Experimental Study on the Damage and Crack Evolution Characteristics of TS-RCR Structural Bodies under Cyclic Loading and Unloading

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Introduction
Te three-soft coal seam [1][2][3][4] refers to the coal seam with a soft roof, soft foor, and soft coal quality.Te following requirements should be met: (1) According to [5,6], coal seams with a sturdiness coefcient f < 1 are classifed as weak coal seams (Platt's coefcient f, also known as the rock sturdiness coefcient and fastening coefcient, is 1/10 of the ultimate uniaxial compressive strength of rocks and is dimensionless); (2) the uniaxial compressive strength of direct roof rocks of the coal seam σ ≤ 25 MPa or joint spacing d ≤ 60 cm; (3) the uniaxial compressive strength of the coal seam foor σ ≤ 25 MPa.Te roof, coal, and foor of the lower three-soft-protected coal seams form a TS-RCR structure in the pressure-relief mining process of the close-distance seam group.Infuenced by engineering activities such as mining of upper protected coal seams, blasting of the adjacent chamber, roadway driving, and mining of the working face, the TS-RCR structure bears cyclic loads [7,8].However, the formed TS-RCR structural body also presents diferent mechanical properties due to the diferent roof and foor lithology of the close-distance seam group.It is diferent from the mechanical properties of single coal and rock.Te mechanical properties of the TS-RCR structural body under cyclic loading and unloading are diferent from those under uniaxial compression.
Ben Ammar et al. [9] studied the efects of periodic stifness, hysteresis loop, energy dissipation, and damping on the damage density of sandwich-rich structures in different cyclic loading and unloading tests.Shkuratnik et al. [10] discussed the acoustic-emission characteristics of coal samples under diferent loading paths and analyzed the relationship among acoustic-emission parameters, stress, and strain.He et al. [11] and Bagde and Petroš [12] investigated the efects of cyclic loading and unloading frequencies, amplitudes, and loading rates on the fatigue mechanical properties of sandstone.
Cai et al. [13] compared the mechanical properties and acoustic-emission characteristics of coal and rock monomers, primary coal and rock masses, and synthetic coal and rock masses under uniaxial compression.Yu et al. [14] studied the uniaxial compressive strength of loose coal-rock masses.Dou et al. [15,16] used acoustic emissions and electromagnetic radiation to monitor the uniaxial compression experiment of coal and rock masses.Besides, they analyzed the relationship between the content and strength of rocks in the coal-rock structure and the occurrence of rock bursts.
Zuo et al. [17,18] studied the acoustic-emission characteristics of rock monomers, coal monomers, and coal-rock masses under the uniaxial compression test and the triaxial compression test.Chen et al. [19] conducted uniaxial compression tests on roof sandstone-coal pillar structures with diferent height ratios and studied the relationship among macrofailure initiation stress, uniaxial compressive strength, elastic modulus, and the coal-rock height ratio of roof-coal pillar structures.Wang et al. [20] used acoustic emission and microseismic to monitor the uniaxial compression test process of diferent coal-rock composite samples.Besides, they studied the relationship between the intensity of acoustic-emission and microseismic signals and the uniaxial compressive strength and bursting liability of the combined samples with diferent height ratios when impact failure occurred.
Li et al. [21] studied the infuence law of coal thickness in coal-rock masses on acoustic-emission characteristics during their fracture process.Ran [22] used nuclear magnetic resonances and acoustic emissions to study the dynamic mechanical properties and damage evolution characteristics of red sandstone under cyclic impact loads.Hua [23] used an electron microscope scanner and nuclear magnetic resonance equipment to study the mechanical properties and damage evolution mechanisms of weakly consolidated sandstone under uniaxial cyclic loading and unloading.
Researchers have studied the fatigue damage of coal-rock structures under cyclic loading and unloading, their mechanical properties, and their energy evolution characteristics.However, the following problems should be solved: Research on mechanical properties and damage evolution under cyclic loading and unloading mainly focuses on a single rock or coal.Tere are few studies on the damage variables and cumulative damage variables of rock-coal-rock structures under cyclic loading and unloading, as well as the mechanical properties and damage evolution of TS-RCR structures with diferent lithologies in Guizhou Province, China, under cyclic loading and unloading.Terefore, it is necessary to study the mechanical and damage characteristics of the TS-RCR structural body with diferent lithologies in close-distance seam groups under cyclic loading and unloading.Uniaxial cyclic loading and unloading tests were carried out on the TS-RCR structural body of the lower protected coal seam with diferent lithologies to study its mechanical and damage deformation characteristics.

Engineering Background
Te minefeld of the Xiangshui Coal Mine of Guizhou Pannan Coal Development Co., Ltd. is located in Xiangshui, south of Panzhou, Guizhou, China (Figure 1).Tere are 13 layers of commercial coal seams from bottom to top in the minefeld with a total thickness of 19.94 m and 5 layers of commercial coal seams in the whole minefeld with a thickness of 12.11 m.Most commercial coal seams have 4 layers with a thickness of 3.90 m.Te local commercial coal seam has 4 layers with a thickness of 3.93 m.Te mine has 745.41 million tons of geological raw coal reserves, 579.51 million tons of industrial reserves, and 387.61 million tons of recoverable reserves according to the exploration.According to the outcrop profle and core data analysis of the 13 th layer of the commercial coal seams of the Longtan Formation in Xiangshui Coal Mine, Guizhou, China, the combination lithology of the roof and foor of the main commercial coal seam refers to weak siltstone, argillaceous siltstone, silty mudstone, and mudstone, which belong to the mining conditions of three-soft coal seams.

Lithofacies of the Coal Measures and
Mechanical Model of the TS-RCR Structural Body  , and 28 # .Figure 1 lists the lithofacies assemblages of the roof and foor of commercial coal seams based on the analysis of the outcrop profle and core data of the 13 th commercial coal seam of the Longtan Formation in the Botu mining area of the Xiangshui Coal Mine Area.

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Te combined lithology of the roof and foor of the main commercial coal seams in Xiangshui Coal Mine is mainly composed of weak siltstone, argillaceous siltstone, silty mudstone, mudstone, and coal seam, which belong to fne deposits [24][25][26][27] in general.Tere are many coal layers, but most of them are of small thickness, generally less than 1.3-3.5 m.Te roof and foor are dominated by thin layers of mudstone.Te structure of coal seam is complex, and mudstone and carbargilite are the main sources of mining dirt.Te lithofacies assemblage is relatively closer to the sea and usually develops in the tidal-fat sedimentary system.It is difcult for the combination to form thick sedimentary rocks and thick coal seams because water changes too frequently.Te combination is easily afected by seawater, so thin coal seams or thinner coal seams with complex multilayer structures are developed.

Mechanical Model of the TS-RCR Structural Body.
Te geological conditions of Liupanshui Mine Field in Guizhou, China, are complex, with a wide distribution of high-gas coal and gas-outburst hazardous coal seams.Coal seams with the soft roof, soft foor, and soft coal are common.Common joint fractures of coal masses are developed, with low strength and loose and changeable coal masses.Te immediate roof and immediate foor of the coal seam have fracture development and soft crushing.It is disturbed by engineering activities such as mining of the protected upper coal seams, blasting of the adjacent chamber, roadway driving, and mining of the working face in the mining process of the close-distance seam group.It makes the TS-RCR structural body of the lower protected coal seam subject to cyclic loading.
Te TS-RCR structural body formed by it also presents diferent mechanical properties due to the diferent lithologies of the roof and foor of the close-distance seam group, which are diferent from the mechanical properties of single coal and rocks.Te mechanical properties of the TS-RCR structural body under cyclic loading and unloading are diferent from those under uniaxial compression.It makes the protected coal seam rock fracture evolution, gas seepage, coal-rock pressure-relief range, and other characteristics diferent.Protected coal seams, roof, and foor are simplifed into a TS-RCR structure.When both rocks and coal are at the linear elastic stage, the equivalent elastic modulus E of the parts of rocks and coal in series can be calculated as follows [28][29][30][31][32][33]: where

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Equation ( 1) is used to obtain the following equations [28][29][30][31][32][33]: where σ j is the stress acting on the TS-RCR structural body and ε j is the strain generated by the TS-RCR structural body.Equations ( 2) and ( 3) are combined to obtain the following equation [28][29][30][31][32][33]: When the lithology of coal and rocks constituting the TS-RCR structural body is determined, the elastic modulus E of the TS-RCR structural body is inversely proportional to the volume ratio of coal to the structural body (equation ( 2)).When the lithology of coal and rocks forming the TS-RCR structural body is determined, the strain ε j produced by the TS-RCR structural body is proportional to the volume ratio of coal in the structural body under certain stress σ j (equation ( 4)).
Te original rock stress balance of the lower protected coal seam will be destroyed, and surrounding rock stress will be redistributed under engineering disturbances such as upper protection coal seam, adjacent roadway excavation, chamber excavation, and working face mining.Coal, roof, and foor rocks are assumed to be at the linear elastic stage.Te TS-RCR structural body can be regarded as a series of roof rocks, coal, and foor rocks of the lower protected coal seam, which forms a series-combination structure.Te TS-RCR structural body of the lower protected coal seam can be regarded as the model in Figure 2.
Strain produced under the same stress (σ 0 � σ m � σ j � σ y ) is ε m , ε j , and ε y (Figure 3), respectively, after the stress of coal, the TS-RCR structural body, and rocks is redistributed.Te equilibrium principle of elastic mechanics is used to obtain the following [28][29][30][31][32][33]: Strain ε j produced on the TS-RCR structural body is compared with strain ε m produced on the coal mass under the same stress (σ 0 � σ m � σ j � σ y ) to obtain the following equation [28][29][30][31][32][33]: Stress σ j acting on the TS-RCR structural body is compared with stress σ y acting on the coal mass are compared to obtain the following equation [28][29][30][31][32][33]: According to the coal-bearing strata of the Xiangshui coal mine in Guizhou, the elastic modulus of coal is lower than that of roof and foor rocks, that is, According to equations ( 8) and ( 10), According to equation ( 6),

Coal
Coal According to equations ( 9) and ( 10), Ten, 1 Equation ( 6) is used to obtain Ten, ε j > ε y .Based on the above analysis, strain generated under the same stress (σ 0 � σ m � σ j � σ y ) is ε m , ε j , and ε y , respectively, when the stress of coal, the TS-RCR structural body, and rock is redistributed.Strain ε y generated on rocks is less than strain ε j generated on the TS-RCR structural body, and strain ε j generated on the TS-RCR structural body is less than strain ε m generated on the coal mass; that is, ε m > ε j > ε y .Te lower three-soft-protected coal seams are subjected to the same engineering disturbance stress, and the strain generated in the coal mass is maximum.

Cyclic Loading and Unloading Test-Specimen
Production and Scheme Design

Fabrication of the Structural Body.
According to the analysis of the rock facies of the coal-bearing rock system of Xiangshui Coal Mine in Guizhou, the roof and foor of the main coal mining layer are mainly weak siltstone, siltstone, siltstone, and mudstone.Terefore, four representative rockcoal-rock combination structures are selected (Figure 4).Te deformations of the surrounding rocks before instability and ruptures are mainly concentrated in coal masses for mining working faces.Te larger the size of the coal mass in the combined sample, the more favorable it is for observing the deformation.However, the roof and foor rocks transmit the upper and lower loads and release energy outward when the rupture occurs, so the proportion of coal masses cannot be infnitely large.Figure 4 shows the size and combination mode of the fnal combined sample.All coal and rock samples were taken from the same place to reduce the infuence of the discreteness of coal and rock samples on the test.Coal and rock samples used in the test were taken from about 250 m of the travelling roadway of the 120519 working face of the Botu mining area of Xiangshui Coal Mine.Te preparation process for the structural body specimen is as follows: Coal, siltstone, argillaceous siltstone, silty mudstone, and mudstone were drilled into a cylinder with a diameter of 50 mm with a drilling coring machine in the laboratory through on-site sampling.Ten the cylinder was cut into a rock specimen with a height of 30 mm and a coal specimen with a height of 40 mm by a cutting machine.Ten the combination was glued with marble glue.A grinding machine was used to grind the upper and lower end faces of the specimen into a standard one with a height ratio of 3 : 4 : 3 (with a diameter of 50 mm and a height of 100 mm).Te composite structural bodies of argillaceous siltstone, coal, and siltstone were recorded as 1-1 # ; those of silty mudstone, coal, and silty mudstone were recorded as 2-1 # ; those of argillaceous siltstone, coal, and mudstone were recorded as 3-1 # ; and those of mudstone, coal, and mudstone were recorded as 4-1 # .Each group of composite structural bodies was made of 3 specimens (Figure 5 for the sample processing).

Cyclic Loading and Unloading Test System.
Te cyclic loading and unloading test system mainly included the RMT-301 rock and concrete mechanics test system, the DS5 full information acoustic-emission signal analyzer, and the MicroMR12-040V nuclear magnetic resonance analyzer (Figure 6 for the test system).Advances in Civil Engineering structural body is placed in the middle of the test machine platform, and the initial load of the specimens is 1 kN to ensure stable contact between the specimen and the surface of the upper plate of the test machine.Te displacement increment cyclic loading method with a loading rate of 0.06 mm/min is adopted.Te increment of each cyclic loading is 0.1 mm and then it is unloaded to 0.01 mm.Load to 0.2 mm and unload to 0.01 mm for the next time.Repeat the above process until the structural specimen is destroyed.
Figure 7 shows loading and unloading waveforms.

Mechanical Properties of the Composite Structural Body Specimen under Cyclic Loading and Unloading
Figure 8 shows the stress-strain curves of TS-RCR structural bodies with diferent lithologies during uniaxial cyclic loading and unloading tests.Te stress-strain curves of TS-RCR structures with diferent lithologies under cyclic loading and unloading show a change rule of sparsity, density, and sparsity with the increased cycles.Te deformation of TS-RCR structures under cyclic loading and unloading can be divided into four stages: (1) Te deformation rate of TS-RCR structures is small and the curve is sparse at the slow deformation stage.However, the number of cycles in this stage is small.(2) Te deformation and failure processes of the TS-RCR structure account for a large proportion at the uniform deformation stage.Te curves in the cyclic loading and unloading processes are dense, and the deformation generated in each cycle is small and stable.(3) Te strain rate of the TS-RCR structure at the accelerated deformation stage is larger than that at the frst two stages.Te stress-strain curves change When the height ratios of TS-RCR structural bodies are the same, the higher the strength of the upper and lower rocks, the more the number of cycles of the TS-RCR structural body formed, and the denser the cyclic loading and unloading curve.On the contrary, the lesser the number of cycles of the TS-RCR structural body, the sparser are the cyclic loading and unloading curves.When the TS-RCR height ratio is constant, the strength of the roof, foor, and coal seam of the protected coal seam is greater, and the axial compression deformation of the structural body requires greater axial stress.It reduces the damage efect of axial stress on the whole TS-RCR structural body and increases the peak strength of the whole structural body to a certain extent.Te peak strength of the TS-RCR structural body composed of siltstone and argillaceous siltstone is the largest, and the deformation caused by each cycle is small.More cycles under cyclic loading and unloading make the cyclic loading and unloading curve denser.Mudstone has low strength and large deformations.Te peak strength of the TS-RCR structural body is small, and the deformation generated by each cycle is large.Te number of cycles under cyclic loading and unloading is less, which makes the cyclic loading and unloading curve sparse.

Structural Damage and Crack Evolution
Characteristics under the Cyclic Loading and Unloading  Advances in Civil Engineering and unloading stages of TS-RCR-structure samples under uniaxial cyclic loading and unloading.
Te test operation steps are as follows: (1) Put the checked oil sample and TS-RCR structure into the magnet box successively to set the system parameters.(2) Establish marking lines with calibration samples to improve the accuracy of structural testing.(3) Put the structure into the magnet box, and use the test and analysis interface to determine the T 2 spectral curve of the structure.(4) Input measured T 2 spectral curve data, and then input the volume of the structure, that is, the porosity and pore size distribution of the structure.Te structure is removed from the loading and unloading test stand after each loading and unloading.Fill with water for 1 hour, and wrap with waterproof tape to repeat the MRI test.
Pore diameters increase with increased relaxation time, and the evolution process of pore diameter distribution can be refected by changes in the T 2 spectrum curve shapes of TS-RCR structural bodies with diferent lithologies [34][35][36][37].Te distribution of the porosity and pore diameters of watersaturated TS-RCR structural bodies was tested by nuclear magnetic resonance after each cyclic loading and unloading to analyze the variation of the pore structure of the TS-RCR structural body with diferent lithologies.Figure 9 shows the relationship between signal intensity and relaxation time T 2 of structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # after loading and unloading.
According to the test results, the pore size of the TS-RCR structure is divided as follows [38,39]: small pore size (r < 0.1 μm), medium pore size (0.1 μm < r < 1 μm), and large pore size (r > 1 μm).Te pores and cracks in the structural samples are assumed to be spherical or columnar-pipe.Ten the relationship between pore radius r and relaxation time T 2 in the structure can be obtained as follows [40,41]: where ρ 2 is the relaxation strength of the particle surface T 2 spectrum and the ρ 2 value is related to the lithology of specimens.Te value of porous medium materials is generally 1-10 μm/ms, and the value is 5 μm/ms in the work.S is the surface area of the pore, 10 −4 m 2 ; V is the pore volume, 10 −6 m 2 ; F kx is the pore geometry factor of the specimens, 3 for spherical pores and 2 for columnar pores; and r is the pore radius of the specimens, μm.r � 0.1 T 2 is obtained by simplifying equation (15); that is, the pore radius is proportional to the T 2 value.Te smaller the T 2 value, the smaller the pore.
Te T 2 spectrum curves of TS-RCR structural bodies with diferent lithologies are monitored around 1 and 10 ms before cyclic loading and unloading, and two obvious peaks are detected (Figure 9).Peak points appear near 1 and 10 ms, and the corresponding pore radius is about 0.1 and 1 μm, indicating many micropores and mesopores in structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # with diferent lithologies.Te nuclear-magnetic inversion diagram of the TS-RCR structural body changes under cyclic loading and unloading.Te pore structure of the TS-RCR structural bodies with diferent lithologies changes signifcantly with the increased number of cycles.Te signal intensity of the TS-RCR structural body with diferent lithologies at the relaxation time of 1 ms increases with the increased axial displacement of cyclic loading and unloading.
Micropores in the TS-RCR structural body are constantly developing under cyclic loading and unloading, and the peak value appears in the 2 nd and 3 rd cycles.Te number of internal macropores is the minimum, and the TS-RCR structural body enters the continuous crack-growth stage.When the relaxation time of the TS-RCR structural body is between 10 and 100 ms, signal strength decreases.It refects the increasing axial displacement of cyclic loading and unloading as well as the continuous compression and closure of internal pores in the TS-RCR structural body.
Te signal strength of the structure increases slightly in the 4 th and 5 th cycles when the relaxation time is between 100 and 1,000 ms.Cracks inside the TS-RCR structural body enter the accelerated crack-propagation stage.Pore penetration intensifes, and the number of pores with large diameters increases continuously.Te internal porosity of the TS-RCR structural body reaches the peak, and the damage degree reaches the maximum, indicating that the TS-RCR structural body is about to be destroyed.

Porosity Variation Characteristics of the TS-RCR Structural Body Based on Nuclear Magnetic Resonance.
Uniaxial cyclic loading and unloading tests were carried out on structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # with diferent lithologies, and then nuclear magnetic resonance tests were carried out.Figure 10 shows the NMR scanning images of the TS-RCR structural bodies with diferent lithologies, and Figure 11 presents the variation curves of the porosity of TS-RCR structural bodies after inversion with the number of cycles.
Pores exist among the grains of TS-RCR structural body specimens with diferent lithologies, and the grain boundaries are clear (Figure 10).Te internal pores of the TS-RCR structural body increase with the increased axial displacement of loading and unloading.As the strength of the upper and lower rock masses decreases, pore development and expansion are more likely to occur under the same coaxial displacement.Pores inside structural body 1-1 # mainly occur in the central coal body, while the upper argillaceous siltstone and lower siltstone have fewer pores (Figure 10(a)).Pores inside the structural bodies 2-1 # , 3-1 # , and 4-1 # mainly occur in the central coal body, and the upper and lower rock bodies also have pores (Figures 10(b)-10(d)).Te pore scope increases with the weakened strength of upper and lower rock bodies.
Te pore diameter distribution of structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # with diferent lithologies is dominated by micropores (0-0.1 μm), which account for 7.23-8.46%,6.22-8.12%,7.87-8.94%,and 6.19-8.11% of the total volume, respectively (Figure 11).As the TS-RCR structural body is subjected to cyclic loading and unloading, changes in micropores, mesopores, and macropores of the TS-RCR structural body with diferent lithologies are as follows:  (3) Te proportion of macropores (1-25 μm) in the structure frst decreases and then increases.Te fnal increment values of macropores in structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # with diferent lithologies are 0.02, 0.11, 0.09, and 0.74%, respectively.Te fnal macropores of the TS-RCR structural body increase gradually with decreased lithological strength.Mesopores inside the TS-RCR structural body are compressed and destroyed with the increased load, and a short downward trend occurs.As the load continues to increase, mesopores are destroyed and combined to form macropores. Besides, it is more prone to damage and failure with the decreased lithologic strength of the TS-RCR structural body, which results in the expansion and penetration of internal cracks and increased porosity.

Crack Evolution Characteristics of the TS-RCR Structural
Body Based on Acoustic Emissions.Te DS5-8b acoustic emission detector was used to collect the waveform variation characteristics of the structure during the test and to analyze the growth of diferent microcracks in the structure under cyclic loading and unloading.Acoustic emission sensors were symmetrically arranged at the upper and lower ends of samples, with blue dots 1-8 # in Figure 12.
A coupling agent was used for coupling to prevent acoustic-emission signals from losing signals due to poor contact, and elastic tapes were used for fxing to prevent dropping in the test process.Te waveform characteristics of the cracking process were also collected for mechanical signals and acoustic-emission data during the test.Te acoustic-emission sampling frequency was 3 MHz, with a threshold value of 50 dB and a preamplifer of 40 dB.
According to the experimental study of Wu et al. [42][43][44], initial microcracks inside the structure are under shear stress, and their tip expands.Te AF value of the detected wave signal collected by acoustic emissions was low, and vice versa.Te AF value [45][46][47][48][49][50] is defned as the ratio of acoustic-emission ringing count to duration, and its unit is KHz. Figure 13 shows the time history of acoustic-emission AF values of the TS-RCR structural bodies with diferent lithologies.
According to Chong et al. [51][52][53][54][55][56], the acousticemission detection wave properties of concrete specimens under four-point bending conditions are studied based on the resonant sensor.Te acoustic-emission- According to Figure 13, the damage and failure process of the structural body is roughly divided into three periods according to the time-history analysis of the acousticemission AF (the average frequency of a wave signal) value of the structural body: the active period with the relatively low AF value (AF < 60 kHz) in the 1 st and 2 nd cycles, the rising period with the relatively stable increased AF value in the 3 rd and 4 th cycles, and the explosive period with the relatively high AF value (AF ≥ 60 kHz) in the 5 th cycle.
(1) Te crack evolution law in the active period of the AF value is as follows: Te AF values of structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # with diferent lithologies increase with the increased axial displacement of loading and unloading.Te gradual expansion, intersection, and compaction of pores, microcracks, and defects occur inside the structural body, with a small number of new cracks.Te damage cracks of structural bodies 1-1 # , 2-1 # , 3-1 # , and 4-1 # are mainly tensile, and a few shear cracks occur in the inner part of structural body 4-1 # .(2) Te crack evolution law at the ascending period of the AF value is as follows: As the axial displacement of loading and unloading increases, the Te RA value [57,58] is defned as the ratio of the rise time to the amplitude of the acoustic-emission-detection wave, ms/V.Figure 14 shows the acoustic-emission RA time histories of structures with diferent lithologies.
According to the time-history diagram analysis of the acoustic-emission RA values of structures with diferent lithologies in Figure 14, the acoustic-emission RA values of 1-1 # , 2-1 # , 3-1 # , and 4-1 # structures with diferent lithologies are less than 2 ms•v −1 .Only the 4-1 # structure has RA values slightly greater than 2 ms•v −1 during the 4 th -5 th cyclic loading and unloading.Terefore, the structural damage and failure process can be roughly divided into three periods.Te period of the relatively high RA value in the frst 1-2 cycles is divided into the active period; the period of the relatively stable rise of the RA value in the middle 3-4 weeks is divided into the rising period; and the period of the peak RA value in the last ffth cycle is divided into the transition period.
According to the relationship between the RA value and time, the evolution law of the RA value and AF value of structures with diferent lithologies in diferent periods under cyclic loading and unloading is similar.Te damage and failure processes of structures can be roughly divided into the active period, ascending period, and transition period.Tere is a slight diference between the RA value interval and the RA value due to the changes in the discriminant way of dividing the AF value interval.Specifcally, the transition period of the RA value is changed to the burst period of the AF value.
According to the RA value, the value of damage failure tensile and shear cracks in the structure during cyclic loading and unloading is much lower than that of the AF value.Te main reason is that the way to defne and delimit the range of the AF value is diferent from that of the RA value.Te AF value and RA value are signifcantly diferent under multiple factors, such as ringing count-duration time and rising time amplitude.Combined with the comparative analysis of the macroscopic fracture characteristics of structures, the evolution law of the RA and AF values is consistent with the macroscopic fracture characteristics of structures.Te acoustic-emission damage location times of the TS-RCR structural body with diferent lithologies increase under uniaxial cyclic loading and unloading (Figure 12).TS-RCR structural bodies with diferent lithologies are more prone to internal damage and deformations under cyclic loading and unloading as the strength of roof and foor rocks becomes weaker.It promotes crack propagation and accelerates the instability and failure of the TS-RCR structural bodies.

Damage Location Analysis of the Structural Body
Te dense location and the high-energy location are mainly distributed in the middle of the TS-RCR structural body.Te upper and lower location points of the TS-RCR structural body gradually increase with the weakened strength of the roof and foor rock masses.It is also refected in another aspect that lithology strength afects the damage Advances in Civil Engineering deformation and crack propagation of the TS-RCR structural body with diferent lithologies after cyclic loading and unloading in the uniaxial cycle.

Conclusions
(1) Te lithofacies assemblages of the roof and foor of the commercial coal seam were listed based on the analysis of the outcrop profle and core data of the 13 th commercial coal seam of the Longtan Formation in Xiangshui Coal Mine Area, Guizhou, China.Four representative TS-RCR structural bodies were selected for research, and the mechanical model of the TS-RCR structural body was proposed.(2) Te stress-strain curves of TS-RCR structures with diferent lithologies under cyclic loading and unloading show a change rule of sparsity, density, and sparsity with the increased cycles.Te deformation of TS-RCR structures under cyclic loading and unloading can be divided into four stages.Te results of the cyclic loading and unloading test showed that when the height ratio of the TS-RCR structural body was the same, the higher was the strength of the upper and lower rocks, the more was the number of cycles of the TS-RCR structural body, and the denser were the cyclic loading and unloading curves.On the contrary, the lesser the number of cycles of the TS-RCR structural body, the sparser were the cyclic loading and unloading curves.14 Advances in Civil Engineering (4) Te results of the acoustic-emission test showed that the damage and failure processes of the TS-RCR structural body was roughly divided into three periods according to the time-history analysis of the acoustic-emission AF value of the TS-RCR structural body: the active period with the relatively low AF value (AF < 60 kHz) in the 1 st and 2 nd cycles, the rising period with the relatively stable increased AF value in the 3 rd and 4 th cycles, and the explosive period with the relatively high AF value (AF ≥ 60 kHz) in the 5 th cycles.According to the relationship between the RA value and time, the evolution law of the RA value and AF value of structures with diferent lithologies in diferent periods under cyclic loading and unloading is similar.

Figure 1 :
Figure 1: Combined lithology of the roof and foor of the main commercial coal seams in the Xiangshui coal mine.

Figure 6 :Figure 7 :
Figure 6: Compositions of the test system of the TS-RCR structural-body specimen.

8 Advances in Civil Engineering ( 1 )
Micropores inside the TS-RCR structural body generally show fuctuations of decreasing, rising, decreasing, and rising.As the lithology strength of the TS-RCR structural body gradually decreases, fuctuations become more obvious.Te fuctuation of structural body 1-1 # is weak, and the fuctuations of structural bodies 2-1 # , 3-1 # , and 4-1 # are strong.As the lithologic strength of the TS-RCR structural body gradually decreases, micropores are more easily transformed into mesopores under cyclic loading and unloading, which decreases micropores.New micropores are generated with the further increased load, which increases micropores.It repeats until the structure is destroyed.(2)Mesopores (0.1-1 μm) inside the TS-RCR structural body generally show fuctuations of rising, decreasing, rising, and decreasing.Tis characteristic is opposite to the fuctuation of micropores.Te micropores of the structural TS-RCR body gradually change into mesopores under cyclic loading and unloading, which increases mesopores.As the load further increases, mesopores change into macropores.It repeats until the structure is destroyed.

( 3 )
Te results of the nuclear magnetic resonance test showed that the T 2 spectrum curves of the TS-RCR structural bodies were monitored at about 1 and 10 ms, and two obvious peaks were detected.Micropores and mesopores existed in large volumes in the TS-RCR structural body.Te signal intensity increased in the relaxation time range of 1 ms, decreased at 10-100 ms, and increased slightly at 100-1,000 ms.Te pore structure of the TS-RCR structural body with diferent lithologies changed signifcantly with the increased cycle number.
E, E y, and E m are the elastic moduli of the TS-RCR structural body, rocks, and coal, respectively.H y and H m are the thickness of rocks and coal, respectively, H � H y + H m .