1. Introduction
Energy is the foundation of the continuous development of the world economy and human civilization [
1]. With the rapid progress of human society in the past century, sustainable development has become the common choice of mankind. Coal occupies a large proportion of the primary energy consumption; however, the initial coal mining and utilization process is harsh [
2]. Although the process meets the energy supply requirement of a development stage, it is not conducive to long-term sustainable development. Therefore, at the present stage, when alternative energy has not completely replaced coal energy, on one hand, we must attempt to vigorously enhance the efficiency of alternative energy, and on the other hand, we must focus our attention on fine mining and the efficient utilization of coal resources. In particular, the progress of support, mining equipment, and mining technology has provided a solid foundation for improving the efficiency of coal mining. At the same time, the progress of research methods has also provided an important optimization direction for various designs in coal mining [
3]. Based on the research hotspot in China’s mining industry, roof cutting pressure releasing gob-side entry retaining (RCPRGER) technology, and the results of field practice, this paper proposes, for the first time, an efficient design method of blasting parameters, which can considerably shorten the test section length and test workload by using a mathematical method, namely, a neural network algorithm [
4]. The research results have definite guiding significance for the development of the novel gob-side entry retaining technology and fine mining in the industrial domain of coal.
Gob-side entry retaining technology has been an important development direction for coal mining science in China, since it was first used in the 1950s [
5]. This technology has many advantages, such as an increase in the coal mining rate in the mining area, prolongation of the mine service life, reduction in the workload of roadway excavation, and simplification of the succession procedure of working faces. In particular, with increasing tension concerning coal resources in the recent years, the gob-side entry retaining technology has become a key aspect of coal mining technology research in China [
6,
7]. Based on this technique, in 2009, a new gob-side entry retaining technology by roof cutting and pressure releasing was proposed by Professor He Manchao, an academician of the Chinese Academy of Sciences [
8]. As shown in
Figure 1, the new entry retaining technology is based on the short beam theory, and its core principle is as follows: Under the condition of roof reinforcement by constant-resistance large-deformation anchorage cables, the roof of the retaining entry on the side adjacent to the goaf along the working face mining direction is cut by bidirectional concentrated tension blasting (BCTB). Consequently, the horizontal stress transmission between the roof of the retaining entry and the goaf can be terminated. After the working face mining, the goaf roof collapses along the roof cutting surface, and the gangue falling from the goaf roof adjacent to the retaining entry can support the overlying strata effectively after its broken expansion, which can stabilize the retained entry surrounding the rocks [
9]. Compared with conventional means of gob-side entry retention, the new technology eliminates the need for filling materials, simplifies the technological process, and considerably improves the application range of gob-side entry retention. After development in recent years, this technology has been successfully tested and popularized in Baijiao, Tangshangou, Hecaogou, and other mines [
10,
11,
12].
As the key link of the RCPRGER technology, roof cutting blasting is required to cut off the original long cantilever beam of the entry roof in the tendency direction of the working face, and to construct the short cantilever beam for the retained entry [
13]. This process is directly related to the success of entry retention. In the existing research, the key parameters of the roof cutting blasting in the tested mines, such as the charge weight per blasthole, charge structure, and blasthole spacing, are all determined through field blasting tests. The test process is tedious and affects the process continuity of working face mining, blasthole drilling, and other related processes. Furthermore, the roof cutting effect in the blasting test section is uncertain, which affects the quality of roof cutting and entry retention [
14]. In the early stages of the novel entry retaining technology becoming popular, because of the lack of field experience, field blasting testing was a crucial process. However, the technology has currently been popularized and applied to more than 20 mines under different roof conditions. Because the total length of the retained entry is presently more than 200,000 m, more convenient and effective methods to determine the key parameters of roof cutting blasting are required. To this end, in this study, based on practical experience, the authors attempt to use a mathematical analysis method to simplify the design process of the key parameters of roof cutting blasting.
2. Mechanics Process Mechanism with Roof Cutting
The principle of the RCPRGER technology involves terminating the horizontal stress transfer between the goaf roof and gob-side entry roof by roof cutting, and by utilizing the broken rocks in the goaf to support the goaf overburden after the working face mining has advanced and goaf roof has collapsed and expanded after breaking, that is, it is a form of no-pillar mining, as shown in
Figure 1 [
15]. In the entry retaining process, to prevent roof subsidence, a bolt-cable support of the entry is required immediately after the entry is excavated. Moreover, the supporting material includes constant-resistance large-deformation anchorage cables, and the roof near the roof cutting slit is the key supporting area. This is because when the goaf roof collapses after roof cutting and working face mining, the roof of the retained entry presents a cantilever beam structure, and the roof cutting side of the entry roof is prone to subsidence deformation. In addition, the roof cutting height is designed according to the bulking coefficient of the roof in RCPRGER technology, as follows [
13]:
where
HF is the roof cutting height,
HM is the mining height, △
H1 is the amount of roof subsidence, △
H2 is the amount of bottom heave, and
Kb is the bulking coefficient of the gob roof. After the goaf roof collapses and lags behind the working face beyond a certain distance, the caved gangue will gradually be compacted and play an effective role in supporting the overlying strata. Next, the subsidence of the goaf roof and the retained entry roof will be reduced considerably compared with the subsidence in the no-roof-cutting condition.
Through the field application and theoretical research corresponding to the new gob-side entry retaining technology, the stress evolution rule of the surrounding rocks in the entry retaining process can be summarized. Compared with the conventional long wall mining method, in which the section protect coal pillars must be retained, the major difference in the stress evolution of the surrounding rocks in the new method is the addition of the roof cutting stress state. According to rock mass mechanics, the stress state of any point in the rock can be simplified as three orthogonal stresses—the maximum, intermediate, and minimum principal stresses—according to the magnitude of the stress [
16]. In this study, the analysis of the stress state evolution of the entry-surrounding rock is performed primarily to reflect the stress change process of the surrounding rock under the RCPRGER technology, and then to extract the stress state changes before and after roof cutting, which provides the basis for determining the key factors affecting the roof cutting process. Therefore, the analysis focuses on the main changes in the stress value of the surrounding rock, and the direction of the stress is not clearly defined. Taking point A as an example, which is located on the goaf roof closed to the roof cutting surface, its stress evolution can be summarized based on the Mohr stress circle theory, as shown in
Figure 2.
(a) Original rock stress state
Before the entry excavation, the entry surrounding rock is in the natural stress state, unaffected by the processes of entry excavation, coal mining, roof cutting, and so on. Thus, point A is in the original rock stress state and is subjected to the three principal stresses (i.e.,
σ10,
σ20 and
σ30), which denote the maximum, intermediate, and minimum principal stresses, respectively (
σ10 >
σ20 >
σ30). The normal stress,
σ, and shear stress, τ, in any direction of point A can be expressed as follows [
17]:
where
θ is the angle between
σ and the maximum principal stress. The strength envelope curve can be approximately expressed as follows:
where
c and
φ are the cohesive force and internal friction angle of the rock, respectively.
(b) Entry excavation stress state
After the entry excavation, the surrounding rock stress gets redistributed. The maximum principal stress of point A increases and the minimum principal stress decreases. That is, σ10 increases to σ11 and σ30 decreases to σ31. At this time, the stress circle of point A is beyond the range of the strength envelope curve, and thus some parts of the surrounding rock are destroyed by the excavation.
(c) Bolt-cable support stress state
The bolt-cable support is usually installed after entry excavation. The bolt-cable support is a type of active support, that is, the steel strand cable is pretightened during installation. Therefore, the surrounding rock of the roof is subjected to the active pressure of the bolt cable after the installation of the bolt-cable support, and the roof rock changes from a two-way stress state to a three-way stress state. As a result, the strength of the surrounding rocks is enhanced, as follows: The bolt-cable support can not only provide a certain support force to the roof, but it can also change the stress state of the internal rock mass within the surrounding rocks to increase the strength of the surrounding rocks themselves [
18]. The support force to the roof causes the minimum principal stress to increase, and the strength increase of the surrounding rocks themselves causes the range of the envelope curve to expand. In particular, owing to the influence of the pretightening force of the bolt-cable support, the minimum principal stress of point A increases, that is,
σ31 increases to
σ32. Meanwhile, the bolt-cable support also influences the mechanical properties of the surrounding rock, as follows: The cohesive force
c increases to
c’, and the internal friction angle
φ increases to
φ’. Thus, the strength envelope curve turns from pink to green, as shown in
Figure 2c. At this time, the stress circle of point A returns to the range of the strength envelope curve, and the entry surrounding rock is stable.
(d) Roof cutting stress state
The roof cutting stress state is a special stress state, specific to the RCPRGER technology. The roof cutting height is designed according to the rock stratum property and stratigraphic structure, and the cutting can terminate the horizontal stress transmission between the goaf roof and retaining entry roof to a certain extent, which influences the pressure release in the rock around the cutting slit. Therefore, the main stress change due to the roof cutting to point A is the decrease in the maximum principal stress, that is, the decrease of σ12 to σ13. Thus, the stress state is more stable than the previous stage.
(e) Premining stress state
The premining stress state refers to the stress state of the surrounding rocks in the area affected by the stress concentration in advance of the working face, and the affected area is usually within approximately 30 m ahead of the working face [
19]. With the working face mining advancing in this stage, the effect of stress concentration in the front of the working face to point A becomes increasingly significant.
σ13 increases to
σ14 and the stress state of point A gradually turns unstable.
(f) Postmining stress state
The postmining stress state refers to the stress state when the coal seam is mined out and the surrounding rocks are adjacent to the goaf. The stress state of point A turns into the postmining stress state after the working face mining until it is reached. The point is exposed on the surface of the goaf roof at this stage owing to the mining out of the coal seam, and the minimum principal stress σ34 is reduced to σ35 = 0. At this point, the stress circle of point A is beyond the range of the envelope curve, and the goaf roof begins to break down.
In conclusion, the stress evolution process of the surrounding rock can be divided into six stages in the RCPRGER, namely, the original rock stress state, entry excavation stress state, bolt-cable support stress state, roof cutting stress state, premining stress state, and postmining stress state. Among them, the first three stages occur before the roof cutting and are not affected by the cutting. The latter three stages are critical to whether the goaf roof can collapse smoothly and be separated from the entry roof completely after the roof cutting and coal mining. In this section, point A, which is located on the surface of the goaf roof near the roof cutting slit, is taken as an example. During the entire stress evolution process, point A is subjected to unstable disturbances three times until it collapses, that is, in the entry excavation state, premining stress state, and postmining stress state. However, the effect of mining on the retained entry roof is reduced greatly after the roof cutting.
Through the above-mentioned analysis, it can be concluded that the collapse of the goaf roof near the roof cutting slit undergoes the above six stress evolution states in the RCPRGER technology under all geological conditions are fulfilled. Furthermore, roof cutting directly affects the collapse form of the goaf roof and the entry retention. Meanwhile, roof cutting is also the most controllable process, therefore, the parameters of the roof cutting blasting should be designed appropriately, as they can determine the success or failure of the entry retention.