Effect of Face Length in Initial Mining Stage of Fully Mechanized Top-Coal Caving Face with Large Mining Height

Based on the field measurement of the end resistance of the support during the initial weighting of the basic roof and the macroscopic mine pressure behavior during the weighting period of 101, 22,211, 103 and 301 fully mechanized caving face in Changchun Xing Coal Mine, the mine pressure law of the working face is summarized and compared. Furthermore, the relationship between the working face length, the working resistance of the support (the weighting strength) and the macroscopic mine pressure behavior is obtained. In the range of face length 126–230 m, with the increase of face length, the end-of-cycle resistance of the support gradually increases, the dynamic load coefficient of the support gradually increases, and the strata behavior of the working face changes from strong to very strong. When the face length is short (126–140.5 m), the hanging roof area is too large to cause hurricanes when the working face is pressed, which threatens and damages the personal safety and equipment of the working face staff. Based on the above research, the problem of optimizing the surface length is proposed, and the surface length is determined to be within the range of 140–230 m.


Introduction
Underground mining of thick coal layers with a thickness of more than 4.5 m has operational, technical, safety and economic problems. When the mining height exceeds 5.0 m, it is difficult to optimize the productivity and control surface stability due to the limitation of equipment design and surface conditions (Hebblewhite et al. 2002(Hebblewhite et al. , 2013. Top caving coal mining provides a method that can extract 80 % in the thickness range of 5-9 m, and the development cost per ton will be significantly reduced (Mohammad et al. 1997).
According to the classcial mining pressure theory (Hoek et al. 1995;Brady and Brown 2004): the direct top thickness in fully mechanized caving mining face is related to how much top coal is released. The roof structure of fully mechanized caving mining face is mainly divided into two types. As shown in Fig. 1, one is the general structure model, the direct top of this model is thicker, and the structure with a transfer force above. The other is a beam structure, the direct top of which is thinner, the upper hard rock layer is thicker, and the direct top has no small structure, as shown in Fig. 2.
The top load of the support-rock relationship under the general structural model includes the action of top coal, direct top and main roof (Xie and Zhao 2009;Vakili and Hebblewhite 2010). The support under beam structure should bear the direct force of top coal and direct top, the impact force of direct top fracture (Alehossein and Poulsen 2010).
Because of the advantage of caving mining, caving mining has been widely used in the world. Khanal introduce the various processes of designing Longwall Top Coal Caving (LTCC) technology in India Singh Ni Coal Co., Ltd (SCCL) mining area. A comprehensive analysis of geological and geophysical data in the mining area is carried out by the project, and a detailed geotechnical framework is developed for the assessment of LTCC technology (Khanal et al. 2014).
Humphries used several proprietary and commercial modeling packages, as well as on-site observations and expert advice to investigate LTCC assessment of coal mining in Australia. The project investigated in detail the various parameters known to affect LTCC performance in China and adjusted these observations to develop a method that is easy to apply to potential mining resources or reserves LTCC in Australia (Humphries et al. 2006). Basarir et al. (2015) analyzed the magnitude and direction of the principal stresses around the stoping roadways caused by the fully mechanized caving by employing numerical simulation with finite difference. Yasitli and Unver (2005) adopted a strain softening model in association with the Mohr-Coulomb yield criterion to simulate the strata behavior of longwall mining with top-coal caving, the caved roof rocks are simulated by time dependent elastic.
China is a big coal producing country in the world. After 30 years of continuous development and progress, Fully Mechanized Top-coal Caving (FMTC) has been widely popularized and used in thick and extra thick coal seam mining in China, especially in large mining high comprehensive caving mining technology (Wang et al. 2014a(Wang et al. , b, 2015. It has made important contribution to high yield, high efficiency and intensive production in thick coal seam mining area in China (Wang 2014a, b). In recent years, with the continuous popularization and use of FMTC technology, the abnormal occurrence of mine pressure in FMTC mining face is also puzzling its safety (Wang and Zhang 2015), such as severe pressure at the top of the basic roof, obvious roof subsidence and serious coal wall falling.
In FMTC mining face, the effects of coal caving are closely related to the degree of damage to the top-coal which is determined by the stress state therein. Therefore, research into the evolution of mininginduced stress field is the key to understanding failure mechanisms of top-coal. By taking the FMTC face in Wangjialing Coal Mine (China) as the engineering background and using field measurement and numerical simulation, Huo et al. (2020) studied the evolution of the principal stress field in FMTC under high horizontal stress. According to the fragment dimension theory of coal and refuse (Huang and Liu 2006), inversion analysis was used to research the caving process of top coal and roof rock. (Zhang et al. 2016) performed inversion of top coal and roof rock caving to analyze the flow pattern and top coal loss. This study aims to provide bases for the further improvement of top coal recovery and the formulation of corresponding technical measures.
According to the analysis of relevant literature, scholars have analyzed the mining pressure law of fully mechanized caving face from the aspects of field measurement, numerical simulation and theminingtical basis. The analysis stage is mostly periodic pressure stage, and the relationship between surface length and mining pressure law during the initial mining period of fully mechanized caving face is less studied.
In this paper, the mining pressure of 22 coal face 101, 22,211, 103 and 301 in Changchun Xing Coal Mine are measured. The cyclic end resistance of the support at different parts of the four fully mechanized caving face is counted, the mine pressure manifestation law of the working face is analyzed, mainly for the main roof initial pressure and the first two periodic pressure macroscopic mining pressure. Finally, the variation law of working resistance, dynamic load coefficient and macroscopic mining pressure of the main roof support under different surface length of fully mechanized caving are obtained.

Mine Overview
The coverage of the mine field in Changchun Xing Coal Mine is about 20 km 2 , the annual production capacity of the integrated mine is about 2.4 million tons, and the design service life of the mine is 67 a. The specific location of the mine is near Zhou Dazhuang to Yinggezhai Village, Zuoyun County, Datong, Shanxi Province. Changchun Xing Coal Mine belongs to Datong coalfield, and its surface landform is low hills. The No.22 coal seam is stable and basically recoverable in the whole area, which is the main coal seam, and its reserves account for 77.3 % of the whole mine. Changchun Xing coal mine adopts belt conveyor coal transport. The No. 22 Coal 4 mining face relative position diagram see Fig. 3.

Working Coditions
The 101 working face is the first mining face of the mine, the length of the working face is 140.5 m in front of the jump face, the buried depth is 239-291 m, and the thickness of the coal seam is 9.61-11.94/ 10.75 m. The influence of working face mining on surface building and other aspects: after working face mining, the ground above goaf will sink and form cracks. When the working face advances to 650 m away from the cut, due to the influence of large drop fault, the working face needs to reopen the cut from 1050 m away from the cut, and continue mining. The face length before jumping is 140.5 m, and the length after jumping is 200 m.
The 22,211 working face is the second working face of the mine, the length of the working face is 126-230 m (126 m before mining the face is connected, 230 m after the face is connected). The thickness of the coal seam is 10.30-12.00/11.2 m. According to the faults exposed in the working face, the working face is designed to connect at 544 m from the cut, and withdraw at 1256 m from the cut. The length of the working face before and after the connection is 126 m/230m.
The 103 working face is the third mining face of the mine, the face length is 230 m, the coal thickness is 9.61-14.52/12.24 m, the coal seam inclination angle is 0°-7°, average 3°.

Equipment Configurations
The type and matching of the equipment used in the 4 working faces of the mine are the same, see below for details: The middle hydraulic support adopts the ZF13000/ 25/38 type support cover type low position caving coal support, the support height is 2500-3800 mm, the center distance is 1750 mm, the support initial support force is 10128kN (P = 31.5 MPa), the rated working resistance 13000kN (40.43 MPa). Transition hydraulic support model is ZF13000/ 27.5/42, height (minimum/maximum) 2500/3800 mm, center distance 1750 mm, support strength 1.16 MPa.
The coal mining machine type is MG500/1180-WD, the front scraper conveyor adopts SGZ1000/1400 scraper conveyor, and the rear scraper conveyor adopts SGZ1200/1400 scraper conveyor. Transfer machine selects model SZZ1200/700 double-strand scraper transfer machine. Crushing model is PCM400 type, belong to hammer crusher.

Working Resistance Monitoring method
Face pressure monitoring adopts the real-time on-line monitoring system of KJ564 mine pressure produced by Qingdao Benmo Rock Control Technology Co., Ltd. The equipment layout of the monitoring system area is shown in Fig. 4. In the working face head, middle and tail support range, according to the principle that one monitoring sub-station is responsible for 5-10 supports, each sub-station contains three monitoring channels, the first and second channels are respectively connected with the front and rear columns of the support, and the third channel is connected to the adjacent supports. Each sub-station support is defined as a monitoring line, numbered in turn.
By monitoring and analyzing the mining pressure data of each area of the working face, the roof motion law and the mining pressure manifestation law of each area are obtained.

Macroscopic Pressure Monitoring Method
The macroscopic mining pressure monitoring in the working face mainly includes macroscopic and quantitative mining pressure phenomena. It mainly includes the working resistance of each column of the support, the shrinkage of the living column and the  Each 7-9 supports are divided into a group along the face length direction, each group selects a support measuring point observation.

Mine Pressure Results Acquisition Method
(1) Working resistance monitoring: the mine pressure monitoring system automatically records the working resistance of the support in real time and transmits the data to the mine database through the system. Using computer software to analyze the aggregate data, the roof pressure and dynamic load coefficient are analyzed.
(2) Macroscopical pressure monitoring: manual observation underground coal mine every 1-3 days,, the roof subsidence, roof fall height, support living column shrinkage, coal wall side and bottom drum quantity at each monitoring site of working face are measured and recorded.
During the roof weighting of the working face, follow-up observation of the above content, and record the support number and opening pressure of the working face support safety valve. The remaining representative live column shrinkage and support safety valve opening state are collected and recorded by the coal mining team.

Mining Pressure Law During Initial Mining of 101 Working Face
The mining process of 101 working face is affected by the fault, it is necessary to open the hole again after jumping mining, so there are two initial mining in this face. A total of 15 measuring lines were arranged in 101 working face, and the monitoring sub-station data at No. 10,No. 20,No. 30,No. 40,No. 50,No. 60 and No. 70 supports are selected to analyze the mining pressure manifestation law.  The pressure value of the main roof pressure at the 7 key parts before and after jumping mining are shown in Tables 1 and 2, and the comparison is shown in Fig. 6.
It can be seen from Tables 1 and 2, and Fig. 6: (1) The mean value of dynamic load coefficient is 1.45 before jumping and 1.43 after jumping. The first weighting of the main roof of the working face is obvious and strong.
(2) When weighting, the intensity of face pressure varies from region to region, the working face pressure of No.40-No.70 support before jumping is strong, that is, the middle and tail pressure of the working face is strong. However, after jumping, the pressure of the working face of No.20-No.40 support is strong, that is, the pressure of the head and middle is strong. (3) The working resistance of the support before and after the face jumping is equal before weighting, and the difference of the working resistance of the support before and after face jumping is large when weighting. There was no significant change in the adjacent roadways of the working face, no floor heave, and no surface change.
After jumping mining The initial weighting of the No. 1-42 support in the working face is relatively strong, resulting in the shear failure of the four-link pin of the No. 17-23 support in the working face. A rear column cylinder of the No. 20 support bursts, a rear column cylinder piston bends, and the front column seal of the tail side of the No. 37 support machine is damaged. The stroke of the No.24-41 support column decreases by 500-600 mm. Hurricane occurs in the head and middle of the working face. There is no big change in the adjacent roadways of the working face, no floor heave, and no change in the surface.

Macroscopic Mining Pressure Behavior of the
First Two Periodic Weighting Before jumping mining In the first cycle of pressure, No. 1-40 basic roof pressure is relatively strong, resulting in No. 17 and No. 18 support tail beams were broken, No. 20 support column pin was broken, the working face no hurricane.
In the second cycle, the basic roof pressure of No. 1-50 support is relatively strong, the maximum decrease of support column is 500 mm, resulting in the destruction of the pin of No. 18 support tail beam, the working face no hurricane. After jumping mining No. 55-82 support roof pressure is strong during the first cycle, the column stroke decreased 350-700/560mm, No. 55-58 support tail beam tubing broken, no hurricane in the working face.

Mining Pressure Law During Initial Mining of 301 Working Face
The length of 301 working face is 230 m, a total of 13 measuring lines are arranged. The mine pressure monitoring sub-stations of No.20,No.30,No.50,No.55,No.65,No.70,No.90,No.110 and No.120 supports are selected to analyze the law of the basic roof weighting. The change curves of the end-of-cycle resistance and the initial support force of the supports at the five measuring lines of No.50,No.60,No.70,No.90 and No.110 when advancing 110 m are shown in Fig. 7.
The end-of-cycle resistance and dynamic load coefficient at 9 key parts of the working face are shown in Table 3 and compared in Fig. 8. Table 3 shows the average end-of-cycle resistance of the working face is 12,935 kN. Figure 8 shows that the pressure distribution is uneven. The end-of-cycle resistance in the middle of the working face (No. 30-70 support) is the largest, reaching 14,265 kN, and the average pressure is 13,567 kN. Table 3 shows the average dynamic load coefficient of the working face is 1.79 which shows that the mining pressure behavior is strong, especially in the middle of the working face.

Comparison of the End-of-Cycle Resistance and Dynamic Load Coefficient
The end-of-cycle resistance and dynamic load coefficient of the support at the first weighting of the basic roof of the four working faces are compared in Table 4, Fig. 9.   Fig. 9 shows: (1) When the face length is in the range of 126-230 m, with the increase of the face length, the end-of-cycle resistance of the support gradually increases, the dynamic load coefficient of the support gradually increases, and the strata behavior of the working face gradually increases.
(2) When the face length is in the range of 126-230 m, under different face lengths, at the end of the cycle, the maximum resistance of the bracket in multiple positions exceeds the rated working resistance of 13 000 kN, exceeding a large range. It shows that the rated working resistance of the bracket cannot meet the support needs of some parts after the increase of the face length.
b Fig. 7 The change curve of 7 monitoring site support cycle end working resistance and setting load in 301 working face

Comparison of Macroscopic Strata Behavior
When the length of the working face is 126-140.5 m, the macroscopic pressure of the working face appears intense when first weighting occurs, especially the hurricane in the working face, which not only moves the equipment forward, but also threatens the personal safety of the working face staff. The reason for this phenomenon is that the surface length is short, the first weighting step is large, and the hanging roof of the goaf is large, which causes the macroscopic mine pressure to appear violently when the basic roof first breaks.
When the face length is 230 m, working face gangs, roof fall is more serious, but no hurricane.
According to the comparison of four different working faces, it can be seen from the aspects of support pressure and dynamic load coefficient: When the face length is in the range of 126-230 m, with the increase of face length, the support pressure at the first weighting of the basic roof increases, the dynamic load coefficient increases, and the weighting degree increases.
From the macroscopic strata behavior law above, it can be obtained that: when face length is too small (126-140.5 m), there are hurricanes, harm to personnel and equipment. When face length increases (140.5-230 m), spalling, roof fall and other phenomena will aggravate.  Fig. 8 Comparison of end resistance of support circulation for 301 working face

Discussion
(1) When the face length is 230 m, the pressure value and dynamic load coefficient of the main roof are both higher than the rated value. Without changing the support, the face length of the working face needs to be optimized. (2) Macroscopic strata behavior of working face with shorter face length (126-140.5 m) shows that too short face length also threatens the safety production of working face. (3) According to the measured results, the reasonable working face length for the existing support should be within the range of 140-230 m.

Conclusions
(1) Working resistance of working face support. 22,211, 101, 103 and 301 fully mechanized caving face support end cycle resistance are 12331 kN, 12764 kN, 12478 kN, 12395 kN. The mean value of the maximum end-cycle resistance of the support is 14,079 kN, and the pressure is large. The dynamic load coefficients of the bracket are 1.53, 1.59, 1.87 and 2.18, respectively. The weighting strength changes from strong to very strong.
When the working face length is 126-140.5 m; during the first weighting and periodic weighting period of the basic roof, the macroscopic mine pressure appears violent and the working face produces hurricanes. When the face length is longer (230 m), gangs and roof fall are more serious, but there is no hurricane. (3) Relationship between working face length and weighting strength.
In the range of 126-230 m, with the increase of the face length, the end-of-cycle resistance, the dynamic load coefficient of the support gradually increases, and the mininig pressure behavior of the working face gradually increases. When the length of the working face is short (126-140.5 m), hurricane will occur when weighting.