Study on Lateral Deformation and Failure Characteristics of Coal Based on Different Confining Pressures

To study the lateral deformation characteristics of coal under different confining pressures, coal compression experiments with confining pressures of 0MPa, 3MPa, 5MPa, and 7MPa were conducted under the same loading rate by using the TAW-2000 electrohydraulic servo rock mechanics experimental machine. ,e results of the study showed the following: at the initial stage of loading, the lateral strain of coal was about 12.22%–46.9% of the axial strain at the elastic deformation stage and 41.18%–64.96% of the axial strain at the inelastic deformation to peak stress stage. Compared with the experiment under 0MPa confining pressure, the growth rate of the lateral strain of the coal under 3MPa, 5MPa, and 7MPa confining pressures was much smaller than that of the corresponding axial strain. When the coal was damaged under different confining pressures, the lateral strain was maintained at about 0.6×10. Based on the field verification, we proposed that the lateral strain during the coal failure and the nonlinear region of the lateral axis ratio changing with time can be used as potential parameters for predicting the coal failure.


Introduction
Because of the impact of excavation, the coal wall of the coal roadway and the coal wall of the working face change from the initial three-way force to the one-way force so that the coal wall of the roadway and the coal near the coal wall of the working face bear the weight of rock strata above the excavation space. e weight causes an increase in the supporting pressure of the coal near the coal wall of the roadway and the working face, causing the coal near the coal wall to expand to the side of the goaf, ultimately resulting in the spalling of roadway coal wall and working face coal wall.
at is, under the support pressure, the coal body on one side of the goaf of the coal wall will produce lateral expansion deformation. In practical engineering applications, it is of great significance to study the lateral deformation and lateral deformation characteristics of coal under different surrounding rock pressures to control the expansion deformation of coal rib, to guide roadway support, and to grout reinforcement of coal rib. Scholars in China and abroad have carried out a lot of research on the lateral deformation characteristics of rocks. Based on the triaxial test results of granite under different confining pressures, Jingxiang et al. analyzed the lateral deformation characteristics of granite and proposed that the brittleness characteristics of rock samples can be comprehensively reflected from the change law and magnitude of brittleness [1]. Guojun et al. believed that, under the continuous action of cyclic loading and unloading, the failure of rock is a cumulative process. e growth trend of axial and lateral cumulative irreversible strains under cyclic loading is linear, and the increase of confining pressure can effectively limit the lateral cumulative irreversible strain of rock [2]. Based on the triaxial unloading seepage test results under different initial confining pressures and unloading rates, Rubin et al. obtained the lateral deformation characteristics. In these tests, volumetric dilatancy characteristics of sandstone changed significantly with the increase of initial confining pressure and unloading rate during unloading failure of sandstone [3]. Using an MTS815 electrohydraulic servo-controlled rigid testing machine, Memetyusup et al. conducted unloading/decreasing confining pressure tests with constant axial deformation. e test results indicated that, with the decrease of the confining pressure, the lateral expansion of rock samples continued, and the axial stress decreased [4]. Dong et al. proposed that the axial stress of rock can be determined by its axial strain before the peak, and it is controlled by lateral deformation after the peak, and it gradually decreases to residual strength with the increase of lateral deformation [5].
Compared with rock, coal has different characteristics in different directions. It also has low strength and undergoes a large deformation under a given loading process. Zhenqian et al. investigated the lateral deformation characteristics of coal under different loading rates by using the TAW-2000 electrohydraulic servo rock mechanics experimental system. ey concluded that the lateral strain was the same when the first axial stress dropped under different loading rates, and the lateral strain was used as the control variable to predict the failure of coal [6]. Yongping carried out uniaxial and triaxial compression tests on typical soft and hard coal rock samples by MTS815 electrohydraulic servo rock mechanics testing machine. e results demonstrated that the strength, yield axial strain, and lateral deformation characteristics of soft and hard coal rock were greatly different due to the different internal structures of soft and hard coal rock and the different development degrees of pores and cracks. e internal damage evolution characteristics of coal and rock can be characterized by lateral plastic deformation damage [7]. Mingqing observed that there was a typical step-like drop after the peak stress of hard coal with impact tendency, and the significant increase of lateral axis ratio was mainly after the peak stress [8]. Tarasov and Potvin believed that the existence of surrounding rock can affect the brittleness of rock [9]. Guo et al. investigated the lateral deformation of strip coal pillar in Tangkou Coal Mine with a mining depth of over 1000 m. ey concluded that, in deep mining, the accumulation and release of energy cause discontinuous damage in the heterogeneous coal mass, and the lateral deformation of coal pillar shows discontinuity and mutation characters [10][11][12].
In summary, the lateral deformation characteristics and laws of rock under triaxial compression [13][14][15] and the axial deformation characteristics of coal under different confining pressures and loading speeds [16,17] have been investigated in depth in China and abroad, and some great results have been achieved. However, there are few studies regarding the lateral deformation of coal under the same loading rate and different confining pressures. In this study, the TAW-2000 electrohydraulic servo rock mechanics testing machine was employed to examine the deformation characteristics of coal samples under the same loading rate and different confining pressures. Furthermore, the influence of the confining pressure on the lateral deformation characteristics of the coal body was analyzed. e research results combined with the actual measurement results in the underground mine are of great significance to the analysis of the deformation characteristics of the coal body after excavation at a certain buried depth and the selection of reasonable support schemes. At the same time, they can also be utilized as a basis for evaluating the damage of the coal body [18][19][20][21].

Sample Preparation and Test Method
e TAW-2000 electrohydraulic servo rock mechanics testing machine was used in this research. e overall stiffness of the test frame was 10 GN/m, the maximum axial force was 2000 kN, and the maximum confining pressure was 100 MPa. e dynamic strain monitor and the triaxial test pressure chamber are shown in Figure 1.
e coal samples were selected from the standard specimen made of coal blocks in a coal mine in Shanxi Province of China. e coal blocks obtained from the underground were processed into standard cylindrical coal samples with a diameter of 50 mm and a height of 100 mm by using drilling, cutting, and grinding [22][23][24]. e experimental coal samples are shown in Figure 2. e experimental data were monitored by a high-precision extensometer. Since the extensometer could not resist large deformation, the axial deformation control was adopted in the experiments. e loading speed was set to 0.05 mm/min, and the test confining pressures were 0 MPa, 3 MPa, 5 MPa, and 7 MPa, respectively. e data of time, load, displacement, stress, axial strain, radial strain, and confining pressure change in the whole process were obtained during the tests [25][26][27].
To reduce the influence of the heat shrink tube on the experimental results, the same heat shrink tube was employed to package all coal samples in the experiment. e coal samples after the experiment are exhibited in Figure 3.

Experimental Data Processing.
(1) Peak strength (σ c ): on the stress-strain curve, the maximum stress (i.e., peak stress) [28,29] is obtained through the following formula: where P max is the maximum axial load (N) in the test process and A is the compressed area (m 2 ) of the sample.      In the pressure consolidation and compaction stage of cracks, the microcracks inside the coal body were compacted, and the coal had a negative strain in the radial direction, which was characterized as a radial shrinkage. e experimental results indicated that the radial strain developed forward when the stress exceeded 0.83 MPa at 0 MPa confining pressure, 1.36 MPa at 3 MPa confining pressure, 1.78 MPa at 5 MPa confining pressure, and 1.95 MPa at 7 MPa confining pressure. e coal body entered the linear elastic stage after the pressure consolidation and compaction stage. In this stage, the axial strain and radial strain of the coal body increased according to a certain slope, and the changing trend of the two was consistent. At this stage, the axial and radial stressstrain curves were both straight lines (i.e., the elastic modulus of the coal body was a fixed value). With the increase of stress, the coal entered the nonlinear elastic stage. Compared with the linear elastic stage, the elasticity of coal decreased with the increase of stress, and the strain increased with the increase of stress, which was more obvious with the increase of confining pressure.
With the continuous increase of stress, the weak surface inside the coal body first broke, and the coal body entered the plastic deformation stage. From a large number of experimental test results, we found that there were two types of failure in the loading process of the coal body: (1) e coal body underwent a process of destruction-stabilization-redestruction-restabilizationredestruction caused by the destruction of the weak surface. e stress gradually reached the maximum, and, finally, the main weak surface in the coal body was destroyed, so that the coal body lost its bearing capacity.
(2) When the structure of the coal sample was good, there was no stage of restabilization-redestruction, and the failure bearing capacity of the coal from the main weak surface diminished directly. It can be observed from Figure 4 that the first type of failure was more obvious when the confining pressure was 0 MPa, and when the confining pressure was nonzero, the coal body mostly showed the second type of failure.
After the peak failure, the coal sample entered the postpeak failure stage, in which both the axial strain and the radial strain increased sharply.

Lateral Deformation Characteristics.
To clearly describe the lateral deformation characteristics of the coal under the same loading speed and different confining pressures, the total stress-strain curves of the coal under different confining pressures were plotted. It can be observed from Figure 4 that the axial stress-lateral strain curve and the axial stress-axial strain curve of coal samples under different confining pressures maintained the same changing trend in all four stages. e lateral axis ratio of coal samples under the same loading speed and different confining pressure [31] is shown in Figure 5. e four following stages were observed during the performed tests: (1) e pressure consolidation and compaction stage of the internal cracks in the coal body: the coal body had radial shrinkage in all four cases. e minimum value of lateral deformation at this stage appeared in the uniaxial compression state and was dominated by radial shrinkage. is showed that, no matter whether or not there was confining pressure, there was a pressure consolidation and compaction process in the radial direction of internal cracks in the coal under axial load. e duration of this stage was short, lasting about three seconds.
(2) Elastic stage: according to Figure 4, the lateral strain range of the coal under 0 MPa confining pressure was 0.091 × 10 −3 -3.44 × 10 −3 , and the corresponding axial strain range was 0.377 × 10 −3 -4.01 × 10 −3 . us, the lateral strain was about 24.1%-85.8% of the axial strain. When the confining pressure was nonzero, the range of the lateral strain and the axial strain of the coal body increased, and the range of the lateral strain of the coal body was the same regardless of the confining pressure, and the lateral strain range was 0.045 × 10 −3 -5.06 × 10 −3 . e range of the axial strain under nonzero confining pressure was 0.368 × 10 −3 -12.44 × 10 −3 . erefore, the lateral strain range was approximately 12.22%-46.9% of the axial strain range under a nonzero confining pressure. At this stage, the lateral strain was at a low level, and the specimen was mainly subjected to axial compression. erefore, the lateral strain of the specimen was 41.18%-64.96% of the axial strain under a nonzero confining pressure. at is, the lateral deformation was slightly smaller than the axial strain at 0 MPa; however, it was significantly smaller than the axial deformation when the confining pressure was nonzero. (4) Postpeak expansion stage: the damage of the coal sample at this stage was chiefly manifested as the damage caused by the development of the weak  surface inside the coal body towards the failure surface. At this stage, the strength of the coal body decreased rapidly, and the lateral strain increased quickly. e lateral strain at this stage accounted for more than 50% of the total lateral strain.

Engineering Applications
To verify the reliability of the experimental results, the coal wall of a working face of the No. 5 coal seam of the mine was selected for validation. e deformation of the roadway coal wall in the vertical and horizontal directions was monitored by drilling peep and abscission layer instruments. e experimental location is shown in Figure 9. e test was divided into two groups: In the first group, an anchor net was used with certain confining pressure + steel belt and U-shaped support, respectively, before the lateral strain of the coal wall became equal to 41.18% of the axial strain. In the second group, after the lateral strain of the coal wall became equal to 64.96% of the axial strain, the anchor net + steel belt and U-shaped support were used to observe the deformation and failure of the coal wall [32][33][34]. e destruction of the coal wall is shown in Figure 10 and the monitoring results are displayed in Figure 11. e field test results showed the following: (1) Supporting before reaching the lateral axis ratio of the failure in the experimental study effectively reduced the deformation and failure of coal wall and increased the stability of coal wall. (2) e existence of confining pressure limited the   Shock and Vibration 9 deformation of the coal wall and improved the stability of the coal wall.

Conclusion
(1) e coal bodies under different confining pressures experienced the whole loading process that ultimately led to the destruction of the coal body. e radial deformation of the coal bodies experienced the internal crack consolidation and compaction stage, the elastic deformation stage, the nonlinear elastic deformation stage, and the postpeak plastic stage. is means that the lateral deformation of the coal body underwent four stages, from shrinkage deformation to linear growth and then to postpeak expansion.
(2) e lateral axis ratios of coal under different confining pressures were different, which could be divided into two categories: under 0 MPa confining pressure, the lateral strain of the coal was about 85.5%-96.35% of the axial strain when it was destroyed. When the confining pressure was nonzero, the lateral strain of the coal mass was about 41.18%-64.96% of the axial strain. When the coal body was destroyed, the lateral strain of the coal was maintained at about 0.6 × 10 −2 . (3) e field test results indicated that the lateral axis ratio can be used as a potential parameter for evaluating the deformation and failure of coal wall and as a basis for designing the support parameters. (4) e lateral deformation characteristics of coal under different confining pressures explain why when the same coal seam is mined from shallow to deep, the risk of gas outbursts in coal bodies and the risk of coal wall slicing accidents increase, and the necessity of timely support when excavating underground coal mines becomes more prominent.

Data Availability
e data used to support the findings of the study are available within the article.

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