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Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face
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
Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face Qi Fangkun a,c, Zhou Yuejin b,⇑, Li Jiawei b, Wang Erqian b, Cao Zhengzheng b, Li Ning b a
School of Mines, China University of Mining & Technology, Xuzhou 221116, China State Key Laboratory Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China c Xin’an Coal Co., Ltd., Zaozhuang Mining Group, Zaozhuang 277000, China b
a r t i c l e
i n f o
Article history: Received 27 September 2015 Received in revised form 30 November 2015 Accepted 15 January 2016 Available online xxxx Keywords: Fully mechanized top-coal caving Narrow pillar Gob-side entry Top coal Deformation control
a b s t r a c t A mechanical model to control the top-coal deformation is established in accordance with the structural characters of the gob-side entry surrounding rock for the fully-mechanic top-coal caving; the analytical solution of top coal roof-sag curve is deduced with Winkler elastic foundation beam model. By means of a calculating and analytic program, the top coal roof-sag values are calculated under the conditions of different supporting intensities, widths of narrow pillars and stiffness of top coal; meanwhile, the relationship between the roof-sag values and supporting intensity, width of narrow pillars and stiffness of top coal is analyzed as well. With the actual situation of the gob-side entry taken into consideration, the parameters of top-coal control are determined and a supporting plan is proposed for the top-coal control, which is proved to be reliable and effective by on-site verification. Some theoretical guidance and advice are put forward for the top-coal deformation control in gob-side entry for fully mechanized top-coal caving face. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In the gob-side entry with narrow pillars, the abutment pressure is redistributed because of the subsidence rotary motion of roof strata [1]. The interaction and mutual influences among the top-coal, the narrow pillar, the integrated coal beside the roadway and the floor lead to the surrounding rock deformation and failure. As is known, the deformation amount of the roadway’s sides is often larger than that of the roof and floor. Therefore, how to maintain the stability of the two sides of the roadway, especially that of the narrow pillar walls has been the focus of the research on the deformation and control of roadway driving along goaf [2–5]. Under the condition that the main roof acts on the surrounding rocks of the fully-mechanized caving roadway in a given deformation way, Gao et al. analyzed the deformation mechanism of roadway surrounding rocks [6–8]. However, there has been little research on the relationship between the top-coal deformation and supporting intensity, width of narrow pillars, and stiffness of top coal. Site investigations indicate that the analysis on the topcoal deformation and strained condition is of great significance to study the whole stability of fully-mechanized top-coal caving roadway with narrow pillars. ⇑ Corresponding author. Tel.: +86 13914884696. E-mail address: [email protected] (Y. Zhou).
Based on the geological condition of the 3304 fully-mechanized top-coal caving face of a mine in Zaozhuang, the beam structure model of top-coal is established according to the general surrounding rock structure of the gob-side entry. By means of theoretical analysis, numerical calculation and field measurement, this paper studies the relationship between the caving top-coal deformation and its influencing factors, and puts forward the control method of the roof of caving top-coal of roadway along goaf with narrow pillars; meanwhile the relevant parameters are determined. 2. Top-coal stress and deformation analysis of roadway driving along goaf 2.1. Mechanical model of top-coal One side of the gob-side entry for fully mechanized top-coal caving is the integrated coal beside the roadway while the other side is the narrow pillar. According to the key strata theory and the movement law of overlying strata on the goaf side of fully mechanized caving face, fracture, rotary and subsidence occur in the main roof after the immediate roof caving and sinking in the upper section of the working face; in the lower section, the lateral ‘‘voussoir beam” structure is formed in the coal mass, namely ‘‘the big structure” [9–14]. In this process, a fractured zone with a certain thickness is formed as the marginal coal mass is damaged;
http://dx.doi.org/10.1016/j.ijmst.2016.02.008 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
as the lateral abutment pressure is transferred to the integrated coal beside the roadway, the stress increasing zone is formed; and the stress decreasing zone is formed in the lower part of the big structure. This provides favorable conditions for the roadway driving along goaf where the forced deformation of surrounding rocks is small and easy to control. After the roadway driving along goaf, an organic whole is developed by the top-coal, the floor, the two sides, the narrow pillar and the anchor as the support object of the roadway, called ‘‘the small structure” [13,14]. The roadway support of gob-side entry driving focuses on maintaining the stability of the small structure. The relationship between the roadway driving along goaf with narrow pillars and the overlying strata structure is shown in Fig. 1. The roof subsidence of roadway driving along goaf is mainly composed of the inelastic displacement caused by rock stress release of surrounding rocks in tunnel excavation process and the gravity of overlying strata. For the convenience of study, the inelastic displacement caused by rock stress release is ignored. According to the main roof activity rule, the top-coal of roadway driving along goaf is simplified to the beam structure, and its corresponding mechanical model is established and shown in Fig. 2. Taking the horizontal centerline of the top-coal as the axis, the goaf-side endpoint of top-coal as the origin O, the lateral direction of the integrated coal beside the roadway as the positive direction, we can set up the coordinate. Point A and point B stand for the two walls of the gob-side entry respectively; point C refers to the boundary of the deep stress concentrated zone of the top-coal; the top-coal strata is denoted by OD, and D refers to the random point which is far enough, and can not affect the calculation; the overlying strata stress on the upper part is q1 ðxÞ; under the top coal, the narrow pillar, the gob-side entry and the integrated coal beside the roadway are respectively represented by OA, AB and BD, and their widths are l, L and a þ x0 , respectively; they are acted jointly by the narrow pillar’s holding power q2 ðxÞ, support intensity p and the entity coal’s holding power q3 ðxÞ. The mechanical environment of the fully mechanized top-coal roadway driving along goaf with narrow pillars differs from that of general mining roadways [15,16]. After mining in the upper section, the stress relaxed zone is formed above the gob-side entry; meanwhile the stress concentrated zone appears with the increase of the stress on the two sides of the overlying strata. Thus the quadratic function is employed here. Due to the rotation of the main roof, the narrow pillar enters the plastic state with the bearing capacity dramatically reduced, while the stress concentration on the integrated coal beside the roadway rises higher. The stress concentration coefficient of the BC side of the integrated coal beside the roadway is set as a1 , and that of the top-coal of narrow pillar OA as a2 . The overburdened pressure upon AB above the roof of roadway driving along goaf is regarded as the uniform load, and the relative rock stress coefficient is a3 . Because the roadway excavation has less impact on the top-coal CD side of the virgin coal
Main roof
Fig. 2. Mechanical model of the top-coal of roadway driving along goaf with narrow pillar.
face, the load subjected is still the overlying strata gravity c2 H, where c2 is the average weight, N/m2; and H is the thickness of the overlying strata (m) which can be seen as the buried depth. Therefore, the overlying strata stress q1 ðxÞ, which is acted on the top-coal can be represented by the following piecewise function:
q1 ðxÞ ¼
8 > > > > > <
4ða3 a2 Þc2 H x L20
L20
2
þ a2 c2 H;
06x
a3 c2 H; l6x 4ð1a1 Þc2 H a 2 > > x L0 L1 2 þ a1 c2 H; l þ L 6 x < l þ L þ a > a2 > : c2 H; l þ L þ a 6 x < þ1 ð1Þ
If the coal mass is taken as the isotropic elastic body, the supporting effect of the narrow pillar and the integrated coal beside the roadway on the top-coal can be conducted as the elastic foundation [17]. So
q2 ðxÞ ¼ k1 wðxÞ;
06x6l
ð2Þ
q3 ðxÞ ¼ k2 wðxÞ;
l þ L 6 x < þ1
ð3Þ
where k1 and k2 are the Winkler foundation coefficients reflecting the bearing capacity of the coal-rock mass under the top-coal, which is related with the mechanical properties of the side coal mass and the height of the roadway. Due to the roof subsidence, the two sides enter the plastic state from the coal wall to the coal mass. The side surrounding rocks constantly squeeze and crush the inside roadway, which leads to the reduction of the bearing capacity of the coal-rock mass. The Winkler foundation coefficients go down consequently and then the curve subsidence of the roof is enlarged. In addition, the top-coal of the gob-side entry is affected by the support intensity p, and the whole top-coal is affected by its own gravity c1 h1 , where c1 refers to the unit weight of the coal mass, N/m2; and h1 is the thickness of the top-coal, m. The resultant force, which is called the equivalent load qðxÞ, is composed of the overlying strata stress, the side coal mass’ holding power, the supporting intensity and the self gravity being acted on the top-coal. It can be represented by Eq. (4):
Key block A Key block B Key block C
Immediate roof
Top coal Coal seam Integrated coal beside the roadway
Uncaved top coal
Broken rock
Narrow pillar
Fig. 1. Structure of surrounding rock of roadway driving along goaf with narrow pillar.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
qðxÞ ¼
8 > > > > > <
2 4ða3 a2 Þc2 H x L20 L20
þ a2 c2 H þ c1 h k1 wðxÞ;
06x
a3 c2 H þ c1 h1 þ p; l6x 4ð1a1 Þc2 H > > x L0 L1 2a þ a1 c2 H þ c1 h1 k2 wðxÞ; l þ L 6 x < l þ L þ a 2 > a > : c2 H þ c1 h1 k2 wðxÞ; l þ L þ a 6 x < þ1 ð4Þ
2.2. Top-coal roof-sag curve
3. Analysis of influential factors in top-coal deformation
If the roof coal seam is taken as the homogeneous isotropic linear elastic material, according to the beam theory, the deflection curve equation of beam OD is as follows: 4
EI
d wðxÞ 4
dx
¼ qðxÞ
ð5Þ
where wðxÞ is the deflection of the beam, namely the roof-sag value, 3
as shown in Eq. (6); E the stiffness of top-coal; and I ¼ h1 =12 the moment of inertia of cross-section. Substituting Eq. (4) into Eq. (5), we can get the general solution of Eq. (5) according to the theory of ordinary differential equation.
wðxÞ ¼
8 > > > > > > <
4ða3 a2 Þc2 H k1 l2
Parameters, namely c1 ; c2 ; h0 ; h1 ; H; a1 ; a2 ; a3 ; E; I1 ; L; l; a; p, are the observed quantities obtained from the practical situation of the gob-side entry on site and the laboratory tests. Putting these parameters into Eq. (6), and using Eqs. (6)–(9) calculates the values of c1 ; c2 ; c3 ; . . . ; c15 ; c16 through MATLAB. Then the top-coal roof-sag curve (values) can be obtained.
For better and more precise calculation, this paper takes the 3304 working face of Zaozhuang Mine in Shandong province as a case study. The relationship will be discussed among the roof-sag values, supporting intensity, and narrow pillar width and topcoal stiffness. According to the geological conditions, 3304 working face parameters and field experiment, the following values are obtained: H = 460 m, h0 = 3.10 m, h1 = 5.00 m, L = 5 m, I = 10.417, a1 = 3.00, a2 = 1.50, a3 = 0.30 a = 19.6 m, k1 = 110 MPa, and k2 = 310 MPa. The laboratory experiment shows that E = 1.1 GPa, c1 = 13.50 kN/m3, c2 = 26.00 kN/m3. These values are put into MATLAB to calculate the roof-sag values; and it is found that the
2 c1 h1 x 2l þ a2 c2 Hþ þ ebx ðc1 cos bx þ c2 sin bxÞ þ ebx ðc3 cos bx þ c4 sin bxÞ; k1 a3 c2 Hþc1 h1 þp 4 x 24EI1
þ c5 x þ c6 x þ c7 x þ c8 ; 3
2
06x6l l6x6lþL
2 4ð1a1 Þc2 H c1 h1 > > x l L 2a þ a1 c2 Hþ þ ekx ðc9 cos kx þ c10 sin kxÞ þ ekx ðc11 cos kx þ c12 sin kxÞ; l þ L 6 x 6 l þ L þ a > k2 k2 a2 > > > : c2 Hþc1 h1 þ ekx ðc13 cos kx þ c14 sin kxÞ þ ekx ðc15 cos kx þ c16 sin kxÞ; lþLþa6x61 k2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi where b ¼ 4 k1 =4EI; k ¼ 4 k2 =4EI; and c1 ; c2 ; c3 ; . . . ; c15 ; c16 the undetermined coefficients which are to be determined by the boundary condition of the curve subsidence of top-coal. The outside of the origin (the goaf-side endpoint of top-coal) is the goaf, on which the supporting effect is so minor as to be regarded as the free end, then the boundary condition Eq. (7) can be obtained: 2
x¼0:
d
2
dx 3 d
wðxÞ ¼ 0; ð7Þ
wðxÞ ¼ 0 3 dx In the deep enough position of the integrated coal beside the roadway, the coal seam is still acted upon by the rock stress without displacement and deformation, so it is regarded as the fixed end. The boundary condition Eq. (8) is shown as follows:
x ! 1 : wðxÞ ¼ 0;
ð6Þ
maximum roof-sag value is deviated to the narrow coal pillar. Therefore, the sink value of the side of the narrow coal pillar (x ¼ l) is set as the roof-sag value in the following discussion. 3.1. Relationship among roof-sag value, supporting intensity and narrow pillar width According to the surrounding rock stability theory of big or small structure, the gob-side entry should be located at the outside of the rupture line between the key chunk A and B [13,18,19]. He and Li have calculated out that the narrow coal pillar width can be managed between 3 and 5 m. The relationship among roof-sag
ð8Þ
dwðxÞ ¼0 dx
Since the top-coal is a continuous linear elastic beam, the subsidence value, angle value, shear stress and moment value at points A; B; C should be continuous. Thus the continuity condition Eq. (9) can be shown as follows:
8 wjx!P ¼ wjx!Pþ ; > > > > dw dw > ; ¼ dx > x!P þ < dx x!P d2 w d2 w ¼ dx2 þ ; 2 > > x!P > dx x!P > > > : d3 w d3 w ¼ ; þ dx3 dx3 x!P
x!P
P ¼ A; B; C
ð9Þ
Fig. 3. Relationship between the roof-sag value and narrow pillar width.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
value w, supporting intensity p and narrow pillar width l is shown in Fig. 3 and the results are as follows: (1) Roof-sag value decreases as the supporting intensity increases. They are negative linear correlation. Given the same narrow coal pillar width, the roof-sag value changes comparatively slightly when the supporting intensity increases. This indicates that the supporting intensity has a quite mild influence on the roof-sag value. However, given the same supporting intensity, the roof-sag value changes drastically as the narrow coal pillar width increases. This shows that the roof-sag value is more sensitive to the narrow coal pillar width. Therefore, priority should be given to the narrow coal pillar width in the design of gob-side entry. (2) With the increase of the narrow pillar width, the roof-sag value initially decreases rapidly and then becomes stable. When the pillar width is small (3.0–4.0 m), the roof-sag value changes drastically, indicating that the pillar width has remarkable influence on the roof-sag value. When the width is between 4.0 and 4.5 m, the roof-sag value scarcely changes. When the width is over 4.5 m, the roof-sag value increases slowly as the pillar width increases, indicating that the pillar width between 4.0 and 4.5 m makes the gob-side entry locate in the stress decreasing zone where the roof is easiest to control. Therefore, the pillar width should be between 4.0 and 4.5 m. In view of the production equipment and rock mechanical parameters, the allowable maximum top-coal deformation is set at 100 mm. Fig. 3 shows that when the narrow coal pillar width is 4.0–4.5 m, the needed supporting intensity p is 0.4 MPa. 3.2. Relationship between the roof-sag values and top-coal stiffness According to the supporting theory, the key to working face tunnel support is to maintain the stability of the ‘‘small structure” in gob-side entry [20–22]. In addition to the positive reinforcement measures, including anchor, anchor cable, anchor net support, and grouting, reinforcement in the top-coal of the gob-side entry is also adopted. The injecting paste material is infused into the cracks of the weak plane of the top-coal to improve the stiffness of topcoal, thus further improving the structural integrity and stability of the gob-side entry. In that sense, the stiffness of top-coal is manageable. If the width of narrow pillar is set as L0 = 4.0 m, the relationship between the roof-sag values w and top-coal stiffness E (Fig. 4) under different supporting intensities p can be obtained through
MATLAB. Fig. 4 shows that the relationship between roof-sag values and top-coal stiffness is inversely proportional basically. When E < 2.0 GPa, the roof-sag values decrease rapidly with the increase of the coal stiffness. This indicates that the change of coal stiffness has great influence on the roof-sag values. When E P 2.0 GPa, the roof-sag values decrease slowly, indicating that the influence of the changing stiffness on the roof-sag values becomes weaker and weaker and the stiffness increase cannot reduce the roof-sag values remarkably. If the allowable top-coal deformation is set at 100 mm, the width of the narrow pillar l as 4.0–4.5 m, and supporting intensity p as 0.4 MPa, the minimum stiffness of top coal should be 1.5 GPa. In order to meet the requirement for the stiffness, comprehensive measures such as the anchor cable, anchor net and grouting are adopted to control the roof-sag values and reinforce the top-coal to increase its stiffness to 1.5 GPa. 4. Engineering application 4.1. Project profile The roadway driving along goaf for 3304 fully mechanized topcoal caving face is located in the No. 33 mining area which is 300 m under the earth. The unexploited area is at its left side and 3302 working face under exploitation is at the right side. The special layout is shown in Fig. 5. The 243 m long working face is to mine the Shanxi Formation 3# coal seam. This coal seam is about 460 m under the ground and its average depth is 8.1 m with 7° dip angle. The length of the roadway driving along goaf is 828 m. The exploitation manner is to tunnel along the floor and the designed width and height of the tunnel is 5 and 3.1 m respectively. The immediate roof is the mudstone or sandy mudstone with an average depth of 3.9 m. The main roof is fine sandstone with an average depth of 9.8 m. The coal and rock mass is banded structure with fracture development. The stiffness of the top-coal, between 0.83 and 0.99, is relatively small. 4.2. Supporting pattern According to the above mentioned study, the width of the narrow pillar is set at 4 m, supporting intensity 0.4 MPa and the stiffness of the top-coal 1.5 GPa. The parameters of the anchor cables, anchor net and grouting are as follows: (1) The anchor net and tissot are adapted to support the roof of the tunnel. Seven M20L2400 mm deformed-steel-bar anchor stocks of high intensity are employed, whose designed torque are 200 Nm and preload P78.4 kN. The two sides of
Open-off cut
Roadway
Narrow pillar
Roadway driving along goaf
Key block A
Key block C
Key block B
Working face 3304
Fig. 4. Relationship between the roof-sag value and top-coal stiffness.
Goaf of working face 3302
Fig. 5. Positional relationship of roadway driving along goaf in working face.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
Fig. 6. Curve of the roof-sag value.
5
foundation model, the analytical solution of the curve of the roof-sag values is conducted. (2) A calculating analytical program is drawn up based on the analytical solution. With the actual engineering project as a case study, the roof-sag values of the top-coal under different supporting intensities, widths of narrow pillar and stiffness of top-coal are obtained. Meanwhile the relationships among roof-sag values, supporting intensities, widths of narrow pillar and stiffness are analyzed. It can be concluded that the width of narrow pillar and stiffness has more obvious influence on the roof-sag value than that of supporting intensity. (3) In view of the practical mining condition, the support parameters of the top-coal of the gob-side entry are determined; in addition, some suggestions are put forward as to the application of comprehensive support including anchor cable, anchor net as well as grouting. The field experiment proves that the scheme is reliable and effective. To sum up, the conclusion has good reference significance for the deformation control of the roof as well as the safety and maintenance under similar conditions.
Acknowledgments This paper are funded by the National Natural Science Foundation of China (No. 51374201, 51323004) the State Key Development Program for Basic Research of China (No. 2013CB227900), the College Student’s Program for Innovation of China University of Mining and Technology of China (No. 201507). Fig. 7. Curve of deformation value of roadway’s sides.
the roadway are supported by 5 M20L1800 mm deformed steel bar high intensity anchor stocks on each side. Each anchor stock, whose designed torque are 200 Nm and preload P58.8 kN, is anchored by two rolls of MSZ2350 (M20L500 mm) resin cartridge with the array pitch 900 mm and the interval 800 mm. (2) Where the top coal is broken and the plane growth is weak, grouting is adopted to guarantee the top-coal stiffness above 1.5 GPa. For the area where the top coal is seriously broken, unified anchor grouting is conducted with chemical grouts. 4.3. Supporting effect To check out the reliability and effectiveness of the supporting scheme, the ‘‘cross-stationing” is adopted to monitor the deformation values of the top-coal as well as rib-side convergence in the gob-side entry of the 3304 working face. By setting the finish time of the gob-side entry support as the start time, the curve of the roof-sag values and the curve of deformation values of roadway’s side are shown as in Figs. 6 and 7. As is shown in Figs. 6 and 7, deformation value of the tunnel approaches to the maximum and remains stable after 32 days of roadway driving along goaf. The maximum roof-sag value of the top-coal is 91 mm and the maximum deformation value of roadway’s sides is 145 mm. The deformation of roadway driving along goaf is within the allowable range, which proves that the abovementioned comprehensive supporting scheme is reliable and effective. 5. Conclusions (1) According to the surrounding rock characteristics of the roadway driving along goaf, a mechanical model of top-coal is established. By means of the Winkler elastic
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Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face Qi Fangkun a,c, Zhou Yuejin b,⇑, Li Jiawei b, Wang Erqian b, Cao Zhengzheng b, Li Ning b a
School of Mines, China University of Mining & Technology, Xuzhou 221116, China State Key Laboratory Geomechanics & Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China c Xin’an Coal Co., Ltd., Zaozhuang Mining Group, Zaozhuang 277000, China b
a r t i c l e
i n f o
Article history: Received 27 September 2015 Received in revised form 30 November 2015 Accepted 15 January 2016 Available online xxxx Keywords: Fully mechanized top-coal caving Narrow pillar Gob-side entry Top coal Deformation control
a b s t r a c t A mechanical model to control the top-coal deformation is established in accordance with the structural characters of the gob-side entry surrounding rock for the fully-mechanic top-coal caving; the analytical solution of top coal roof-sag curve is deduced with Winkler elastic foundation beam model. By means of a calculating and analytic program, the top coal roof-sag values are calculated under the conditions of different supporting intensities, widths of narrow pillars and stiffness of top coal; meanwhile, the relationship between the roof-sag values and supporting intensity, width of narrow pillars and stiffness of top coal is analyzed as well. With the actual situation of the gob-side entry taken into consideration, the parameters of top-coal control are determined and a supporting plan is proposed for the top-coal control, which is proved to be reliable and effective by on-site verification. Some theoretical guidance and advice are put forward for the top-coal deformation control in gob-side entry for fully mechanized top-coal caving face. Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction In the gob-side entry with narrow pillars, the abutment pressure is redistributed because of the subsidence rotary motion of roof strata [1]. The interaction and mutual influences among the top-coal, the narrow pillar, the integrated coal beside the roadway and the floor lead to the surrounding rock deformation and failure. As is known, the deformation amount of the roadway’s sides is often larger than that of the roof and floor. Therefore, how to maintain the stability of the two sides of the roadway, especially that of the narrow pillar walls has been the focus of the research on the deformation and control of roadway driving along goaf [2–5]. Under the condition that the main roof acts on the surrounding rocks of the fully-mechanized caving roadway in a given deformation way, Gao et al. analyzed the deformation mechanism of roadway surrounding rocks [6–8]. However, there has been little research on the relationship between the top-coal deformation and supporting intensity, width of narrow pillars, and stiffness of top coal. Site investigations indicate that the analysis on the topcoal deformation and strained condition is of great significance to study the whole stability of fully-mechanized top-coal caving roadway with narrow pillars. ⇑ Corresponding author. Tel.: +86 13914884696. E-mail address: [email protected] (Y. Zhou).
Based on the geological condition of the 3304 fully-mechanized top-coal caving face of a mine in Zaozhuang, the beam structure model of top-coal is established according to the general surrounding rock structure of the gob-side entry. By means of theoretical analysis, numerical calculation and field measurement, this paper studies the relationship between the caving top-coal deformation and its influencing factors, and puts forward the control method of the roof of caving top-coal of roadway along goaf with narrow pillars; meanwhile the relevant parameters are determined. 2. Top-coal stress and deformation analysis of roadway driving along goaf 2.1. Mechanical model of top-coal One side of the gob-side entry for fully mechanized top-coal caving is the integrated coal beside the roadway while the other side is the narrow pillar. According to the key strata theory and the movement law of overlying strata on the goaf side of fully mechanized caving face, fracture, rotary and subsidence occur in the main roof after the immediate roof caving and sinking in the upper section of the working face; in the lower section, the lateral ‘‘voussoir beam” structure is formed in the coal mass, namely ‘‘the big structure” [9–14]. In this process, a fractured zone with a certain thickness is formed as the marginal coal mass is damaged;
http://dx.doi.org/10.1016/j.ijmst.2016.02.008 2095-2686/Ó 2016 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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as the lateral abutment pressure is transferred to the integrated coal beside the roadway, the stress increasing zone is formed; and the stress decreasing zone is formed in the lower part of the big structure. This provides favorable conditions for the roadway driving along goaf where the forced deformation of surrounding rocks is small and easy to control. After the roadway driving along goaf, an organic whole is developed by the top-coal, the floor, the two sides, the narrow pillar and the anchor as the support object of the roadway, called ‘‘the small structure” [13,14]. The roadway support of gob-side entry driving focuses on maintaining the stability of the small structure. The relationship between the roadway driving along goaf with narrow pillars and the overlying strata structure is shown in Fig. 1. The roof subsidence of roadway driving along goaf is mainly composed of the inelastic displacement caused by rock stress release of surrounding rocks in tunnel excavation process and the gravity of overlying strata. For the convenience of study, the inelastic displacement caused by rock stress release is ignored. According to the main roof activity rule, the top-coal of roadway driving along goaf is simplified to the beam structure, and its corresponding mechanical model is established and shown in Fig. 2. Taking the horizontal centerline of the top-coal as the axis, the goaf-side endpoint of top-coal as the origin O, the lateral direction of the integrated coal beside the roadway as the positive direction, we can set up the coordinate. Point A and point B stand for the two walls of the gob-side entry respectively; point C refers to the boundary of the deep stress concentrated zone of the top-coal; the top-coal strata is denoted by OD, and D refers to the random point which is far enough, and can not affect the calculation; the overlying strata stress on the upper part is q1 ðxÞ; under the top coal, the narrow pillar, the gob-side entry and the integrated coal beside the roadway are respectively represented by OA, AB and BD, and their widths are l, L and a þ x0 , respectively; they are acted jointly by the narrow pillar’s holding power q2 ðxÞ, support intensity p and the entity coal’s holding power q3 ðxÞ. The mechanical environment of the fully mechanized top-coal roadway driving along goaf with narrow pillars differs from that of general mining roadways [15,16]. After mining in the upper section, the stress relaxed zone is formed above the gob-side entry; meanwhile the stress concentrated zone appears with the increase of the stress on the two sides of the overlying strata. Thus the quadratic function is employed here. Due to the rotation of the main roof, the narrow pillar enters the plastic state with the bearing capacity dramatically reduced, while the stress concentration on the integrated coal beside the roadway rises higher. The stress concentration coefficient of the BC side of the integrated coal beside the roadway is set as a1 , and that of the top-coal of narrow pillar OA as a2 . The overburdened pressure upon AB above the roof of roadway driving along goaf is regarded as the uniform load, and the relative rock stress coefficient is a3 . Because the roadway excavation has less impact on the top-coal CD side of the virgin coal
Main roof
Fig. 2. Mechanical model of the top-coal of roadway driving along goaf with narrow pillar.
face, the load subjected is still the overlying strata gravity c2 H, where c2 is the average weight, N/m2; and H is the thickness of the overlying strata (m) which can be seen as the buried depth. Therefore, the overlying strata stress q1 ðxÞ, which is acted on the top-coal can be represented by the following piecewise function:
q1 ðxÞ ¼
8 > > > > > <
4ða3 a2 Þc2 H x L20
L20
2
þ a2 c2 H;
06x
a3 c2 H; l6x
If the coal mass is taken as the isotropic elastic body, the supporting effect of the narrow pillar and the integrated coal beside the roadway on the top-coal can be conducted as the elastic foundation [17]. So
q2 ðxÞ ¼ k1 wðxÞ;
06x6l
ð2Þ
q3 ðxÞ ¼ k2 wðxÞ;
l þ L 6 x < þ1
ð3Þ
where k1 and k2 are the Winkler foundation coefficients reflecting the bearing capacity of the coal-rock mass under the top-coal, which is related with the mechanical properties of the side coal mass and the height of the roadway. Due to the roof subsidence, the two sides enter the plastic state from the coal wall to the coal mass. The side surrounding rocks constantly squeeze and crush the inside roadway, which leads to the reduction of the bearing capacity of the coal-rock mass. The Winkler foundation coefficients go down consequently and then the curve subsidence of the roof is enlarged. In addition, the top-coal of the gob-side entry is affected by the support intensity p, and the whole top-coal is affected by its own gravity c1 h1 , where c1 refers to the unit weight of the coal mass, N/m2; and h1 is the thickness of the top-coal, m. The resultant force, which is called the equivalent load qðxÞ, is composed of the overlying strata stress, the side coal mass’ holding power, the supporting intensity and the self gravity being acted on the top-coal. It can be represented by Eq. (4):
Key block A Key block B Key block C
Immediate roof
Top coal Coal seam Integrated coal beside the roadway
Uncaved top coal
Broken rock
Narrow pillar
Fig. 1. Structure of surrounding rock of roadway driving along goaf with narrow pillar.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
qðxÞ ¼
8 > > > > > <
2 4ða3 a2 Þc2 H x L20 L20
þ a2 c2 H þ c1 h k1 wðxÞ;
06x
a3 c2 H þ c1 h1 þ p; l6x
2.2. Top-coal roof-sag curve
3. Analysis of influential factors in top-coal deformation
If the roof coal seam is taken as the homogeneous isotropic linear elastic material, according to the beam theory, the deflection curve equation of beam OD is as follows: 4
EI
d wðxÞ 4
dx
¼ qðxÞ
ð5Þ
where wðxÞ is the deflection of the beam, namely the roof-sag value, 3
as shown in Eq. (6); E the stiffness of top-coal; and I ¼ h1 =12 the moment of inertia of cross-section. Substituting Eq. (4) into Eq. (5), we can get the general solution of Eq. (5) according to the theory of ordinary differential equation.
wðxÞ ¼
8 > > > > > > <
4ða3 a2 Þc2 H k1 l2
Parameters, namely c1 ; c2 ; h0 ; h1 ; H; a1 ; a2 ; a3 ; E; I1 ; L; l; a; p, are the observed quantities obtained from the practical situation of the gob-side entry on site and the laboratory tests. Putting these parameters into Eq. (6), and using Eqs. (6)–(9) calculates the values of c1 ; c2 ; c3 ; . . . ; c15 ; c16 through MATLAB. Then the top-coal roof-sag curve (values) can be obtained.
For better and more precise calculation, this paper takes the 3304 working face of Zaozhuang Mine in Shandong province as a case study. The relationship will be discussed among the roof-sag values, supporting intensity, and narrow pillar width and topcoal stiffness. According to the geological conditions, 3304 working face parameters and field experiment, the following values are obtained: H = 460 m, h0 = 3.10 m, h1 = 5.00 m, L = 5 m, I = 10.417, a1 = 3.00, a2 = 1.50, a3 = 0.30 a = 19.6 m, k1 = 110 MPa, and k2 = 310 MPa. The laboratory experiment shows that E = 1.1 GPa, c1 = 13.50 kN/m3, c2 = 26.00 kN/m3. These values are put into MATLAB to calculate the roof-sag values; and it is found that the
2 c1 h1 x 2l þ a2 c2 Hþ þ ebx ðc1 cos bx þ c2 sin bxÞ þ ebx ðc3 cos bx þ c4 sin bxÞ; k1 a3 c2 Hþc1 h1 þp 4 x 24EI1
þ c5 x þ c6 x þ c7 x þ c8 ; 3
2
06x6l l6x6lþL
2 4ð1a1 Þc2 H c1 h1 > > x l L 2a þ a1 c2 Hþ þ ekx ðc9 cos kx þ c10 sin kxÞ þ ekx ðc11 cos kx þ c12 sin kxÞ; l þ L 6 x 6 l þ L þ a > k2 k2 a2 > > > : c2 Hþc1 h1 þ ekx ðc13 cos kx þ c14 sin kxÞ þ ekx ðc15 cos kx þ c16 sin kxÞ; lþLþa6x61 k2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffi where b ¼ 4 k1 =4EI; k ¼ 4 k2 =4EI; and c1 ; c2 ; c3 ; . . . ; c15 ; c16 the undetermined coefficients which are to be determined by the boundary condition of the curve subsidence of top-coal. The outside of the origin (the goaf-side endpoint of top-coal) is the goaf, on which the supporting effect is so minor as to be regarded as the free end, then the boundary condition Eq. (7) can be obtained: 2
x¼0:
d
2
dx 3 d
wðxÞ ¼ 0; ð7Þ
wðxÞ ¼ 0 3 dx In the deep enough position of the integrated coal beside the roadway, the coal seam is still acted upon by the rock stress without displacement and deformation, so it is regarded as the fixed end. The boundary condition Eq. (8) is shown as follows:
x ! 1 : wðxÞ ¼ 0;
ð6Þ
maximum roof-sag value is deviated to the narrow coal pillar. Therefore, the sink value of the side of the narrow coal pillar (x ¼ l) is set as the roof-sag value in the following discussion. 3.1. Relationship among roof-sag value, supporting intensity and narrow pillar width According to the surrounding rock stability theory of big or small structure, the gob-side entry should be located at the outside of the rupture line between the key chunk A and B [13,18,19]. He and Li have calculated out that the narrow coal pillar width can be managed between 3 and 5 m. The relationship among roof-sag
ð8Þ
dwðxÞ ¼0 dx
Since the top-coal is a continuous linear elastic beam, the subsidence value, angle value, shear stress and moment value at points A; B; C should be continuous. Thus the continuity condition Eq. (9) can be shown as follows:
8 wjx!P ¼ wjx!Pþ ; > > > > dw dw > ; ¼ dx > x!P þ < dx x!P d2 w d2 w ¼ dx2 þ ; 2 > > x!P > dx x!P > > > : d3 w d3 w ¼ ; þ dx3 dx3 x!P
x!P
P ¼ A; B; C
ð9Þ
Fig. 3. Relationship between the roof-sag value and narrow pillar width.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
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value w, supporting intensity p and narrow pillar width l is shown in Fig. 3 and the results are as follows: (1) Roof-sag value decreases as the supporting intensity increases. They are negative linear correlation. Given the same narrow coal pillar width, the roof-sag value changes comparatively slightly when the supporting intensity increases. This indicates that the supporting intensity has a quite mild influence on the roof-sag value. However, given the same supporting intensity, the roof-sag value changes drastically as the narrow coal pillar width increases. This shows that the roof-sag value is more sensitive to the narrow coal pillar width. Therefore, priority should be given to the narrow coal pillar width in the design of gob-side entry. (2) With the increase of the narrow pillar width, the roof-sag value initially decreases rapidly and then becomes stable. When the pillar width is small (3.0–4.0 m), the roof-sag value changes drastically, indicating that the pillar width has remarkable influence on the roof-sag value. When the width is between 4.0 and 4.5 m, the roof-sag value scarcely changes. When the width is over 4.5 m, the roof-sag value increases slowly as the pillar width increases, indicating that the pillar width between 4.0 and 4.5 m makes the gob-side entry locate in the stress decreasing zone where the roof is easiest to control. Therefore, the pillar width should be between 4.0 and 4.5 m. In view of the production equipment and rock mechanical parameters, the allowable maximum top-coal deformation is set at 100 mm. Fig. 3 shows that when the narrow coal pillar width is 4.0–4.5 m, the needed supporting intensity p is 0.4 MPa. 3.2. Relationship between the roof-sag values and top-coal stiffness According to the supporting theory, the key to working face tunnel support is to maintain the stability of the ‘‘small structure” in gob-side entry [20–22]. In addition to the positive reinforcement measures, including anchor, anchor cable, anchor net support, and grouting, reinforcement in the top-coal of the gob-side entry is also adopted. The injecting paste material is infused into the cracks of the weak plane of the top-coal to improve the stiffness of topcoal, thus further improving the structural integrity and stability of the gob-side entry. In that sense, the stiffness of top-coal is manageable. If the width of narrow pillar is set as L0 = 4.0 m, the relationship between the roof-sag values w and top-coal stiffness E (Fig. 4) under different supporting intensities p can be obtained through
MATLAB. Fig. 4 shows that the relationship between roof-sag values and top-coal stiffness is inversely proportional basically. When E < 2.0 GPa, the roof-sag values decrease rapidly with the increase of the coal stiffness. This indicates that the change of coal stiffness has great influence on the roof-sag values. When E P 2.0 GPa, the roof-sag values decrease slowly, indicating that the influence of the changing stiffness on the roof-sag values becomes weaker and weaker and the stiffness increase cannot reduce the roof-sag values remarkably. If the allowable top-coal deformation is set at 100 mm, the width of the narrow pillar l as 4.0–4.5 m, and supporting intensity p as 0.4 MPa, the minimum stiffness of top coal should be 1.5 GPa. In order to meet the requirement for the stiffness, comprehensive measures such as the anchor cable, anchor net and grouting are adopted to control the roof-sag values and reinforce the top-coal to increase its stiffness to 1.5 GPa. 4. Engineering application 4.1. Project profile The roadway driving along goaf for 3304 fully mechanized topcoal caving face is located in the No. 33 mining area which is 300 m under the earth. The unexploited area is at its left side and 3302 working face under exploitation is at the right side. The special layout is shown in Fig. 5. The 243 m long working face is to mine the Shanxi Formation 3# coal seam. This coal seam is about 460 m under the ground and its average depth is 8.1 m with 7° dip angle. The length of the roadway driving along goaf is 828 m. The exploitation manner is to tunnel along the floor and the designed width and height of the tunnel is 5 and 3.1 m respectively. The immediate roof is the mudstone or sandy mudstone with an average depth of 3.9 m. The main roof is fine sandstone with an average depth of 9.8 m. The coal and rock mass is banded structure with fracture development. The stiffness of the top-coal, between 0.83 and 0.99, is relatively small. 4.2. Supporting pattern According to the above mentioned study, the width of the narrow pillar is set at 4 m, supporting intensity 0.4 MPa and the stiffness of the top-coal 1.5 GPa. The parameters of the anchor cables, anchor net and grouting are as follows: (1) The anchor net and tissot are adapted to support the roof of the tunnel. Seven M20L2400 mm deformed-steel-bar anchor stocks of high intensity are employed, whose designed torque are 200 Nm and preload P78.4 kN. The two sides of
Open-off cut
Roadway
Narrow pillar
Roadway driving along goaf
Key block A
Key block C
Key block B
Working face 3304
Fig. 4. Relationship between the roof-sag value and top-coal stiffness.
Goaf of working face 3302
Fig. 5. Positional relationship of roadway driving along goaf in working face.
Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008
F. Qi et al. / International Journal of Mining Science and Technology xxx (2016) xxx–xxx
Fig. 6. Curve of the roof-sag value.
5
foundation model, the analytical solution of the curve of the roof-sag values is conducted. (2) A calculating analytical program is drawn up based on the analytical solution. With the actual engineering project as a case study, the roof-sag values of the top-coal under different supporting intensities, widths of narrow pillar and stiffness of top-coal are obtained. Meanwhile the relationships among roof-sag values, supporting intensities, widths of narrow pillar and stiffness are analyzed. It can be concluded that the width of narrow pillar and stiffness has more obvious influence on the roof-sag value than that of supporting intensity. (3) In view of the practical mining condition, the support parameters of the top-coal of the gob-side entry are determined; in addition, some suggestions are put forward as to the application of comprehensive support including anchor cable, anchor net as well as grouting. The field experiment proves that the scheme is reliable and effective. To sum up, the conclusion has good reference significance for the deformation control of the roof as well as the safety and maintenance under similar conditions.
Acknowledgments This paper are funded by the National Natural Science Foundation of China (No. 51374201, 51323004) the State Key Development Program for Basic Research of China (No. 2013CB227900), the College Student’s Program for Innovation of China University of Mining and Technology of China (No. 201507). Fig. 7. Curve of deformation value of roadway’s sides.
the roadway are supported by 5 M20L1800 mm deformed steel bar high intensity anchor stocks on each side. Each anchor stock, whose designed torque are 200 Nm and preload P58.8 kN, is anchored by two rolls of MSZ2350 (M20L500 mm) resin cartridge with the array pitch 900 mm and the interval 800 mm. (2) Where the top coal is broken and the plane growth is weak, grouting is adopted to guarantee the top-coal stiffness above 1.5 GPa. For the area where the top coal is seriously broken, unified anchor grouting is conducted with chemical grouts. 4.3. Supporting effect To check out the reliability and effectiveness of the supporting scheme, the ‘‘cross-stationing” is adopted to monitor the deformation values of the top-coal as well as rib-side convergence in the gob-side entry of the 3304 working face. By setting the finish time of the gob-side entry support as the start time, the curve of the roof-sag values and the curve of deformation values of roadway’s side are shown as in Figs. 6 and 7. As is shown in Figs. 6 and 7, deformation value of the tunnel approaches to the maximum and remains stable after 32 days of roadway driving along goaf. The maximum roof-sag value of the top-coal is 91 mm and the maximum deformation value of roadway’s sides is 145 mm. The deformation of roadway driving along goaf is within the allowable range, which proves that the abovementioned comprehensive supporting scheme is reliable and effective. 5. Conclusions (1) According to the surrounding rock characteristics of the roadway driving along goaf, a mechanical model of top-coal is established. By means of the Winkler elastic
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Please cite this article in press as: Qi F et al. Top-coal deformation control of gob-side entry with narrow pillars and its application for fully mechanized mining face. Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.02.008