Research on “Playing Football” Type Roof Control in Fully-Mechanized Mining Face with a Super-Large Mining Height under the Background of 5G+ Big Data

Abstract

With the increase of mining height at the working face, the influence range of roof fractures in the goaf increases, the advanced supporting pressure on the coal wall increases, ground pressure becomes more intense, and roof support becomes more difficult. Based on the analysis of ground pressure behavior in the first mining and caving stage, the normal mining stage, and the final mining breakthrough stage of the fully-mechanized mining face near 12,404, the relationship between conveyor current and coal speed is studied and compared. Based on the intelligent control system of the fully-mechanized mining face with a super-high mining height of 12,404 and the structure of the football team, the “playing football” roof control mode of the fully-mechanized mining face with super-high mining height under the background of 5G+ big data is put forward. The conclusions are as follows: In 12,404, the ground pressure was first mined. During normal mining, when the roof with a buried depth of more than 200 m is broken, the speed of the coal machine is kept within 12 m/min, and the full guard defends and controls the roof, pulling the lead frame through the area with severe ground pressure. When the roof is good, it is necessary to speed up the coal cutting and get rid of the pressure. When it is less than 200 m, it will overcome the local weighting, and show an offensive trend to speed up and increase production. In the final mining breakthrough stage, the speed of the coal machine should be controlled within 8 m/min, with attention to defense, guarding against roof leakage, and reducing waste rock.

1. Introduction

With the continuous advancement of shallow resource mining, the coal mining depth is increasing at a rate of 10 m to 25 m per year, along with a gradual increase in the mining height. Featured by rich coal reserves, shallow coal seams, and simple geological conditions, the Shendong Mining Area has a number of 10-million-ton coal mines, including several working faces with a large mining height. However, under the influence of special geological and mining conditions, there are still gangue leakage and roof accidents [1,2], which pose a serious threat to the normal production of coal mines and personnel safety. Given these accidents in the Shendong Mining Area, Ju Jinfeng [3] surveyed the coal pillar mining and support crushing accidents in shallow coal seams and took preventive measures; Ju J [4] studied the force-induced rotation of key blocks in the roof strata by analyzing the thickness of overlying coal pillars and the thickness of spacing between coal seams, and proposed a method for passing pressure zones supported by coal pillars; Wei Like [5] probed into the interaction between special rock beams and supports in the room-type coal mining method; Xie Xingzhi [6] explored the characteristics of mining pressure behavior in the room-type coal mining method; Xu Jialin studied the terrain passing ravines and valleys [7,8] and found that this terrain is the most prone to roof collapse and the most influential mining condition in the Shendong Mining Area, while abnormal mining pressure behavior, as well as geological conditions and roof control, are the main influencing factors for roof accidents [9]. In recent years, roof collapse and support crushing accidents [10,11] have occurred in some fully-mechanized mining faces of the Shendong Mining Area in the retracement stage, resulting in the stoppage of working faces and seriously affecting the retracement and safe production of working faces. With the active exploration [12,13] in the working faces with a super-large mining height in the Shendong Mining Area, the normal production of fully-mechanized mining faces with a super-large mining height of 8.8 m has been realized. Zhang Lihui [14] researched preventing roof collapse in working faces with a large mining height through engineering analogy, involving burial depth, roof characteristics, and difficulties in roof control. Liu Husheng [15] analyzed the mechanism of roof collapse at super-large mining height and proposed multiple measures for preventing this roof collapse. However, with the increase of mining height in a working face and the application of full caving method for the goaf roof, the influencing range of goaf roof fracture increases, the advanced abutment pressure of the coal wall increases, the mine pressure behavior is more intense, the degree of coal sheeting on the coal wall and gangue sheeting on the roof increases, and the difficulty of supporting the roof by the working face support and protecting the coal wall by the face guard increases [16,17,18]. As a result, the roof of the working face collapses and bulk gangues on the roof fall from the coal wall at the coal sheeting in front of the support to the scraper conveyor along the face guard. This poses a serious threat to the operator, causes hidden safety hazards to the equipment in the working face, and restricts the safe production of the fully-mechanized working face. Hence, the roof control of working faces with a super-large mining height is an urgent problem to be solved.

To comply with China’s “dual carbon” policy, Shanxi Coking Coal Group actively adopts a top-level design, investing in intelligent construction planning [19]. Currently, the intelligent construction of coal mines is ready to take off, with strong market vitality [20]. Wang Guofa [21] reported the connotation of intelligent coal mines and pointed out that building smart mines is inevitable for the development of the coal industry. Cai Feng analyzed the problems in the intelligent construction of coal mining enterprises and put forward measures and suggestions to accelerate the research and development of key technologies and equipment as well as technological innovation [22]. Huangling No. 1 Coal Mine has completed the first intelligent unmanned working face in a 1.4–2.2 m thin coal seam [23]; Xiaobaodang Coal Mine [24] has completed the theoretical, equipment matching, and equipment research and development work for a 450 m ultra-long intelligent mining project; Caojiatan Coal Mine [25] has even developed an intelligent fully-mechanized coal mining technology and its supporting system for a 10 m super-large mining height, promoting the development of intelligent working faces towards higher and more independent directions; The CHN Energy Investment Group [26] vigorously develops mining robots, gradually achieving the ambitious goal of “few human mining and no human in dangerous areas” in coal mines; Shandong Energy Group has also actively responded to the national energy development strategy and accelerated investment in intelligent development and construction [27]. Shaanxi Yanchang Petroleum and Balasu Coal Industry Company is committed to building a world-class intelligent mine, with complete infrastructure construction facilities, such as a 5G mine and industrial Internet [28].

Intelligent mining is a necessary path for the safe and efficient development of resources [29,30], and data governance is of great significance for accelerating the intelligent construction of coal mines [31]. Tangjiahui Coal Mine has pioneered transparent and intelligent mining at working faces based on digital twin intelligent “fully-mechanized caving mining” technology [32]. Based on video and image processing technology, Halagou Coal Mine of Shendong Company, China Shenhua Energy, has established a transportation system model to match the speed of conveyors with the amount of coal. [33] Based on embedded WEB servers, Lv Jiachen has established a simulation model and built up an experimental device for shearer positioning systems, delivering high test positioning accuracy. [34] By analyzing basic scaffold structures and electro-hydraulic control systems, Lu Xuliang has constructed a control model for scaffolds’ positions and postures, and developed an autonomous stent control system. [35] Zhou Zijian has studied the speed and tracking accuracy of shearers by using AMESim 2021.1 software, finding that the optimal speed range of automation is 10~13 m/s [36]. By analyzing the movement characteristics of shearers, W. Shibo et al. [37], based on the Kalman filter, greatly reduced the large positioning error of shearers in each cycle of cutting. Based on the fuzzy control theory, Xu, ZP [38] has proposed a memory-based cutting technology, which can effectively identify coal gangue and automatically adjust the traction speed and roller height. Liu Qing et al. have used the SAM2.0 centralized control system to greatly reduce the number of underground coal mine workers, increasing the production efficiency from 5% to 20%. [39]. However, the advantages of intelligent working surfaces are rarely leveraged for roof control in the coal production process. At present, most of the existing research on roof control methods focuses on the analysis of causes and post-prevention and treatment of roof leakage accidents. Few reports are about the prevention and control methods based on roofs of intelligent working faces. Based on the intelligent construction of CHN Energy’s Shendong Shangwan Coal Mine and the 5G+ big data platform, this paper proposes a “playing football” type roof control method in combination with intelligent equipment for the working face, providing a reference for the intelligent roof control of fully-mechanized mining faces with a super-large mining height.

2. Introduction to Working Face with a Super-Large Mining Height and Difficulty in Roof Control

2.1. Overview of Working Face

Taking the 12,404 fully-mechanized mining face with a super-large mining height of 8.8 m in the Shendong Shangwan Coal Mine as an example, Shangwan Coal mine is located in Wulan Mulun Town, Yijin Horo Banner, Ordos City, Inner Mongolia Autonomous Region. The width of the 12,404 fully-mechanized mining face is 280.6 m, the coal seam inclination is 1~5°, the advance length is 5309 m, and the designed mining height is 8.6 m; the ground elevation of this fully-mechanized mining face is 1154~1318 m, and the floor elevation of the coal seam is 1044.61~1073.81 m. There is a lime ditch diagonally crossing the connecting surface of working faces 10–21. The thickness of the overlying bedrock in the ditch is 95–124 m. This ditch is a seasonal ditch flow and a large flood discharge channel in this area, with a normal flow rate of 60 m³/h. In the mining area of the 12,404 working face, there are 18 ponds, with a water accumulation of about 61,500 m³ and good water yield.

The characteristics of the roof and floor in the 12,404 fully-mechanized coal face are shown in

Table 1. Characteristics of roof and floor in 12,404 fully-mechanized coal face.

Roof and Floor	Rock Name	Thickness (m)	Lithological Characteristics
Main roof	Fine-grained sandstone	5.68~20.34 16.89	Grey-white, gray-black stratification plane, fine-grained sandy texture, argillaceous cementation, with horizontal bedding, wavy bedding, and cross-bedding
Immediate roof	Sandy mudstone	0.8~13.49 7.48	Grey-white, sandy pelitic texture, ragged, flat, dense and hard fracture, containing pyrite nodules, with cross-bedding and wavy bedding
False roof	Carbon mudstone	0.3~0.10 0.2	Black, argillaceous cementation, massive structure
Immediate floor	Mudstone	1.15~5.75 2.19	Black gray, pelitic texture, flat and dense fracture, massive structure, containing phytolith

.

The main roof is fine-grained sandstone, with a natural compressive strength of about 17.62~23.68 MPa and a Protodyakonov coefficient of about 2.16~2.37. The immediate roof is sandy mudstone, with a natural compressive strength of about 19.58~40.16 MPa and a Protodyakonov coefficient of about 2.18~3.74. The false roof is carbon mudstone, with a natural compressive strength of about 23.26~29.00 MPa and a Protodyakonov coefficient of about 2.33~2.9. The immediate floor is mudstone, with a natural compressive strength of about 19.23~21.83 MPa and a Protodyakonov coefficient of about 2.18.

The positive side of the rubber transport roadway for the 12,404 fully-mechanized mining face with a super-large mining height is equipped with Φ 27 × 2400 mm fiberglass anchor rods at a row spacing of 1 m, 5 rods per row at a spacing of 1–1.1 m. The positive side of the air-return roadway is equipped with Φ 32 × 2400 mm fiberglass anchor rods at a row spacing of 1 m, 5 rods per row at a spacing of 1–1.1 m.

A total of 120 supports are arranged in the 12,404 fully-mechanized mining face with a super-large mining height, including 5 face-end supports (3 in the nose and 2 in the tail), and 4 transition supports (2 in the nose and 2 in the tail). The setting load of the support is 25.2 MPa. The supports are closely arranged, and the top beam between the supports cannot exceed 100 mm. The support spacing is controlled by the side pushing jacks during the moving process.

When the pressure is large, coal cutting measures are taken after the roof is protected by pre-support.

Three cylinders that have a rated resistance of 9000 kN are used for advanced supporting of the gate road for the rubber conveyor belt. Starting from the position 7 m away from the working face, one cylinder is placed every 7 m at the pedestrian side of the reversed loader bridge section, with a supporting distance of 21 m. For the return air gate road, ZFDC81100/30.5/55 D advanced supports are selected, with a total of 4 supports arranged in 2 groups, which advance in a stepping manner.

2.2. Introduction to Difficulty in Roof Control in a Working Face with a Super-Large Mining Height

The great change in burial depth of the coal seam in the 12,404 fully-mechanized mining face with a super-large mining height and the differences in roof lithology and depth of stratum makes it difficult to predict the mine pressure behavior of the fully-mechanized mining face. Due to the large mining height and the fluctuation of the coal seam, the thickness of bedrock in shallow buried working face varies greatly, and the bedrock ratio also varies widely, so that the mine pressure behavior in the working face shows great differences at different burial depths. Through the engineering analogy method, the mine pressure behavior and roof damage conditions of the 12,403 working face adjacent to the 12,404 fully-mechanized mining face are analyzed to take corresponding countermeasures for the 12,404 fully-mechanized mining face under similar engineering conditions.

1.

Mine pressure behavior after the first weighting

After the first weighting, the periodic weighting conditions of the working face are shown in

Table 2. Statistics of periodic weighting.

Weighting Frequency	Start of Weighting	End of Weighting	Non-Pressure Persistence Length	Persistence Length after Weighting	Step
1	90.6	98.6	9.1	8	17.1
2	106.9	117.2	8.3	10.3	18.6
3	127.9	141.2	10.7	13.3	24
4	153	161.2	11.8	8.2	20
5	163.7	179.3	2.5	15.6	18.1
6	185.1	191.7	5.8	6.6	12.4
7	197.4	204.4	5.7	7	12.7
8	221.7	226.5	17.3	4.8	22.1
9	233.7	239.3	7.2	5.6	12.8
10	249.2	259	9.9	9.8	19.7
Statistics of average weighting			8.83	8.92	17.75
Statistics of average weighting (number of cuts)			11.0	11.2	22.2

and Figure 1. The weighting step is 17.75 m, the average weighting is 8.9 m, and the non-pressure persistence length is 8.8 m, with little difference. The reason for the long weighting step is that the working face shows continuous weighting, which is manifested as local weighting after weighting, and there is intermittent pressure in the nose, middle, and tail areas.

Until the first periodic weighting after first weighting, the working face shows a serious spalling phenomenon. The coal seam is gradually laminated into a concave shape. The spalling depth is large in the 121–125# support area of the tail. The three-level face guard is not connected after it is driven.

During the weighting period, the coal seam in the working face shows serious spalling that reaches 3 m at the deepest part. The beam end spacing is large. Generally, there are still two paces after the advanced support is pulled and there is a large cargo capacity when the face guard is taken back. The spalling of 121~124# supports in the tail is the deepest. The three-level face guard has not been connected after it is driven, which is the same during the non-pressure period. Because of the slow advance speed of the working face, burial depth exceeding 240~250 m, large mining height, and large advanced abutment pressure of the working face, the coal wall has been crushed in advance at 3~5 m. With the advance of stoping, the crushed coal wall shows spalling. When the working face advances to 300 m, the advanced pressure increases, the mine pressure behavior of the return air gate road is serious and the upper anchor cable is continuously broken. During the later stoping period, the 121~124# supports show bulk gangue leakage phenomenon (Figure 2).

2.

Mine pressure behavior during normal mining period

The periodic weighting law for burial depth greater than 200 m is as follows: The periodic pressure step is about 16.4 m, showing a “two large and one small” law. The large and small periodic weightings alternate, with a maximum pressure of 51~55 MPa (Figure 3). The weightings are strong, generally 30–100#, as shown in Figure 3. The coal wall spalling at the working face is severe, showing a “crescent” shape. Furthermore, due to the prolonged duration of the weightings, the impact range gradually expands in the working face. The subsidence of the support live column is 30~200 mm, and the opening rate of the support safety valve is 5–26%.

The periodic weighting law for burial depth less than 200 m is as follows: The periodic weighting step is about 12.58 m. The mine pressure behavior is not obvious compared with that when the burial depth exceeds 180 m. The pressure, generally 35–40 MPa, is not intense and is localized. The weighting duration is short, with less spalling of the coal walls, and the coal walls at the head and tail of the machine are generally straight, as shown in Figure 4.

3.

Mine pressure behavior in final mining

With the remaining 200 m of final mining advancement, the weighting frequency of the working face is 8 in total, the average weighting step is approximately 17.5 m, and the continuous distance of weighting is approximately 5.8 m. In addition, the conveyor is crushed due to the overload of leakage current, seriously affecting the normal production of the working face (Figure 5 and Figure 6).

3. Application of 5G+ Big Data in Fully-Mechanized Mining Face with a Super-Large Mining Height

Coal has long been an important supporting energy source for national development. The coal industry is facing a shortage of human resources. Currently, “mechanization of labor” has been widely adopted. The workers still have high labor intensity when operating coal mining equipment in confined working spaces, and “automation to reduce manpower” needs to be further implemented.

The intelligent control system of the 12,404 fully-mechanized mining face with a super-large mining height applies “5G + working face three-dimensional digital twin system + automatic speed control system for three machines + precise positioning technology for the shearer + electro-hydraulic control and attitude monitoring system of support equipped with HarmonyOS + shearer automatic coal cutting system + centralized lubrication for the three machines, centralized lubrication for the shearer, and intelligent spray system + centralized control system for the fully-mechanized mining face” to create a fully-mechanized mining face with a super-large mining height and build an underground centralized control center. A 4G/5G dual redundant security network communication system has been established for the project. All employees can read the system data at any time, and check the production status of the fully-mechanized mining face, thereby realizing the information sharing and participation of all employees in the management of the intelligent working face.

1.

Three-dimensional digital twin system of working surface

The three-dimensional digital twin system of the working face is a three-dimensional mirrored scene simulation of the fully-mechanized working face, as shown in Figure 7, which integrates process self-optimization, data visualization, and strong human–machine interaction to display the real-time status of the fully-mechanized working face. From the three aspects of data information interaction, physical working face, and digital working face, the on-site fully-mechanized working face is simulated and optimized in terms of process, production workflows, production management, equipment performance, etc., and the “data-centered production model” enables equipment interconnection and reproduction of production scenarios, and accurately guides underground production operations. Intelligent mining face monitoring, planned cutting, abnormal area pre-warning, real-time monitoring of shearer and support data, intelligent restoration of shearer and support, real-time monitoring of transportation equipment conditions, and immersive roaming of the working face scene.

2.

Automatic speed control system for three machines

At present, the automatic speed control system for three machines generally believes that the coal quantity on the conveyor is derived from the coal cut by the shearer. However, in fully-mechanized mining faces with super-large mining heights, particularly in areas with significant mine pressure behavior, the shearer has few cutter holders for its top cutter, and most of the coal blocks on the top of the coal wall fall off. Therefore, the traditional flow estimation method is no longer applicable for fully-mechanized mining faces with a super-large mining heights. In order to accurately estimate the coal quantity on the conveyor, the automatic speed control system for the three machines in such mining faces utilizes laser cross-section scanning based on video recognition. This system can not only assess the coal quantity on the conveyor but also quickly identify when large pieces of the coal wall fall across the conveyor, causing a blockage and resulting in an overload of the conveyor. It can then rapidly take action to prevent the three machines from overloading, thereby enhancing the reliability of the system. In the centralized control interface of the three machines, adjustments can be made to the speed control interval, speed control range, and speed of the triangular coal conveyor at the head and tail of the shearer based on different geological and mining conditions, and the aforementioned logic enables stepless speed regulation. When the roof leaks and large pieces of coal and gangue fall onto the conveyor, the load on the conveyor will increase, causing increases in the traction currents of the three machines. At this time, the speed can be turned up to accelerate the running of the conveyor, thus preventing the accumulation of large pieces. The automatic speed control system for the three machines in the fully-mechanized mining face with a super-large mining face can respond to various complex working conditions, reduce labor intensity, standardize the management of electromechanical equipment, and minimize equipment wear.

3.

Precise positioning technology for the shearer

The shearer encoder is employed to position the shearer. The position of the shearer encoder can be directly read by the mine pressure control system to guide the support following the shearer, allowing for accurate positioning of the shearer without being affected by equipment and geological conditions. Reading the position of the shearer encoder directly through the mine pressure control system is an important breakthrough in support following the shearer at the automated working face of medium and thick coal seams. With this technology, the automated coal cutting and support following the shearer are no longer affected by the working face floor, the shearer position shifting, or automated unexpected stops. The use of the shearer encoder to position the shearer is an important supplement to the use of infrared technology to position the shearer, filling a gap in domestic technology, preventing shearer misalignment due to infrared sensor system failures, and improving the automation rate. The optional installation of infrared sensors in the automated working face saves production costs, reduces maintenance personnel, and saves labor costs.

4.

Electro-hydraulic control and attitude monitoring system of support equipped with HarmonyOS

The electro-hydraulic control and attitude monitoring system of the support equipped with HarmonyOS can achieve functions such as precise personnel positioning, information interaction, and support control. It can perform self-diagnosis of faults and execute self-regulation actions when the equipment is damaged. Combined with the precise positioning technology of the shearer, centralized follow-up automation can be achieved. In risk conditions such as end triangular coal bevel cutting and middle bevel cutting, it supports remote control, wireless remote control, automatic pressure replenishment, and backwashing, ensuring production safety in real time [40]. In normal production, important data such as column pressure, displacement stroke, and position and direction of the shearer are monitored in real time and transmitted to the three-dimensional digital twin system of the working face. In order to facilitate safe production in emergency situations, the sound and light alarm, emergency stop, locking, status display reminder, and other functions of the electro-hydraulic control system can be manually operated, and the parameters can be adjusted and set online [41]. The ZE07-05E Ethernet electro-hydraulic control system is stable, reliable, and powerful, as shown in Figure 8.

With the help of HarmonyOS, technologies such as high-precision personnel positioning, video monitoring systems, and mining equipment Internet of Things have been implemented. The video monitoring system for the 12,404 fully-mechanized mining face has been established as shown in Figure 9, which displays the environment and equipment information of the intelligent working face in all aspects. Technologies such as visualization, simulation, virtual-real interaction, perception, Internet of Things, and cloud computing have been applied to create a fully simulated model of the working face equipment, access massive amounts of data, and display equipment operation data, and support posture in real time. Through multi-view mode and roaming mode, the working face situation is restored from multiple angles, and the operation data of each piece of equipment in the working face is displayed, thereby aiding in the continuous optimization of the processes at the working face.

The Harmony platform models the key underground areas and equipment, personnel, as well as safe production status, and achieves a seamless connection between production management and digital twin on-site management, which allows managers to grasp information more quickly. The subsystems of the fully-mechanized mining face have achieved linkage integration through the use of Internet of Things and industrial communication network technology, greatly improving the efficiency of production control, achieving lean production and cost reduction and efficiency improvement on the working face, and laying a solid technical foundation for the IoT interaction of future working face equipment.

5.

Automatic coal cutting system of shearer

Automated coal cutting employs memory cutting techniques. The automated coal-cutting system of the shearer records parameters such as the drum rocker arm angle and traction speed, enabling it to automatically and repeatedly learn the cutter’s drum trajectory. First, the relevant parameters of memory coal cutting are set, followed by the execution of the memory coal cutting process. A complete cycle of demonstration cutter learning is carried out starting from one end of the working face and completing a full cycle of coal cutting across the entire working face according to the actual coal mining process [42]. This can reduce the number of shearer drivers working in high-risk areas, decrease the labor intensity of operators, and help achieve the ultimate goal of “safety through absence of personnel” in underground coal mines, thereby providing high safety benefits.

6.

Centralized lubrication of three machines, centralized lubrication of shearer, and intelligent spray system

The centralized lubrication system for the three machines adjusts the oil supply parameters through the mine control box, enabling single-point control and point-by-point monitoring of all lubrication points to meet the lubrication requirements. The system is composed of a pneumatic pump, mine control box, mine valve type oil feeder, and an oil supply pipeline, etc., to achieve precise lubrication of the three machine’s lubrication points at a fixed time, fixed point, and fixed quantity.

The centralized lubrication system for the shearer adjusts the oil supply parameters through the mine control box, enabling single-point control and point-by-point monitoring of all lubrication points to meet the lubrication requirements. The system is composed of a pneumatic pump, mine control box, mine valve type oil feeder, and an oil supply pipeline, etc., to effectively reduce the wear rate and failure rate of the shearer.

The intelligent spray system transmits information to the controller through the vibration sensor. The controller then opens the valve to spray water for dust suppression, and the spray automatically stops when the equipment is turned off. The device consists of a controller, vibration sensor, control valve, and spray component, to effectively reduce the dust concentration on the working surface.

7.

Centralized control center of fully-mechanized mining face

The centralized control center is arranged at the tail of the self-moving machine of the fully-mechanized mining face, as shown in Figure 10. The control authority of the main equipment in the fully-mechanized mining face is transferred to the control center. This enables data collection and analysis of the shearer, support, three machines, pump station, mobile substation, comprehensive protector, feed equipment, combined switch, automatic emulsion proportioning equipment, as well as the remote control of these devices. Real-time monitoring and control of the working status of each subsystem are realized. Based on the centralized control system, the console driver can monitor the working status of each subsystem in real time. The system is characterized by fault display and recording functions, which can help maintenance personnel quickly find problems and greatly improve maintenance efficiency. Furthermore, the integration of “three posts into one” is realized on the basis of the centralized control system [43].

The centralized control center can remotely monitor and control the main production equipment of the fully-mechanized mining face in real time. By applying information technology, it is a mine safety monitoring system integrating “worker, machine, environment, and management” to improve mine safety. It can reduce the number of workers in high-risk areas and help achieve the ultimate goal of “safety through absence of personnel” in underground coal mines, thereby providing high safety benefits.

4. “Playing Football” Type Roof Control in Fully-Mechanized Mining Face with a Super-Large Mining Height

To address the issue of roof control in the 12,404 fully-mechanized mining face with a super-large mining height, the “playing football” type roof control mode of the fully-mechanized mining face with a super-large mining height is proposed under the background of 5G+ big data.

4.1. “Playing Football” Type Structure Composition

A football team is mainly composed of the coaching team, footballers, and logistics support.

1.

Coaching team

The coaching team is the core part of the organizational structure. It is essential for the coaching team to have a thorough understanding of each member and excel in the division of labor, guiding the players to maximize their potential. In the 12,404 fully-mechanized mining face with a super-large mining height, the three-dimensional digital twin system of the working face plays the role of the coaching team. This system enables the intelligent mining face to achieve a “data-centered production model”, facilitating equipment interconnection and reproducing production scenarios, thus accurately guiding underground production operations.

2.

Footballer

Footballers are the core strength of a football team. They need to perform tasks such as defending, organizing attacks, passing, and shooting in football matches. The excellence and coordination of the footballers directly influence the combat capacity of the football team.

Defender: The defender is responsible for passing the ball in a football game, and the speed of the defender’s passes affects the pace of the game. When the attacking conditions are favorable, the attacking rhythm can be accelerated to seize the initiative. Conversely, when the conditions are not favorable, the rhythm can be slowed down to seek a better opportunity. In the fully-mechanized mining face with a super-large mining height, the automatic speed control system of the three machines can effectively control the coal output from the working face. When the roof conditions are good, the speed of the three machines and the shearer can be increased to enhance coal production. Conversely, when the roof conditions are poor, the speed of the three machines and the shearer can be reduced, and the advanced support workers can cooperate to ensure effective protection and control of the roof.

Midfielder: The midfielder plays a crucial role in organizing the attack in a football game. In a fully-mechanized mining face with a super-large mining height, the support operator of the working face lags behind the shearer by 2~3 supports to timely adjust the support and protect the roof, thereby ensuring the safety of the roof of the working face. Based on the precise positioning technology for the shearer and the electro-hydraulic control and attitude monitoring system of support equipped with HarmonyOS, the position of the shearer encoder can be directly read by the mine pressure control system to guide the support and shield following the shearer for alignment, allowing for accurate positioning of both the shearer and personnel, information interaction, and the support control function of the centralized follow-up automation, end triangular coal bevel cutting, middle bevel cutting, and remote control and wireless remote control, without being affected by equipment and geological conditions. When the roof conditions of the fully- mechanized mining face are favorable, based on the precise positioning technology for the shearer and the electro-hydraulic control and attitude monitoring system of the support equipped with HarmonyOS, the support can be delayed to enhance the production of the working face. Conversely, when the electro-hydraulic control and attitude monitoring system of the support equipped with HarmonyOS detects a deterioration in roof conditions, the production speed will be slowed down, and timely support will be provided to ensure roof safety.

3.

Striker

The striker is responsible for scoring goals in a football game. In a fully-mechanized mining face with a large mining height, when the coal wall of the working face is straight and the roof remains intact, the shearer driver acts as a “striker”, cuts coal at a normal speed, and utilizes the automated coal cutting system of the shearer to reduce the damage rate to the roof and equipment caused by coal cutting, thus safeguarding the roof management of the working face.

4.

Logistics support

In the structure of a football team, logistics support is in charge of the physical health and rehabilitation of the footballers. In a fully-mechanized mining face with a super-large mining height, centralized lubrication of the three machines and shearers can maintain the production and extend the service life of the machinery and equipment. The intelligent spray system can reduce dust during the production process and improve the clarity of the vision of the working face. The centralized control system of the fully-mechanized mining face plays a role in coordination and cooperation. When other members of the team do not notice large pieces of roof falling off or conveyor overloading, this system can restrict equipment use at the working face, adjust the operating status of the equipment, and prevent the development of disadvantages on the roof of the working face.

4.2. Roof Control in Fully-Mechanized Mining Face with a Super-Large Mining Height

The 12,404 fully-mechanized mining face has just commenced initial mining. Currently, we are only able to analyze the conveyor current, mine pressure, and shearer speed during the initial mining stage, and apply the general rules in the initial mining period to other stages. Additionally, throughout the mining process, we continually distill new insights and rules, which can be utilized for further research and application.

1.

Initial mining and strong release stage

In the early stage of mining, the 12,403 fully-mechanized mining face employed water pressure pre-splitting and triangular zone blasting forced caving technology. Two boreholes were arranged for water pressure pre-splitting, and each borehole was fractured into 9 sections. Four blasting holes in the triangle were constructed in the rubber conveyor belt, and five were constructed in the return air gate road. After the working face was put into operation, the roof of the middle goaf did not obviously collapse after caving at both ends initially. When advancing to 10~20 m, the roof gradually collapsed and filled the goaf. When advancing to 44 m, the initial weighting occurred in the main roof, with the weighting occurring on the 80~105# supports at the tail of the machine, exerting a pressure of 40~50 MPa. Among these, the 90~105# supports experienced higher pressure, and the working face suffered severe spalling. After four consecutive cuts, the spalling weakened. Subsequently, the weighting occurred on the 25~70# supports at the head of the machine, with the local pressure of the working face exceeding 45 MPa, leading to increased spalling of the coal wall. After three consecutive cuts, weighting occurred over a large area on the 25~110# supports at the working face, resulting in severe spalling of the working face. The beam end distance was large, and there was still a beam end distance before and after the advancement was completed. The mine pressure behavior of 50~70# and 90~100# supports was strong, and the coal wall became brittle. The initial weighting ended after advancing 57.4 m.

According to the analysis of the mine pressure-heat diagram (Figure 11), the current change diagram of the conveyor (Figure 12) and the speed change diagram of the shearer (Figure 13) during the initial mining period of 12,404, at the initial weighting stage, as shown in stage I of Figure 13, i.e., the weighting step is 7.2 m, the persistence length is 4 m, the non-pressure persistence length is 3.2 m, the weighting time is between 11–20, 10:54:56 and 11–21, 02:53:21, corresponding to stage I in Figure 12, there is a period of time when the current is 0, which is analyzed with the operation situation during the production period. During the shutdown and maintenance period, the current at this stage is not analyzed and considered. The rated current of the conveyor is 325 A, which exceeds 85% of the rated current of the conveyor, that is, 276.25 A, and results in overload. Therefore, the conveyor has a crushing risk. In interval A, the maximum current of the conveyor reaches 414.29 A, which is far more than the overload current of the equipment. At the same time, the speed change diagram of the shearer is compared and analyzed. As shown in stage a of Figure 13, in interval α, the speed of the shearer soars, and the maximum value reaches 15 m/min. Compared with the speed of the shearer in the same stage, most of them are below 10 m/min. The speed of some shearers is in the stage of 0, which is the time of shutdown and maintenance without analysis. According to the comparison of the time axis, it is observed that the high-speed stage of the shearer speed is ahead of the overload stage of the conveyor. The reason is that the shearer cuts coal at high speed, more coal blocks are broken down to the conveyor, and even large blocks are crossed between the coal wall and the cable groove to increase the resistance. Due to the excessive speed of the shearer, the amount of coal in the conveyor is too large, resulting in the overload of the conveyor. Therefore, during the weighting period of the working face, in order to prevent the roof from being supported in time due to the overload of the conveyor, it is necessary to monitor the working face through the three-dimensional digital twin system of the working face, and control the shearer with the automatic coal cutting system of the shearer. The speed of the shearer is controlled below 11 m/min, and it is best to control it at 10 m/min, which can not only control the roof but also keep up with the output. In view of the initial mining and strong caving stage of 12,403, it is difficult for the roof to collapse for the first time when 12,404 starts the mining stage of the working face. At this stage, the mining speed should not be too large to prevent the overhanging distance from being too large. Taking the three-dimensional digital twin system of the working face as the core guidance, through the three-machine automatic speed control system, the coal transportation volume of the conveyor is limited, and the position of the shearer is accurately positioned with the precise positioning technology of the shearer and the automatic coal cutting technology. The position of the shearer is accurately positioned, and the roof and floor are cut flat to ensure the support effect of the support and to ensure that the fully-mechanized mining face will not cut the roof and leak the roof. Then, with the help of the electro-hydraulic control and attitude monitoring system of the support equipped with HarmonyOS, the support is timely erected to support the roof and ensure that all the supports reach the initial support force of 25.2 MPa. When the fully-mechanized mining face is advanced to about 40 m from the main side of the open-off cut, the support reduction should be minimized when the support is moved. The support that can be moved under pressure or support that contacts the roof during movement can be employed, and the chasing support or advanced support should be employed when there is a sign of roof leakage.

2.

Normal mining stage

When the burial depth is greater than 200 m, all act as “defenders”.

At this stage, the working face exhibits severe mine pressure behavior, strong weighting, and serious spalling of the fully-mechanized mining face. Generally, the weighting ranges from 30 to 100#, characterized by a wide range and prolonged duration. The coal wall is severely crushed, exhibiting serious spalling in a “crescent” shape. The upper spalling can reach a maximum depth of 2~3.8 m. The subsidence of the support live column is 30~200 mm, and the opening rate of the support safety valve is 5~26%. Due to large burial depths and poor roof conditions, the conveyor’s current is the data shown in Figure 14. In Stage II, the maximum current in Section B far exceeds the overcurrent threshold, reaching 464.28 A, with a long time of exceeding the overload current section. Therefore, the roof control is difficult. A comparison of the speeds of the coal machines in the same time period, as shown in Figure 15, shows that the maximum speed of the coal machines is 17 m/min. When the speed of the coal machines is lower than 12 m/min, the current of the conveyor is normal. Therefore, at this stage, when the roof condition is poor, the speed of coal machines should be strictly controlled, so that it is lower than 12 m/min. Strict control of roof safety is required. Centralized lubrication of the three machines and the shearer ensures the reliability of the equipment. By utilizing precise positioning technology for the shearer, the coal-cutting speed is strictly controlled. In the event of a roof collapse risk, all personnel assist in timely roof and side protection, reducing the amount of gangue entering the conveyor, while simultaneously decreasing the shearer speed and controlling the cargo volume to prevent conveyor overload. In the case of roof separation, the “roof contacting and moving method” is employed based on the electro-hydraulic control and attitude monitoring system of the system equipped with HarmonyOS. When the roof is broken or collapsed, the pre-support method should be adopted. When weighting occurs but the roof conditions are stable, it is essential to strengthen the support and advance quickly to alleviate the pressure, as shown in Figure 16.

When the burial depth is less than 200 m, organize the attack normally.

The periodic weighting step is about 12.58 m. The overall intensity of periodic pressure is weakened, with pressure of generally 35–40 MPa, and less exceeding 40 MPa. The pressure is locally weak, with a short duration. There is no serious spalling on the working face, the depth of localized spalling in the middle section is 0.3–1.5 m, and the coal wall at the head and tail of the machine is basically straight. At this stage, the currents of conveyors are shown in Figure 17. The currents are all within the rated current range, and the three-machine system operates well. The speeds of coal machines are shown in Figure 18, which indicates that the speeds of coal machines are within 12 m/min. When the coal walls of working faces are straight and the roofs are complete, coal machine drivers would act as a “vanguard”, who can, according to the situation of roofs, appropriately increase the speed to cut the coal, so as boost the output. Based on the electro-hydraulic control and attitude monitoring system of the support equipped with HarmonyOS, the control scheme of supports pulling following the coal shearer as a “midfielder”, and lags behind the shearer by 2–3 supports to move the support for roof protection in time; the rest act as “defenders”, responsible for advancing support, adjusting the support type, pushing the support, etc. Based on the centralized control system of the fully-mechanized mining face, the mine pressure and roof conditions are observed in time, and the equipment operating status of the fully-mechanized mining face can be adjusted at any time in special circumstances, as shown in Figure 19.

3.

Final mining progress stage

The 12,403 fully-mechanized mining face experienced 10 weighting events as it transitioned from the remaining 200 m to the remaining 31.1 m, with an average weighting step of 17.3 m and an average continuous distance of weighting of 6.2 m. The pressure position should be at the remaining 24.8 m and 7.3 m. Due to cut-through to the last mining work surfaces, the difficulty of roof control increases; gangue falling becomes frequent, resulting in an increased load of conveyors; and the frequency of conveyors’ currents exceeding the rated current would also increase, as shown in Figure 20 and Figure 21. The maximum current may even reach 565.71 A; in this case, conveyors are very easy to become crushed, thus seriously affecting safe and efficient production. Similarly, the 12,404 fully-mechanized mining face strictly controlled the coal-cutting speed of the shearer based on the automated coal-cutting technology of the shearer. When there is a risk of roof collapse, the electro-hydraulic control and posture monitoring system of the support equipped with HarmonyOS, enables all personnel to assist in protective measures, ensuring timely roof and side protection, prioritizing the advancement of support, reducing the amount of gangue entering the conveyor, and at the same time reducing the speed of the shearer and controlling the cargo volume to prevent the conveyor from overloading. When the roof conditions deteriorate, close attention must be paid to the status of the fully-mechanized mining face, and preparations for real-time remote intervention and adjustment should be made if necessary, as shown in Figure 22.

5. Conclusions

1.

According to the analysis of the roof management in the working faces adjacent to the 12,404 fully-mechanized mining face with a super-large mining height, it has been observed that as the mining height increases, the roof management becomes increasingly complex. Additionally, it has been noted that due to poor roof management in the fully-mechanized mining face with a super-large mining height, roof collapses and accidents involving the support crushing and conveyor crushing frequently occur. The intelligent control system of the 12,404 fully-mechanized mining face with a super-large mining height is superior to the traditional operating system. This system is capable of automatically positioning the shearer and enhancing roof support.

2.

Based on an analysis of the mining pressure and roof conditions during normal mining, passing lime ditches, and final mining breakthrough in the 12,404 fully-mechanized mining face with a super-large mining height, and in combination with the intelligent construction system of the mine, this paper provides the following suggestions: during normal mining, when the buried depth is greater than 200 m and the roof is fractured, it is necessary to reduce the coal cutting speed and pull the advanced support to pass through the area with intense mining pressure behavior; when the roof is in good condition, coal cutting needs to be accelerated to shake off the pressure. When the buried depth is less than 200 m, local pressure should be overcome, followed by accelerating production to increase output. When passing the lime ditch, coal cutting should be at normal speed. In the final mining stage, it is neccessary to be cautious of roof leakage, reduce gangue, and improve the reliability of roof control in fully-mechanized mining faces with a super-large mining height.

3.

This study has analyzed the mine pressure and roof situation in the normal mining stage and the “cut-through to the last mining work surfaces” stage at the 12,404 super-high fully-mechanized mining face. In combination with the intelligent construction system of the mine, during the normal mining period, when the buried depth is greater than 200 m and the roof is broken, it is necessary to slow down the speed of coal cutting and control the speed of coal machines within 12 m/min, with all staff in a defense state. Forepoling shall be installed over the regions with dramatic mine pressure. When the roof is in good condition, coal cutting shall be sped up, so as to throw off the pressure. When it is less than 200 m, the production can be accelerated after overcoming the local pressure, so as to boost the output. In the “cut-through to the last mining work surfaces” stage, the speed of coal machines shall be controlled within 8 m/min, with attention paid to defense against roof falling, while reducing gangues. In this way, the reliability of roof management for super-high fully-mechanized mining faces can be improved.