original 3 m 2 m (width height) roadways and 20 m 6 m 2 m (length width height) goafs. The coal pillars cut by the roadways and goafs are the residual coal pillars, with lengths and width of 20 m and 11 m, respectively. Model calculations were performed until a balance was achieved. The coal shearing is simulated by stepped excavation of the repeated mining face. Due to the limitations of the model’s dimension, the excavation step size is set to 1 m, the sheared excavation height is set to 2 m, and the support coal caving is set to 2.23 m. The simulated excavation plan is shown in Fig. 2. In the simulation, to characterize the continuous advancement of the mining face and the delayed impact of caving on the mining face advancement, 98% of the maximum balance force release rate [13,14] is selected to limit the number of calculation steps between two cavings of the mining face. 3.3. Analysis of simulation results 3.3.1. Repeated mining face passing residual coal pillar The curves of vertical stresses and deformation associated with the repeated mining face roof with various advancing step sizes when passing a residual coal pillar are shown in Fig. 3. Due to the old residual coal pillars and goafs, the roof vertical stress and deformation curves in front of the repeated mining face generally display wavy variations. Fig. 3a shows that after exposing an old residual coal pillar and as the mining face continues to advance, the vertical stress in the roof above the coal pillar in front of the mining face gradually decreases. The roof vertical stress above the second coal pillar in front of the mining face gradually decreases after an initial increase. The roof stress gradually increases starting from the third coal pillar, and the amount of increased stress decreases with distance from the mining face. Moreover, as shown in Fig. 3b, with continuous advancement of the repeated mining face, the roof deformation in front of the mining face displays a gradual increasing trend; however, abnormality appears at the second coal pillar in front of the face. As the mining face advances, the deformation at the second coal pillar gradually increases. When the advancement of the mining face into the coal pillar exceeds 6 m, the deformation of the roof above the first pillar and goaf increases sharply, whereas the deformation of the second coal pillar starts to decrease and displays a roof rock ‘‘rebound” phenomenon. Fig. 4 shows variations, with various sizes of advancement steps, in the extent of elastic deformation in the repeated mining face roof rock upon passing the residual coal pillar. As shown in Fig. 4, due to the impact of repeated mining face advance pressure bearing, the exposed coal pillar is already in a state of elastic deformation as the face exposes the pillar. At the initial stage of repeated mining face excavation, there is no clear
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Table 1 Rock formation mechanics parameters. Lithology
Thickness (m)
Medium-grained sandstone Sandy mudstone Fine sandstone Sandy mudstone 3# coal Mudstone Medium-grained sandstone
Bulk density (kg/ m3)
Shear modulus (GPa)
Bulk modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
6.25
2600
3.10
5.70
30
3.0
2.3
11.60 17.30 4.50 4.23 4.30 5.20
2400 2550 2400 1400 2350 2600
3.10 5.90 3.20 0.61 2.30 5.60
5.65 8.20 6.20 0.92 5.90 7.80
28 40 28 22 30 30
2.6 7.8 2.4 1.7 2.6 8.9
2.0 4.6 1.8 0.8 2.3 5.2
Overlying strata Main roof
Coal pillar 11 m
2m
Hydraulic support Boundary pillar 40 m
Immediate roof Residual coal
Boundary pillar 6m
Floor strata
Old goaf
Repeated mining face
Fig. 2. Simulated excavation plan of repeated mining face.
change in the extent of elastic deformation to the roof. As the face advances 6 m into the coal pillar, the elastic deformation suddenly increases. As the mining face continues to advance, the elastic deformation increases significantly above the first goaf and the second coal pillar. In summary, it can be inferred that the main roof rock strata starts to display bending and rotation as the repeated mining face passes though the coal pillar. When the face advances 6 m into the coal pillar, the load borne by the remaining width of the pillar in front of the face reaches the limit under the impact of the advance pressure bearing. As the face advancement continues, failure of the entire coal pillar occurs, and the main roof atop the goaf suddenly fractures. Due to this fracturing, the roof rock strata in front of the fracture zone appears to ‘‘rebound”.
4. Model of roof movement and fracturing associated with repeated mining 4.1. Roof structure model as the mining face passes coal pillar Based on the numerical simulation results, the old mining residual coal pillar has already experienced elastic failure under the force of the long-term load of the overlying strata. As the repeated mining face advances, the width of the coal pillar is gradually reduced. The failure of the pillar will occur as a result of forces induced by the repeated mining face and bearing pressures,
35
0.45
30
0.40 Advancing direction of the face
25 20 15 10
Deformation of roof (m)
Roof vertical stress (MPa)
3.3.2. Repeated mining face while passing through a goaf The vertical stress and deformation curves of the repeated mining face roof while passing through a goaf at various advancement step sizes are shown in Fig. 5.
As shown in Fig. 5, the four stress curves and the four deformation curves essentially coincide. In other words, as the mining face advances, the changes in vertical stress and deformation in the roof in front of the mining face are very small; i.e., the impact of excavation on the roof stress distribution and deformation is relatively low as the repeated mining face passes through the goaf. The extent of elastic deformation in the roof rock as the repeated mining passes through the goaf with various step sizes is shown in Fig. 6. As with the variations of trends in the roof vertical stresses and deformations, the roof elastic deformation does not vary as the mining face advances. It can be further inferred that the face roof in the goaf already contains fractures as the repeated mining face passes through the coal pillar; the collapse of the residual coal layer above the goaf and of the roof rock in certain region around the goaf has already occurred. Furthermore, a relatively stable ‘‘Voussoir beam” structure forms above the goaf [15–17], and the repeated mining face passing through the goaf is affected only by the collapsed coal and rock in the goaf; the effect on the main roof rock strata movement above is relatively small.
0.35 0.30
Repeated mining face exposes the coal pillar Repeated mining face advances 2 m in coal pillar Repeated mining face advances 4 m in coal pillar Repeated mining face advances 6 m in coal pillar Repeated mining face advances 8 m in coal pillar Repeated mining face advances 10 m in coal pillar
0.25
Advancing direction of the face
0.20 0.15 0.10
5 0.05 0
6 m 11 m
Location of the face corresponding to the stress curve
Coal pillar Old goaf
(a) Vertical stress in roof with advancement of mining face
Coal pillar
6 m 11 m
0 Location of the face corresponding to the deformation curve
Old goaf
(b) Deformation of roof with advancement of mining face
Fig. 3. Repeated mining face roof vertical stress and deformation curves when passing coal pillars.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
(a) Face exposing residual coal pillar
(b) Face advancing 2 m into residual coal pillar
(c) Face advancing 4 m into residual coal pillar
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n Tension-n shear-p tension-p Tension-n tension-p Tension-p (d) Face advancing 6 m into residual coal pillar
(f) Face with completed residual coal pillar excavation
(e) Face advancing 8 m into residual coal pillar
Fig. 4. Elastic deformation in the repeated mining face roof rock while passing a residual coal pillar.
35
0.20
Repeated mining face exposes the old goaf Repeated mining face advances 2 m in the goaf Repeated mining face advances 4 m in the goaf Repeated mining face advances 6 m in the goaf
25
Deformation of roof (m)
Roof vertical stress (MPa)
30
20 15 10
0.15
0.10 Advancing direction of the face 0.05
5 0
6 m 11 m
Advancing direction of the face
0
Location of the face corresponding to the deformation curve 6 m 11 m
-5 Old goaf Coal pillar Location of the face corresponding to the stress curve
Old goaf
(a) Vertical stresses in roof
Coal pillar
(b) Roof deformation
Fig. 5. Repeated mining face roof vertical stress and deformation when passing through a goaf.
None Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p Tension-p
(a) Face exposing goaf
(b) Face advancing 2 m into goaf
(c) Face advancing 4 m into goaf
None Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p Tension-n tension-p Tension-p
(d) Face upon exiting goaf
Fig. 6. Elastic deformation in roof rock as the repeated mining face passes through a goaf.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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thereby causing a sudden increase in the length of the unsupported roof. The roof above the original goaf will fracture prematurely and create an enlarged rock block above the repeated mining face and the support beams, as shown in Fig. 7. The main roof enlarged rock block B is the key block body that affects the development of the repeated mining face ground pressure and the stress in the support. The stability of rock block B controls the intensity of the repeated mining face ground pressure and the stability of the supports [18].
Immediate roof
A
B-1
The residual coal layer above an old goaf and parts of the intermediate roof in small coal mines are subjected to long-term overlying rock loads. Due to these effects and the forces associated with the repeated mining face, caving damage has already occurred, and blocks are scattered in the goaf. The supports are exposed in the goaf as the repeated mining face passes through, rendering these supports easy to excavate as the main roof collapses. It is also easy for the loose coal and rock blocks in the goaf to enter the mining area, thereby affecting coal production. To mitigate these adverse conditions, the excavation-after-filling technique [19] is typically used as the repeated mining face passes through the goaf. Based on the numerical simulation, when the repeated mining face passes through the coal pillar, enlarged main roof rock block B is formed (Fig. 7). As the face continues to advance, the width of the coal pillar between the face and the goaf in front is gradually reduced. Enlarged rock block B in the roof above the face will contact the waste rock during its rotation, thereby creating a twopoint support. Under the force of the overlying rock load, it is very possible that rock block B may fracture again [20], forming main roof rock blocks B-1 and B-2, as shown in Fig. 8. The rotational deformation of main roof rock block B-1 is the main factor controlling the face ground pressure development and support stress.
B-2
Goaf
Coal pillar Grouting reinforcement
4.2. Model of the roof structure as the mining face passes the old goaf
Main roof
Repeated mining face
Fig. 8. Roof structure behavior as the repeated mining face passes a coal pillar.
the main roof that moves with the roof, q1 is the pillar support force imposed on the roof in front of the face, and a is the remaining coal pillar width in front of the face. The thickness of the main roof rock block B is h. Based on the unit widths of the supports of main roof rock block B, an analysis based on the force and moment balance condition yields the following equation:
P
P P
9 > =
Fx ¼ TA TB ¼ 0 Fy ¼ Q A þ q1 a þ P0 ql0 F 0 Q B ¼ 0 M o ¼ M q þ M F 0 þ M Q B M q1 M P 0 M T B ¼ 0
ð1Þ
> ;
The moments acting on rock block B produced by the forces on the rotational axis are
9 > > > > > > > > > =
2
Mq ¼ 12 ql0 cos h
MF 0 ¼ 12 F 0 l0 cos h MQ B ¼ Q B l0 cos h þ h 2b sin h > Mq1 ¼ q1 a l1 2a > > > > > > MP0 ¼ P0 ðl1 þ l2 Þ > > ; MT B ¼ T B ðh b l0 sin hÞ
ð2Þ
5. Mechanical analysis of repeated mining face roof-support structure
The heights of the contacts between block B and the two adjacent rocks can be calculated using the following equation [21]:
5.1. Roof-support structure as the face passes a coal pillar
b¼
Based on the model of the main roof movement and fracturing, a simplified mechanical model of the roof-support structure during passage of a repeated mining face through a coal pillar was developed to analyze the stress state of key block B, as shown in Fig. 9. In the above figure, l0 is the length of key block B, l1 is the distance between the advance fracture location of main roof rock block B and the face coal wall, l2 is the distance between the support resistance force point and the coal wall, h is the rotational angle of key rock block B, b is the contact location heights of rock blocks A and B, P0 is the support resistance force through the top coal or the intermediate roof in rock block B, F0 is the weight of block B, q is the load on block B imposed by the soft rock layer atop
The formula for calculating the force TB by rock block C on rock block B is [17]
h l0 sin h 2
TB ¼
ð3Þ
l0 ðql0 þ F 0 Þ 2ðh l0 sin hÞ
ð4Þ
The length of enlarged rock block B, l0, is exactly the periodic pressure appearance step size. The weight of rock block B, F0, can be calculated using the equation:
F 0 ¼ l0 dhc
ð5Þ
l0 q
A
Main roof
B
Immediate roof
Coal pillar
Scattered coal and rock
Coal pillar
Main roof C
Goaf
A b/2
Immediate roof
Coal pillar
TA
F0
O QA
q1
Repeated mining face
Fig. 7. Roof structure as the repeated mining passes the coal pillar.
P0
b/2 TB
θ
Coal pillar
Scattered coal and rock
a Old goaf
QB
B
l2
l1
Fig. 9. Mechanical model of the roof as the repeated mining face passes a coal pillar.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
in which d is the width of the face hydraulic support, in units of m; and c is the average bulk density of the rock overlying the repeated mining face, in units of kN/m3. Due to the age of such small room and pillar mines, serious elastic failure of the residual coal pillar has already occurred as a result of long-term overlying rock loads and goaf flooding. Therefore, the roof supporting force q1 provided by the remaining coal pillar can be taken as approximately the limit residual strength. Neglecting the compressional deformation of the repeated mining face coal layer and intermediate roof rock layer and assuming that rock block B has not touched the waste rock in the goaf, the support to rock block B by the goaf waste rock may be neglected. After substituting Eqs. (2)–(5) in Eq. (1), the results are
l0 ðql0 þ F 0 Þ TA ¼ 2ðh l0 sin hÞ QA ¼
Enlarged rock block B is the key block that controls the development of ground pressure and support stress as the repeated mining face passes through the coal pillars. The roof control by the repeated mining face is the primary factor preventing the sliding instability of main roof rock block B [22,23]. To prevent the sliding instability of block B, the following condition must be met [21]:
ð8Þ
in which f is the friction coefficient between the fractured rocks and is typically assigned a value of 0.2. After substituting Eq. (7) in Eq. (8), simplification yields the support resistance through the top coal layer and intermediate roof to rock block B, P0, which is needed to prevent sliding instability of rock block B and must satisfy the following condition: P0 P
ðql0 þ F 0 Þ½l0 cos h þ ð2h bÞ sin h q1 a½2l0 cos h þ ð2h bÞ sin h ð2l1 aÞ 2l0 cos h þ ð2h bÞ sinh 2ðl1 þ l2 Þ þ
l0 ðql0 þ F 0 Þ½2ðh b l0 sin hÞ 2l0 f cos h f ð2h bÞsin h 2ðh l0 sinhÞ½2l0 cos h þ ð2h bÞ sinh 2ðl1 þ l2 Þ ð9Þ
5.2. Mechanical analysis of roof-support structure as the mining face passes through the goaf Similarly, based on the main roof movement and fracture model when the repeated mining face passes through the goaf, a simplified roof-support structure mechanical model was developed to analyze the stress state of key block B, as shown in Fig. 10. Assuming that the secondary fracturing of enlarged main roof rock block B is located at the center of the rock, the lengths of rock 0 blocks B-1 and B-1 after fracturing are the same; both are l0 , 0 l0 ¼ 0:5l0 . Rock block B-1 contains advance fractures above the goaf. Due to the forces and rotational movements associated with the original and repeated mining, the old goaf is filled with the fallen residual top coal and intermediate roof rock. This fallen coal and rock, however, is piled loosely and provides relatively little support to the remaining roof and thus can be ignored. Repeating the derivation of Eqs. (1)–(8), the support resistance through the top coal layer and intermediate roof to rock block B, P 00 , needed to prevent sliding instability of rock block B, must satisfy the following condition:
T A'
F0'
O
b' /2
B-1
θ'
Q A'
Immediate roof
Q B'
b'/2 TB'
P0'
Coal pillar l 1'
l2
Fig. 10. Model of roof behavior as the repeated mining face passes through the goaf.
P00 P
2l0 cos h þ ð2h bÞ sin h l0 ðql0 þ F 0 Þðh b l0 sin hÞ þ þ ql0 þ F 0 q1 a Pð7Þ 0 ðh l0 sin hÞð2l0 cos h þ 2h sin h b sin hÞ
A
Main roof
ð6Þ
2 q1 að2l1 aÞ þ 2P0 ðl1 þ l2 Þ ql0 cos h F 0 l0 cos h
TA f P Q A
l 0' q
0
0
0
ðql0 þ F 00 Þ½l0 cos h0 þ ð2h b Þ sin h0 0 0 0 2l0 cos h0 þ ð2h b Þ sin h0 2ðl1 þ l2 Þ 0
þ
0
0
0
0
0
l0 ðql0 þ F 00 Þ½2ðh b l0 sin h0 Þ 2l0 f cos h0 f ð2h b Þ sin h0 0 0 0 0 2ðh l0 sin h0 Þ½2l0 cos h0 þ ð2h b Þ sin h0 2ðl1 þ l2 Þ ð10Þ
5.3. Reasonable support working resistance calculation example Using the repeated mining face of the integrated mine as an example, the reasonable support working resistance was calculated. Based on the geologic conditions in the repeated mining face in this particular project, l0 ¼ 12 m, h ¼ 10 , d ¼ 1:5 m, c ¼ 25 kN/ m3, l1 ¼ 10 m, l2 ¼ 1:5 m, a ¼ 5 m, h ¼ 5:4 m, f = 0.2, q ¼ 400 kN/ 0
m, and q1 ¼ 1:0 103 kN/m. In addition, l0 ¼ 6 m, h0 ¼ 20 , and 0 l1 ¼ 4 m. After substituting these values in Eqs. (9) and (10) and combining the results with Eqs. (3) and (5), the support resistance through the top coal layer and intermediate roof to rock block B, which is needed to prevent sliding instability of rock block B, must satisfy the inequality P0 P 10; 435 kN as the repeated mining face passes the residual coal pillar. In addition, the total thickness of the coal layer is M ¼ 4:3 m; the repeated mining face is formed by way of fully mechanized caving, and the coal caving height is 2 m. Therefore, the thickness of the top coal above the face support is Md = 2.3 m. The intermediate roof thickness is hz = 4.5 m, the support roof control distance is lk = 4.5 m, and the bulk density of the coal layer is approximately cM = 13 kN/m3. Consequently, the weight load of the overlying coal and intermediate roof above the face support is given by Eq. (11):
P1 ¼ lk dMd cM þ lk dhz c ¼ 977 kN
ð11Þ
When the repeated mining face passes the residual coal pillar and goaf, to ensure that sliding instability of the key rock block above the face does not occur and to ensure the stability of the face roof, the minimum face support working resistance must meet the conditions given by the following Eqs. (12) and (13):
PS ¼ P 0 jmin þ P 1 ¼ 10435 þ 977 ¼ 11; 412 kN P0S ¼ P 00 jmin þ P 01 ¼ 9766 þ 977 ¼ 10; 743 kN
ð12Þ ð13Þ
Because residual goafs and coal pillars are typically side by side in residual coal after the old room and pillar mining, adequate resistance of face supports should be ensured when performing repeated mining of coal. The absence of sliding instability of key rock blocks above the face should be ensured when the repeated mining face passes through residual coal pillars and goafs, thereby achieving stable control of the face roof and avoiding accidents such as roof falls and frame pressing.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx 11000
Resistance of hydraulic support (kN)
10000 9000 8000 7000 6000 Residual coal pillar
5000
6m
Old goaf
11 m
4000 0
4
8
12
16
20
24
28
32
36
40
Advancing distance of repeated mining face (m)
Fig. 11. Average end-of-cycle resistance of repeated mining face support.
6. On-site project verification The repeated mining face in the case study used for verification involved fully mechanized caving coal hydraulic support via model ZF12000/18.5/35 and used the KJ34 mining support monitoring system to monitor the face support resistance. Before performing the repeated mining, three KJ345 model mechanized caving coalsupport pressure-monitoring stations were installed. Two hydraulic supports were each positioned at the upper, middle, and lower portions of the face for monitoring. The monitoring results were averaged, and the results are shown in Fig. 11. Fig. 11 shows that as the repeated mining face passed the first residual coal pillar, the width of the residual coal pillar was reduced to 3.0 m, and as the repeated mining face advanced, the coal pillar suddenly lost stability and failed. The roof rock created fracturing above the old goaf, thereby causing the end-of-cycle support resistance to suddenly increase. When the repeated mining face passed the second residual pillar, a loss-of-balance failure occurred when the remaining width of the residual pillar reached 4.8 m. The final support resistance also suddenly increased, which is essentially consistent with the results of the numerical calculations. The figure also indicates that there was a consistent increase in the support resistance as the repeated mining face passed the old goaf. This result verified the conclusion that the secondary fracturing will occur during the rotational movement and the contacting of the waste rock with the old enlarged roof rock block above the face. The monitoring results indicate that the maximum working resistance values of the support as the repeated mining face passes the residual coal pillar and goaf are 10,521 kN and 9983 kN, respectively. These values are slightly less than the minimum working resistance values obtained from the theoretical calculations. Therefore, both the mechanical model and the support resistance calculation formulas are reasonable. The support-type and working-resistance settings based on the calculations can satisfy the requirement of a stable roof during the re-working of an old mine by way of a repeated mining face. 7. Conclusions This study focused on roof movements and fracture characteristics associated with a repeated mining face of residual coal after room and pillar mining based on numerical simulation and theoretical analysis. A roof structure mechanical model was developed, and analysis revealed the mechanisms controlling roof behavior associated with advancement of the repeated mining face. The main conclusions are as follows: (1) As the repeated mining face passes a coal pillar, there is gradual reduction of the coal pillar width, and as the bearing
7
limit of the remaining coal pillar is reached, instability of and damage to the pillar will occur, causing the headspace length of the main roof to suddenly increase. The main roof rock strata will develop fractures above the goaf, and thus the roof rock in front of the fracture will ‘‘rebound”. After this advance fracturing of the main roof, a relatively stable ‘‘Voussoir beam” structure will form above the goaf. The repeated mining face passing the goaf is affected only by the fallen coal and rock in the goaf; the impact on the main roof movement is relatively limited. (2) A roof-fracture structural model was constructed for a repeated mining face that passes a residual coal pillar and goaf. When the face passes a coal pillar, the sudden loss of stability of the coal pillar will cause the length of the main roof headspace to suddenly increase. The main roof rock strata will develop advance fractures above the goaf, thereby creating an enlarged rock block in the main roof. After rotational movement and contact of the waste rock with the enlarged rock block in the roof and with the continuing advancement of the face and the load imposed by the overlying rock, the rock block will develop secondary fracturing. (3) Based on the roof-fracture structural model, roof mechanical models for a repeated mining face passing an old residual coal pillar and goaf were developed. Based on key-block theory, the support and protection conditions for ensuring roof stability were derived. (4) Combined with the specific geologic condition of a repeated mining face in an integrated mine in Shanxi, the minimum reasonable support resistance when the repeated mining face passes a residual coal pillar and goaf was calculated. The on-site monitoring indicated that the mechanical model and the support load calculation are both reasonable. The results provide an important basis for selecting supports and working resistance settings in a repeated mining face of residual coal after the old room and pillar mining and provide references for roof control in similar conditions involving a repeated mining face.
Acknowledgments Financial support for this work, provided by the National Basic Research Program of China (No. 2014CB046905), Innovation Project for Graduates in Jiangsu Province (No. KYLX15_1405), the National Natural Science Foundation of China (Nos. 51274191 and 51404245), and the Doctoral Fund of Ministry of Education of China (No. 20130095110018), is gratefully acknowledged.
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[8] Zhang BS. Study on the surrounding rock control theory and technology of ultra-close multiple-seams mining. Taiyuan: Taiyuan University of Technology; 2008. [9] Zhu T, Zhang BS, Feng GR, Zhang XY, Kang LX. Roof structure and control in the lower seam mining field in the ultra-close multiple seams. J China Coal Soc 2010;35(2):190–3. [10] Bai JB, Hou CJ. Research on principle of roof stability of abandoned workings and supporting technology. J China Coal Soc 2005;30(1):8–11. [11] Zhang ZZ, Bai JB, Han ZT, Wang XY, Xu Y, Wang M. Roof mechanics analysis and backfill technology for abandoned roadway. J Min Safety Eng 2013;30 (2):194–8. [12] Liu DP, Tang HB, Zhao YS. Numerical simulation study on stress distribution in the roof and floor for repeated mining of residue coal after pillar and chamber mining. J Taiyuan Univ Technol 2014;45(3):389–93. [13] Vlachopoulos N, Diederichs MS. Improved longitudinal displacements profiles for convergence confinement analysis of deep tunnel. Rock Mech Rock Eng 2009;42:131–46. [14] Lu YL, Wang LG, Zhang B. Roadway failure and support in a coal seam underlying a previously mined coal seam. Int J Min Sci Technol 2012;22 (5):619–24. [15] Xu JL, Ju JF. Structural morphology of key strata its influence on strata behavior in fully-mechanized face and with super great mining height. Chinese J Rock Mech Eng 2011;30(8):1547–56.
[16] Han CL, Zhang N, Yao YH, Zhang NC, Ji M. Transfer bearing mechanism of thick composite roof in gob-side entry retaining. Rock Soil Mech 2013;34 (S1):318–23. [17] Lu JF, Xu JL. Structural characteristics of key strata and strata behaviour of a fully mechanized longwall face with 7.0 m height chocks. Int J Rock Mech Min Sci 2013;58(2):46–54. [18] Su H, Bai JB, Yan S, Chen Y, Zhang ZZ. Study on gob-side entry retaining in fully-mechanized longwall with top-coal caving and its application. Int J Min Sci Technol 2015;25(3):503–10. [19] Ma ZG, Fan JQ, Sun K, Zhao GZ, Pan YG. Study on stole stability repeated mining with fully-mechanized solid filling technology in residual coal pillar area. J Min Safety Eng 2011;28(4):499–504. [20] Tan YL, Jiang JQ, Song Y. The preliminary study of the second fracturing of hard roof over coalface. J Shandong Min Inst 1990;9(2):133–8. [21] Qian MG, Miao XX, Li LJ. Mechanism foe the fracture behaviours of main floor in longwall mining. Chinese J Geotech Eng 1995;17(6):55–62. [22] Li YF, Hua XZ. Mechanical analysis of stability of key blocks of overlying strata for gob-side entry retaining and calculating width of roadside backfill. Rock Soil Mech 2012;33(4):1134–40. [23] Lu HF, Yao DX, Shen D, Cao JY. Fracture mechanics solution of confined water progressive intrusion height of mining fracture floor. Int J Min Sci Technol 2015;25(1):99–106.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining Jiang Bangyou, Wang Lianguo ⇑, Lu Yinlong, Sun Xiaokang, Jin Gan State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 27 November 2015 Received in revised form 14 January 2016 Accepted 22 February 2016 Available online xxxx Keywords: Residual coal after room and pillar mining Repeated mining Fully mechanized caving face Roof control Support resistance
a b s t r a c t To investigate the abnormal ground pressures and roof control problem in fully mechanized repeated mining of residual coal after room and pillar mining, the roof fracture structural model and mechanical model were developed using numerical simulation and theoretical analysis. The roof fracture characteristics of a repeated mining face were revealed and the ground pressure law and roof supporting conditions of the repeated mining face were obtained. The results indicate that when the repeated mining face passes the residual pillars, the sudden instability causes fracturing in the main roof above the old goaf and forms an extra-large rock block above the mining face. A relatively stable ‘‘Voussoir beam” structure is formed after the advance fracturing of the main roof. When the repeated mining face passes the old goaf, as the large rock block revolves and touches gangue, the rock block will break secondarily under overburden rock loads. An example calculation was performed involving an integrated mine in Shanxi province, results showed that minimum working resistance values of support determined to be reasonable were respectively 11,412 kN and 10,743 kN when repeated mining face passed through residual pillar and goaf. On-site ground pressure monitoring results indicated that the mechanical model and support resistance calculation were reasonable. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction For many years, the major working of small coal mines in China involved the room and pillar mining method. This mining configuration involved great degrees of risk and uncertainty, with numerous abandoned coal pillars and a low recovery rate. Consequently, large coal resources were abandoned. A relatively unique mining method, the repeated mining of residual coal, has been proposed for the retrieval of residual coal deposits that were damaged and abandoned by this old-style mining method [1,2]. This repeated mining is performed in a residual coal deposit damaged by disorderly mining. However, the abnormal roof fracturing and movement in such repeated mining practices have resulted in serious threats to the safety of workers and equipment, such as those associated with mine pillars [3,4]. In repeated mining of residual coal, there have been frequent accidents by way of abnormal roof collapse such as mine-face roof falls and support failures, resulting in great economic losses and casualties [5]. To address the challenges of abnormal pressure characteristics of the face roof and its control, Zhang et al. [6] conducted theoret-
⇑ Corresponding author. Tel.: +86 516 83885205. E-mail address: [email protected] (L. Wang).
ical analysis of a main roof fracture step size based on Ref. [7]. Believing that the roof rock in the residual coal zone rock is in a state of coexisting ‘‘complete-not completely collapse-complete collapse”, Zhang et al. proposed four structures for the roof above a residual-coal repeated mining face and constructed corresponding mechanical models. Ref. [8] developed a ‘‘dispersion-block” structural model for the repeated mining face roof and studied the roof structure instability and roof-fall mechanism associated with a residual pillar based on dispersion and block theories. Building on a study of Ref. [8], Zhu et al. [9] developed a force model for a repeated mining face support system associated with a ‘‘dispersion-block” roof structure and derived a calculation equation for support loading. It is inevitable that repeated mining of residual coal will encounter empty areas resulting from the original mining. Based on key-block theory, Bai and Hou [10] and Zhang et al. [11] established a mechanical model for an unsupported roadway roof and proposed a high-water-content backfill material and a roof-control method when the mining face passes through empty roadways. Liu et al. [5] developed a fracture mechanical model for a repeated mining face that passes under an unsupported roadway roof to reveal the basic initial fracture mechanism; they also derived calculation equations for support loading to represent conditions before and after the repeated mining face passes through an empty roadway.
http://dx.doi.org/10.1016/j.ijmst.2016.05.017 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
However, current research on the damage characteristics of roof movements and control mechanism when a repeated mining face passes through old mining residual pillars and goafs is relatively scarce. Old mining residual pillars and goafs alternate in small coal mines, which significantly increases the roof fracture complexity and control difficulties in repeated mining. To address issues such as ground pressure abnormalities and roof control difficulties in a repeated mining fully mechanized caving face of residual coal after small scale room and pillar mining, a specific residual coal repeated mining project in an integrated mine in Shanxi province was studied. The repeated mining face roof movements and fracture characteristics were analyzed, and a mechanical model of the roof-support structure for a mining face that passes through old residual pillars and goafs was developed. The study results yielded repeated mining face roof-control mechanisms, a reasonable resistance calculation formula for the working resistance of support, and a basis for selecting supports and working resistance settings in a repeated mining face. 2. Geological overview of the project The integrated coal mine in Shanxi consists of five small coal mines and has a design production capacity of 0.60 Mt/a, primarily from mining of 3# coal seam. This coal layer is at an average depth of 370 m and has a thickness of 3.06–5.02 m, with an average of 4.23 m. The coal is sufficiently stable for mining. The intermediate roofs are composed mostly of siltstone and mudstone with a thickness of 15.0–18.1 m. The roof rocks are thinly to moderately thickly bedded, and this particular level contains abundant plant debris and relatively well-developed fissures. The floor is composed of black mudstone with some siltstone. Limited by coal mining technology and equipment, small coal mines have been using room and pillar mining for the 3# coal seam prior to integration, with a criss-crossing roadway layout. A site survey revealed that the mining of small coal mines has usually included manual construction of cut-in holes in the floor of 3# coal seam. These holes are 2 m wide, 2 m high, and approximately 20 m long, and the pillar spacing is approximately 15 m. Subsequently, rib expansion was performed on the cut-in holes a distance of 2– 3 m on either side. A residual coal pillar was formed between adjacent goafs after mining; these residual pillars are roughly 20 m long and 11 m wide, as shown in Fig. 1. The mine roadways are approximately 3 m wide and 2 m high. This type of old small coal mine room and pillar mining poses many difficulties for control of the repeated mining face roof. 3. Numerical simulation of the repeated mining face 3.1. Model construction The numerical calculation software product FALC3D was used to construct a numerical model. The model takes into account 6m
11 m
2m
4.23 m
3m
20 m
Old goaf
Old roadway
6m Residual coal pillar
Fig. 1. Small coal mine room and pillar mining layout (m).
the geologic conditions of repeated mining of the 3rd-layer residual coal in such a mine. The dimensions of the model are 222 m 158 m 69.5 m. The model includes displacement control boundaries. The side horizontal displacement is set to zero, and the vertical and horizontal displacements of the bottom are both set to zero. The top surface is allowed to move freely. A uniform vertical load of 7.5 MPa is exerted on the top surface to simulate the load from the weight of the overlaying strata. The model uses Mohr–Coulomb constitutive relations. Due to the original mining damage and the long-term effects of the overlying rock load, the residual coal layer is modeled as a lowelasticity-modulus medium with a Poisson’s ratio of nearly zero [12]. Specific parameters are shown in Table 1. 3.2. Simulated plan After constructing the initial model, a null command is used to model a series of original 3 m 2 m (width height) roadways and 20 m 6 m 2 m (length width height) goafs. The coal pillars cut by the roadways and goafs are the residual coal pillars, with lengths and width of 20 m and 11 m, respectively. Model calculations were performed until a balance was achieved. The coal shearing is simulated by stepped excavation of the repeated mining face. Due to the limitations of the model’s dimension, the excavation step size is set to 1 m, the sheared excavation height is set to 2 m, and the support coal caving is set to 2.23 m. The simulated excavation plan is shown in Fig. 2. In the simulation, to characterize the continuous advancement of the mining face and the delayed impact of caving on the mining face advancement, 98% of the maximum balance force release rate [13,14] is selected to limit the number of calculation steps between two cavings of the mining face. 3.3. Analysis of simulation results 3.3.1. Repeated mining face passing residual coal pillar The curves of vertical stresses and deformation associated with the repeated mining face roof with various advancing step sizes when passing a residual coal pillar are shown in Fig. 3. Due to the old residual coal pillars and goafs, the roof vertical stress and deformation curves in front of the repeated mining face generally display wavy variations. Fig. 3a shows that after exposing an old residual coal pillar and as the mining face continues to advance, the vertical stress in the roof above the coal pillar in front of the mining face gradually decreases. The roof vertical stress above the second coal pillar in front of the mining face gradually decreases after an initial increase. The roof stress gradually increases starting from the third coal pillar, and the amount of increased stress decreases with distance from the mining face. Moreover, as shown in Fig. 3b, with continuous advancement of the repeated mining face, the roof deformation in front of the mining face displays a gradual increasing trend; however, abnormality appears at the second coal pillar in front of the face. As the mining face advances, the deformation at the second coal pillar gradually increases. When the advancement of the mining face into the coal pillar exceeds 6 m, the deformation of the roof above the first pillar and goaf increases sharply, whereas the deformation of the second coal pillar starts to decrease and displays a roof rock ‘‘rebound” phenomenon. Fig. 4 shows variations, with various sizes of advancement steps, in the extent of elastic deformation in the repeated mining face roof rock upon passing the residual coal pillar. As shown in Fig. 4, due to the impact of repeated mining face advance pressure bearing, the exposed coal pillar is already in a state of elastic deformation as the face exposes the pillar. At the initial stage of repeated mining face excavation, there is no clear
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Table 1 Rock formation mechanics parameters. Lithology
Thickness (m)
Medium-grained sandstone Sandy mudstone Fine sandstone Sandy mudstone 3# coal Mudstone Medium-grained sandstone
Bulk density (kg/ m3)
Shear modulus (GPa)
Bulk modulus (GPa)
Friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
6.25
2600
3.10
5.70
30
3.0
2.3
11.60 17.30 4.50 4.23 4.30 5.20
2400 2550 2400 1400 2350 2600
3.10 5.90 3.20 0.61 2.30 5.60
5.65 8.20 6.20 0.92 5.90 7.80
28 40 28 22 30 30
2.6 7.8 2.4 1.7 2.6 8.9
2.0 4.6 1.8 0.8 2.3 5.2
Overlying strata Main roof
Coal pillar 11 m
2m
Hydraulic support Boundary pillar 40 m
Immediate roof Residual coal
Boundary pillar 6m
Floor strata
Old goaf
Repeated mining face
Fig. 2. Simulated excavation plan of repeated mining face.
change in the extent of elastic deformation to the roof. As the face advances 6 m into the coal pillar, the elastic deformation suddenly increases. As the mining face continues to advance, the elastic deformation increases significantly above the first goaf and the second coal pillar. In summary, it can be inferred that the main roof rock strata starts to display bending and rotation as the repeated mining face passes though the coal pillar. When the face advances 6 m into the coal pillar, the load borne by the remaining width of the pillar in front of the face reaches the limit under the impact of the advance pressure bearing. As the face advancement continues, failure of the entire coal pillar occurs, and the main roof atop the goaf suddenly fractures. Due to this fracturing, the roof rock strata in front of the fracture zone appears to ‘‘rebound”.
4. Model of roof movement and fracturing associated with repeated mining 4.1. Roof structure model as the mining face passes coal pillar Based on the numerical simulation results, the old mining residual coal pillar has already experienced elastic failure under the force of the long-term load of the overlying strata. As the repeated mining face advances, the width of the coal pillar is gradually reduced. The failure of the pillar will occur as a result of forces induced by the repeated mining face and bearing pressures,
35
0.45
30
0.40 Advancing direction of the face
25 20 15 10
Deformation of roof (m)
Roof vertical stress (MPa)
3.3.2. Repeated mining face while passing through a goaf The vertical stress and deformation curves of the repeated mining face roof while passing through a goaf at various advancement step sizes are shown in Fig. 5.
As shown in Fig. 5, the four stress curves and the four deformation curves essentially coincide. In other words, as the mining face advances, the changes in vertical stress and deformation in the roof in front of the mining face are very small; i.e., the impact of excavation on the roof stress distribution and deformation is relatively low as the repeated mining face passes through the goaf. The extent of elastic deformation in the roof rock as the repeated mining passes through the goaf with various step sizes is shown in Fig. 6. As with the variations of trends in the roof vertical stresses and deformations, the roof elastic deformation does not vary as the mining face advances. It can be further inferred that the face roof in the goaf already contains fractures as the repeated mining face passes through the coal pillar; the collapse of the residual coal layer above the goaf and of the roof rock in certain region around the goaf has already occurred. Furthermore, a relatively stable ‘‘Voussoir beam” structure forms above the goaf [15–17], and the repeated mining face passing through the goaf is affected only by the collapsed coal and rock in the goaf; the effect on the main roof rock strata movement above is relatively small.
0.35 0.30
Repeated mining face exposes the coal pillar Repeated mining face advances 2 m in coal pillar Repeated mining face advances 4 m in coal pillar Repeated mining face advances 6 m in coal pillar Repeated mining face advances 8 m in coal pillar Repeated mining face advances 10 m in coal pillar
0.25
Advancing direction of the face
0.20 0.15 0.10
5 0.05 0
6 m 11 m
Location of the face corresponding to the stress curve
Coal pillar Old goaf
(a) Vertical stress in roof with advancement of mining face
Coal pillar
6 m 11 m
0 Location of the face corresponding to the deformation curve
Old goaf
(b) Deformation of roof with advancement of mining face
Fig. 3. Repeated mining face roof vertical stress and deformation curves when passing coal pillars.
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(a) Face exposing residual coal pillar
(b) Face advancing 2 m into residual coal pillar
(c) Face advancing 4 m into residual coal pillar
None Shear-n Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n Tension-n shear-p tension-p Tension-n tension-p Tension-p (d) Face advancing 6 m into residual coal pillar
(f) Face with completed residual coal pillar excavation
(e) Face advancing 8 m into residual coal pillar
Fig. 4. Elastic deformation in the repeated mining face roof rock while passing a residual coal pillar.
35
0.20
Repeated mining face exposes the old goaf Repeated mining face advances 2 m in the goaf Repeated mining face advances 4 m in the goaf Repeated mining face advances 6 m in the goaf
25
Deformation of roof (m)
Roof vertical stress (MPa)
30
20 15 10
0.15
0.10 Advancing direction of the face 0.05
5 0
6 m 11 m
Advancing direction of the face
0
Location of the face corresponding to the deformation curve 6 m 11 m
-5 Old goaf Coal pillar Location of the face corresponding to the stress curve
Old goaf
(a) Vertical stresses in roof
Coal pillar
(b) Roof deformation
Fig. 5. Repeated mining face roof vertical stress and deformation when passing through a goaf.
None Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p Tension-p
(a) Face exposing goaf
(b) Face advancing 2 m into goaf
(c) Face advancing 4 m into goaf
None Shear-n shear-p Shear-n shear-p tension-p Shear-n tension-n shear-p tension-p Shear-p Shear-p tension-p Tension-n shear-p tension-p Tension-n tension-p Tension-p
(d) Face upon exiting goaf
Fig. 6. Elastic deformation in roof rock as the repeated mining face passes through a goaf.
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thereby causing a sudden increase in the length of the unsupported roof. The roof above the original goaf will fracture prematurely and create an enlarged rock block above the repeated mining face and the support beams, as shown in Fig. 7. The main roof enlarged rock block B is the key block body that affects the development of the repeated mining face ground pressure and the stress in the support. The stability of rock block B controls the intensity of the repeated mining face ground pressure and the stability of the supports [18].
Immediate roof
A
B-1
The residual coal layer above an old goaf and parts of the intermediate roof in small coal mines are subjected to long-term overlying rock loads. Due to these effects and the forces associated with the repeated mining face, caving damage has already occurred, and blocks are scattered in the goaf. The supports are exposed in the goaf as the repeated mining face passes through, rendering these supports easy to excavate as the main roof collapses. It is also easy for the loose coal and rock blocks in the goaf to enter the mining area, thereby affecting coal production. To mitigate these adverse conditions, the excavation-after-filling technique [19] is typically used as the repeated mining face passes through the goaf. Based on the numerical simulation, when the repeated mining face passes through the coal pillar, enlarged main roof rock block B is formed (Fig. 7). As the face continues to advance, the width of the coal pillar between the face and the goaf in front is gradually reduced. Enlarged rock block B in the roof above the face will contact the waste rock during its rotation, thereby creating a twopoint support. Under the force of the overlying rock load, it is very possible that rock block B may fracture again [20], forming main roof rock blocks B-1 and B-2, as shown in Fig. 8. The rotational deformation of main roof rock block B-1 is the main factor controlling the face ground pressure development and support stress.
B-2
Goaf
Coal pillar Grouting reinforcement
4.2. Model of the roof structure as the mining face passes the old goaf
Main roof
Repeated mining face
Fig. 8. Roof structure behavior as the repeated mining face passes a coal pillar.
the main roof that moves with the roof, q1 is the pillar support force imposed on the roof in front of the face, and a is the remaining coal pillar width in front of the face. The thickness of the main roof rock block B is h. Based on the unit widths of the supports of main roof rock block B, an analysis based on the force and moment balance condition yields the following equation:
P
P P
9 > =
Fx ¼ TA TB ¼ 0 Fy ¼ Q A þ q1 a þ P0 ql0 F 0 Q B ¼ 0 M o ¼ M q þ M F 0 þ M Q B M q1 M P 0 M T B ¼ 0
ð1Þ
> ;
The moments acting on rock block B produced by the forces on the rotational axis are
9 > > > > > > > > > =
2
Mq ¼ 12 ql0 cos h
MF 0 ¼ 12 F 0 l0 cos h MQ B ¼ Q B l0 cos h þ h 2b sin h > Mq1 ¼ q1 a l1 2a > > > > > > MP0 ¼ P0 ðl1 þ l2 Þ > > ; MT B ¼ T B ðh b l0 sin hÞ
ð2Þ
5. Mechanical analysis of repeated mining face roof-support structure
The heights of the contacts between block B and the two adjacent rocks can be calculated using the following equation [21]:
5.1. Roof-support structure as the face passes a coal pillar
b¼
Based on the model of the main roof movement and fracturing, a simplified mechanical model of the roof-support structure during passage of a repeated mining face through a coal pillar was developed to analyze the stress state of key block B, as shown in Fig. 9. In the above figure, l0 is the length of key block B, l1 is the distance between the advance fracture location of main roof rock block B and the face coal wall, l2 is the distance between the support resistance force point and the coal wall, h is the rotational angle of key rock block B, b is the contact location heights of rock blocks A and B, P0 is the support resistance force through the top coal or the intermediate roof in rock block B, F0 is the weight of block B, q is the load on block B imposed by the soft rock layer atop
The formula for calculating the force TB by rock block C on rock block B is [17]
h l0 sin h 2
TB ¼
ð3Þ
l0 ðql0 þ F 0 Þ 2ðh l0 sin hÞ
ð4Þ
The length of enlarged rock block B, l0, is exactly the periodic pressure appearance step size. The weight of rock block B, F0, can be calculated using the equation:
F 0 ¼ l0 dhc
ð5Þ
l0 q
A
Main roof
B
Immediate roof
Coal pillar
Scattered coal and rock
Coal pillar
Main roof C
Goaf
A b/2
Immediate roof
Coal pillar
TA
F0
O QA
q1
Repeated mining face
Fig. 7. Roof structure as the repeated mining passes the coal pillar.
P0
b/2 TB
θ
Coal pillar
Scattered coal and rock
a Old goaf
QB
B
l2
l1
Fig. 9. Mechanical model of the roof as the repeated mining face passes a coal pillar.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
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B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
in which d is the width of the face hydraulic support, in units of m; and c is the average bulk density of the rock overlying the repeated mining face, in units of kN/m3. Due to the age of such small room and pillar mines, serious elastic failure of the residual coal pillar has already occurred as a result of long-term overlying rock loads and goaf flooding. Therefore, the roof supporting force q1 provided by the remaining coal pillar can be taken as approximately the limit residual strength. Neglecting the compressional deformation of the repeated mining face coal layer and intermediate roof rock layer and assuming that rock block B has not touched the waste rock in the goaf, the support to rock block B by the goaf waste rock may be neglected. After substituting Eqs. (2)–(5) in Eq. (1), the results are
l0 ðql0 þ F 0 Þ TA ¼ 2ðh l0 sin hÞ QA ¼
Enlarged rock block B is the key block that controls the development of ground pressure and support stress as the repeated mining face passes through the coal pillars. The roof control by the repeated mining face is the primary factor preventing the sliding instability of main roof rock block B [22,23]. To prevent the sliding instability of block B, the following condition must be met [21]:
ð8Þ
in which f is the friction coefficient between the fractured rocks and is typically assigned a value of 0.2. After substituting Eq. (7) in Eq. (8), simplification yields the support resistance through the top coal layer and intermediate roof to rock block B, P0, which is needed to prevent sliding instability of rock block B and must satisfy the following condition: P0 P
ðql0 þ F 0 Þ½l0 cos h þ ð2h bÞ sin h q1 a½2l0 cos h þ ð2h bÞ sin h ð2l1 aÞ 2l0 cos h þ ð2h bÞ sinh 2ðl1 þ l2 Þ þ
l0 ðql0 þ F 0 Þ½2ðh b l0 sin hÞ 2l0 f cos h f ð2h bÞsin h 2ðh l0 sinhÞ½2l0 cos h þ ð2h bÞ sinh 2ðl1 þ l2 Þ ð9Þ
5.2. Mechanical analysis of roof-support structure as the mining face passes through the goaf Similarly, based on the main roof movement and fracture model when the repeated mining face passes through the goaf, a simplified roof-support structure mechanical model was developed to analyze the stress state of key block B, as shown in Fig. 10. Assuming that the secondary fracturing of enlarged main roof rock block B is located at the center of the rock, the lengths of rock 0 blocks B-1 and B-1 after fracturing are the same; both are l0 , 0 l0 ¼ 0:5l0 . Rock block B-1 contains advance fractures above the goaf. Due to the forces and rotational movements associated with the original and repeated mining, the old goaf is filled with the fallen residual top coal and intermediate roof rock. This fallen coal and rock, however, is piled loosely and provides relatively little support to the remaining roof and thus can be ignored. Repeating the derivation of Eqs. (1)–(8), the support resistance through the top coal layer and intermediate roof to rock block B, P 00 , needed to prevent sliding instability of rock block B, must satisfy the following condition:
T A'
F0'
O
b' /2
B-1
θ'
Q A'
Immediate roof
Q B'
b'/2 TB'
P0'
Coal pillar l 1'
l2
Fig. 10. Model of roof behavior as the repeated mining face passes through the goaf.
P00 P
2l0 cos h þ ð2h bÞ sin h l0 ðql0 þ F 0 Þðh b l0 sin hÞ þ þ ql0 þ F 0 q1 a Pð7Þ 0 ðh l0 sin hÞð2l0 cos h þ 2h sin h b sin hÞ
A
Main roof
ð6Þ
2 q1 að2l1 aÞ þ 2P0 ðl1 þ l2 Þ ql0 cos h F 0 l0 cos h
TA f P Q A
l 0' q
0
0
0
ðql0 þ F 00 Þ½l0 cos h0 þ ð2h b Þ sin h0 0 0 0 2l0 cos h0 þ ð2h b Þ sin h0 2ðl1 þ l2 Þ 0
þ
0
0
0
0
0
l0 ðql0 þ F 00 Þ½2ðh b l0 sin h0 Þ 2l0 f cos h0 f ð2h b Þ sin h0 0 0 0 0 2ðh l0 sin h0 Þ½2l0 cos h0 þ ð2h b Þ sin h0 2ðl1 þ l2 Þ ð10Þ
5.3. Reasonable support working resistance calculation example Using the repeated mining face of the integrated mine as an example, the reasonable support working resistance was calculated. Based on the geologic conditions in the repeated mining face in this particular project, l0 ¼ 12 m, h ¼ 10 , d ¼ 1:5 m, c ¼ 25 kN/ m3, l1 ¼ 10 m, l2 ¼ 1:5 m, a ¼ 5 m, h ¼ 5:4 m, f = 0.2, q ¼ 400 kN/ 0
m, and q1 ¼ 1:0 103 kN/m. In addition, l0 ¼ 6 m, h0 ¼ 20 , and 0 l1 ¼ 4 m. After substituting these values in Eqs. (9) and (10) and combining the results with Eqs. (3) and (5), the support resistance through the top coal layer and intermediate roof to rock block B, which is needed to prevent sliding instability of rock block B, must satisfy the inequality P0 P 10; 435 kN as the repeated mining face passes the residual coal pillar. In addition, the total thickness of the coal layer is M ¼ 4:3 m; the repeated mining face is formed by way of fully mechanized caving, and the coal caving height is 2 m. Therefore, the thickness of the top coal above the face support is Md = 2.3 m. The intermediate roof thickness is hz = 4.5 m, the support roof control distance is lk = 4.5 m, and the bulk density of the coal layer is approximately cM = 13 kN/m3. Consequently, the weight load of the overlying coal and intermediate roof above the face support is given by Eq. (11):
P1 ¼ lk dMd cM þ lk dhz c ¼ 977 kN
ð11Þ
When the repeated mining face passes the residual coal pillar and goaf, to ensure that sliding instability of the key rock block above the face does not occur and to ensure the stability of the face roof, the minimum face support working resistance must meet the conditions given by the following Eqs. (12) and (13):
PS ¼ P 0 jmin þ P 1 ¼ 10435 þ 977 ¼ 11; 412 kN P0S ¼ P 00 jmin þ P 01 ¼ 9766 þ 977 ¼ 10; 743 kN
ð12Þ ð13Þ
Because residual goafs and coal pillars are typically side by side in residual coal after the old room and pillar mining, adequate resistance of face supports should be ensured when performing repeated mining of coal. The absence of sliding instability of key rock blocks above the face should be ensured when the repeated mining face passes through residual coal pillars and goafs, thereby achieving stable control of the face roof and avoiding accidents such as roof falls and frame pressing.
Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017
B. Jiang et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx 11000
Resistance of hydraulic support (kN)
10000 9000 8000 7000 6000 Residual coal pillar
5000
6m
Old goaf
11 m
4000 0
4
8
12
16
20
24
28
32
36
40
Advancing distance of repeated mining face (m)
Fig. 11. Average end-of-cycle resistance of repeated mining face support.
6. On-site project verification The repeated mining face in the case study used for verification involved fully mechanized caving coal hydraulic support via model ZF12000/18.5/35 and used the KJ34 mining support monitoring system to monitor the face support resistance. Before performing the repeated mining, three KJ345 model mechanized caving coalsupport pressure-monitoring stations were installed. Two hydraulic supports were each positioned at the upper, middle, and lower portions of the face for monitoring. The monitoring results were averaged, and the results are shown in Fig. 11. Fig. 11 shows that as the repeated mining face passed the first residual coal pillar, the width of the residual coal pillar was reduced to 3.0 m, and as the repeated mining face advanced, the coal pillar suddenly lost stability and failed. The roof rock created fracturing above the old goaf, thereby causing the end-of-cycle support resistance to suddenly increase. When the repeated mining face passed the second residual pillar, a loss-of-balance failure occurred when the remaining width of the residual pillar reached 4.8 m. The final support resistance also suddenly increased, which is essentially consistent with the results of the numerical calculations. The figure also indicates that there was a consistent increase in the support resistance as the repeated mining face passed the old goaf. This result verified the conclusion that the secondary fracturing will occur during the rotational movement and the contacting of the waste rock with the old enlarged roof rock block above the face. The monitoring results indicate that the maximum working resistance values of the support as the repeated mining face passes the residual coal pillar and goaf are 10,521 kN and 9983 kN, respectively. These values are slightly less than the minimum working resistance values obtained from the theoretical calculations. Therefore, both the mechanical model and the support resistance calculation formulas are reasonable. The support-type and working-resistance settings based on the calculations can satisfy the requirement of a stable roof during the re-working of an old mine by way of a repeated mining face. 7. Conclusions This study focused on roof movements and fracture characteristics associated with a repeated mining face of residual coal after room and pillar mining based on numerical simulation and theoretical analysis. A roof structure mechanical model was developed, and analysis revealed the mechanisms controlling roof behavior associated with advancement of the repeated mining face. The main conclusions are as follows: (1) As the repeated mining face passes a coal pillar, there is gradual reduction of the coal pillar width, and as the bearing
7
limit of the remaining coal pillar is reached, instability of and damage to the pillar will occur, causing the headspace length of the main roof to suddenly increase. The main roof rock strata will develop fractures above the goaf, and thus the roof rock in front of the fracture will ‘‘rebound”. After this advance fracturing of the main roof, a relatively stable ‘‘Voussoir beam” structure will form above the goaf. The repeated mining face passing the goaf is affected only by the fallen coal and rock in the goaf; the impact on the main roof movement is relatively limited. (2) A roof-fracture structural model was constructed for a repeated mining face that passes a residual coal pillar and goaf. When the face passes a coal pillar, the sudden loss of stability of the coal pillar will cause the length of the main roof headspace to suddenly increase. The main roof rock strata will develop advance fractures above the goaf, thereby creating an enlarged rock block in the main roof. After rotational movement and contact of the waste rock with the enlarged rock block in the roof and with the continuing advancement of the face and the load imposed by the overlying rock, the rock block will develop secondary fracturing. (3) Based on the roof-fracture structural model, roof mechanical models for a repeated mining face passing an old residual coal pillar and goaf were developed. Based on key-block theory, the support and protection conditions for ensuring roof stability were derived. (4) Combined with the specific geologic condition of a repeated mining face in an integrated mine in Shanxi, the minimum reasonable support resistance when the repeated mining face passes a residual coal pillar and goaf was calculated. The on-site monitoring indicated that the mechanical model and the support load calculation are both reasonable. The results provide an important basis for selecting supports and working resistance settings in a repeated mining face of residual coal after the old room and pillar mining and provide references for roof control in similar conditions involving a repeated mining face.
Acknowledgments Financial support for this work, provided by the National Basic Research Program of China (No. 2014CB046905), Innovation Project for Graduates in Jiangsu Province (No. KYLX15_1405), the National Natural Science Foundation of China (Nos. 51274191 and 51404245), and the Doctoral Fund of Ministry of Education of China (No. 20130095110018), is gratefully acknowledged.
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Please cite this article in press as: Jiang B et al. Ground pressure and overlying strata structure for a repeated mining face of residual coal after room and pillar mining. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.05.017