Research on Control of Rib Spalling Disaster in the Three-Soft Coal Seam

Energy School, Xi’an University of Science and Technology, Xi’an 710054, China Key Laboratory of Western Mines and Hazard Prevention of China Ministry of Education, Xi’an 710054, China Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, Shaanxi Coal Geology Group Co, Ltd., Xi’an 710021, China Shaanxi Coal Industry and Chemical Technology Research Institute Co., Ltd., Xi’an 710010, China


Introduction
ree-soft coal seam refers to the soft roof, soft coal seam, and soft floor. Due to factors such as soft coal quality and low firmness coefficient, rib spalling often occurs in the threesoft coal seam face during the mining process. On the one hand, it affects the advancing speed of the working face. On the other hand, it is more likely to cause serious accidents such as roof fall accidents [1][2][3]. erefore, it is of great significance to research on control of rib spalling disaster in the three-soft coal seam.
Existing studies show that there are various forms of rib spalling. In nearly horizontal and gently inclined coal seams, the roof pressure, the physical and mechanical properties of the coal are the main influencing factors of rib spalling. Rip spalling form of soft coal is mainly shearsliding failure [4,5]. Fang et al. [6] established a mechanical model of the rib spalling in soft coal fully mechanized caving face and proposed a control plan for using wood bolts in the severe areas of the rib spalling. Pang and Wang [7] established the tensile-sliding mechanical model of rib spalling of the hard and thick coal seam and obtained the relationship between the fracture depth and width of the coal wall, the strength of coal, and the mining height, and proposed the concept of "Critical Protection Force." Yang and Kong [8] studied the deformation and strength characteristics of brown rope coal samples with different diameters and proposed a flexible reinforcement technology to prevent and control rib spalling and a combination of wood bolting at the upper end of the coal wall and water injection in the coal seam of the three-soft unstable face, and comprehensive measures to strengthen the management and control of the supportsurrounding rock relationship [9,10]. Zhou [11] obtained the change rule of the mechanical properties under the loose coal and verified the good technical results of the coal wall shallow hole water injection to prevent rib spalling through the field industrial test. e bolt-grouting support theory was researched on the seepage diffusion mechanism of the bolt-grouting liquid, and the bolt-grouting support effect of the roadway was improved [12,13]. e mechanism of rib spalling and disaster-causing mechanism of soft steeply dipping coal fully mechanized caving face were researched [14,15]. Yin [16] established a cusp catastrophe model and an arc sliding instability model for a fully mechanized caving face with large mining height and proposed a "Solid-liquid Synchronization" fault detection method to control rib spalling with the core of reducing the failure ratio of the support.
e compression rod mechanics model of the large mining height rib spalling in the three-soft coal seam was established, the influence of different factors was analyzed, and advanced grouting to strengthen the coal wall with large mining height was proposed [17][18][19]. By studying the failure mechanism of the key strata of the overburden, a method to determine the grouting drilling angle was proposed, and the complex engineering practice on-site verified the scientific nature of the theory [20][21][22]. e above-mentioned scholars have done a lot of research on the mechanism of rib spalling and proposed different control measures, but they have not conducted indepth discussions on on-site grouting engineering practices. In the past ten years, grouting reinforcement materials have achieved considerable development and have been widely used in the coal minefield. Kang and Feng [23] summarized the common materials for downhole grouting at present: cement-based, chemical, and cement and chemical composite grouting materials, and pointed out that chemical grouting materials have the characteristics such as fast curing, high strength, and good permeability, which can timely deal with coal accidents such as rib spalling and roof fall quickly and effectively. e polymer grouting reinforcement material for mines was a rapidly developing new application technology in the coal minefield, which can timely and efficiently deal with geological disasters encountered in underground mining and production [24]. However, serious safety problems appeared in the actual application process, such as spontaneous combustion of the material itself and poor reinforcement effect. erefore, the in-depth research and promotion of polymer grouting materials are also continuing [25].
In this paper, the three-soft coal seam face was taken as the engineering background and based on analyzing the rib spalling form of Heyang Coal Mine 1508 Working Face, rib spalling and grouting reinforcement mechanism were explored. A similar simulation experiment was used to study the characteristics of roof stress evolution under three-soft coal seam mining conditions. A numerical simulation experiment was used to study the change law of the grouting borehole stress field under different grouting pressures. e research results to the site were applied, and the effect of grouting reinforcement was verified through different monitoring methods.
is research has a good reference significance for another three-soft coal seam antirib spalling disaster prevention and control work.

Mining Status of 1508 Working Face.
e 1508 Working Face is located in the middle of the first mining area of the Heyang Coal Mine, and the coal seam is a three-soft coal seam. e west is the 1506 Working Face to be excavated, and the east is the 1509 Working Face to be excavated. e average inclination angle of the coal seam is 5°; the inclined length of the working face is 150 m, and the strike length is 1721 m; the average coal thickness is 4.5 m, and the design mining height is 3.8 m.
e roof management adopts ZY4200/18/38 hydraulic support to support the roof, comprehensive mechanized mining, and the roof in the old goaf area adopts all collapse treatment. e working face layout is shown in Figure 1(a). At present, the most serious area of rib spalling in 1508 Working Face is located at the 25#∼35# support, as shown in Figure 1(b). Rib spalling was accompanied by a large number of falling roof rocks, which made it difficult to guarantee the safety of face production personnel. e falling of roof rocks increases the difficulty of advancing the working face, leaving the exposed coal wall in a state of high stress for a long time, which will increase the amount of rib spalling and cause the vicious circle of "rib spalling-slow advancement-high stress-rib spalling." e columnar diagram of overlying strata at 1508 Working Face is shown in Figure 1(c).

Type of Rib Spalling in
ree-Soft Coal Seam. e firmness coefficient of coal is between 0.3 and 0.4. e coal is crisp and dry, with low moisture content and poor water retention. e upper coal seam is loose and broken, and the bedding is disordered; the middle layer contains gangue and falls along with the mining; the lower is relatively stable. Due to the development of cracks, joints, and beddings in the roof and coal walls, it is easy to rib spalling and roof fall, which brings great hidden dangers to the production of fully mechanized mining face. e coal about 1.5 m above is soft with poor integrity, which will cause direct roof and roof fall. Rib spalling at the 25#∼35# supports is particularly serious, as shown in Figure 2(a). Based on the analysis of the actual rib spalling on-site, the rib spalling in the three-soft coal seam is sliding-type, as shown in Figure 2

Mechanism of Rib Spalling.
For the three-soft coal seam, the cracks of coal wall with mining continue to develop under the coal's gravity and the overlying roof load. Under the joint action of the support and the surrounding rock, the coal wall was fully broken, resulting in the sliding-type rib spalling. e actual shear-sliding surface is a curved surface, which is simplified as a plane since the height difference between the coal wall and rib spalling is small, as shown in Figure 3. In Figure 3, G is the gravity of the coal; Q 1 is the gravity of the top coal seam and the direct roof; Q 2 is the load transmitted by the main roof and the overlying rock; Q 3 is the advanced supporting pressure on the coal wall of the working face; P 1 is the support protection belt resistance; P 2 is the support resistance; R x and R y are the frictional resistance and support reaction force of the broken coal by the lower coal; R is the support reaction force of the unbroken coal. ds is the unit area of the linear sliding-type surface; α is the angle between the sliding surface of the blade and the coal wall; β is the angle between the top beam of the support and the roof. N is the axial concentration force of the slidingtype surface; S is the tangential concentration force of the sliding surface.

Shock and Vibration 3
According to the Mohr-Coulomb criterion, the angle α between the sliding-type surface of rib spalling and the coal wall is as follows: where φ is the friction angle in the coal body. N and S are obtained by mechanical analysis: en, the critical condition for rib spalling of the threesoft coal seam is as follows: where μ is the friction coefficient of coal. Based on the above analysis, the main influencing factors of rib spalling in the three-soft coal seam are the load of the coal, roof and the overlying rock, internal friction angle of coal, the cohesion, the support effect of the support, the shear strength of the coal, and the rotation angle of the main roof.

Mechanism of Grouting.
Grouting reinforcement refers to the use of grouting pumps and other means to inject materials with strong permeability into the broken rock Linear sliding-type surface mass according to a certain pressure.
e following explains the mechanism of grouting reinforcement from three aspects.

Improve the Strength and Rigidity of the Coal Wall
Structure Plane. When the structural plane in the coal is relatively developed, its strength and stiffness are mainly controlled by the structural plane. When the strength and rigidity of the structural plane are low, it is relatively easy to separate, sliding or open along the weak structural plane, which leads to a decrease in the strength of the coal and an increase in its volume, thereby causing the rib spalling. After the reinforcement material is injected, the cracks in the structural plane are filled by the grouting material, which increases the cohesion and friction factor of the cracks, and thus the strength and rigidity of the coal wall are improved. It greatly reduces the probability of damage to the surrounding rock and coal wall along the weak structural plane, thereby increasing the overall strength of the coal wall of the working face and improving its physical and mechanical properties and self-bearing load capacity.

Enriched and Compacted Coal Cracks.
When the grouting materials are injected into the coal wall, following the flow characteristics of the liquid, cracks inside the coal wall are filled, thereby reducing the porosity in the coal wall and reducing or disappearing the stress concentration of the cracks. e coal mass around the cracks is transformed from a two-dimensional stress state to a three-dimensional stress state, which significantly improves the stress distribution around the cracks, transforms the mechanical failure mechanism of the broken coal and rock mass. Figure 4 is a comparison diagram of the mechanical properties of coal before and after grouting reinforcement. It can be seen from the figure that after grouting, the physical and mechanical characteristics of coal, such as coal compressive strength, cohesion, and internal friction angle, increase significantly, which leads to a reduction in the amount of rib spalling.

Transform Coal Failure Mechanism.
According to the theory of fracture mechanics, when there are cracks in the coal body, the maximum stress concentration will appear at the ends of the cracks, and the stress concentration factor I: where C is crack length; ρ is the radius of the crack end; and W is the ratio of the two to the rock mass size. erefore, the coal failure is the result of crack instability and expansion. After the cracks in the coal are filled with grouting materials, coupled with the binding effect of the grouting materials, that will greatly weaken the stress concentration at the ends of the cracks and then transform the failure mechanism of the coal and rock mass. Besides, when the coal wall is broken, the normal stress of the crack plane is 0, and when the crack is filled with grouting materials, it becomes a three-dimensional stress state. e expression of the principal stress of the Coulomb criterion is as follows: where σ 1 is the principal stress in the limit state; σ 3 is the third principal stress in the limit state, MPa; θ is the angle between the fracture surface and the largest principal plane; and R c is uniaxial compression strength.
Comprehensive analysis shows that, on the one hand, grouting reinforcement can enrich the cracks in the coal body and improve the strength and rigidity of the coal wall; on the other hand, it can reduce the stress concentration of the cracks and change the coal body failure mechanism. ereby, it can prevent and control the disaster of rib spalling.

Similarity Simulation Experiment
It can be seen from Section 3.1 that the rib spalling of the three-soft coal seam is mainly closely related to the roof load. erefore, a similar simulation experiment for the mining of the three-soft coal seam is carried out to grasp the characteristics of the roof load change.

Text Design.
Taking the 1508 Working Face of Heyang Coal Mine as the engineering background, a similar simulation experiment of three-soft coal seam grouting was set up. e experiment was adopted a plane strain model frame with an external dimension (length × width × height) � 3 × 0.2 × 1.6 m. According to the similarity theorem, the geometric similarity ratio is 1 : 250. During the work process, the model is comprehensively monitored by the support pressure sensor, as shown in Figure 5.

Analysis of Roof Stress.
According to the mining plan, an open cut (2 cm) was made at a 20 cm distance from the left boundary of the model, the support pressure sensor was installed, and the support pressure was increased to the support simulation setting force (4.73 kg). e mining distance was 1 cm each time, and the mining ends until the model 5# coal seam was 20 cm away from the right  Figure 6. It can be seen from formula (4) that the main factors that affect rib spalling include the roof load. And the roof stress at different advancing distances exceeds 25 kg, which is much larger than the initial support force of 4.75 kg, which can easily cause rib spalling. erefore, grouting reinforcement research is urgently needed.

Grouting Seepage Simulation
Model. e grouting fluid diffusion radius is closely related to the grouting pressure, and the grouting fluid diffusion radius is an important basis for judging the grouting effect, so it is very important to choose a reasonable grouting pressure. According to the grouting feedback of the engineering site, the grouting pressure of the working face ranges from 4 to 10 MPa, so FLAC 3D numerical simulation software was used to simulate four grouting pressures of 4 MPa, 6 MPa, 8 MPa, and 10 MPa. And the pore pressure distribution characteristics of grouting holes under different grouting pressures were analyzed to select a reasonable grouting pressure. e grouting model is shown in Figure 7. Figure 8, the cloud diagram is used to show the pore pressure distribution characteristics of different grouting pressure schemes along the grouting hole direction after 1 hour of grouting. e radial pore pressure has the highest pressure in the center of the borehole. e pore pressure around the borehole is circular, and the greater the grouting pressure, the smaller the area of the stress circle formed by the radial pore pressure.

Pore Pressure Distribution. As shown in
As shown in Figure 9, the pore pressure along the axial direction of the grouting hole is the largest at the center of the borehole. e farther away from the center of the borehole, the smaller the pressure, and it also decays continuously along the radius of the borehole. And the greater the grouting pressure, the smaller the range of the stress field formed by the axial pore pressure. According to the numerical simulation results, 10 MPa is the best grouting pressure.

Grouting Material.
Silicate modified polymer grouting reinforcement material (SMPGM) is a new type of mining polymer grouting reinforcement material that has developed rapidly in recent years. Kang and Feng [26] pointed out that compared with traditional polyurethane grouting materials (PGM), through the study of silicate modification, the new materials not only retain the excellent mechanical properties of PGM themselves but also avoid the shortcomings of flammability and are suitable for grouting reinforcement of the surrounding rock of roadways. Table 1 shows the physical performance comparison between SMPGM selected in this experiment and traditional PGM.

Shock and Vibration
In terms of flame retardant properties, SMPGM is completely flame retardant and does not need to add any harmful flame retardants, while traditional PGM is a flame retardant that is naturally free from fire and needs to be added with harmful elements such as chlorine, bromine, and phosphorus. Because the compressive elastic modulus of SMPGM is higher than that of traditional PGM, the apparent mechanical performance is embodied as brittleness, which is conducive to the restoration of the overall structure of the broken coal and rock mass and the enhancement of the overall bearing capacity, which is more conducive to underground using.   Besides, SMPGM also has the following characteristics: (1) Strong permeability and high bonding force; (2) High strength, excellent toughness, and it can adapt to surrounding rock; (3) It has strong applicability, and its reactivity and strength are not affected in any water environment; (4) e construction is quick and easy.

Scheme Design.
Because of the current disaster situation in the 1508 Working Face, it was decided to carry out the grouting reinforcement experiment at the 25#∼35# support. Between the 25#∼35# supports, the boreholes are arranged 1.5 m below the roof, with an elevation angle of 20°to make the bottom of the hole close to the roof of the coal seam to facilitate the reinforcement of the coal seam. e designed hole depth is 6 m, and the hole diameter is 42 mm.
Because the theoretical diffusion radius of SMPGM is 189.0 cm, the diameter is 3.78 m, the design hole spacing is 3 m to ensure the grouting effect. Grouting is carried out after the construction of one hole, and all the holes are filled in sequence. After every 4 m of the working face, drill again according to the above procedures to ensure the normal grouting and reinforcement work of the coal seam. e grouting hole layout design is shown in Figure 10.
According to the grouting design, the reinforcement area of each borehole is approximately 6 m in length, 3 m in width, and 3 m in height, and the volume of reinforcement is 6 × 3 × 3 � 54 m 3 . e porosity is calculated at about 2.2%, as the grouting volume is 54 × 2.2% � 1.08 m 3 , the unilateral usage is 1.15 t, and the usage of reinforcement material per drill hole is about 1.242 t. e next round of grouting is carried out after every 4 m of advancement of the working face, which is carried out in a cycle. e required equipment and materials are shown in Table 2.

Experiment Process.
e SMPGM is divided into A and B components. A two-liquid injection mixing gun was used to realize automatic mixing of A, B components, which can achieve the dual purpose of mixing and uniform mixing through the mixing gun and be sucked out by the grouting pump and injected into the coal by a grout pipe. e diffusion radius of the grouting materials under normal fracture opening and porosity is 1.5∼2.5 m. When a large amount of grout seeps from the borehole, it means that the grout has filled the surrounding coal, the grouting was stopped. e two grouting times and grouting positions are shown in Table 3. To ensure the grouting effect, the average grouting volume of a single borehole is 1.32 t. Figure 11 is a schematic diagram of grouting for coal wall in 1508 Working Face.

Analysis of Coal Wall Stability.
In order to understand the actual grouting reinforcement effect and reinforcement radius, a series of observation holes were drilled around the grouting hole, and the YSZ(B) optical borehole sight was used to detect the borehole to fully understand the slurry penetration information. As shown in Figure 12, in the observation hole 4.5 m away from the grouting borehole, there is a clear white grouting material solidified and hardened body, and the reinforcing material at the hole wall is circled by a red coil. Since the coal body at the free surface of the coal wall is relatively soft, the image of the reinforcement effect at a distance from the orifice is more obvious. e hardened white reinforcement material can always be seen in the entire observation hole. It can be seen that the slurry has a better penetration effect in the coal body, and the grouting influence radius is at least 4.5 m. Figure 13 shows the coal wall during the advancement process of the working face before and after grouting reinforcement. It can be seen from Figure 13(a) that the surface of the coal wall was rough before grouting reinforcement, and the coal was severely damaged, causing the remaining coal to accumulate under the support. It can be seen from Figure 13(b) that the coal wall after reinforcement can see the seepage pattern of SMPGM in the coal during the mining process, that is, along the various cracks inside the coal and the microvoids in the bedding. e flow path not only directly proves the bonding effect of the slurry but also highlights the development of internal cracks in the coal. It can be seen from Figure 13(c) that the reinforced coal wall changed from the original soft and was extremely easy to rib spalling to a flat and straight coal wall during the mining process, and the reinforcement effect was good.

Analysis of the Amount of Rib
Spalling. On September 6th, 2019, the first grouting reinforcement was carried out on the coal wall of the 1508 Working Face. e location of this grouting reinforcement was at the 25#, 27#, 29#, 31#, 33# for five supports. Figure 14(a) shows the amount of rib spalling before and after the first grouting. e average rib spalling before grouting is 0.47 m at the working face. Since the grouting started on September 6th, as the working face advanced, the average rib spalling will be reduced to 0.20 m in the next 8 days, which is a decrease of 57.45%. On September 9th, 2019, the second grouting reinforcement was carried out. e location of this grouting reinforcement was 34#, 37#, 40# for three supports. Figure 14(b) shows the amount of rib spalling before and after the second grouting. It can be seen from the figure that the average amount of rib spalling before grouting at the working face is 0.64 m.
Since the grouting started on September 9th, as the working face advanced, the average amount of rib spalling will be reduced to 0.33 m in the next 7 days, which is a decrease of 48.43%. Compared with the first time, the amount of rib spalling has been reduced. On the one hand, the most serious area of rib spalling is located between the 25#∼35# supports, and on the other hand, it is due to the second the number of grouting drilling and amount of grouting materials are relatively small.

Analysis of the Mining Height.
e changes of the mining height before and after grouting at the 1508 Shock and Vibration 9 Working Face were counted and are plotted in Figure 15. From Figure 15(a), it can be seen that before the first grouting, the mining height was reduced due to the severe rib spalling, and the resource recovery rate was low. e average mining height of the working face was 2.72 m, and the average mining height after grouting was 2.87 m. Figure 15(b) shows that the average mining height of the working face before the second grouting was 2.62 m, and the average mining height after the grouting was 2.85 m. Comprehensive analysis shows that the mining height of the working face was increased significantly after grouting reinforcement. Due to the increase in the strength and cohesion of the working face coal wall, the amount of rib spalling was reduced, the roof fall was effectively improved, and the coal mining height was increased, which caused a reduction in resource waste.    Shock and Vibration 13

Conclusions
(1) By analyzing the status of rib spalling in the 1508 Working Face of Heyang Coal Mine, it is concluded that rib spalling in the three-soft coal seam is mainly sliding-type. e mechanical model of sliding-type rib spalling was established, and the main factors affecting rib spalling were determined. And the mechanism of grouting reinforcement technology was analyzed to prevent and control rib spalling from three aspects.
(2) rough similar simulation experiments, the evolution law of roof load under three-soft coal seam mining conditions was obtained. A numerical simulation experiment was used to analyze the pore pressure distribution characteristics of grouting holes under different grouting pressures, and the optimal grouting pressure was determined to be 10 MPa. (3) On-site grouting test was carried out, and SMPGM was injected into the broken coal wall. e stability of the coal wall, the amount of rib spalling, the mining height were analyzed before and after the two grouting sections of the 1508 Working Face, and the results showed that after grouting reinforcement, the coal wall became flat and straight, the amount of rib spalling was significantly reduced, and the mining height was increased. e disasters of coal rib spalling have been effectively controlled. 9.01 9.02 9.03 9.04 9.05 9.06 9.07 9.08 9.09 9.10 9.11 9.12 9.13 9.14 9.04 9.05 9.06 9.07 9.08 9.09 9.10 9.11 9.12 9.13 9.14 9.16 9.15

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.