Research On Water-Immersion Softening Mechanism of Coal Rock Mass Based on Split Hopkinson Pressure Bar Experiment

: The coal mining process is affected by multiple sources of water such as groundwater and coal seam 10 water injection. Understanding the dynamic mechanical parameters of water-immersed coal is helpful to the safe 11 production of coal mines. The impact compression tests were performed on coal with different moisture contents 12 by using the ϕ 50 mm Split Hopkinson Pressure Bar (SHPB) experimental system, and the dynamic characteristics 13 and energy loss laws of water-immersed coal with different compositions and water contents were analyzed. 14 Through analysis and discussion, it is found that: (1) When the moisture content of the coal sample is 0%, 30%, 15 60%, the stress, strain rate and energy first increase and then decrease with time; (2) When the moisture content of 16 the coal sample increases from 30% to 60%, the stress "plateau" of the coal sample disappears, resulting in an 17 increase in the interval of the compressive stress and a decrease in the interval of the expansion stress. (3) The 18 increase of the moisture content of the coal sample will affect its impact deformation and failure mode. When the 19 moisture content is 60%, the incident rod end and the transmission rod end of the coal sample will have obvious 20 compression failure, and the middle part of the coal sample will also experience expansion and deformation. (4) The coal composition ratio suitable for the impact experiment of coal immersion softening is optimized.


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Owing to the development of the coal mining industry, the depth of coal mining continues to increase, and the 26 dynamic disasters of coal rock in coal mines are becoming more and more serious [1] . Understanding the dynamic 27 mechanical parameters of coal rock is of great significance for preventing and reducing the occurrence of disasters. [2] . 28 Coal is a porous, non-uniform and discontinuous medium composed of multiple mineral components. When the 29 underground water level in the mining area rises, the coal is immersed in the water, and the free water penetrates into the pores and fissures of the coal. This promotes the expansion and connection of pores and fissures, changes the water content and permeability of the coal and rock mass, and reduces or even destroys the bearing capacity and 32 strength of coal [3,4] . 33 In order to develop laboratory coal samples consistent with the properties of raw coal under complex geological 34 conditions, a large number of researchers have studied the composition, production process, and mechanical 35 properties of the formed coal samples. GU et al. [5] investigated the influence of coal particle size on briquette through 36 forming experiments on the change of particle size before and after the forming of pulverized coal and the forming 37 of raw materials with different particle sizes. XU et al. [6] pointed out that the smaller the particle size of the coal 38 sample, the larger the fractal dimension of the pore structure of the briquette, and the higher the mechanical strength 39 disaster prevention and dust reduction. 94 2 Design of dynamic mechanical test of water immersed coal 95 2.1 Split Hopkinson pressure bar test system 96 The SHPB test device (shown in Figure 1) includes a pressure bar, super dynamic strain gauges, an oscilloscope 97 and a data acquisition system. The diameter of the strut is 50 mm, and the material of the bullet, strut, and absorption 98 rod are the same. The elastic modulus is 206 MPa, the density is 7850 kg/m 3 , the bullet length is 500 mm, the 99 incident and projection rod length is 3000 mm, and the wave velocity is 7143 m/s. The principle of the SHPB test 100 is as follows. At different impact speeds, the punch acts on the incident rod, and a stress wave is generated on the 101 incident rod. After the stress wave contacts the specimen, the reflected wave and the incident wave are generated 102 on the incident rod and the transmission rod, respectively, and the data acquisition system records the data of the 103 strain gauges on each compression bar. The pulverized coal was taken from the N2808 working face of the 8# anthracite coal seam of Yuyang Coal 109 Mine of Chongqing Songzao Coal and Electricity Co., Ltd. The specific parameters of the coal mass are shown in 110 In order to study the relationship between the immersion softening mechanism and mechanical parameters of 113 coal, coal samples with different mechanical properties are prepared by configuring different coal sample 114 components in this paper. Cement, sand, activated carbon, and coal powder of different particle sizes are used to 115 prepare coal samples with a relatively uniform pore and fissure structure compared with that of the raw coal [28] . 116

Preparation of water-immersed coal sample for dynamic mechanics test
118 In order to study the mechanism of coal immersion softening, and to make the effect of immersion softening 119 more obvious, three immersion schemes with a large gradient are designed: dry coal sample, coal sample with a 120 moisture content of 30% and coal sample with a moisture content of 60%. In order to avoid the influence of residual 121 moisture in the production process, the coal sample in Section 2.1 is first dried, and then the coal sample that needs 122 to be immersed is weighed. During the immersion process, the water does not need to be pressurized, that is, the 123 coal sample is immersed in the water container. According to the moisture absorption capacity of the coal sample, 124 a certain amount of water is absorbed to reach the moisture content required by the experiment. Finally, the soaked 125 coal samples are wrapped in plastic wrap and put all into the storage box ready for the SHPB test. 126

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This test is mainly to study the influence of coal composition and water immersion on coal softening 129 mechanism. Therefore, each type of water-bearing coal sample consists of 5 groups of coal samples with different 130 components, and each group of 3 coal samples is subjected to repeated tests. After the SHPB test is conducted on 131 the coal samples with different moisture contents, the stress, strain, and strain rate of the samples are calculated by 132 the "dual wave method" [44] . Through data processing, the changes in the stress, strain, strain rate and energy of the where C is the elastic wave velocity, E is the elastic modulus, A is the cross-sectional area of the compression bar, 136 l0 is the length of the test sample, A0 is the cross-sectional area of the test sample, εr is the measured strain from 137 the reflected waves, is the measured stress .    Compared with the change rule of the dry coal sample, the characteristic rule of the four parameters of the coal 162 sample is more obvious. From the overall analysis, the effect of coal immersion is regular, especially the evolution 163 of the strain of the coal sample over time [33] . In Figure 4 (a), a "plateau" appears for the #1, #2, and #3 coal samples 164 after the swelling stress is generated in the coal sample, indicating that the internal moisture of the coal sample has 165 a certain buffer effect on the deformation of the coal sample. Figure 4 ( afterwards, the coal samples of 5 different compositions have a trend of slow increase again [37] . Figure 4 (c) indicates 168 that the strain rate change curve of the coal sample is similar to that of the dry coal sample. The strain rate of the 169 coal sample is in a slow increase stage within the first 45us, and then enters a rapid increase stage until 130 us. 170 Further analysis of the strain rate of the coal sample indicates that the strain rate changes in the slow growth zone, 171 the rapid growth zone and the rapid decline zone are more obvious than those of the dry coal sample. This means 172 that water can promote an increase of internal deformation of the coal sample.  becomes longer, and the expansion stress stage becomes shorter. In addition, the failure stress stage is also 186 significantly shortened. By comparison of Figure 4 (b) and Figure 5 (b), it is found that with the increase of the 187 water content, the first slow increase stage of the strain becomes longer, and the rapid increase stage does not change 188 much, but the second slow increase stage disappears. Figure 5 (c) shows a more obvious interval variation and the 189 variation patterns of the five coal samples are also more uniform. From the change of strain rate with time alone, 190 the effect of water immersion on the coal sample with a water content of 60% is more obvious than that on the coal 191 samples with a water content of 0% and 30%. Figure 5 (d) shows the energy dissipation curve of coal sample 192 destruction. When the water content increases to 60%, the first slow increase interval of energy dissipation increases, 193 and the rapid increase interval and the second slow increase interval decrease, indicating that the energy consumed 194 during the destruction process of coal mass is reduced after the coal mass is immersed in water. 195 Under the condition of triaxial stress, σi * (i=1, 2, 3) [13] , when there is no fluid inside the rock, the triaxial stress 203 forms an effective stress, and thus the corresponding strain εi * (i=1, 2, 3) [47] is generated. According to Hooke's law: 204

Analysis of test results
where μ is the Poisson's ratio of the rock, E is the initial elastic modulus, and ε1 is the axial strain of the rock. 206 Then the effective damage stress of the rock is [48] : 207 where D is the statistical damage variable. 209 The statistical damage variable D is defined as follows: 210 where N is the number of micro-units that the rock can be divided into, and Na is the number of damaged micro-212 units in the rock. 213 Assuming that the micro-units obey the Weibull distribution, the density function of the number of damaged micro-units in the rock is: 215 where F0 and m are the Weibull distribution parameters, and F1 is the strength variable of the rock micro-unit at the 217 first failure point. 218 Then the damage of dF1 extends to the inside of the rock. At this time, the failure interval of the rock is (F1， 219 F1+dF1), and the number of micro-units damaged inside the rock is NP(x), that is, the total number of damaged 220 micro-units when the rock is stressed is: 221 After Equation (7) is substituted into Equation (6), the damage variable D of the rock can be obtained as [30] : 223 Therefore, further substituting Equation (4) into Equation (3), we can obtain: 225 Combining Equation (9) and Equation (4) together, we get: 227 When the coal mass is immersed in water, the water moves in the fissure structure of the coal mass in a laminar 229 flow, and performs capillary or diffusion movement in the smaller pores. Therefore, capillary force or self-suction 230 force is introduced into the water-immersed coal mass. Assuming that water produces capillary force inside the 231 pores of the coal sample and surface tension on the surface of the water, the force of water will exist in the form of 232 "liquid bridge force". This means that when moisture condenses in the pores between the pulverized coal particles, 233 the moisture and the particles form a common micro-unit force body. With more and more water in the pores, the 234 thickness of the water film between the particles increases, and the formed liquid bridge force also increases, thereby 235 increasing the cohesive force between the pulverized coal particles. However, there is a certain upper limit for the 236 self-suction of the pores. When the water film increases to a certain thickness, the change in this cohesive force 237 decreases. From a microscopic point of view, there are many influencing factors, such as the viscosity coefficient 238 of the liquid and the distance between the pulverized coal particles. In the case of an infinitesimal body, the liquid 239 bridge force inside the infinitesimal body is simplified to: 240 where σw is the liquid bridge force inside the micro-unit body, σw1 is the static liquid bridge force of the micro-unit where φ is the distance between the pulverized coal particles, ω is the contact angle between the pulverized coal 247 particles and water, and υ is the viscosity coefficient of water. 248 Usually the dimensionless tension parameter Ca is used to measure the ratio of dynamic liquid bridge force to 249 static liquid bridge force: 250 Assuming that the temperature is 20℃. At this temperature, the surface tension coefficient γ of water is 252 0.07275 N/m, the viscosity coefficient μl is 1.01×10 3 N·s·m 2 , and the maximum value of the relative velocity 253 between particles vr is 2.084 m/s. In this paper, only the capillary force, i.e., the static liquid bridge force is 254 considered in the calculation of the liquid bridge force. Therefore, assuming that the adhesion force between 255 pulverized coal particles and water is a liquid bridge force, the calculation equation is: 256 where σγ is the surface tension of water, and is the contact angle between coal particles and water. 258 The calculation equation of the liquid bridge force is further transformed into: 259 where a is the radius of the pulverized coal particles, H is the length of the liquid bridge or the distance between 261 two pulverized coal particles, and d is the immersion height of the liquid bridge or the height of the pulverized 262 coal particles that can be wrapped by water to remove the surface tension. 263

Analysis of macroscopic strength failure based on microscopic coal immersion softening 264
Combined with the strength analysis of the rock micro-unit body, the macro-strength criterion of the 265 unimmersed coal sample is derived as follows. Using the Lemaitre equivalent strain principle, we obtain: 266 Assuming σ2=σ3=0, that is, the coal sample is subjected to uniaxial stress. At this time, without considering the 273 influence of water, we obtain the strength damage model of the coal sample [45] : 274

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(21) 281 Figure 6 shows the transformation relationship between compressive stress, swelling stress and failure stress 282 of the coal samples with different water contents. When the water content of the coal sample increases from 0% to 283 60%, the failure stress interval of the five coal samples decreases while the expansion stress interval increases. 284 However, there is no uniform relationship between changes in the compressive stress interval. This means that after 285 the coal sample is immersed in water, the water inside the coal sample helps increase the swelling stress interval of 286 the coal sample. For the #2 and #5 coal samples, the above-mentioned change characteristics are particularly obvious. 287 For different moisture contents, as the moisture increases from 0% to 30%, the coal particles are gradually fissure structures of the coal mass is increased, and thus the compressive strength of the briquette is enhanced to a certain extent. When the moisture content of the coal sample is 60%, the pulverized coal particles are gradually 295 surrounded by the moisture. As a result, the cohesion is reduced, and the compressive strength of the coal sample 296 may be also reduced. The above impact compression test shows that the compressive strength of coal samples and 297 the overall proportion of different stages are closely related to moisture. In addition, the ratio of activated carbon to 298 pulverized coal has an important influence on the compressive stress, expansion stress and failure stress of the coal 299 sample. The greater the ratio of activated carbon to pulverized coal, the more obvious the transformation of the three 300 stresses, as shown in Figure 6. 301 302 Figure 6 Influence of moisture content of coal sample on transformation of stress properties 303 Figure 7 shows the impact failure modes of the coal samples with different moisture contents. It can be clearly 304 seen that when the water content is 0%, 30% and 60%, the #1-#5 coal samples all undergo longitudinal compression 305 failure. And from one end of the incident rod, obvious cracks were generated, until the coal sample was completely 306 destroyed. However, when the moisture content of the coal sample increases, the coal sample is impacted by the 307 incident rod, the middle part of the coal sample begins to expand and deform, and one end of the transmission rod 308 also begins to break. The failure mode changes from damage on one side to damage on both sides. 309 According to the theoretical analysis of microscopic coal mass water soaking softening, when the amount of 310 moisture added to the dry coal particles reaches a reasonable range, the pulverized coal particles and water are 311 combined with each other, thereby promoting the agglomeration of coal particles and increasing the overall cohesive 312 force of the coal [15] . When the pulverized coal particles are subjected to an impact force, greater force is required to 313 separate the particles. The above is the process of transforming the macroscopic impact force of the coal mass into 314 the microscopic separation force of the pulverized coal particles. When the amount of water added to the dry coal 315 particles exceeds the reasonable range, the volume of the liquid bridge formed between the particles increases. 316 However, the volume of the pore structure between the particles is ultimately limited, and thus more moisture will 317 gradually wrap the particles, allowing the liquid to penetrate. This process reduces the cohesive force between the 318 particles. If the coal sample is subjected to an external impact load, it is more prone to instability and damage. From increases, the interval of the slow growth area increases while the interval of the stable growth area decreases. 332 Especially when the water content of the coal sample is 60%, there is no stable growth area in the #3 and #4 coal 333 samples. From the analysis of the composition of the coal samples, the proportions of coal particles and sand in the 334 #1 and #5 coal samples are the same, but the cement component gradually increases. Therefore, when the coal 335 sample has high moisture content, the deformation of the coal sample is affected. 336 From the analysis of the energy of the coal sample, the energy change of the coal sample during the entire 339 destruction process is obvious, and it mostly occurs after 50 us. In the early stage, there is a process of energy 340 accumulation. After the coal sample is destroyed, the energy is rapidly reduced. This experiment shows that 341 reasonable moisture can promote the agglomeration of dry coal particles. When the moisture exceeds certain content, 342 the agglomeration effect of moisture on coal particles is weakened. 343 In summary, the composition ratios of the #3 and #4 coal samples are not suitable for water immersion 344 experiments on coal with high water content. In the case of the three water contents, the slow growth interval of 345 Coal Sample #1 is relatively long, and thus Coal Sample #1 is used for the coal immersion softening experiment 346 because it has the best composition ratio. 347

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In this paper, the SHPB experiment was carried out on five coal samples with three different moisture contents, 349 and the dynamic characteristics and energy dissipation of water-immersed coal with different compositions and 350 water contents were analyzed. After analysis and discussion, the following conclusions are drawn: 351 (1) When the moisture content of the coal sample is 0%, 30%, 60%, the stress, strain rate, and energy dissipation 352 of the coal sample first increase and then decrease with time while the strain of the coal sample almost increases all 353 the time with slow growth stages and rapid growth stages. 354 (2) When the water content of the coal sample increases from 30% to 60%, the stress "plateau" of the coal 355 sample disappears, the interval of the compressive stress increases, and the interval of the expansion stress decreases. 356 (3) The increase of water content of coal will affect the impact deformation and failure mode of coal. When 357 the water content is 0% and 30%, the coal sample undergoes compression deformation and destruction from one 358 end of the incident rod; but when the water content is 60%, the middle part of the coal sample shows expansion and 359 deformation. 360 (4) The best coal composition ratio for this impact experiment of coal immersion softening is: "No. 425 361