Mechanical Mechanism of Water Injection to Enhance the Stability of Soft Coal

)e physical and mechanical properties of soft coal body constitute one of the most important factors inducing coal wall spalling. In order to explore the mechanical essence of coal instability disaster and stability enhancement of water injection, the 7 coal in Huainan mining area is taken as the research object. Firstly, the distribution characteristics of coal particle size, point-load strength, original water content, microstructure characteristics, and shear strength of coal under different water contents are measured by laboratory tests.)en, based on the test results, the cementationmorphology and force evolution law of granular coal water in coal body are analyzed using liquid bridge theory.)e results show the following: (1)With the increase of particle size, the mass ratio of granular coal increases gradually. )e percentage of particle coal with particle size less than 2.5mm accounts for 47.157%, fractal dimension is 2.172, and uniaxial compressive strength and tensile strength are 3.822MPa and 0.165MPa, respectively. (2))e coal body is dry (the original moisture content is 1.336%), containing a large number of loose particles, pores, fissures, and other microfabrics. )is “low water content and multiporosity” feature is the essential reason for its low strength, fragmentation, and instability and disaster. (3) In the process of water content increasing from 0.966% to 26.580%, the shear stress-displacement curve of coal body gradually changes from softening type to hardening type, and the failure type transitions from brittleness to ductility.)e cohesive force increases first and then decreases, while the angle of internal friction almost has no change. (4) After reasonable water injection, the shape of liquid bridge in coal body changes into capillary tube, and the liquid bridge force reaches themaximum value, which transforms from a highly unstable bulk to a stable continuum.)e research results have important theoretical significance and practical value for the safe and efficient mining of soft coal seams.


Introduction
Coal slip and roof fall often occur when mining the work face with large heights, large inclination angles, and soft coal seams, which seriously affect the mining efficiency, cause economic losses, and threaten human safety. In order to control the stability of the surrounding rock of the working face, a lot of research on the characteristics and control techniques of instability of coal wall have been carried out. Wang et al. [1], Xu et al. [2], and Pang et al. [3] found that the plastic failure zone in front of the coal wall in the highcutting fully mechanized caving mining is obviously greater than that of the large-cutting fully mechanized mining. A suitable protective plate structure and a reasonable protective method are proposed by analyzing the characteristics of coal wall slabs in the working face with super large mining height. Zhang [4] established a mechanical model for analyzing the slab side of the coal wall in a fully mechanized caving face with large mining height in extra-thick coal seams and revealed the mechanical mechanism of the slab side. Based on the main influencing factors of the flanks, reasonably determining the height of the machine mining, appropriately increasing the advance speed, and determining a reasonable coal cutting speed were put forward to prevent coal siding. Li et al. [5] established a mechanical model of the coal wall sliding slab and judged the stability of the coal body on the sliding surface. Jin et al. [6] established a cusp catastrophe model of coal wall instability and analyzed the conditions of the slabs. Reducing the pressure on the coal wall, making full use of the protection board, and strengthening the coal wall were proposed to prevent coal siding. Wu et al. [7] researched the mechanism of coal wall fragmentation and discussed the catastrophic mechanism of the "support-surrounding rock" system on the background of fully mechanized caving face with large inclination soft coal. Hua and Xie [8] found 12 factors of the coal wall slabs based on limit equilibrium theory. Dip mining, hydraulic support with high working resistance, speeding up the advancement of the working face, strengthening the coal wall, and moving frame with pressure in time were proposed to control the coal slab. Ning [9] proposed the coal wall force analysis model and the mechanism of coal slab gangs as revealed: under the action of mining stress, the coal wall first undergoes minor flexural deformation. When the mining stress reaches the peak value, cracks occur at the upper part of the coal wall at 0.35 times the mining height, and then all the coal walls fall off. Yin et al. [10] established a mechanical model of coal wall instability with different slab trajectories in a fully mechanized caving face with high-cutting height and a mechanical model of coal slab siding with weak inclusions. In addition, a safety evaluation system for coal slab slabs was developed.
e previous research mainly focused on the working face with large height and hard coal seam. e measures to prevent coal slip were proposed on the basis of clarifying the factors influencing the instability of the coal wall. However, the physical and mechanical properties of the soft coal must be one of the most important factors inducing the coal slip. Field engineering shows that water injection into the coal seam can improve the physical and mechanical properties of coal and prevent accidents, which has been successfully applied in many coal mines [11][12][13][14]. In order to study the mechanical characteristics of soft coal after water injection, experimental research on the strength and deformation characteristics of coal with different water contents was carried out. e compression test of 8# coal of the Luling Coal Mine in Huaibei under different moisture content was carried out with TSZ10 triaxial instrument, and it was found that the cohesion and shear strength of the coal can be improved by injecting reasonable water into the coal [15]. e uniaxial and triaxial compression tests of the coal body of the Xinzhuangzi Coal Mine in Huainan under different water content conditions were also carried out. Results show that the strength and deformation characteristics of the coal are related to the water content and the strength of the coal increases after reasonable water injection [16]. Wang et al. [17] measured the angle of repose of coal particles with different particle sizes and water content. e relationship between the angle of repose and the fluidity of coal powder and the cohesion between particles was discussed. It is also found that the physical and mechanical properties of soft coal can be significantly improved after water injection, which has a positive effect on controlling the stability of coal. However, the mesomechanical mechanism of water injection to enhance coal stability is still unclear. erefore, in the present study, the particle size distribution, point-load strength, water content, and mesostructural characteristic of loose coal in Huainan were tested. e shear strength of coal under different water contents was tested to analyze the physical and mechanical properties of soft coal. e cementation form of coal particles and water in different water-bearing states were discussed based on the liquid bridge force theory, and the mechanical mechanism of water injection to enhance coal stability was revealed.

Preparation for Raw Coal.
e 7# coal seam in Huainan is a typical soft coal seam, which is rich in folds and varying in shape. e coal body with well-developed structure has much band-shaped, granular, scaly, and powder-like primary fissures. ere is almost no cohesiveness in the extremely fragile coal. e cut loose coal is extremely small, with many large pores, and it is muddy ( Figure 1). During the mining of the 7# coal seam, large-scale splinters and roof fall accidents occurred frequently, which seriously affected production safety. erefore, the 7# coal in the Huainan mining area was selected in the present study [18].

Particle Size Distribution.
Standard sieves with diameters of 0.10 mm, 0.25 mm, 0.45 mm, 0.65 mm, 1.25 mm, 2.50 mm, 5.00 mm, and 10.00 mm were used to separate 3 groups of coal in turn and weigh the coal in each particle size range.

Point-Load Strength.
e soft raw coal cannot be made into a standard sample with a diameter of 50 mm and a height of 100 mm. erefore, the HDH-1 ( Figure 2) pointload device with a maximum loading force of 50 kN and an accuracy of 0.001 kN was used to test the point-load strength of raw coal. In order to ensure the reliability of the test, the height (h) and width (b) of the smallest cross section through the loading point while loading were within the range of 40 ± 5 mm and 50 ± 5 mm, respectively, and the sample length is 1.5 times the minimum cross-sectional height of the loading point ( Figure 3). L and P in Figure 3 represent the sample length and load, respectively. e point-load strength of 15 coal blocks was tested in this study.

Water Content.
Firstly, weigh 10 coal blocks weighing more than 50 g and then dry them at 105°C∼110°C for 24 hours. Secondly, cool the 10 coal blocks and weigh them again.

Mesostructural Characteristic.
A coal block with a relatively flat surface and a size of 5 mm × 5 mm × 10 mm is used to observe the surface image by S-3000N scanning electron microscope.

Shear Strength of Coal with Different Water Contents.
e dry raw coal was fully crushed and then sieved with standard soil sieves to screen out the coal with a particle size of 0.10∼0.25 mm. Eight kinds of coal samples with different water contents were prepared by mixing the same weight of coal particles and different weight of water. ere are 4 coal samples with the same moisture content. e STSJ-5 smart electric quadruple direct shear apparatus ( Figure 4) was used to test the shear strength of coal. e shear rate is 0.02 mm/min, and the normal stress was set to 50 kPa, 100 kPa, 200 kPa, and 400 kPa in turn for coal samples with the same water content. e reading of dynamometer was recorded in real time during the test. e broken coal sample was collected after the test, the actual water content of the coal sample was measured, and the average water content of the same sample was used as the effective experimental data.

Fractal Dimension of Coal Particle
Size. According to the fractional fractal theory, the fractal dimension (D) [19] of coal particle size can be defined as where a is the slope of (M R /M × 100, R) in double logarithmic coordinates, M R is the cumulative weight of fragments with a particle size smaller than R, g; M is the total weight of coal, g.    [20]. e point-load index (I S ) can be defined as follows:

Point-Load Strength
where P max is the peak load, N. e point-load index of irregular specimens can be corrected as follows: e uniaxial compressive strength (R c ) of coal can be written as follows: e uniaxial tensile strength (R t ) of coal can be written as follows:

Shear Strength.
e shear strength (S) of coal can be given as follows: where C is the coefficient of force measuring ring, 1.499 kPa/ 0.01 mm, and R τ is the peak reading of dynamometer, 0.01 mm. Taking the normal pressure (p) and shear strength (S) of the same moisture content sample as the abscissa and ordinate, respectively, the linear fit line was then drawn on the coordinate axis based on different (p, S). e internal friction angle and cohesion can be represented by the inclination of the fitted straight line and the intercept of the line on the vertical axis, respectively.  Figure 5). Among them, the content of coal particles in the range of 2.5∼5 mm is the largest, which is 31.18%, followed by the particle coal with a particle size of 5∼10 mm, which is 21.66%, and the content of coal particles smaller than 0.1 mm is the smallest, which is 1.61%. Generally speaking, the content of coal particles increases with the increase in particle size in the test range. e broken coal contains a lot of fine particles. e content of coal particles with a particle size of less than 2.5 mm is 47.16%. e fractal dimensions of the particle size of the three groups of samples are 2.188, 2.324, and 2.004, respectively, with an average value of 2.172.  (Table 2).

Water Content.
e maximum water content of raw coal is 1.619%, the minimum is 1.131%, and the average is 1.336% (Table 3). On the whole, there are initial defects of different shapes and sizes in the coal body. e distribution of particles, pores, and cracks is uneven and discontinuous, and it contains a large number of loose clusters and flocculent structures. ere are a large number of loose particles, pores, and cracks. e raw coal has the apparent characteristics of high porosity.
In summary, the raw coal is dry and has extremely low macrostrength and a highly porous mesostructure, which is the essential cause of easy instability. Figure 7 shows the shear stress-displacement curve of extremely soft coal with different water contents. It can be seen that when w � 0.966%, as the displacement increases, the shear stress gradually increases and then decreases significantly after reaching the peak. e softening characteristics of coal samples under various normal stresses are obvious, and they have certain brittle failure characteristics. e coal sample with a water content of 3.841% has certain softening characteristics when the normal stress is 50 kPa or 100 kPa. However, when the normal stress is 200 kPa or 400 kPa, as the displacement increases, the shear stress increases slowly and then tends to a progressive value, and the shear stress has no obvious peak value. e hardening characteristics and ductile failure characteristics of the coal specimens gradually became prominent. For a coal sample with a water content of 7.688%, the shear stress-displacement curve shows softening characteristics when the normal pressure is 50 kPa and shows obvious hardening characteristics when the normal pressure is 100 kPa, 200 kPa, or 400 kPa. e shear stress-displacement curves of coal specimens under different normal stresses all have certain hardening characteristics when the water content reaches 12.957%, 17.888%, 20.847%, 22.072%, or 24.526%. In general, with the increase of water content and normal pressure, the shear stress-displacement curve gradually transforms from softening to hardening type, and the failure type slowly transitions from brittleness to ductility. e shear strength of coal under different water contents increases with the increase of normal pressure (Table 4). e internal friction angle of the coal does not change significantly with the water content, being within the range of 12.54°∼14.72°. However, as the water content increases, the cohesion of coal first increases and then decreases (Figure 8). When the water content is 0.966%, the cohesion of coal is the smallest, which is 1.89 kPa, and when the water content is

Discussion
e force inside the coal body after water injection mainly includes two parts: one is the cohesion between particles, and the other is the liquid bridge force generated from the liquid bridge formed by particles and water [21]. e liquid bridge force is caused by capillary pressure, surface tension, and viscous force. ere is no obvious change in the cohesiveness of coal under different water contents. e change of the liquid bridge force with the water content is the main reason for the evolution of the macromechanical characteristics of coal. erefore, it is necessary to clarify the shape of the liquid bridge and the change law of the liquid bridge force inside the coal body to reveal the mechanical mechanism of water injection modification to increase the stability of the coal body. e liquid bridge force can be characterized by the liquid saturation (s) as follows: where V L is the volume of water, cm 3 , and V T is the pore volume of coal, cm 3 . e liquid saturation is determined by water content (w) and pores of coal. e water content of coal can be defined as follows: where M 1 is the weight of raw coal, g, and M 2 is the weight of dried coal, g. e water content of coal can be further given as follows: where M L is the weight of water in the coal, g; ρ 2 is the density of dried coal, g/cm 3 ; and V is the volume of coal, cm 3 . erefore, the volume of water can be written as follows: where ρ 1 is the density of water, 1 g/cm 3 . e porosity of coal (n) can be given as follows: Combining (7)- (12), the liquid saturation can be deduced as follows: It can be found that the liquid saturation increases in direct proportion with the water content of the coal when other conditions are the same.
It is assumed that there are two kinds of particles inside the coal: large and small particles. Based on the linear relationship between liquid saturation and water content, the change of the liquid bridge composed of coal particles and water with the water content can be divided into 4 stages ( Figure 9).
Assuming that f 1 is the liquid bridge force between small particles, f 2 is the liquid bridge force between large particles, and f 12 is the liquid bridge force between large and small particles, the total liquid bridge force (F) can be written as follows: (13) Table 5 shows the variation law of liquid bridge force with water content in different stages. e following can be found: State I: the w and s values are extremely low. e liquid bridges between small particles and between large and small particles are pendulum-shaped or cord-shaped, and the liquid bridges between large particles are pendulum-shaped. With the increase of water content, f 1 , f 12 , f 2 , and F values all increase slowly, and the macroscopic strength of the coal body increases slowly. State II: the w and s values are quite low. e liquid bridges between small particles and between large and small particles are capillary-shaped, and the liquid bridges between large particles are cord-shaped. With the increase of water content, f 1 , f 12 , f 2 , and F values all increase, and the macroscopic strength of the coal body increases correspondingly. State III: the w and s values are moderate. e liquid bridges between small particles and between large and small particles are muddy, and the liquid bridges between large particles are capillary-shaped. With the increase of water content, f 1 , f 12 , f 2 , and F values all increase, and the macroscopic strength of the coal body increases correspondingly. With the increase of water content, f 1 and f 12 gradually decrease, f 2 increases significantly, and the increment of f 2 is much larger than the decrease of f 1 and f 12 . F reaches the peak, the total force inside the coal body reaches the maximum value, and the macroscopic strength is the highest. State IV: the w and s values are quite high. e liquid bridges between different particles are muddy. With the increase of water content, f 1 , f 12 , f 2 , and F values all decrease obviously, and the macroscopic strength of the coal body decreases correspondingly. e mechanical mechanism of water injection to enhance coal stability can be summarized as follows: the loose raw coal contains a large number of fine particles. e liquid bridge composed of soft coal particles and water is mainly capillary-shaped, the liquid bridge force reaches the maximum, and the total internal force is the strongest after reasonable water injection into the coal. Water can promote coal particles which can be agglomerated and cemented by water, causing the coal body to transform from an unstable 8 Advances in Materials Science and Engineering loose body to a stable continuum. e strength and stability of the coal body are thus improved.

Conclusions
e physical and mechanical properties of 7# coal in Huainan were analyzed, and the direct shear test of coal under different water contents was carried out. e mechanical mechanism of water injection in coal to increase its stability was discussed based on the liquid bridge force theory. e main conclusions are as follows: (1) e particle size distribution of the soft coal has obvious regularity, which is in the state of a loose medium and low strength. With the increase of particle size, the weight ratio of coal gradually increases. e weight percentage of particle coal with a particle size of less than 2.5 mm is 47.157%, and the fractal dimension is 2.172. e uniaxial compressive and tensile strengths of the raw coal are 3.822 MPa and 0.165 MPa, respectively. (2) e water content of the raw coal is 1.336%, which is relatively dry. e raw coal contains many loose clusters and flocculent structures. ere are a large number of loosely arranged particles, pores, and cracks, and the raw coal shows the characteristics of high porosity, which is the essential cause of low strength, brokenness, and easy instability and disaster.
(3) e strength and deformation characteristics of the soft coal body change obviously with the water content. In the test water content range (0.966%∼ 26.580%), with the increase of water content, the shear stress-displacement curve of coal gradually transforms from softening to hardening type, and the failure type transitions from brittleness to ductility slowly. e internal friction angle of the coal does not change significantly with the water content; however, the cohesion presents a trend of increasing first and then decreasing. e cohesion reaches the peak value when the moisture content is within the range of 17.888%∼22.072%. erefore, the coal body has the highest strength and the strongest stability when the water content is reasonable. (4) e mechanical mechanism of water injection to enhance coal stability can be summarized as follows: the liquid bridge composed of soft coal particles and water is mainly capillary-shaped, the liquid bridge force reaches the maximum, and the total internal force is the strongest after reasonable water injection into the coal. Water can promote coal particles which can be agglomerated and cemented by the water, causing the coal body to transform from an unstable loose body to a stable continuum. e strength and stability of the coal body are thus improved.

Data Availability
e experimental results used to support the findings of this study are included within the article.