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Soft–strong supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst
International Journal of Mining Science and Technology 24 (2014) 805–810
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Soft–strong supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst Ning Jianguo ⇑, Wang Jun, Liu Xuesheng, Qian Kun, Sun Bi College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China
a r t i c l e
i n f o
Article history: Received 20 February 2014 Received in revised form 19 April 2014 Accepted 17 June 2014 Available online 13 November 2014 Keywords: Deep coal seam Rockburst Gob-side entry retaining Soft–strong supporting body
a b s t r a c t When gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the cementbased supporting body beside roadway will bear greater roof pressure and strong impact load. Then the supporting body may easily deform and fail because of its low strength in the early stage. This paper established the roadside support mechanical model of gob-side entry retaining. Based on this model, we proposed and used the soft–strong supporting body as roadside support in the gob-side entry retaining. In the early stage of roof movement, the soft–strong supporting body has a better compressibility, which can not only relieve roof pressure and strong impact load, but also reduce the supporting resistance and prevent the supporting body from being crushed. In the later stage, with the increase of the strength of the supporting body, it can better support the overlying roof. The numerical simulation results and industrial test show that the soft–strong supporting body as roadside support can be better applied into the gob-side entry retaining in deep coal seams threatened by rockburst. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In order to retain the roadway for the next section working face after the upper section working face has been mined, gob-side entry retaining uses different supporting schemes (filling bodies, rock refuse, single props, etc.) to support the lateral roof in the gob behind the working face or in the roadway [1–5]. Gob-side entry retaining is usually implemented in the internal stress field of abutment pressure around the working face, where it is less influenced by ground pressure and easy to keep stability [6–8]. The implementation of gob-side entry retaining not only has an important role in improving mining rates, decreasing the cost of tunneling and extending the service life of mines, but also optimizes the use of abandoned waste rock and reduces ground pollution, which is consistent with the development of green mining and mining science [9]. When gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the roadside support will bear great roof pressure [10,11]. If cement-based supporting body (using cement as the main cementing material) is used as roadside support, it easily deforms and fails under the roof pressure due to its lower strength at the early stage [12–15]. Therefore, it is urgent to further study how to perform the roadside support when ⇑ Corresponding author. Tel.: +86 15954841060. E-mail address: [email protected] (J. Ning).
gob-side entry retaining is implemented in deep coal seams with the threat of rockburst. Based on the geological and mining conditions in Suncun coal mine of the Xinwen Mining Group, this paper proposed soft–strong supporting body as roadside support when gob-side entry retaining was implemented. Firstly, the mechanical models of cement-based support and soft–strong support were established in order to reveal the roadside supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst. Secondly, the supporting capacity of cement-based supporting body and soft–strong supporting body was analyzed by using numerical simulation. Finally, in order to verify the effects of these two kinds of supporting schemes, an industrial test was carried out in the Suncun coal mine. 2. Mechanical model of roadside support for gob-side entry retaining in deep coal seams threatened by rockburst As shown in Fig. 1a, after the upper section working face has been mined, the immediate roof will cave and fall in the gob, and the basic roof will break inside the coal wall and sag down while rotating. In Fig. 1, the height of the roadway is h, the thickness of immediate roof is mz, the fracture length of basic roof is LA and the distance between outer boundary of supporting body and coal wall is LK. When cement-based supporting body is used as roadside support for gob-side entry retaining, it can be treated as spring
http://dx.doi.org/10.1016/j.ijmst.2014.10.012 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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with greater stiffness due to its smaller shrinkage. The mechanical model is established as shown in Fig. 1b. Fig. 1b shows, in the mechanical model, the deformation of the cement-based supporting body is DS as same as the given roof convergence, the stiffness of the cement-based supporting body and coal seam are K1 and K3, respectively, then the compatibility equation of deformation is as follows:
DP ¼ K 1 DS P ¼ P 0 þ DP Lk ½h mz ðK A 1Þ DS ¼ LA
ð1Þ ð2Þ ð3Þ
G ¼ F DS
where DP is the support resistance increment of the supporting body (kN); DS the amount of compression of the supporting body (m); K1 the stiffness of the supporting body (kN/m); P the support resistance of the supporting body (kN); P0 the setting load of the supporting body (kN); KA the expansion coefficient of gangue in the goaf. As shown in Fig. 2a, when the supporting scheme for gob-side entry retaining uses soft–strong supporting body that contains a combination of the filling bodies A and B, the filling body A is expandable soft material with greater compressibility, and its strength can gradually increase with itself process of compression deformation. However, the filling body B is cement-based supporting material with the characteristics of smaller amount of compression and high strength. In brief, the total amount of compression of the soft–strong supporting body is the sum of those of filling bodies A and B. When filling bodies A and B can be seen as two kinds of springs with different stiffness, and the roof pressure applied on filling body A is equal to that of filling body B, then the compatibility equation of deformation is as follows:
DS ¼ DS2 þ DS3 DP ¼ K 1 DS3 ¼ K 2 DS2 P ¼ P 0 þ DP
ð4Þ ð5Þ ð6Þ
The total stiffness K of soft–strong supporting body is:
K¼
increases rapidly with the increase of the amount of compression. However, when soft–strong supporting body is used as roadside support, its stiffness is smaller than that of the cement-based supporting body and its support resistance increases more slowly with the increase of the amount of compression. The stiffness curves of cement-based supporting body and soft–strong supporting body are shown in Fig. 3. The lateral roof of working face has a great of impact energy on roadside supporting body when gob-side entry retaining is implemented in deep coal mining. So, according to conservation of energy, the energy balance equation can be given by,
DP K 1 K 2 ¼ DS K 1 þ K 2
ð7Þ
where DS is the total amount of compression of the soft–strong supporting body (m); DS3 the amount of compression of filling body B (m); DS2 the amount of compression of filling body A (m); K the total stiffness of the soft–strong supporting body (kN/m); K1 the stiffness of filling body B (kN/m); and K2 the stiffness of filling body A (kN/m). Since all stiffness of filling bodies A and B is positive, from Eq. (7), it should be noted that the total stiffness of soft–strong supporting body is smaller than K1 and K2. From Eqs. (1) and (7), it can be obtained that, when the cementbased supporting body is used as roadside support, the stiffness of the support (K1) is comparatively large and its support resistance
ð8Þ
where G is the impact energy of the overlying roof on the supporting body (kJ); and F the roof pressure on the supporting body (kN). Therefore, the roof pressure on the supporting body is obtained by,
F¼
G DS
ð9Þ
From Eq. (9), it should be noted that, with the increase of deformation of the supporting body, the roof pressure on the support gradually decreases. At this moment, the relationship between the deformation of the support and the roof pressure on the support is depicted in Fig. 4. Fig. 5 is the combination of the Figs. 3 and 4. It can be seen from Fig. 5, when the cement-based supporting body is used as roadside support, the stiffness curve of cement-based supporting body and the roof pressure curve intersect at point A. At this point, the amount of compression of the supporting body is DSa and roof pressure is Fa during the early period of lateral roof movement. It means, while the amount of compression of the supporting body is smaller, the roof pressure is larger, then the body easily deforms and damages under the roof pressure because of the lower early strength of cement-based supporting body, and the gob-side entry retaining fails. However, when soft–strong supporting body is used as roadside support, the stiffness curve of soft–strong supporting body and the roof pressure curve intersect at point B. At this point, the amount of compression of the supporting body is DSb and the roof pressure is Fb. It should be noted that, while the amount of compression of the soft–strong supporting body is larger, the roof pressure is even smaller, compared with cement-based supporting body. Since filling body A has excellent compression deformability and the large amount of compression, it makes a major contribution to the deformation of the total supporting body (DSb), which is helpful to relieve the roof pressure and reduce the impact load generated by strong roof movement. It is also helpful to improve the strength of filling body B. Furthermore, during the later period of lateral roof movement, the improvement of strength of filling body B can support the overlying roof. The support resistance of the total filling body is mainly from the filling body B. Thus, it can be seen that,
LA LA
q
mz
Immediate roof
h
Basic roof
Coal seam
Cement based support Lk (a) Structure model
Floor
S K3
K1
(b) Mechanical model
Fig. 1. Mechanical model of cement-based support for gob-side entry retaining.
F
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J. Ning et al. / International Journal of Mining Science and Technology 24 (2014) 805–810
LA
q
mz
Immediate roof
h
Basic roof
Coal seam
Filling body A Filling body B Lk
K3
S2 S3
K2 K1
S
F
Floor
(b) Mechanical model
(a) Structure model
Support resistance (k N)
Fig. 2. Mechanical model of soft–strong support for gob-side entry retaining.
body is not better. Because the surrounding rock will fall when the displacement of surrounding rock reaches a certain value, like point C, that is called the caving point, as shown in Fig. 5. So, the maximum amount of compression of the roadside support should be less than DSc.
Cement-based support
K1 K "Soft-strong" support Compression of the support(m)
Roof pressure (k N)
Fig. 3. Stiffness curves of roadside support.
F = ϕ (ΔS )
Deformation of the support (m)
Roof pressure ( kN)
Fig. 4. Curve between roof pressure and the deformation of the roadside support.
Cement-based support
Pa Sa Pb Pc
A
Sb Sc
"Soft- strong" support B C
Compression of the support (m) Fig. 5. Curves between roof pressure and compression of the support with different stiffness.
when gob-side entry retaining is implemented in deep coal seams with the threat of rockburst, the roadside support should have a certain compression deformability in order to relieve the roof pressure and strong impact load generated by roof subsidence during the early period of lateral roof movement. During the later period, the roadside support should have a certain bearing capacity in order to support the overlying roof. Consequently, soft–strong supporting body has those characteristics that are suitable for roadside support when gob-side entry retaining is implemented in deep coal seams threatened by rockburst. It should be noted that, during the early period of lateral roof movement, the greater amount of compression of the supporting
3. Numeric simulation of roadside support in deep gob-side entry retaining 3.1. Modeling According to the geological and mining conditions of No.2213 working face in Suncun coal mine of Xinwen Mining Group, a basic numerical model was built by using FLAC3D [14,15]. No.2213 working face uses strike longwall mining technology. It is about 1200 m in depth, 1200 m in strike length, and 160 m in trend length. The average thickness of coal seam is 3.0 m. The coal seam dip angle is 8°. The fractured length of lateral basic roof is 22.5 m, and the thickness of immediate roof is 6 m. The expansion coefficient of gangue in the goaf is 1.35. The mechanical parameters of coal and rock mass are shown in Table 1. The cross-section of the roadway is excavated as a rectangle with 4.0 m in width and 3.0 m in height. The roof is reinforced by bolts and anchor cables. The bolts are made of high strength alloy steel with the tensile strength of 200 kN, 20 mm in diameter and 2.4 m in length. The spacing and row of bolts on the roof is 0.8 m 0.8 m. The anchor cable is made of steel strand with the strength of 353 kN, the diameter of 17.8 mm, and the length of 6.0 m. One anchor cable was installed at the middle of the roof every 5 m along the advancing direction. The spacing and row of bolts on the side is 0.9 m 0.8 m. The numerical model is generated in three dimensions and its size is 80 m 64 m 70 m, corresponding to x, y, z direction, respectively. The in-situ stress is reproduced by setting initial conditions. Lateral boundaries of the model are fixed to eliminate horizontal displacement, and the bottom is fixed to eliminate vertical displacement. The top of the model is applied a vertical stress which is equal to the gravity of overlying stratum (cH), and the horizontal stress coefficient on the lateral sides is set as 0.8. The constitutive relation is Mohr–Coulomb model. Two roadside support schemes are designed. The first one uses a cement-based supporting body with 3 m in height and 2 m in width as roadside support. The second one uses a soft–strong supporting body with 2 m in width as roadside support that consists of the expandable soft material with the height of 0.3 m and the cement-based supporting body with the height of 2.7 m. The cement-based body is composed of cement, coal gauge and water with the proportions of 1:3:0.5. The relationship between the strength and the age of the cement-based body is shown in Table 2. The elastic modulus and Poisson’s ratio of the soft material are 270 MPa and 0.24, respectively. In the numerical model, the soft material uses elastic model, while the coal, rock and cement-based body uses Mohr plastic model. The goaf is seen as a null unit in the
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Table 1 Mechanical parameters of coal and rock mass. Lithology
Thickness (m)
Bulk modulus (GPa)
Shear modulus (GPa)
Internal friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
Fine-grained sandstone Siltstone Sandy mudstone Coal Lime mudstone Sandy mudstone Siltstone Sandy mudstone Fine-grained sandstone Lime mudstone Gritstone
3.0 4.0 3.0 3.0 1.5 4.5 4.5 5.5 10.0 6.5 25.5
13.07 13.05 10.24 4.22 9.36 10.95 11.65 12.65 12.64 13.64 14.54
10.5 11.09 7.43 2.51 6.81 7.61 9.84 10.41 9.41 11.62 12.53
37 36 36 32 34 34 36 38 38 38 39
3.19 3.01 2.76 1.61 2.03 3.13 3.04 3.33 3.91 3.42 4.15
6.1 5.3 4.8 1.0 4.2 4.8 5.3 4.8 6.1 4.2 8.2
Table 2 Uniaxial compressive strength of cement-based supporting body. Age (day)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Strength (MPa) Age (day) Strength (MPa)
0.8 15 27.0
1.2 16 27.8
1.5 17 28.5
2.8 18 29.2
4.8 19 29.8
8.6 20 30.3
12.8 21 30.8
15.4 22 31.3
18.8 23 31.7
22.6 24 32.0
23.6 25 32.0
24.5 26 32.0
25.4 27 32.0
26.3 28 32.0
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 shear-p tension-p
Vertical stress of spporting body (MPa)
Fig. 6. Plastic zone distributing of supporting body with the advancing distance of 8 m.
20 18 16 14 12 10 8 6 4 2 0
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
(a) Advancing 8 m
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 shear-p tension-p
(b) Advancing 24 m
5.95
6.00
6.05 6.10 Step (103)
6.15
Fig. 7. Stress curve of supporting body with the advancing distance of 8 m.
numerical simulation. According to the field monitoring, the distance between the lateral basic roof fracture location and coal wall is 10 m. In order to reflect the influence of immediate roof and basic roof on the supporting body after basic roof fractured, structural units of interface are set in the corresponding position of the numerical model [16]. The simulation processes are as follows. Firstly, the model reaches initial balance under the weight of the overlying strata. Secondly, the roadway is excavated and supported after releasing the stress. Thirdly, the working face advances 4 m each day, the supporting body is filled behind every 4 m of working face advance, and the strength of the filled cement-based supporting body changing to the next stage of strength is shown in Table 2. Finally, the model reaches stress balance at every 4 m of working face advance, and the stress and displacement of the roadway-side support are monitored.
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 shear-p tension-p
(c) Advancing 52 m Fig. 8. Plastic zone of roadside supporting body.
3.2. Analysis of simulation results When the cement-based supporting body is used as roadside support, and the advancing distance of working face is up to 8 m, the plastic zone of the model is shown in Fig. 6, and the stress curve of the supporting body is shown in Fig. 7. When soft–strong supporting body is used, the plastic zone and stress curve of the supporting body are shown in Figs. 8 and 9, respectively.
J. Ning et al. / International Journal of Mining Science and Technology 24 (2014) 805–810
In conclusion, during the early period, great pressure can easily destroy the supporting body in the first scheme with cement-based supporting body. However, the soft–strong supporting body has low strength and great compressibility that can release the roof pressure in the early stage and protect the supporting body from being crushed. The strength of the bottom cement-based body increases with time and becomes high enough to support the roof during the later period.
Vertical stress of supporting body (MPa)
30 25 20 15 10 5 0
809
4
12 20 28 36 44 52 Longwall face advance (m)
60
4. Field observation
Fig. 9. Stress curve of cement-based body in soft–strong supporting body.
2m
Basic point
Roadway Basic point
2m
Basic point Basic point
floo Floor r Fig. 10. Basic measuring points of roadway.
Deformation of surrounding rock (mm)
According to Figs. 6 and 7, when the working face advances 8 m, the supporting scheme with the cement-based supporting body fails. All the stress area on the cement-based supporting body is plastic and the largest stress is 18.42 MPa. According to Fig. 8, when advancing distance of working face is up to 8 m, the second supporting scheme with the soft–strong supporting body, the small part of the supporting body at the side of the gob is into plastic zone. When the advancing is 24 m, part of the supporting body is into plastic zone. When the advancing is 52 m, all the supporting body is into plastic zone. According to Fig. 9, the stress of the bottom cement-based body in the soft– strong supporting body increases slowly at first and quickly later, and the largest stress is 26.4 MPa when the working face advances 52 m. Therefore, with the comparison of two supporting schemes, the compressibility and the uniaxial compressive strength of the cement-based supporting body are small during the early period of lateral roof movement, and it can easily fail. However, when using soft–strong supporting body in the second supporting scheme, the upper soft material has great compressibility that allows the roof to sink during the early period, release some roof pressure and decrease the resistance of the supporting body. During the later period, the soft material cannot be compressed anymore and the strength of the bottom cement-based supporting body is big enough to support the roof, which will realize the stability of roadway.
90 80 70 60 50 40 30 20 10 0
Cement-based support ˃ s roof and floor Roadway˃ s sides of cement-based support Soft-strong support˃ s roof and floor Roadway˃ s sides of soft-strong support
10 20 30 40 50 60 70 80 90 100 Longwall face advanced distance (m)
Deformation of surrounding rock (mm)
Goaf
"Softstrong" support
3m
Roof
To validate the supporting effect of soft–strong supporting body, the industrial test was performed in the No.2213 roadway of the Suncun coal mine (the geological and mining conditions are provided in Section 3.1). The experimental roadway with the length of 300 m was divided into two 150 m length parts. One of them used cement-based body as supporting material, the other used soft–strong body as supporting material with the height of 3 m. The expandable soft material in the soft–strong supporting body was Uroica No.1. According to the Eq. (3), the roof convergence is 240 mm. Because the compression ratio of soft material is 80%, the height of the soft material is 300 mm and the uniaxial compressive strength is 3 MPa after two hours. The strong material of the supporting body was a cement-based body (its proportion is shown in Section 3.1). Its strength could reach about 20 MPa after roof stability [11]. The decussation monitoring method was used to measure the surface displacement of the roadway with the advancing of the working face, including roof-to-floor convergence and two-side displacement. Three monitoring stations were set at 10 m intervals, and steel tacks of 200 mm length were fixed in the sides, roof and floor of the roadway at each station as basic points, as shown in Fig. 10. Measuring guns and poles were used to measure the deformation of the surrounding rock of the roadway, and the horizontal deformation and vertical deformation that are shown in Fig. 11. According to Fig. 11, the deformation of surrounding rock supported by the cement-based supporting body is very low when the working face advances 15 m, and increases rapidly when the advancing more than 15 m. The supporting body fails when the advancing is 40 m. During the early period of lateral roof movement, due to the low compressive strength of the cement-based supporting body and the small compressibility, the cement-based supporting body will bear great roof pressure and strong impact load in the deep coal seam threatened by rockburst. So, the supporting body is easily destroyed, which may lead to supporting scheme failure. However, the deformation of the surrounding rock supported by the soft–strong supporting body is bigger when the advancing is between 0 and 20 m. Because the supporting body has great compressibility that can release the roof pressure in the early stage. Then the strength of the supporting body increases 90 80 70 60 50 40 30 20 10 0
(a) No.1 monitoring station Fig. 11. Deformation of surrounding rock.
10 20 30 40 50 60 70 80 90 100 Longwall face advanced distance (m)
(b) No.2 monitoring station
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J. Ning et al. / International Journal of Mining Science and Technology 24 (2014) 805–810
with the advance of the working face and the deformation of the surrounding rock remains steady after the advancing is up to 60 m, which means the soft–strong supporting body can well adapt the movement of roof in deep gob-side entry retaining threatened by rockburst.
Natural Science Foundation of China (No. 51344009), the Research Award Fund for Outstanding Young Scientists of Shandong Province (No. BS2012NJ007), the Ground Pressure and Strata Control Innovative Team Fund of SDUST (No. 2010KYTD105), and the Natural Science Foundation of Shandong Province (No. ZR2012EEZ002).
5. Conclusions References (1) Because of the great surrounding rock pressure in deep gob-side entry retaining threatened by rockburst, the roadside supporting body should have the characteristics of compressibility to release the impact load in the early stage of lateral roof movement, and great strength to bear the great impact load from the overlying strata and support the roof in the later stage. (2) The supporting scheme with soft–strong supporting body as roadside support is proposed for the deep gob-side entry retaining threatened by rockburst. It is composed of two parts. The upper supporting body is soft material that has low strength and great compressibility that can release the impact pressure and protect the supporting body from being crushed. The bottom part of the supporting body is a cement-based filling body, and its strength is low in the early stage and increase to high enough to support the roof in the later stage. (3) The calculation method for the height of upper expandable soft material of the soft–strong supporting body is proposed. The industrial test and numerical simulation show that using the soft–strong support as roadside support could be suitable for the laws of roof movement when gob-side entry retaining is implemented in deep coal seams threatened by rockburst.
Acknowledgments This research is supported by the National Basic Research Program of China (No. 2010CB226805), the Taishan Scholar Construction Project of Shandong Province, China, the National
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Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Soft–strong supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst Ning Jianguo ⇑, Wang Jun, Liu Xuesheng, Qian Kun, Sun Bi College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China
a r t i c l e
i n f o
Article history: Received 20 February 2014 Received in revised form 19 April 2014 Accepted 17 June 2014 Available online 13 November 2014 Keywords: Deep coal seam Rockburst Gob-side entry retaining Soft–strong supporting body
a b s t r a c t When gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the cementbased supporting body beside roadway will bear greater roof pressure and strong impact load. Then the supporting body may easily deform and fail because of its low strength in the early stage. This paper established the roadside support mechanical model of gob-side entry retaining. Based on this model, we proposed and used the soft–strong supporting body as roadside support in the gob-side entry retaining. In the early stage of roof movement, the soft–strong supporting body has a better compressibility, which can not only relieve roof pressure and strong impact load, but also reduce the supporting resistance and prevent the supporting body from being crushed. In the later stage, with the increase of the strength of the supporting body, it can better support the overlying roof. The numerical simulation results and industrial test show that the soft–strong supporting body as roadside support can be better applied into the gob-side entry retaining in deep coal seams threatened by rockburst. Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In order to retain the roadway for the next section working face after the upper section working face has been mined, gob-side entry retaining uses different supporting schemes (filling bodies, rock refuse, single props, etc.) to support the lateral roof in the gob behind the working face or in the roadway [1–5]. Gob-side entry retaining is usually implemented in the internal stress field of abutment pressure around the working face, where it is less influenced by ground pressure and easy to keep stability [6–8]. The implementation of gob-side entry retaining not only has an important role in improving mining rates, decreasing the cost of tunneling and extending the service life of mines, but also optimizes the use of abandoned waste rock and reduces ground pollution, which is consistent with the development of green mining and mining science [9]. When gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the roadside support will bear great roof pressure [10,11]. If cement-based supporting body (using cement as the main cementing material) is used as roadside support, it easily deforms and fails under the roof pressure due to its lower strength at the early stage [12–15]. Therefore, it is urgent to further study how to perform the roadside support when ⇑ Corresponding author. Tel.: +86 15954841060. E-mail address: [email protected] (J. Ning).
gob-side entry retaining is implemented in deep coal seams with the threat of rockburst. Based on the geological and mining conditions in Suncun coal mine of the Xinwen Mining Group, this paper proposed soft–strong supporting body as roadside support when gob-side entry retaining was implemented. Firstly, the mechanical models of cement-based support and soft–strong support were established in order to reveal the roadside supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockburst. Secondly, the supporting capacity of cement-based supporting body and soft–strong supporting body was analyzed by using numerical simulation. Finally, in order to verify the effects of these two kinds of supporting schemes, an industrial test was carried out in the Suncun coal mine. 2. Mechanical model of roadside support for gob-side entry retaining in deep coal seams threatened by rockburst As shown in Fig. 1a, after the upper section working face has been mined, the immediate roof will cave and fall in the gob, and the basic roof will break inside the coal wall and sag down while rotating. In Fig. 1, the height of the roadway is h, the thickness of immediate roof is mz, the fracture length of basic roof is LA and the distance between outer boundary of supporting body and coal wall is LK. When cement-based supporting body is used as roadside support for gob-side entry retaining, it can be treated as spring
http://dx.doi.org/10.1016/j.ijmst.2014.10.012 2095-2686/Ó 2014 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
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with greater stiffness due to its smaller shrinkage. The mechanical model is established as shown in Fig. 1b. Fig. 1b shows, in the mechanical model, the deformation of the cement-based supporting body is DS as same as the given roof convergence, the stiffness of the cement-based supporting body and coal seam are K1 and K3, respectively, then the compatibility equation of deformation is as follows:
DP ¼ K 1 DS P ¼ P 0 þ DP Lk ½h mz ðK A 1Þ DS ¼ LA
ð1Þ ð2Þ ð3Þ
G ¼ F DS
where DP is the support resistance increment of the supporting body (kN); DS the amount of compression of the supporting body (m); K1 the stiffness of the supporting body (kN/m); P the support resistance of the supporting body (kN); P0 the setting load of the supporting body (kN); KA the expansion coefficient of gangue in the goaf. As shown in Fig. 2a, when the supporting scheme for gob-side entry retaining uses soft–strong supporting body that contains a combination of the filling bodies A and B, the filling body A is expandable soft material with greater compressibility, and its strength can gradually increase with itself process of compression deformation. However, the filling body B is cement-based supporting material with the characteristics of smaller amount of compression and high strength. In brief, the total amount of compression of the soft–strong supporting body is the sum of those of filling bodies A and B. When filling bodies A and B can be seen as two kinds of springs with different stiffness, and the roof pressure applied on filling body A is equal to that of filling body B, then the compatibility equation of deformation is as follows:
DS ¼ DS2 þ DS3 DP ¼ K 1 DS3 ¼ K 2 DS2 P ¼ P 0 þ DP
ð4Þ ð5Þ ð6Þ
The total stiffness K of soft–strong supporting body is:
K¼
increases rapidly with the increase of the amount of compression. However, when soft–strong supporting body is used as roadside support, its stiffness is smaller than that of the cement-based supporting body and its support resistance increases more slowly with the increase of the amount of compression. The stiffness curves of cement-based supporting body and soft–strong supporting body are shown in Fig. 3. The lateral roof of working face has a great of impact energy on roadside supporting body when gob-side entry retaining is implemented in deep coal mining. So, according to conservation of energy, the energy balance equation can be given by,
DP K 1 K 2 ¼ DS K 1 þ K 2
ð7Þ
where DS is the total amount of compression of the soft–strong supporting body (m); DS3 the amount of compression of filling body B (m); DS2 the amount of compression of filling body A (m); K the total stiffness of the soft–strong supporting body (kN/m); K1 the stiffness of filling body B (kN/m); and K2 the stiffness of filling body A (kN/m). Since all stiffness of filling bodies A and B is positive, from Eq. (7), it should be noted that the total stiffness of soft–strong supporting body is smaller than K1 and K2. From Eqs. (1) and (7), it can be obtained that, when the cementbased supporting body is used as roadside support, the stiffness of the support (K1) is comparatively large and its support resistance
ð8Þ
where G is the impact energy of the overlying roof on the supporting body (kJ); and F the roof pressure on the supporting body (kN). Therefore, the roof pressure on the supporting body is obtained by,
F¼
G DS
ð9Þ
From Eq. (9), it should be noted that, with the increase of deformation of the supporting body, the roof pressure on the support gradually decreases. At this moment, the relationship between the deformation of the support and the roof pressure on the support is depicted in Fig. 4. Fig. 5 is the combination of the Figs. 3 and 4. It can be seen from Fig. 5, when the cement-based supporting body is used as roadside support, the stiffness curve of cement-based supporting body and the roof pressure curve intersect at point A. At this point, the amount of compression of the supporting body is DSa and roof pressure is Fa during the early period of lateral roof movement. It means, while the amount of compression of the supporting body is smaller, the roof pressure is larger, then the body easily deforms and damages under the roof pressure because of the lower early strength of cement-based supporting body, and the gob-side entry retaining fails. However, when soft–strong supporting body is used as roadside support, the stiffness curve of soft–strong supporting body and the roof pressure curve intersect at point B. At this point, the amount of compression of the supporting body is DSb and the roof pressure is Fb. It should be noted that, while the amount of compression of the soft–strong supporting body is larger, the roof pressure is even smaller, compared with cement-based supporting body. Since filling body A has excellent compression deformability and the large amount of compression, it makes a major contribution to the deformation of the total supporting body (DSb), which is helpful to relieve the roof pressure and reduce the impact load generated by strong roof movement. It is also helpful to improve the strength of filling body B. Furthermore, during the later period of lateral roof movement, the improvement of strength of filling body B can support the overlying roof. The support resistance of the total filling body is mainly from the filling body B. Thus, it can be seen that,
LA LA
q
mz
Immediate roof
h
Basic roof
Coal seam
Cement based support Lk (a) Structure model
Floor
S K3
K1
(b) Mechanical model
Fig. 1. Mechanical model of cement-based support for gob-side entry retaining.
F
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LA
q
mz
Immediate roof
h
Basic roof
Coal seam
Filling body A Filling body B Lk
K3
S2 S3
K2 K1
S
F
Floor
(b) Mechanical model
(a) Structure model
Support resistance (k N)
Fig. 2. Mechanical model of soft–strong support for gob-side entry retaining.
body is not better. Because the surrounding rock will fall when the displacement of surrounding rock reaches a certain value, like point C, that is called the caving point, as shown in Fig. 5. So, the maximum amount of compression of the roadside support should be less than DSc.
Cement-based support
K1 K "Soft-strong" support Compression of the support(m)
Roof pressure (k N)
Fig. 3. Stiffness curves of roadside support.
F = ϕ (ΔS )
Deformation of the support (m)
Roof pressure ( kN)
Fig. 4. Curve between roof pressure and the deformation of the roadside support.
Cement-based support
Pa Sa Pb Pc
A
Sb Sc
"Soft- strong" support B C
Compression of the support (m) Fig. 5. Curves between roof pressure and compression of the support with different stiffness.
when gob-side entry retaining is implemented in deep coal seams with the threat of rockburst, the roadside support should have a certain compression deformability in order to relieve the roof pressure and strong impact load generated by roof subsidence during the early period of lateral roof movement. During the later period, the roadside support should have a certain bearing capacity in order to support the overlying roof. Consequently, soft–strong supporting body has those characteristics that are suitable for roadside support when gob-side entry retaining is implemented in deep coal seams threatened by rockburst. It should be noted that, during the early period of lateral roof movement, the greater amount of compression of the supporting
3. Numeric simulation of roadside support in deep gob-side entry retaining 3.1. Modeling According to the geological and mining conditions of No.2213 working face in Suncun coal mine of Xinwen Mining Group, a basic numerical model was built by using FLAC3D [14,15]. No.2213 working face uses strike longwall mining technology. It is about 1200 m in depth, 1200 m in strike length, and 160 m in trend length. The average thickness of coal seam is 3.0 m. The coal seam dip angle is 8°. The fractured length of lateral basic roof is 22.5 m, and the thickness of immediate roof is 6 m. The expansion coefficient of gangue in the goaf is 1.35. The mechanical parameters of coal and rock mass are shown in Table 1. The cross-section of the roadway is excavated as a rectangle with 4.0 m in width and 3.0 m in height. The roof is reinforced by bolts and anchor cables. The bolts are made of high strength alloy steel with the tensile strength of 200 kN, 20 mm in diameter and 2.4 m in length. The spacing and row of bolts on the roof is 0.8 m 0.8 m. The anchor cable is made of steel strand with the strength of 353 kN, the diameter of 17.8 mm, and the length of 6.0 m. One anchor cable was installed at the middle of the roof every 5 m along the advancing direction. The spacing and row of bolts on the side is 0.9 m 0.8 m. The numerical model is generated in three dimensions and its size is 80 m 64 m 70 m, corresponding to x, y, z direction, respectively. The in-situ stress is reproduced by setting initial conditions. Lateral boundaries of the model are fixed to eliminate horizontal displacement, and the bottom is fixed to eliminate vertical displacement. The top of the model is applied a vertical stress which is equal to the gravity of overlying stratum (cH), and the horizontal stress coefficient on the lateral sides is set as 0.8. The constitutive relation is Mohr–Coulomb model. Two roadside support schemes are designed. The first one uses a cement-based supporting body with 3 m in height and 2 m in width as roadside support. The second one uses a soft–strong supporting body with 2 m in width as roadside support that consists of the expandable soft material with the height of 0.3 m and the cement-based supporting body with the height of 2.7 m. The cement-based body is composed of cement, coal gauge and water with the proportions of 1:3:0.5. The relationship between the strength and the age of the cement-based body is shown in Table 2. The elastic modulus and Poisson’s ratio of the soft material are 270 MPa and 0.24, respectively. In the numerical model, the soft material uses elastic model, while the coal, rock and cement-based body uses Mohr plastic model. The goaf is seen as a null unit in the
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Table 1 Mechanical parameters of coal and rock mass. Lithology
Thickness (m)
Bulk modulus (GPa)
Shear modulus (GPa)
Internal friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
Fine-grained sandstone Siltstone Sandy mudstone Coal Lime mudstone Sandy mudstone Siltstone Sandy mudstone Fine-grained sandstone Lime mudstone Gritstone
3.0 4.0 3.0 3.0 1.5 4.5 4.5 5.5 10.0 6.5 25.5
13.07 13.05 10.24 4.22 9.36 10.95 11.65 12.65 12.64 13.64 14.54
10.5 11.09 7.43 2.51 6.81 7.61 9.84 10.41 9.41 11.62 12.53
37 36 36 32 34 34 36 38 38 38 39
3.19 3.01 2.76 1.61 2.03 3.13 3.04 3.33 3.91 3.42 4.15
6.1 5.3 4.8 1.0 4.2 4.8 5.3 4.8 6.1 4.2 8.2
Table 2 Uniaxial compressive strength of cement-based supporting body. Age (day)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Strength (MPa) Age (day) Strength (MPa)
0.8 15 27.0
1.2 16 27.8
1.5 17 28.5
2.8 18 29.2
4.8 19 29.8
8.6 20 30.3
12.8 21 30.8
15.4 22 31.3
18.8 23 31.7
22.6 24 32.0
23.6 25 32.0
24.5 26 32.0
25.4 27 32.0
26.3 28 32.0
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 shear-p tension-p
Vertical stress of spporting body (MPa)
Fig. 6. Plastic zone distributing of supporting body with the advancing distance of 8 m.
20 18 16 14 12 10 8 6 4 2 0
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
(a) Advancing 8 m
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 shear-p tension-p
(b) Advancing 24 m
5.95
6.00
6.05 6.10 Step (103)
6.15
Fig. 7. Stress curve of supporting body with the advancing distance of 8 m.
numerical simulation. According to the field monitoring, the distance between the lateral basic roof fracture location and coal wall is 10 m. In order to reflect the influence of immediate roof and basic roof on the supporting body after basic roof fractured, structural units of interface are set in the corresponding position of the numerical model [16]. The simulation processes are as follows. Firstly, the model reaches initial balance under the weight of the overlying strata. Secondly, the roadway is excavated and supported after releasing the stress. Thirdly, the working face advances 4 m each day, the supporting body is filled behind every 4 m of working face advance, and the strength of the filled cement-based supporting body changing to the next stage of strength is shown in Table 2. Finally, the model reaches stress balance at every 4 m of working face advance, and the stress and displacement of the roadway-side support are monitored.
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 shear-p tension-p
(c) Advancing 52 m Fig. 8. Plastic zone of roadside supporting body.
3.2. Analysis of simulation results When the cement-based supporting body is used as roadside support, and the advancing distance of working face is up to 8 m, the plastic zone of the model is shown in Fig. 6, and the stress curve of the supporting body is shown in Fig. 7. When soft–strong supporting body is used, the plastic zone and stress curve of the supporting body are shown in Figs. 8 and 9, respectively.
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In conclusion, during the early period, great pressure can easily destroy the supporting body in the first scheme with cement-based supporting body. However, the soft–strong supporting body has low strength and great compressibility that can release the roof pressure in the early stage and protect the supporting body from being crushed. The strength of the bottom cement-based body increases with time and becomes high enough to support the roof during the later period.
Vertical stress of supporting body (MPa)
30 25 20 15 10 5 0
809
4
12 20 28 36 44 52 Longwall face advance (m)
60
4. Field observation
Fig. 9. Stress curve of cement-based body in soft–strong supporting body.
2m
Basic point
Roadway Basic point
2m
Basic point Basic point
floo Floor r Fig. 10. Basic measuring points of roadway.
Deformation of surrounding rock (mm)
According to Figs. 6 and 7, when the working face advances 8 m, the supporting scheme with the cement-based supporting body fails. All the stress area on the cement-based supporting body is plastic and the largest stress is 18.42 MPa. According to Fig. 8, when advancing distance of working face is up to 8 m, the second supporting scheme with the soft–strong supporting body, the small part of the supporting body at the side of the gob is into plastic zone. When the advancing is 24 m, part of the supporting body is into plastic zone. When the advancing is 52 m, all the supporting body is into plastic zone. According to Fig. 9, the stress of the bottom cement-based body in the soft– strong supporting body increases slowly at first and quickly later, and the largest stress is 26.4 MPa when the working face advances 52 m. Therefore, with the comparison of two supporting schemes, the compressibility and the uniaxial compressive strength of the cement-based supporting body are small during the early period of lateral roof movement, and it can easily fail. However, when using soft–strong supporting body in the second supporting scheme, the upper soft material has great compressibility that allows the roof to sink during the early period, release some roof pressure and decrease the resistance of the supporting body. During the later period, the soft material cannot be compressed anymore and the strength of the bottom cement-based supporting body is big enough to support the roof, which will realize the stability of roadway.
90 80 70 60 50 40 30 20 10 0
Cement-based support ˃ s roof and floor Roadway˃ s sides of cement-based support Soft-strong support˃ s roof and floor Roadway˃ s sides of soft-strong support
10 20 30 40 50 60 70 80 90 100 Longwall face advanced distance (m)
Deformation of surrounding rock (mm)
Goaf
"Softstrong" support
3m
Roof
To validate the supporting effect of soft–strong supporting body, the industrial test was performed in the No.2213 roadway of the Suncun coal mine (the geological and mining conditions are provided in Section 3.1). The experimental roadway with the length of 300 m was divided into two 150 m length parts. One of them used cement-based body as supporting material, the other used soft–strong body as supporting material with the height of 3 m. The expandable soft material in the soft–strong supporting body was Uroica No.1. According to the Eq. (3), the roof convergence is 240 mm. Because the compression ratio of soft material is 80%, the height of the soft material is 300 mm and the uniaxial compressive strength is 3 MPa after two hours. The strong material of the supporting body was a cement-based body (its proportion is shown in Section 3.1). Its strength could reach about 20 MPa after roof stability [11]. The decussation monitoring method was used to measure the surface displacement of the roadway with the advancing of the working face, including roof-to-floor convergence and two-side displacement. Three monitoring stations were set at 10 m intervals, and steel tacks of 200 mm length were fixed in the sides, roof and floor of the roadway at each station as basic points, as shown in Fig. 10. Measuring guns and poles were used to measure the deformation of the surrounding rock of the roadway, and the horizontal deformation and vertical deformation that are shown in Fig. 11. According to Fig. 11, the deformation of surrounding rock supported by the cement-based supporting body is very low when the working face advances 15 m, and increases rapidly when the advancing more than 15 m. The supporting body fails when the advancing is 40 m. During the early period of lateral roof movement, due to the low compressive strength of the cement-based supporting body and the small compressibility, the cement-based supporting body will bear great roof pressure and strong impact load in the deep coal seam threatened by rockburst. So, the supporting body is easily destroyed, which may lead to supporting scheme failure. However, the deformation of the surrounding rock supported by the soft–strong supporting body is bigger when the advancing is between 0 and 20 m. Because the supporting body has great compressibility that can release the roof pressure in the early stage. Then the strength of the supporting body increases 90 80 70 60 50 40 30 20 10 0
(a) No.1 monitoring station Fig. 11. Deformation of surrounding rock.
10 20 30 40 50 60 70 80 90 100 Longwall face advanced distance (m)
(b) No.2 monitoring station
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with the advance of the working face and the deformation of the surrounding rock remains steady after the advancing is up to 60 m, which means the soft–strong supporting body can well adapt the movement of roof in deep gob-side entry retaining threatened by rockburst.
Natural Science Foundation of China (No. 51344009), the Research Award Fund for Outstanding Young Scientists of Shandong Province (No. BS2012NJ007), the Ground Pressure and Strata Control Innovative Team Fund of SDUST (No. 2010KYTD105), and the Natural Science Foundation of Shandong Province (No. ZR2012EEZ002).
5. Conclusions References (1) Because of the great surrounding rock pressure in deep gob-side entry retaining threatened by rockburst, the roadside supporting body should have the characteristics of compressibility to release the impact load in the early stage of lateral roof movement, and great strength to bear the great impact load from the overlying strata and support the roof in the later stage. (2) The supporting scheme with soft–strong supporting body as roadside support is proposed for the deep gob-side entry retaining threatened by rockburst. It is composed of two parts. The upper supporting body is soft material that has low strength and great compressibility that can release the impact pressure and protect the supporting body from being crushed. The bottom part of the supporting body is a cement-based filling body, and its strength is low in the early stage and increase to high enough to support the roof in the later stage. (3) The calculation method for the height of upper expandable soft material of the soft–strong supporting body is proposed. The industrial test and numerical simulation show that using the soft–strong support as roadside support could be suitable for the laws of roof movement when gob-side entry retaining is implemented in deep coal seams threatened by rockburst.
Acknowledgments This research is supported by the National Basic Research Program of China (No. 2010CB226805), the Taishan Scholar Construction Project of Shandong Province, China, the National
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