JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

ISSN 1006-9097

pp 382–385

Vol.14 No.3

Sep. 2008

Hydraulic support stability control of fully mechanized top coal caving face with steep coal seams based on instable critical angle∗ TU Shi-hao(屠世浩), YUAN Yong(袁 永), LI Nai-liang(李乃梁), DOU Feng-jin(窦凤金), WANG Fang-tian(王方田) ( State Key Laboratory of Coal Resource and Mine Safety, School of Mining Engineering, China University of Mining & Technology , Xuzhou

221008, China )

Abstract Analyzed the support instable mode of sliding, tripping, and so on, and believed the key point of the support stability control of fully mechanized coal caving face with steep coal seams was to maintain that the seam true angle was less than the hydraulic support instability critical angle. Through the layout of oblique face, the improvement of support setting load, the control of mining height and nonskid platform, the group support system of end face, the advance optimization of conveyor and support, and the other control technical measures, the true angle of the seam is reduced and the instable critical angle of the support is increased, the hydraulic support stability of fully mechanized coal caving face with steep coal seams is effectively controlled. Keywords steep coal seams, instable critical angle, support stability, fully mechanized top coal caving, control technology

Introduction The mechanized mining of coal seams with larger angle of dip has always been one of the problems and hot spots in mining technical research. In China, there are a large number of coal seams whose angles of dip are larger than 30 degrees. Thus how to exploit these coals safely and efficiently has become the direction for mining workers to make efforts. The hydraulic support stability control is the key point in the mechanized mining of steep coal seams[1]. In the condition of fully mechanized top coal caving, because of the existence of caving processes, there are much uncertainty and complexity in the support stability. The related domestic scholars have done some researches on support sliding, tripping, instability of the tail, etc., under the condition of fully mechanized top coal caving, which are just targeted mainly at light support[2,3] or with support of the auxiliary anti-skidding devices[4]. So the research on heavy ∗ Supported by the National Natrual Science Foundation of China(50504014) Tel: 86-516-83590502, E-mail: [email protected]

support stability without auxiliary anti-skidding devices is not deep, especially, the research of the inherent mechanism and controlling essence of support instability under the above conditions is not deep. Based on the existing research results, this paper makes a deep study on the mechanism of the support instability, the controlling theory and methods according to the seam condition in Dongtan Mine 1301 working face. And some useful conclusions can provide reference for similar coal mining.

1

Project overview

Dongtan Mine 1301 working face has complex geological conditions, fault development, and large head. Its main excavating coal seam of which is 6.8~ 9.1 m in thickness with angle of dip of 0~42° and complex coal structures, the second category coal of which has spontaneous combustion and tendentious impact. With the method of fully mechanized top coal caving, the cutting coal thickness of which is 2.8 m

TU Shihao, et al. Hydraulic support stability control of fully mech-

and the rest is the caving height, and with the ZFS6200/18/35 type top coal support whose main technical parameters are shown in Table 1, the instable Table 1 Project Centre distance (mm)

phenomena like support sliding, dumping, biting and so on will seriously affect the working face production in the initial face exploitation.

Main technical parameters of support

Parameter value 1 500

Project Support height (m)

Parameter value 1.8~3.5

Width (mm)

1 410~1 580

Seam dip ( º)

≤15

Initial supporting load (kN)

5 036~5 527

Pumping station pressure (MPa)

31.5

Working resistance (kN)

6 000~6 250

Supportweight (t)

Supporting strength (MPa)

2

383

0.80~0.86

Analysis of support instability

A number of project practices show that there are mainly forms of support instability on fully mechanized top coal caving face with steep coal seams: sliding, tripping. 2.1 Support sliding As shown in Fig.1, under the function of self-weight W, the weight P of coal rock mass on the cap lid of supports, and total force R from the floor, a single support has the tendency to slide along the bottom slope. In order to keep support stability, the anti-sliding force must be equal to or greater than the sliding force. That is: P cos α f1 + (W + P) cos α f 2 ≥ (W + P)sin α , (1) where, α is the seam angle on the working face; f1 is the coefficient of friction between support headpiece and roof; f2 is the coefficient of friction between support base and floor.

Fig.1 Analysis model of the support stability

From Eq.(1), the instable critical angle of sliding support can be got as: Wf1 ⎤ ⎡ (2) α hl ≤ arctan ⎢( f1 + f 2 ) − . P + W ⎥⎦ ⎣ The minimum initial support force needed by the support to maintain stability in a certain inclination angle is:

25

Bottom plate pressure (MPa)

Qmin ≥

W (sin α − f 2 cos α ) . ( f1 + f 2 ) − tan α

1.4~1.9

(3)

Under normal circumstances, f1 is affected by the impact of the integrity of the roof, its bound is 0.35 to 0.40; f2 is affected by the impact of the integrity and water content of the floor, its bound is 0.22 to 0.82; From the analysis of Eqs.(2) and (3), it can be inferred as follows. (1) α hl has a proportional relationship with P and f2, the larger P and f2 are, the larger the support instable critical angel is. (2) α hl has a inverse proportional relationship with W, the larger W is, the smaller the support instable critical angel. (3) The meaningful conditions of Eq.(3) are: f1 + f 2 > tan α > f 2 , namely: accommodate the support force can control the support stability; if tan α < f 2 , the support can maintain its stability in the state of self weight; if f1 + f 2 < tan α , only accommodate the support force can not control the support stability, support sliding is the certain event. (4) With the analysis of Dongtan Mine 1301 working face, W=250 kN, f1=0.35, f2=0.3, if α < 16° , the support can maintain its stability in the state of self weight; if α > 33°, only accommodate the support force can not control the support stability, support sliding is the certain event; if 16° < α < 33° , accommodate the support force can control the support stability by different seam angle. 2.2 Support dumping Because of the existence of inclination component force of support along coal seams, the resultant force point may depart from the edge of the support, resulting in the collapse of support. Under the condition of torque limit equilibrium, the anti-force point of

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Journal of Coal Science & Engineering (China)

bottom plate is at O. As shown in Fig.1, the critical condition for maintaining support stability is: ( Ph + WC )sin α ≤ ( PB 2 + WB 2 + Pf1h ) cos α , (4)

where, h is the mining height; C is the height of support center of gravity; B is the width of support base. from the analysis of Eq.(4), the dumping critical angle of single support is: ⎡ ( P + W ) B − 2 f1WC ⎤ α dl ≤ arctan ⎢ f1 + (5) ⎥. 2( Ph + WC ) ⎣ ⎦ From the Eqs.(4) and (5), we can get as follows. (1) The larger B and f1 are, the smaller C is, more stable the support is, and vice versa. (2) The relation between 1/h and α is arc tangent function, the large seam angle is, less stable the support is, and vice versa. (3) When the top coal is fragmentized or falling and process of support moving, P is less, even P=0, here, α dl ≤ arctan B / 2C , the stability of support lies on B/2C.

tailentry, the mining was successful even if the support had slid. 3.2 The pseudo-ramp layout can really reduce the true seam dip The pseudo-ramp layout is the effective measure for support stability control of fully mechanized top coal caving face with steep coal seams. As shown in Fig.2, D is the dropping load of the scraper conveyor per web; M is the mining web; S is the advance distance of pseudo-ramp layout; L is the working face length.

3 Principles and measures of support stability control From the analysis of above, based on the conditions of Dongtan Mine 1301 working face, the principles and measures of support stability control of fully mechanized top coal caving face with steep and thick coal seams are as follows. 3.1 Set some space to allow limited support sliding The order of support setting is from the bottom up, the first group support of headentry was set in the interconnection, and one more support was set in the Table 2

Dropping load of the scraper conveyor (mm)

3.3

The pseudo-ramp layout

cos θ = D / M , tan θ = L / S , 2

⎛M ⎞ (6) ⎜ ⎟ − 1. ⎝D⎠ Under the condition of different inclination angles measured in Dongtan Mine, and based on the dropping load of the scraper conveyor, from the Eq.(6), the relationship between inclination angle of working face and the advance distance of pseudo-ramp layout can be perceived in Table 2. S=L

Relation between coal angle and pseudo oblique distance

Inclination angle of coal seams ( º)

Advance distance (m)

Fig.2

16~20

21~25

26~30

31~35

36~40

41~42

26~48

52~78

84~108

112~134

136~160

164~172

8.0~14.8

16.0~24.1

26.0~33.5

34.8~41.8

42.4~50.2

51.5~54.2

Increase the support instability critical angle From the analysis of the Eqs.(2) and (5), we can see that the support instability critical angle can be increased from the aspects as follows. (1) Increase the support initial sustaining force and bolt support resistance. It must be ensured that the pumping station has enough pressure; larger initial supporting force is placed immediately after fixing the support, and the method of “moving with pressure” is adopted while moving the support to realize “less dropping and fast pulling”. (2) Ensure the friction coefficient f between the

support and roof & floor plate to be large. If the floating coal on the floor seam is not completely clear, the roof is broken, water is stored on the floor plate, f will be lowered. Therefore, the quality of coal cutting machine should be improved to ensure that the top and bottom plate be flat. The floating coal in front of the support must be cleared before moving the support to ensure that the support base is closely contacted with the bottom plate. When the roof plate is broken, the advance support should be prepared; and drainage should be paid attention to during the upward-ramp promoting process.

TU Shihao, et al. Hydraulic support stability control of fully mech-

(3) The mining height must be strictly controlled and the promoting speed should be increased appropriately. The higher the mining height is, the easier the effective maintain of support dumping stability is. The appropriate improvement of the advancing speed and the immediate support can efficiently control the increase of the distance between cap lid and empty peak, and can reduce the pressure of the support[5]. (4) Defensive and non-slippery platform, terminal support group layout. When cutting the coal within 5~7 m of the bottom terminal, the bottom coal should be kept appropriately so that the inclination angle of coal seams on working face near the bottom terminal to make the non-slippery platform. The platform can not only prevent the conveyor from sliding but also ensure the reasonable connecting height. Line head and line tail support group can be composed as a whole by 3~5 supports with additional devices. (5) Make good use of side-boards and reduce the distance between adjacent supports. Using side boards of adjacent supports to limit the position and timely adjust the distance and direction of supports can prevent inclination of leakage pole between supports. Supports have full access to the roof so that they can bear equal forces, avoiding the phenomena of empty roof, exposed roof, and emitted roof in case that they are instable because of rear torsion. (6) Unidirectional working, the order should strictly be from the bottom up for pushing the scraper conveyor and moving the supports in group and interval.

4

Analysis of controlling effects

The principles and measures of stability control of the above supports have been tested on 1301 working face of Dongtan Mine. The results show as follows. (1) The distribution of the support initial sustain force and working resistance is in the state of normal distribution, which means that the distribution of the support initial sustain force and working resistance is a uniform distribution. The average effective resistance of front leg is 176 kN/m, while the rear support leg is 138 kN/m, so the former is larger than the latter by 22%, which has no significant change compared with the common situation that the former pole is larger than the latter by 16%[6]. The maximum resistance of working face support is 5 592 kN, and the average resistance is 5 355 kN, which are respectively 93.2% and 89.3% of the rated working resistance. The support work-

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ing resistance has been made use of effectively so that the support is in good sustaining state. (2) The support sliding, dumping and scraper conveyor sliding were effectually controlled, and the working time use capability was evidently improved. (3) During the period of back quarrying of this working face, there are 2.4 million tons coal quarried safely with the average production of 498 400 tons coal, the average monthly footage of 208 m, the largest monthly production of 55.618 2 million tons coal, the maximum daily production of 25 869 tons, and the back quarrying efficiency of 114.7 tons per worker. Thus there are notable technical and economic results. References [1]

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