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Stability of roof structure and its control in steeply inclined coal seams
International Journal of Mining Science and Technology 27 (2017) 359–364
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Stability of roof structure and its control in steeply inclined coal seams Li Xiaomeng a,⇑, Wang Zhaohui b, Zhang Jinwang b a b
College of Mining and Geomatics Engineering, Hebei University of Engineering, Handan 056038, China College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 11 January 2016 Received in revised form 2 May 2016 Accepted 3 September 2016 Available online 21 February 2017 Keywords: Steeply inclined coal seam Inclined masonry structure Overlying strata Structure instability
a b s t r a c t To improve the effectiveness of control of surrounding rock and the stability of supports on longwall topcoal caving faces in steeply inclined coal seams, the stability of the roof structure and hydraulic supports was studied with physical simulation and theoretical analysis. The results show that roof strata in the vicinity of the tail gate subside extensively with small cutting height, while roof subsidence near the main gate is relatively assuasive. With increase of the mining space, the caving angle of the roof strata above the main gate increases. The characteristics of the vertical and horizontal displacement of the roof strata demonstrate that caved blocks rotate around the lower hinged point of the roof structure, which may lead to sliding instability. Large dip angle of the coal seam makes sliding instability of the roof structure easier. A three-hinged arch can be easily formed above both the tail and main gates in steeply inclined coal seams. With the growth in the dip angle, subsidence of the arch foot formed above the main gate decreases significantly, which reduces the probability of the roof structure becoming unstable as a result of large deformation, while the potential of the roof structure’s sliding instability above the tail gate increases dramatically. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Steeply inclined coal seams widely exist in many coalproducing areas, such as Xinjiang, Ningxia, Shanxi, Guizhou, Chongqing, Huainan, Gansu and Beijing [1–3]. As the major mining area has moved to western China, where half of the mines exploit steeply inclined coal seams, research on mining steeply inclined seam has become a high priority [4,5]. There are remarkable differences in mining method, structure of overlying strata, rules of mine pressure and stability control of supports between mining steeply inclined seams and horizontal seams [6–12]. The emphasis on surrounding rock control is determined by the structure of the overlying strata along the inclination, which is one of the main bases of stope support design. Domestic scholars have made great research on the problem of surrounding rock control in steeply inclined panels. Wang [13,14] studied the fracture mode and evaluation of the main roof in a steeply inclined thick seam based on elastic mechanics. Xie [8] studied the interaction characteristics between strata movement and the support system around a large mining height fully-mechanized face in a steeply inclined seam. Yin [15] obtained the ground pressure of the surrounding rock on a large dip angle face by photo-elastic ⇑ Corresponding author. E-mail address: [email protected] (X. Li).
experiment. Tu [16] studied deformation and fracture features in asymmetrical filling along an incline based on the theory of thin plates. Through comprehensive methods such as similar simulation, Wu [17–19] proposed the inclined masonry structure of dip direction and anti-dip direction pile types, and pointed out that the unbalanced movement of these structures was the primary factor in the instability of the ‘‘R-S-F” system. Nevertheless, the principal feature by which the steeply inclined face is different from an approximately horizontal face lies in the discrepancy in structure of the surrounding rock along an incline caused by the dip angle. There is a lack of in-depth study on the structural forms, instability forms and causational rules of mine pressure. Taking panel 1201 of Dayuan coal mine as engineering background, the rules of overlying strata movement along the inclination and roof structure in mining steeply inclined seam are researched through similar simulation experiment. The instability conditions of the roof structure in the dip direction were studied by theoretical analysis and was verified through in-situ rules of mine pressure. The research achievements are expected to have significance for designing roof control in mining steeply inclined seams. 2. Engineering situation The depth of #2 coal seam of panel 1201 in Dayuan coal mine is from 195.6 m to 242.6 m and the panel width is 60 m. The average
http://dx.doi.org/10.1016/j.ijmst.2017.01.018 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1390 11-1 11-2 11-3 11-4 11-5 11-6 11-7 11-8 11-9 11-10 11-11 11-12
# 13 1° 4
90 m 80 m
9-3
9-4
9-5
9-6
9-7
9-8
9-9
8-3
8-4
8-5
8-6
8-7
8-8
8-9 8-10
7-1
7-2
7-3
7-4
7-5
7-6
7-7
7-8
7-9
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
5-1
5-2
5-3
5-4
5-5
5-6
5-7
4-1
4-2
4-3
4-4
4-5
4-6
3-1
3-2
3-3
3-4
3-5
2-1
2-2
2-3
2-4
1-1
1-2
1-3
60 m
1360
°
1340
9-2 8-2
70 m
50 m
46
7# ° 28 Tranport entry
9-1 8-1
9-10 9-11
Siltstone
40 m 1350 Panel Caving Floor Coal seam zone
Base line
1340
Medium sandstone Fine sandstone Coal #2
20 0
1350
16 #
1360
10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12
1380
1370
24 40 # ° 31 52 # °
1370
Ventilation entry
1200
3 47 4# °
1380
Fig. 1. Schematic diagram of the roadway arrangement in panel 1201.
1800
thickness of the coal seam is 6.8 m and the average dip angle is 45°. A fully-mechanized longwall caving method is applied in the panel. The coal-cutting height is 2.3 m, and the caving height is 4.5 m. The immediate roof is an 8 m thick siltstone with a Protodyakonov coefficient f from 4 to 6.14. The basic roof is a medium sandstone from 3 to 5 m thick and a Protodyakonov coefficient f of 6.31. The direct floor includes carbon mudstone and siltstone with a thickness varying between 3.8 and 5.4 m. The basic floor is a 12 m thick medium sandstone. The ventilation entry was laid out in the floor and the transport entry is also in the floor [20]. The joint section is a circular arc, as shown in Fig. 1. 37 supports from the top down were installed along the tilt direction. The dip angle in the position of No. 31 support reaches its maximum value of 52°. During the three-month period of equipment installation, the supports in the upper end have worse stability including sliding, toppling and squeezing. The face advanced not more than 10 m from open-off cut.
Unit: (mm)
Fig. 2. Arrangement of measuring line.
ac of volume-weight is 1.6:1 and the ratio of time similarity is 10. The physical and mechanical parameters of rock strata are shown in Table 1. The model is 180 cm long, 16 cm wide and 120 cm in height. The remaining weight of the overlying strata was uploaded by the compensation method. The displacement observation points were arranged from the bottom up after layout of the model. The layout density was 10 cm 10 cm. There were 11 layers in total, which are shown in Fig. 2. Coal cutting and drawing in different panel widths were simulated after the strata reached its scheduled strength. Coal cutting of 2.3 m mining height in 100 m panel widths was firstly simulated based on practical production and then coal drawing was simulated to observe the rules of overlying strata caving. 3.2. Presentation of results
3. Physical simulation of overlying strata movement 3.1. Program of similar simulation In mining steeply-inclined coal seams, the emphasis on stability control of supports is determined by the rules of overlying strata movement and their forms of instability. For the extraction of an approximately horizontal seam, the roof structure in the strike direction is largely understood as a result of research. However, in steeply inclined seams, it is of greater significance to study the failure mode of the roof and its structure in the tilt direction. The differences in structure of overlying strata and its instability mode are factors causing great discrepancy in the ‘support and surrounding rock’ system between mining steeply inclined seams and approximately horizontal seams. Therefore, a plane analog simulation was applied in the experimental research on overlying strata movement and its structural instability in the tendency direction. The experiment was designed on the basis of similarity theory, including geometric similarity, kinematic similarity and dynamic similarity. The ratio of geometric similarity, aL is 100:1, the ratio,
3.2.1. Rules of overlying strata movement To be consistent with practical production, after the materials reached their scheduled strength, an initial advance of 2.3 m without drawing was simulated. Excavation started 20 m from the bottom boundary to simulate the rules of overlying strata movement on a fully-mechanized caving steeply-inclined seam. The simulated width of each panel was 100 m. The vertical displacement in different layers after coal drawing is presented in Fig. 3. Fig. 3 demonstrates that the horizontal displacement after mining a steeply-inclined seam is mainly positive. The maximum vertical and horizontal displacement is biased towards the bottom end of the panel. The displacement of measuring lines 50 m and 60 m suddenly increase to respectively 50 m and 60 m away from the bottom end. The vertical displacement arrives at about 5.0 m and 5.5 m. The horizontal and vertical displacement of strata above the 80 m measuring line is very small. The strata above have bed separation from the lower layer. The rock strata within 80 m of the base line subsides extensively. Under the effect of forces perpendicular to the rock strata, the broken articulated rock moves
Table 1 Physical-mechanical parameters of rock strata. No.
Lithology
Thickness (m)
Compressive strength (MPa)
Simulated strength (MPa)
Natural density (g/cm3)
Simulated density (g/cm3)
8 7 6 5 4 3 2 1
Fine sandstone Medium sandstone Siltstone Medium sandstone Siltstone #2 coal Siltstone Medium sandstone
13.2 5.3 4.2 4 8 6.8 4.6 12
21.3 21.3 49.2 63.1 50.7 1.2 52 56
0.133 0.133 0.308 0.394 0.317 0.008 0.325 0.350
2.67 2.63 2.64 2.63 2.64 1.40 2.64 2.63
1.67 1.64 1.65 1.64 1.65 0.88 1.65 1.64
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X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364 4.0 0 90 m 80 m 70 m 60 m 50 m 40 m
3.0 2.5 2.0 1.5 1.0 0.5
-1 Vertical displacement (m)
Horizontal displacement (m)
3.5
-2 -3 -4 -5
0 -40
-20
0
20
40
60
-6 -40
80
Horizontal distance to bottom (m) (a) Horizontal displacement of different layers
-20 0 20 40 60 Horizontal disttance to bottom (m) (b) Vertical displacement of different layers
80
Fig. 3. Horizontal and vertical displacement of overburden strata after coal caving.
65°
(a) Cutting coal 100 m
45
40
37
Inclined masonry structure
54°
65°
4
°
65°
65
37
51°
Inclined masonry structure (b) Caving 40 m
57°
Inclined masonry structure (c) Caving 80 m
57 °
Structural instability
(d) Caving 100 m
Fig. 4. Movement process of overlying strata along inclination.
horizontally towards the upper end, which means the caved block rotates around the bottom hinged point. This is totally different from that of faces mining gently dipping seams in which caved blocks move towards the gob center. The mutation of vertical displacement in the upper end demonstrates that an inclined masonry structure is formed 60 m above the measuring line while the rock below the structure has sliding instability and glides towards the bottom, which increases the vertical displacement. After drawing, sliding down of the caved blocks leads to extensive activity in the upper roof. Broken roof in the upper end has a larger space for movement, which results in a dynamic load acting on the supports. Furthermore, roof exposure in the upper end will decrease the anti-sliding force and aggravate the sliding and squeezing of supports.
3.2.2. Caving features of rock strata The overlying strata movement in different coal drawing stages is shown in Fig. 4. Fig. 4 indicates that the overlying strata movement is small after cutting 100 m along the tilt direction. The upper end, with a caving angle of 65°, has caved sufficiently more than the bottom end with a caving angle of 51°. A three-hinged arch is formed at both the upper and bottom end. The rock strata extrude each other along the tilt direction (Fig. 4a). In the process of drawing coal, broken roof above the softer coal breaks into small rock pieces and slides down the panel. The caving angle at the bottom end increases to 57° and the upper end remains at 65°. Experimental results show that rock strata in the upper end comes to full caving in the initial mining period, while the breaking of strata in the bottom end develops with the increase in extraction space. When the panel width is 100 m, the movement range of overlying strata is 37 m in the normal direction of rock strata. When drawing coal at 80 m, the range is 40 m. By this time, the layer in which an
inclined masonry structure forms develops upwards and the lower rock strata moves towards the space from which coal is drawn. When drawing coal at 100 m, the movement range increases to 45 m and the lower inclined masonry structure becomes unstable again (Fig. 4d). Research indicates that the middle-upper part in the tilt direction of a steeply-inclined stope is the active zone of strata movement. Through observing the characteristics of strata movement in a similar simulation, sliding instability of an inclined masonry structure in the upper end being the main instability mode of a roof breaking structure is verified intuitively. A three-hinged arch structure is formed when mining a steeply inclined seam both in the upper and bottom end. The difference is that the bottom structure is supported by sliding rock and the caving range increases with larger extraction space. After drawing coal, a roof exposure zone exists below the masonry structure in the upper end (Fig. 4c and d) and the degree of caving reaches its maximum after initial coal-cutting. Coal drawing results in the roof exposure being shielded by the inclined masonry structure in the upper end. Strata under the structure slides down after caving and is very active. The instability of this structure leads to roof exposure and dynamic load in the upper end, which aggravates the sliding and toppling of supports.
4. Instability mode of an inclined masonry structure and site verification 4.1. Sliding instability After extraction of the steeply-inclined coal seam, the immediate roof collapses, slides down and fills the bottom end and the basic roof curves and breaks. The inclined arch structure (Fig. 5b), which is different from a horizontal Voussoir beam (Fig. 5a) is
X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364
T2
362
Q=qL
L
=q
h
R
2
Q
h
T
L
T
L R1=qL/2
α
R
1
T1
R2=qL/2
(a) Horizontal arch
(b) Inclined arch
Fig. 5. Structure comparison between steeply inclined and horizontal seam.
formed in both the upper and bottom end of the tendency direction. On the basis of the equilibrium condition of the arch structure, reaction forces T1, R1, T2, R2 in the upper and bottom arch springing can be obtained as follows:
1.2
1 2 1 qL cos a þ qhL sin a =h 8 2 1 2 1 qL cos a qhL sin a =h T2 ¼ 8 2 1 R1 ¼ qL cos a þ qh sin a 2 1 R2 ¼ qL cos a qh sin a 2
0.4
0
ð1Þ
-0.2 -0.4
0
10
20
30
40
50
60
Dip angle α (°)
ð2Þ
bility of the masonry structure formed by inclined rock is obviously affected by the dip angle of the coal seam. With increasing dip angle, the upper rock more readily suffers from sliding instability, which results in dynamic loading of the upper supports. 4.2. Deformation instability During the revolving deformation of the inclined masonry structure, a plastic condition is more likely to develop in the lower rock due to the larger compressive stress in the lower rock and partial stress concentration. Fig. 7 is used for calculating rotary deformation. Eq. (4) can be obtained by calculating the torque relative to point O.
a l a þ ql cos a ¼ T 1 h T3 D þ 2 2 2
ð4Þ
a/2
Q Δ
Note that, when k multiplied by tanu is larger than 1, the structure will maintain stability. Where T is the thrust in spring line direction, kN; u is the friction angle among rocks, °; i is the ratio of rock length L/2 to thickness h of the basic roof. And i can be calculated by Eq. (3).
Fig. 6. Relationship between dip angle and sliding stability coefficient k of upper arch foot.
ð3Þ
a/2
θ ql
l
h
O
α
T1
where RT and q are respectively the tensile strength and load of roof. The upper rock structure will not experience sliding instability with an increase in the coefficient k. Fig. 6 shows that the sliding stability coefficient k is approximately 1.1 when the dip angle is 0° (horizontal coal seam). The inclined masonry structure is more stable. As the dip angle decreases to about 55°, k diminishes to zero, which demonstrates that the thrust T2 of the upper rock is zero and the sliding instability phenomenon occurs in the upper rock structure. For mining steeply inclined seams, the sliding sta-
T3
1 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi RT i¼ 2q cos a
0.6
0.2
where q is the load intensity of arch structure, kN/m; a is the dip angle of coal seam, °; L is the maximum span of roof, m; h is the thickness of roof, m; T1 and T2 are, respectively, thrust in the tendency direction in both arch springing, kN; R1 and R2 are, respectively, the friction force in both arch springings, kN. The roof structure in a steeply-inclined panel, which shows obvious stress asymmetry of structure, is different from that in a gently dipping seam. The dip angle of a coal seam decreases the pressure perpendicular to the roof. The unipolarity of the component force of the overlying strata pressure leads to an increase in thrust T1 at the bottom end and a decrease in T2 at the upper end. Therefore, sliding instability probably occurs in the upper end. Defining the sliding stability coefficient k as T divided by R. Choose upper thrust T2 in a steeply inclined seam.
i i tan a i tan a
0.8
k
T1 ¼
k¼2
1.0
R1
Fig. 7. Deformation instability of rotary rock.
X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364
Mohr-Coulomb criterion, the hinged point in the lower arch structure is more likely to break than the horizontal arch and the dip angle of the coal seam decreases the normal load of the inclined masonry structure perpendicular to the rock strata. Fig. 8 shows that the rotary angle of the rock decreases with an increase in dip angle of the coal seam, which means that both the deflection in the biting point and the degree of deformation instability decrease with an increase in dip angle. In conclusion, the instability mode of an inclined masonry structure, formed after roof breaks in a steeply-inclined seam, is mainly sliding instability.
0.38 0.36
sin θ
0.34 0.32 0.30 0.28 0.26 0.24 0
10
20
30
40
50
60
4.3. Verifying the structural instability modes by working resistance
Dip angle α (°)
Fig. 8. Relationship between the rotation angle of lower arch foot and dip angle of coal seam.
where T1 equals T3 plus qlsina. The rotary angle of the rock can be obtained using a method similar to analyzing the deformation instability of horizontal rock.
sin h ¼
1 tan a þ i 4K0i2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 9 tan2 a þ 48K 0 i tan a þ 48K 0 i
ð5Þ
12K 0 i
2
where K0 equals n times K times K; n is the ratio of compressive strength to tensile strength; K, determined by the conditions to be clamped or simply supported, is approximately between 1/2 and 1/3; K is the ratio of compressive stress to the compressive strength of rock. When K equals 1/2, n equals 10 and K equals 2, the effects of dip angle on the rotary angle of the rock are obtained, as shown in Fig. 8. Eq. (1) indicates that the shear stress R1 in the inclined masonry structure decreases with an increase in dip angle, and the normal stress, which is the inclined thrust T1, increases. Based on the
1.10 1.05 Volatility index (MPa)
363
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65
Lower #1
Lower #9
Middle #19
Upper #29
Upper #37
There are 37 supports in panel 1201, which include 34 basic caving supports of ZFY4800/17/28 and 3 transitional caving supports of ZFG4800/18/32 with one support in the upper end and two supports in the bottom. The working resistance of supports in the upper, middle and bottom panel was monitored as was the support working resistance of #29 and #37 in the upper end, #19 in the middle and #1 and #9 in the upper end. A(m) is defined as the volatility index of the working resistance of support No.m. Its value is an indicator for evaluating the variability of working resistance at different positions in the panel. The larger the value of A(m) indicates more obvious dynamic load effects.
AðmÞ ¼
n 1X Þ2 ðai a n i¼1
ð6Þ
is the average where ai is the working resistance of day i, MPa; a value of working resistance within n days, MPa. Due to small differences in the working resistance in the tilt direction, the volatility index A(m) is used as the indicator to describe the variability of working resistance in the tendency direction. Fig. 9 shows that the volatility index of the upper segment is greater than the bottom segment along the face line direction. That is to say, the sliding instability of the inclined masonry structure results in an obvious variability, which is the reason for high dynamic load in the upper end whilst load intensity caused by deformation instability is smaller in bottom end. Therefore, the sliding instability of the inclined masonry structure in the upper end and its sliding down causes roof exposure and intensive loading. The sliding, toppling and squeezing of the upper supports threatens the stability of supports in the middle and bottom end, which greatly affect the safe and effective production in steeply inclined panels. 5. Engineering application and results
Working resistance
Fig. 9. Volatility index of working resistance along inclination.
Due to the large dip angle of panel 1201 in Dayuan coal mine, the normal and tangential force of gravity of roof and supports
Backing board Upper roadway
(a) Backing board in roof exposure zone
(b) Control results of supports
Fig. 10. Result of control of supports after application of the backing board in the upper end.
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X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364
varies with an increase in dip angle, which in turn results in a larger tangential force and smaller normal force. Hence, the loading of the support system in the panel decreases and the dynamic load causing instability of the support system in the upper end increases. This will intensify the toppling and squeezing of supports, which finally results in insufficient support to roof. To achieve stability of the supports in the whole panel caused by instability of support and surrounding rock system in the upper end when roof exposure occurs, the exposed roof is filled with plate-girder and wicker fence to prevent further roof exposure and ensure sufficient contact with the roof, as shown in Fig. 10a. At the same time, when the roof is broken, the supports are advanced and the telescopic girder extended after cutting coal and the face guard opened to support the roof and coal rib. The supports are moved without pressure relief and the setting load of supports set at more than 20 MPa before moving to guarantee the integrity of the roof. The monthly production of coal increased to 80,000 t through the technical measurements described above. The stability of supports in the upper end is controlled and sliding, toppling and squeezing of supports is prevented. As a result, good working conditions of the support system is achieved. 6. Conclusions (1) Strata displacement in a steeply inclined panel is significantly different from that of a gently horizontal panel. The characteristics of horizontal and vertical displacement demonstrate that the broken block rotates around the bottom hinged point and forms an inclined masonry structure in the vicinity of both tail and main gates. Due to the larger moving space after coal seam extraction, strata movement at the upper end is more strenuous than that at the bottom end. The sliding stability of the upper arch springing of the inclined masonry structure causes obvious dynamic load on the upper supports. (2) In mining a steeply inclined seam, the sliding stability of the masonry structure formed by inclined rock is affected extensively by the dip angle of the coal seam. The probability of sliding stability of the upper hinged point becomes small and the deflection of the bottom block diminishes with the increase of dip angle. The introduction of a volatility index A(m) demonstrated that the dynamic load effect becomes more obvious than that of the middle and bottom of the panel which verifies the results of theoretical analysis. The instability of the upper inclined masonry structure can easily cause roof exposure in mining a steeply inclined seam and, thus causes a dramatic decrease of support stability along the whole panel. (3) By filling the upper exposed zone with backing board and timber crib, combined with moving supports ahead without pressure relief and increasing the setting load of adjacent supports, the bad effects of upper roof exposure on the
stability of supports in middle and bottom panel are minimized. Monthly coal production is improved and the stability and good condition of supports are ensured.
Acknowledgments The financial support from the Joint Funds of the National Natural Science Foundation of China (No. U1361209) and the National Basic Research Program of China (No. 2013CB227903) is greatly appreciated. References [1] Wu YP, Liu KZ, Yun DF, Xie PS, Wang HW. Research progress on the safe and efficient mining technology of steeply dipping seam. J China Coal Soc 2014;39 (8):1611–8. [2] Deng YH, Wang SQ. Feasibility analysis of gob-side entry retaining on a working face in a steep coal seam. Int J Min Sci Technol 2014;24(4):499–503. [3] Shi PW. The complexity of basic roof’s breaking movement in highly inclined seam. Mine Press Roof Manage 1999;Z1:26–8. [4] Zhang Y, Zhang B, Li L. Study on the effect of roof fracture development on gas drainage in steep full-mechanized caving mining. J Min Saf Eng 2014;31 (5):809–13. [5] Xie PS, Wu YP, Wang HW. Study on space activity law of overburden strata above longwall coal mining face in high inclined seam. Coal Sci Technol 2012;40(9):1–5. [6] Xin YJ, Gou PF, Ge FD. Analysis of stability of support and surrounding rock in mining top coal of inclined coal seam. Int J Min Sci Technol 2014;24(1):63–8. [7] Ma R, Li GC, Zhang N, Liu C, Wei YH, Zhang M. Analysis on mechanism and key factors of surrounding rock instability in deeply inclined roadway affected by argillation and water seepage. Int J Min Sci Technol 2015;25(3):465–71. [8] Xie PS, Wu YP, Wang HW, Ren SG. Interaction characteristics between strata movement and support system around large mining height fully-mechanized face in steeply inclined seam. J Min Saf Eng 2015;32(1):14–9. [9] Wu YP. Key technologies and strategies of mechanized mining in highly inclined seam. Mine Press Roof Manage 2002;1:60–3. [10] He MC, Peng YY, Zhao SY, Shi HY, Wang N, Gong WL. Fracture mechanism of inversed trapezoidal shaped tunnel excavated in 45inclined rock strata. Int J Min Sci Technol 2015;25(4):531–5. [11] Li HP, Pu WL, Guo SQ. Probe into the technical developing direction of optimum mining in great inclined seam. Coal Min Technol 2005;10(1):20–2. [12] Shao XP, Shi PW, He GC. Analysis of load transmitting arch of roof in steeply inclined seam. J Univ Sci Technol Beijing 2007;5:447–51. [13] Wang JA, Zhang JW, Gao XM, Wen JD, Gu YD. Fracture mode and evolution of main roof stratum above longwall fully mechanized top coal caving in steeply inclined thick coal seam (I). J China Coal Soc 2015;40(6):1353–60. [14] Wang JA, Zhang JW, Gao XM, Wen JD, Gu YD. Fracture mode and evolution of main roof stratum above longwall fully mechanized top coal caving in steeply inclined thick coal seam (II): periodic fracture. J China Coal Soc 2015;40 (8):1737–45. [15] Yin GZ, Li XS, Guo WB. Photo-elastic experimental and field measurement study of ground pressure of surrounding rock of large dip angle working coalface. Chin J Rock Mech Eng 2010;29(1):3336–43. [16] Tu HS, Tu SH, Chen F. Study on the deformation and fracture feature of steep inclined coal seam roof based on the theory of thin plates. J Min Saf Eng 2014;31(1):49–54. [17] Wu YP, Xie PS, Ren SG. Analysis of asymmetric structure around coal face of steeply dipping seam mining. J China Coal Soc 2010;35(2):182–4. [18] Wu YP. Dynamic equation of system ‘‘roof(R)-support(S)-floor(F)” in steeply dipping seam mining. J China Coal Soc 2005;30(6):736–41. [19] Wu YP, Xie PS, Wang HW. Incline masonry structure around the coal face of the steeply dipping mining seam. J China Coal Soc 2010;35(8):1252–6. [20] Wang ZQ, Zhao JL, Li ZQ. Determination of height of ‘‘three zone” in the stope with stagger position and internal misaligned roadway layout. J Min Saf Eng 2013;30(2):4–10.
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International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Stability of roof structure and its control in steeply inclined coal seams Li Xiaomeng a,⇑, Wang Zhaohui b, Zhang Jinwang b a b
College of Mining and Geomatics Engineering, Hebei University of Engineering, Handan 056038, China College of Resources and Safety Engineering, China University of Mining and Technology, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 11 January 2016 Received in revised form 2 May 2016 Accepted 3 September 2016 Available online 21 February 2017 Keywords: Steeply inclined coal seam Inclined masonry structure Overlying strata Structure instability
a b s t r a c t To improve the effectiveness of control of surrounding rock and the stability of supports on longwall topcoal caving faces in steeply inclined coal seams, the stability of the roof structure and hydraulic supports was studied with physical simulation and theoretical analysis. The results show that roof strata in the vicinity of the tail gate subside extensively with small cutting height, while roof subsidence near the main gate is relatively assuasive. With increase of the mining space, the caving angle of the roof strata above the main gate increases. The characteristics of the vertical and horizontal displacement of the roof strata demonstrate that caved blocks rotate around the lower hinged point of the roof structure, which may lead to sliding instability. Large dip angle of the coal seam makes sliding instability of the roof structure easier. A three-hinged arch can be easily formed above both the tail and main gates in steeply inclined coal seams. With the growth in the dip angle, subsidence of the arch foot formed above the main gate decreases significantly, which reduces the probability of the roof structure becoming unstable as a result of large deformation, while the potential of the roof structure’s sliding instability above the tail gate increases dramatically. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Steeply inclined coal seams widely exist in many coalproducing areas, such as Xinjiang, Ningxia, Shanxi, Guizhou, Chongqing, Huainan, Gansu and Beijing [1–3]. As the major mining area has moved to western China, where half of the mines exploit steeply inclined coal seams, research on mining steeply inclined seam has become a high priority [4,5]. There are remarkable differences in mining method, structure of overlying strata, rules of mine pressure and stability control of supports between mining steeply inclined seams and horizontal seams [6–12]. The emphasis on surrounding rock control is determined by the structure of the overlying strata along the inclination, which is one of the main bases of stope support design. Domestic scholars have made great research on the problem of surrounding rock control in steeply inclined panels. Wang [13,14] studied the fracture mode and evaluation of the main roof in a steeply inclined thick seam based on elastic mechanics. Xie [8] studied the interaction characteristics between strata movement and the support system around a large mining height fully-mechanized face in a steeply inclined seam. Yin [15] obtained the ground pressure of the surrounding rock on a large dip angle face by photo-elastic ⇑ Corresponding author. E-mail address: [email protected] (X. Li).
experiment. Tu [16] studied deformation and fracture features in asymmetrical filling along an incline based on the theory of thin plates. Through comprehensive methods such as similar simulation, Wu [17–19] proposed the inclined masonry structure of dip direction and anti-dip direction pile types, and pointed out that the unbalanced movement of these structures was the primary factor in the instability of the ‘‘R-S-F” system. Nevertheless, the principal feature by which the steeply inclined face is different from an approximately horizontal face lies in the discrepancy in structure of the surrounding rock along an incline caused by the dip angle. There is a lack of in-depth study on the structural forms, instability forms and causational rules of mine pressure. Taking panel 1201 of Dayuan coal mine as engineering background, the rules of overlying strata movement along the inclination and roof structure in mining steeply inclined seam are researched through similar simulation experiment. The instability conditions of the roof structure in the dip direction were studied by theoretical analysis and was verified through in-situ rules of mine pressure. The research achievements are expected to have significance for designing roof control in mining steeply inclined seams. 2. Engineering situation The depth of #2 coal seam of panel 1201 in Dayuan coal mine is from 195.6 m to 242.6 m and the panel width is 60 m. The average
http://dx.doi.org/10.1016/j.ijmst.2017.01.018 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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1390 11-1 11-2 11-3 11-4 11-5 11-6 11-7 11-8 11-9 11-10 11-11 11-12
# 13 1° 4
90 m 80 m
9-3
9-4
9-5
9-6
9-7
9-8
9-9
8-3
8-4
8-5
8-6
8-7
8-8
8-9 8-10
7-1
7-2
7-3
7-4
7-5
7-6
7-7
7-8
7-9
6-1
6-2
6-3
6-4
6-5
6-6
6-7
6-8
5-1
5-2
5-3
5-4
5-5
5-6
5-7
4-1
4-2
4-3
4-4
4-5
4-6
3-1
3-2
3-3
3-4
3-5
2-1
2-2
2-3
2-4
1-1
1-2
1-3
60 m
1360
°
1340
9-2 8-2
70 m
50 m
46
7# ° 28 Tranport entry
9-1 8-1
9-10 9-11
Siltstone
40 m 1350 Panel Caving Floor Coal seam zone
Base line
1340
Medium sandstone Fine sandstone Coal #2
20 0
1350
16 #
1360
10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12
1380
1370
24 40 # ° 31 52 # °
1370
Ventilation entry
1200
3 47 4# °
1380
Fig. 1. Schematic diagram of the roadway arrangement in panel 1201.
1800
thickness of the coal seam is 6.8 m and the average dip angle is 45°. A fully-mechanized longwall caving method is applied in the panel. The coal-cutting height is 2.3 m, and the caving height is 4.5 m. The immediate roof is an 8 m thick siltstone with a Protodyakonov coefficient f from 4 to 6.14. The basic roof is a medium sandstone from 3 to 5 m thick and a Protodyakonov coefficient f of 6.31. The direct floor includes carbon mudstone and siltstone with a thickness varying between 3.8 and 5.4 m. The basic floor is a 12 m thick medium sandstone. The ventilation entry was laid out in the floor and the transport entry is also in the floor [20]. The joint section is a circular arc, as shown in Fig. 1. 37 supports from the top down were installed along the tilt direction. The dip angle in the position of No. 31 support reaches its maximum value of 52°. During the three-month period of equipment installation, the supports in the upper end have worse stability including sliding, toppling and squeezing. The face advanced not more than 10 m from open-off cut.
Unit: (mm)
Fig. 2. Arrangement of measuring line.
ac of volume-weight is 1.6:1 and the ratio of time similarity is 10. The physical and mechanical parameters of rock strata are shown in Table 1. The model is 180 cm long, 16 cm wide and 120 cm in height. The remaining weight of the overlying strata was uploaded by the compensation method. The displacement observation points were arranged from the bottom up after layout of the model. The layout density was 10 cm 10 cm. There were 11 layers in total, which are shown in Fig. 2. Coal cutting and drawing in different panel widths were simulated after the strata reached its scheduled strength. Coal cutting of 2.3 m mining height in 100 m panel widths was firstly simulated based on practical production and then coal drawing was simulated to observe the rules of overlying strata caving. 3.2. Presentation of results
3. Physical simulation of overlying strata movement 3.1. Program of similar simulation In mining steeply-inclined coal seams, the emphasis on stability control of supports is determined by the rules of overlying strata movement and their forms of instability. For the extraction of an approximately horizontal seam, the roof structure in the strike direction is largely understood as a result of research. However, in steeply inclined seams, it is of greater significance to study the failure mode of the roof and its structure in the tilt direction. The differences in structure of overlying strata and its instability mode are factors causing great discrepancy in the ‘support and surrounding rock’ system between mining steeply inclined seams and approximately horizontal seams. Therefore, a plane analog simulation was applied in the experimental research on overlying strata movement and its structural instability in the tendency direction. The experiment was designed on the basis of similarity theory, including geometric similarity, kinematic similarity and dynamic similarity. The ratio of geometric similarity, aL is 100:1, the ratio,
3.2.1. Rules of overlying strata movement To be consistent with practical production, after the materials reached their scheduled strength, an initial advance of 2.3 m without drawing was simulated. Excavation started 20 m from the bottom boundary to simulate the rules of overlying strata movement on a fully-mechanized caving steeply-inclined seam. The simulated width of each panel was 100 m. The vertical displacement in different layers after coal drawing is presented in Fig. 3. Fig. 3 demonstrates that the horizontal displacement after mining a steeply-inclined seam is mainly positive. The maximum vertical and horizontal displacement is biased towards the bottom end of the panel. The displacement of measuring lines 50 m and 60 m suddenly increase to respectively 50 m and 60 m away from the bottom end. The vertical displacement arrives at about 5.0 m and 5.5 m. The horizontal and vertical displacement of strata above the 80 m measuring line is very small. The strata above have bed separation from the lower layer. The rock strata within 80 m of the base line subsides extensively. Under the effect of forces perpendicular to the rock strata, the broken articulated rock moves
Table 1 Physical-mechanical parameters of rock strata. No.
Lithology
Thickness (m)
Compressive strength (MPa)
Simulated strength (MPa)
Natural density (g/cm3)
Simulated density (g/cm3)
8 7 6 5 4 3 2 1
Fine sandstone Medium sandstone Siltstone Medium sandstone Siltstone #2 coal Siltstone Medium sandstone
13.2 5.3 4.2 4 8 6.8 4.6 12
21.3 21.3 49.2 63.1 50.7 1.2 52 56
0.133 0.133 0.308 0.394 0.317 0.008 0.325 0.350
2.67 2.63 2.64 2.63 2.64 1.40 2.64 2.63
1.67 1.64 1.65 1.64 1.65 0.88 1.65 1.64
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X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364 4.0 0 90 m 80 m 70 m 60 m 50 m 40 m
3.0 2.5 2.0 1.5 1.0 0.5
-1 Vertical displacement (m)
Horizontal displacement (m)
3.5
-2 -3 -4 -5
0 -40
-20
0
20
40
60
-6 -40
80
Horizontal distance to bottom (m) (a) Horizontal displacement of different layers
-20 0 20 40 60 Horizontal disttance to bottom (m) (b) Vertical displacement of different layers
80
Fig. 3. Horizontal and vertical displacement of overburden strata after coal caving.
65°
(a) Cutting coal 100 m
45
40
37
Inclined masonry structure
54°
65°
4
°
65°
65
37
51°
Inclined masonry structure (b) Caving 40 m
57°
Inclined masonry structure (c) Caving 80 m
57 °
Structural instability
(d) Caving 100 m
Fig. 4. Movement process of overlying strata along inclination.
horizontally towards the upper end, which means the caved block rotates around the bottom hinged point. This is totally different from that of faces mining gently dipping seams in which caved blocks move towards the gob center. The mutation of vertical displacement in the upper end demonstrates that an inclined masonry structure is formed 60 m above the measuring line while the rock below the structure has sliding instability and glides towards the bottom, which increases the vertical displacement. After drawing, sliding down of the caved blocks leads to extensive activity in the upper roof. Broken roof in the upper end has a larger space for movement, which results in a dynamic load acting on the supports. Furthermore, roof exposure in the upper end will decrease the anti-sliding force and aggravate the sliding and squeezing of supports.
3.2.2. Caving features of rock strata The overlying strata movement in different coal drawing stages is shown in Fig. 4. Fig. 4 indicates that the overlying strata movement is small after cutting 100 m along the tilt direction. The upper end, with a caving angle of 65°, has caved sufficiently more than the bottom end with a caving angle of 51°. A three-hinged arch is formed at both the upper and bottom end. The rock strata extrude each other along the tilt direction (Fig. 4a). In the process of drawing coal, broken roof above the softer coal breaks into small rock pieces and slides down the panel. The caving angle at the bottom end increases to 57° and the upper end remains at 65°. Experimental results show that rock strata in the upper end comes to full caving in the initial mining period, while the breaking of strata in the bottom end develops with the increase in extraction space. When the panel width is 100 m, the movement range of overlying strata is 37 m in the normal direction of rock strata. When drawing coal at 80 m, the range is 40 m. By this time, the layer in which an
inclined masonry structure forms develops upwards and the lower rock strata moves towards the space from which coal is drawn. When drawing coal at 100 m, the movement range increases to 45 m and the lower inclined masonry structure becomes unstable again (Fig. 4d). Research indicates that the middle-upper part in the tilt direction of a steeply-inclined stope is the active zone of strata movement. Through observing the characteristics of strata movement in a similar simulation, sliding instability of an inclined masonry structure in the upper end being the main instability mode of a roof breaking structure is verified intuitively. A three-hinged arch structure is formed when mining a steeply inclined seam both in the upper and bottom end. The difference is that the bottom structure is supported by sliding rock and the caving range increases with larger extraction space. After drawing coal, a roof exposure zone exists below the masonry structure in the upper end (Fig. 4c and d) and the degree of caving reaches its maximum after initial coal-cutting. Coal drawing results in the roof exposure being shielded by the inclined masonry structure in the upper end. Strata under the structure slides down after caving and is very active. The instability of this structure leads to roof exposure and dynamic load in the upper end, which aggravates the sliding and toppling of supports.
4. Instability mode of an inclined masonry structure and site verification 4.1. Sliding instability After extraction of the steeply-inclined coal seam, the immediate roof collapses, slides down and fills the bottom end and the basic roof curves and breaks. The inclined arch structure (Fig. 5b), which is different from a horizontal Voussoir beam (Fig. 5a) is
X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364
T2
362
Q=qL
L
=q
h
R
2
Q
h
T
L
T
L R1=qL/2
α
R
1
T1
R2=qL/2
(a) Horizontal arch
(b) Inclined arch
Fig. 5. Structure comparison between steeply inclined and horizontal seam.
formed in both the upper and bottom end of the tendency direction. On the basis of the equilibrium condition of the arch structure, reaction forces T1, R1, T2, R2 in the upper and bottom arch springing can be obtained as follows:
1.2
1 2 1 qL cos a þ qhL sin a =h 8 2 1 2 1 qL cos a qhL sin a =h T2 ¼ 8 2 1 R1 ¼ qL cos a þ qh sin a 2 1 R2 ¼ qL cos a qh sin a 2
0.4
0
ð1Þ
-0.2 -0.4
0
10
20
30
40
50
60
Dip angle α (°)
ð2Þ
bility of the masonry structure formed by inclined rock is obviously affected by the dip angle of the coal seam. With increasing dip angle, the upper rock more readily suffers from sliding instability, which results in dynamic loading of the upper supports. 4.2. Deformation instability During the revolving deformation of the inclined masonry structure, a plastic condition is more likely to develop in the lower rock due to the larger compressive stress in the lower rock and partial stress concentration. Fig. 7 is used for calculating rotary deformation. Eq. (4) can be obtained by calculating the torque relative to point O.
a l a þ ql cos a ¼ T 1 h T3 D þ 2 2 2
ð4Þ
a/2
Q Δ
Note that, when k multiplied by tanu is larger than 1, the structure will maintain stability. Where T is the thrust in spring line direction, kN; u is the friction angle among rocks, °; i is the ratio of rock length L/2 to thickness h of the basic roof. And i can be calculated by Eq. (3).
Fig. 6. Relationship between dip angle and sliding stability coefficient k of upper arch foot.
ð3Þ
a/2
θ ql
l
h
O
α
T1
where RT and q are respectively the tensile strength and load of roof. The upper rock structure will not experience sliding instability with an increase in the coefficient k. Fig. 6 shows that the sliding stability coefficient k is approximately 1.1 when the dip angle is 0° (horizontal coal seam). The inclined masonry structure is more stable. As the dip angle decreases to about 55°, k diminishes to zero, which demonstrates that the thrust T2 of the upper rock is zero and the sliding instability phenomenon occurs in the upper rock structure. For mining steeply inclined seams, the sliding sta-
T3
1 2
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi RT i¼ 2q cos a
0.6
0.2
where q is the load intensity of arch structure, kN/m; a is the dip angle of coal seam, °; L is the maximum span of roof, m; h is the thickness of roof, m; T1 and T2 are, respectively, thrust in the tendency direction in both arch springing, kN; R1 and R2 are, respectively, the friction force in both arch springings, kN. The roof structure in a steeply-inclined panel, which shows obvious stress asymmetry of structure, is different from that in a gently dipping seam. The dip angle of a coal seam decreases the pressure perpendicular to the roof. The unipolarity of the component force of the overlying strata pressure leads to an increase in thrust T1 at the bottom end and a decrease in T2 at the upper end. Therefore, sliding instability probably occurs in the upper end. Defining the sliding stability coefficient k as T divided by R. Choose upper thrust T2 in a steeply inclined seam.
i i tan a i tan a
0.8
k
T1 ¼
k¼2
1.0
R1
Fig. 7. Deformation instability of rotary rock.
X. Li et al. / International Journal of Mining Science and Technology 27 (2017) 359–364
Mohr-Coulomb criterion, the hinged point in the lower arch structure is more likely to break than the horizontal arch and the dip angle of the coal seam decreases the normal load of the inclined masonry structure perpendicular to the rock strata. Fig. 8 shows that the rotary angle of the rock decreases with an increase in dip angle of the coal seam, which means that both the deflection in the biting point and the degree of deformation instability decrease with an increase in dip angle. In conclusion, the instability mode of an inclined masonry structure, formed after roof breaks in a steeply-inclined seam, is mainly sliding instability.
0.38 0.36
sin θ
0.34 0.32 0.30 0.28 0.26 0.24 0
10
20
30
40
50
60
4.3. Verifying the structural instability modes by working resistance
Dip angle α (°)
Fig. 8. Relationship between the rotation angle of lower arch foot and dip angle of coal seam.
where T1 equals T3 plus qlsina. The rotary angle of the rock can be obtained using a method similar to analyzing the deformation instability of horizontal rock.
sin h ¼
1 tan a þ i 4K0i2
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 9 tan2 a þ 48K 0 i tan a þ 48K 0 i
ð5Þ
12K 0 i
2
where K0 equals n times K times K; n is the ratio of compressive strength to tensile strength; K, determined by the conditions to be clamped or simply supported, is approximately between 1/2 and 1/3; K is the ratio of compressive stress to the compressive strength of rock. When K equals 1/2, n equals 10 and K equals 2, the effects of dip angle on the rotary angle of the rock are obtained, as shown in Fig. 8. Eq. (1) indicates that the shear stress R1 in the inclined masonry structure decreases with an increase in dip angle, and the normal stress, which is the inclined thrust T1, increases. Based on the
1.10 1.05 Volatility index (MPa)
363
1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65
Lower #1
Lower #9
Middle #19
Upper #29
Upper #37
There are 37 supports in panel 1201, which include 34 basic caving supports of ZFY4800/17/28 and 3 transitional caving supports of ZFG4800/18/32 with one support in the upper end and two supports in the bottom. The working resistance of supports in the upper, middle and bottom panel was monitored as was the support working resistance of #29 and #37 in the upper end, #19 in the middle and #1 and #9 in the upper end. A(m) is defined as the volatility index of the working resistance of support No.m. Its value is an indicator for evaluating the variability of working resistance at different positions in the panel. The larger the value of A(m) indicates more obvious dynamic load effects.
AðmÞ ¼
n 1X Þ2 ðai a n i¼1
ð6Þ
is the average where ai is the working resistance of day i, MPa; a value of working resistance within n days, MPa. Due to small differences in the working resistance in the tilt direction, the volatility index A(m) is used as the indicator to describe the variability of working resistance in the tendency direction. Fig. 9 shows that the volatility index of the upper segment is greater than the bottom segment along the face line direction. That is to say, the sliding instability of the inclined masonry structure results in an obvious variability, which is the reason for high dynamic load in the upper end whilst load intensity caused by deformation instability is smaller in bottom end. Therefore, the sliding instability of the inclined masonry structure in the upper end and its sliding down causes roof exposure and intensive loading. The sliding, toppling and squeezing of the upper supports threatens the stability of supports in the middle and bottom end, which greatly affect the safe and effective production in steeply inclined panels. 5. Engineering application and results
Working resistance
Fig. 9. Volatility index of working resistance along inclination.
Due to the large dip angle of panel 1201 in Dayuan coal mine, the normal and tangential force of gravity of roof and supports
Backing board Upper roadway
(a) Backing board in roof exposure zone
(b) Control results of supports
Fig. 10. Result of control of supports after application of the backing board in the upper end.
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varies with an increase in dip angle, which in turn results in a larger tangential force and smaller normal force. Hence, the loading of the support system in the panel decreases and the dynamic load causing instability of the support system in the upper end increases. This will intensify the toppling and squeezing of supports, which finally results in insufficient support to roof. To achieve stability of the supports in the whole panel caused by instability of support and surrounding rock system in the upper end when roof exposure occurs, the exposed roof is filled with plate-girder and wicker fence to prevent further roof exposure and ensure sufficient contact with the roof, as shown in Fig. 10a. At the same time, when the roof is broken, the supports are advanced and the telescopic girder extended after cutting coal and the face guard opened to support the roof and coal rib. The supports are moved without pressure relief and the setting load of supports set at more than 20 MPa before moving to guarantee the integrity of the roof. The monthly production of coal increased to 80,000 t through the technical measurements described above. The stability of supports in the upper end is controlled and sliding, toppling and squeezing of supports is prevented. As a result, good working conditions of the support system is achieved. 6. Conclusions (1) Strata displacement in a steeply inclined panel is significantly different from that of a gently horizontal panel. The characteristics of horizontal and vertical displacement demonstrate that the broken block rotates around the bottom hinged point and forms an inclined masonry structure in the vicinity of both tail and main gates. Due to the larger moving space after coal seam extraction, strata movement at the upper end is more strenuous than that at the bottom end. The sliding stability of the upper arch springing of the inclined masonry structure causes obvious dynamic load on the upper supports. (2) In mining a steeply inclined seam, the sliding stability of the masonry structure formed by inclined rock is affected extensively by the dip angle of the coal seam. The probability of sliding stability of the upper hinged point becomes small and the deflection of the bottom block diminishes with the increase of dip angle. The introduction of a volatility index A(m) demonstrated that the dynamic load effect becomes more obvious than that of the middle and bottom of the panel which verifies the results of theoretical analysis. The instability of the upper inclined masonry structure can easily cause roof exposure in mining a steeply inclined seam and, thus causes a dramatic decrease of support stability along the whole panel. (3) By filling the upper exposed zone with backing board and timber crib, combined with moving supports ahead without pressure relief and increasing the setting load of adjacent supports, the bad effects of upper roof exposure on the
stability of supports in middle and bottom panel are minimized. Monthly coal production is improved and the stability and good condition of supports are ensured.
Acknowledgments The financial support from the Joint Funds of the National Natural Science Foundation of China (No. U1361209) and the National Basic Research Program of China (No. 2013CB227903) is greatly appreciated. References [1] Wu YP, Liu KZ, Yun DF, Xie PS, Wang HW. Research progress on the safe and efficient mining technology of steeply dipping seam. J China Coal Soc 2014;39 (8):1611–8. [2] Deng YH, Wang SQ. Feasibility analysis of gob-side entry retaining on a working face in a steep coal seam. Int J Min Sci Technol 2014;24(4):499–503. [3] Shi PW. The complexity of basic roof’s breaking movement in highly inclined seam. Mine Press Roof Manage 1999;Z1:26–8. [4] Zhang Y, Zhang B, Li L. Study on the effect of roof fracture development on gas drainage in steep full-mechanized caving mining. J Min Saf Eng 2014;31 (5):809–13. [5] Xie PS, Wu YP, Wang HW. Study on space activity law of overburden strata above longwall coal mining face in high inclined seam. Coal Sci Technol 2012;40(9):1–5. [6] Xin YJ, Gou PF, Ge FD. Analysis of stability of support and surrounding rock in mining top coal of inclined coal seam. Int J Min Sci Technol 2014;24(1):63–8. [7] Ma R, Li GC, Zhang N, Liu C, Wei YH, Zhang M. Analysis on mechanism and key factors of surrounding rock instability in deeply inclined roadway affected by argillation and water seepage. Int J Min Sci Technol 2015;25(3):465–71. [8] Xie PS, Wu YP, Wang HW, Ren SG. Interaction characteristics between strata movement and support system around large mining height fully-mechanized face in steeply inclined seam. J Min Saf Eng 2015;32(1):14–9. [9] Wu YP. Key technologies and strategies of mechanized mining in highly inclined seam. Mine Press Roof Manage 2002;1:60–3. [10] He MC, Peng YY, Zhao SY, Shi HY, Wang N, Gong WL. Fracture mechanism of inversed trapezoidal shaped tunnel excavated in 45inclined rock strata. Int J Min Sci Technol 2015;25(4):531–5. [11] Li HP, Pu WL, Guo SQ. Probe into the technical developing direction of optimum mining in great inclined seam. Coal Min Technol 2005;10(1):20–2. [12] Shao XP, Shi PW, He GC. Analysis of load transmitting arch of roof in steeply inclined seam. J Univ Sci Technol Beijing 2007;5:447–51. [13] Wang JA, Zhang JW, Gao XM, Wen JD, Gu YD. Fracture mode and evolution of main roof stratum above longwall fully mechanized top coal caving in steeply inclined thick coal seam (I). J China Coal Soc 2015;40(6):1353–60. [14] Wang JA, Zhang JW, Gao XM, Wen JD, Gu YD. Fracture mode and evolution of main roof stratum above longwall fully mechanized top coal caving in steeply inclined thick coal seam (II): periodic fracture. J China Coal Soc 2015;40 (8):1737–45. [15] Yin GZ, Li XS, Guo WB. Photo-elastic experimental and field measurement study of ground pressure of surrounding rock of large dip angle working coalface. Chin J Rock Mech Eng 2010;29(1):3336–43. [16] Tu HS, Tu SH, Chen F. Study on the deformation and fracture feature of steep inclined coal seam roof based on the theory of thin plates. J Min Saf Eng 2014;31(1):49–54. [17] Wu YP, Xie PS, Ren SG. Analysis of asymmetric structure around coal face of steeply dipping seam mining. J China Coal Soc 2010;35(2):182–4. [18] Wu YP. Dynamic equation of system ‘‘roof(R)-support(S)-floor(F)” in steeply dipping seam mining. J China Coal Soc 2005;30(6):736–41. [19] Wu YP, Xie PS, Wang HW. Incline masonry structure around the coal face of the steeply dipping mining seam. J China Coal Soc 2010;35(8):1252–6. [20] Wang ZQ, Zhao JL, Li ZQ. Determination of height of ‘‘three zone” in the stope with stagger position and internal misaligned roadway layout. J Min Saf Eng 2013;30(2):4–10.