Strength Damage and Acoustic Emission Characteristics of Water-Bearing Coal Pillar Dam Samples from Shangwan Mine, China

Abstract

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam’s standing sustainability. In the paper, the coal pillar dam body of the 2² layers of coal in the Shangwan Coal Mine is studied (2² represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions.

1. Introduction

The coal resources in the western mining areas in China are relatively abundant, the water resources are extremely scarce, and the space allocation between water and coal is not coordinated, which has become the principal element influencing green mining for coal resources from the western region [1]. The construction of underground reservoirs provides a new technology for solving the conflict of water–coal co-mining in western mineral areas [2]. This technology uses the goaf formed by coal mining to store water and constructs an artificial dam body to connect with dams of coal columns and block mine water, to realize the conservation and usage of mine water supply. The dam body is also equipped with water diversion facilities to realize the coordination of mine water mining and water source conservation and usage [3,4]. After this groundwater reservoir of the coal mining has been built for operation, the coal pillar dam is in a water-saturated condition. The erosion of water not only causes the dynamic change of the coal pillar water content but also has an obvious deterioration of the mechanical performance on the coal body [5].

Water–rock interaction has been used as the focus of research in the field of rock mechanics, and the weakening of rocks by water has a significant impact in engineering practice [6]. In mining engineering, coal rock bodies are inevitably exposed to water-rich environments, and under the multiple influences of water intrusion [7], dynamic and static loading [8], and cyclic loading [9], rock stability control becomes a major challenge. Since the successful application of the groundwater reservoir technique in domestic and international coal mines, an evaluation of coal pillar dam stability as well as coal–water interaction has become a focal issue. The mechanical response [10], permeability [11], internal hole fabric [12], and damage features [13] in the coal pillar dams will change after water intrusion. For the past few years, many researchers have studied the interaction between coal and water. It was shown that the rise in water-containing capacity made the samples exhibit stronger plastic characteristics, while the compression strength and modulus of elasticity both dropped to various extents [14,15]. Yao et al. systematically studied the water content/time, mechanical properties, and acoustic–thermal evolution law of coal-measure sedimentary rocks between dry and water-saturated states, which provided the basis for future research on rock mechanical properties under water action [16]. When Chen and other colleagues investigated the impact of repeated water immersion exposure on fracture damage in coal samples, they found the fracture development threshold did not alter as the quantity of water immersion increased [17]. Energy dissipation theory offers a new way to quantitatively characterize rock damage [18]. Peng et al. found that the compressive strength increased with increasing circumferential pressure, but the dissipation energy remained the same when studying the destructive characteristics of the coal samples at different depths in triaxial compression [19]. The above literature has extensively studied the damage of the samples according to moisture content and the dry–moisture cycles, but the internal mechanism of strength damage and energy dissipation features under various aqueous distribution conditions are not widely reported for the samples, and the relevant mechanisms are not clear.

Acoustic emission is defined as an elastic wave propagating because of the speedy discharge of power within the substance and is already extensively applied to lossless detection and lesion estimation for cracking [20]. Qin et al. obtained information on the strength-softening properties and breakdown precursors for the coal samples at various moisture levels by uniaxial compression experiments under acoustic emission monitoring [21]. Ren et al. verified the feasibility of the AE technique with the moment tensor to evaluate fracture damage in true triaxial rock samples, concluding that the damage difference between shear and tensile microcracks is proportional to the loading rate after unloading [22]. Xia investigated the fracture evolution characteristics in coal samples under acoustic emission monitoring by simulating coal pillar reinforcement with fiber-reinforced polymers and described the stress–strain relationships for strain hardening, strain softening, and sudden failure of CFRP sandwich coal specimens [23]. In addition, the damage process of rock materials is always accompanied by energy transfer and evolution. Therefore, the research method of energy dissipation has been widely used to describe the rock damage and failure process in recent years [24]. According to the energy dissipation characteristics of uniaxially compressed coal-rock mass, the dissipation coefficient peaks at the yield point of the sample. The restrictions on energy reserves and the accumulated power from acoustic emission allow for comprehensively characterizing the crack growth behavior of the sample [25]. Dissipated energy is also an important indicator for calculating rock damage variables. According to the evolution characteristics of dissipated energy, the damage evolution equation can be redefined [26]. Throughout the above literature, the research mainly focuses on the fracture damage and the acoustic emission properties for coal samples in various loading methods. However, there are few reports about damage characteristics and AE features in the coal samples at diverse moisture contents as well as the moist and dry cycle conditions.

In this paper, the coal column dam in the subsurface reservoir of the 2² coal at Shangwan Coal Mine has been studied. The strength damage characteristics under different water distribution states are explored from the laboratory scale. There are two types of circulation, and the internal mechanism of mechanical damage under the action of coal water is analyzed by means of the evolution law of acoustic emission and energy evolvement characteristic parameters of water-containing coal samples. Therefore, studying the change law of mechanical properties of coal samples in a dynamic aquiferous state, monitoring the key indicators of deformation and failure process can not only provide quantitative evaluation for the strength damage of the coal pillar dam but also provide the possibility for monitoring and evaluating the standing stability of the aquiferous coal column dam.

2. Material and Methodology

2.1. Preparation of Materials

Shangwan Coal Mine is in Ejin Horo Banner, Ordos City, Inner Mongolia Autonomous Area. This area has little precipitation and a large amount of evaporation. If the mine water cannot be effectively stored, the discharge of mine water after reaching the standard may aggravate the current situation of serious waste of water resources and adversely affect the long-term ecological protection of the region [27]. Based on this, Shangwan Coal Mine used the groundwater reservoir technique at the coalfield to reach the conservation and usage of water sources in coal mines (Figure 1). According to Figure 2, the field of stress in the coal column dam body mainly includes the vertical pressure in the caprock, lateral pressure in the falling gangue, and water storage pressure. At the same time, the coal pillar dam body is also subjected to long-term erosion by mine water, which is mainly manifested by immersion and repeated scouring. The bottom of the coal column dam is permanently immersed within the mine water, and its middle and upper parts are constantly in a state of dry saturation of dynamic water content because of the repetitive scouring effect caused by the change of the water level [28]. Therefore, it is necessary to investigate the standing mechanism for coal column dams due to water impact in terms of its standing erosive effect on coal column dams.

The 2² coal layer is the coal layer of the underground reservoir that is being operated by the Shangwan Coal Mine (2² represents No. 2 coal seam of Shangwan Coal Mine). In order to ensure that the experimental conditions are identical to the practical operating environments, we chose the 2² coal seam for the subject of this study, which is presented in Figure 3. The natural mean water content is determined to be 7.59%. Based on the requirements from the International Society of Rock Mechanics Test Code [29], the raw coal was machined to 30 standard cylindrical samples of 50 mm × 100 mm with excellent completeness without surface fractures. Because it is inconvenient to measure the moisture content in coal column dam in goaf, four groups of different water content (reflecting the dynamic change of standing water content of coal column dam) and three groups of dry–wet circulating coal samples (reflecting repeated flushing action of water) are designed and numbered according to the form of “grouping-water content/number of cycles-number of blocks”. As an illustration, A-1-1 indicates the first coal sample in the water content group with 0% water content, and B-1-2 indicates dry–wet cycle. Information on the grouping of coal samples is shown in

Table 1. Specimen numbering sheet.

Label	A-1	A-2	A-3	A-4
Water content	0.00%	4.35%	8.12%	12.32%
Label	A-1-1~A-1-3	A-2-1~A-2-3	A-3-1~A-3-3	A-4-1~A-4-3
Dry–wet cycles	1	2	3
Label	B-1-1~B-1-3	B-2-1~B-2-3	B-3-1~B-3-3

.

2.2. Methodology

The test consists of three sections: (1) non-destructive soaking test of coal samples to measure the natural water level in the coal sample as well as the change in the moisture content pattern through time; (2) uniaxial compression experiment together with acoustic emission experiment are conducted on coal samples in diverse aqueous states to determine the change patterns in physical parameters including tension and pressure in coal samples.

The drying equipment is 101-2 electrothermal dry box (Cortes Experimental Instruments Co., Dongying, China), according to Figure 4. The setting of drying condition is 110 °C, and the drying time is fixed to 12 h [30]. After drying, the coal sample is packed firmly in cling film immediately to protect it from absorbing water from the environment and influencing the precision of the test. HL-8-1WS stone water rate sequential weighing instrument (China University of Mining & Technology, Xuzhou, China) is selected as the soaking equipment to soak the coal sample [31]. The experimental equipment is shown in Figure 4. The coal sample is sufficiently exposed to replete moist steam and free water absorption inside the thermo-constant humid state box, which also avoids damaging the coal sample via too much direct water infiltration stress, thus protecting the completeness and originality of the coal sample to a good degree.

An electrohydraulic servo universal testing machine, MTS, is adopted for the loading mechanism. The loading method is conducting shift loading with a velocity of 0.3 mm/min. A PCI-II acoustic emission device from PAC (Princeton Junction, NJ, USA), an American acoustic physics corporation, is adopted for monitoring the acoustic emission signal throughout the process of the test. Four sensors are installed on the top and bottom 1/3 of the four surfaces of the coal sample barrel, two on each of the top and bottom water planes, and distributed at 90°, so as to guarantee accurate acoustic emission detection [32]. The testing system is shown in Figure 5.

3. Results

3.1. Strength Damage Law of Aquiferous Coal Sample

As shown in Figure 6, according to the slope of the water content–immersion time curve, that is, the variation law of the water intake rate of coal samples, the entire water-absorbing process is separated into four phases: I—accelerating growth stage (0–2 h); II—uniform growth stage (3–4 h); III—decelerating growth stage (5–9 h); IV—water saturation stage (10–51 h). According to the relation between the moisture content of the coal sample and the soaking time, the soaking time points of coal samples in the following single-axis compressive experiments at diverse moisture contents are determined as: 0 h, 1 h, 4 h, and 51 h; the corresponding water content: 0.00%, 4.35%, 8.12%, and 12.32%.

The water content of coal samples is related to the soaking time as a logarithmic function, and the fitting function is as follows:

$$w=3.31\ln \left(-18.1lnt\right)\hspace{1em}{R}^2=0.98$$

(1)

In Equation (1), w is the water content; t is the soaking time (unit: h); R² refers to the related coefficient of the matched curve.

According to Figure 7a, the pressure–strain curve of the coal sample is classified into five stages: the first stage is the compression stage; the second stage is the flexible distortion stage; the third stage is the stable development of microcracks; the fourth stage is the unstable damage stage; and finally, the fifth stage is the post-damage stage. In the compression phase, the profile is foveate, which shows that the primary fracture in the coal sample is gradually closed under pressure, and the pressure grows as the loading shift increases, and a fracture compaction phase of the coal sample becomes relatively long as the water content rises. The higher the moisture content, the lower the slope of the curve, which is mainly due to the coupling effect of hydrogen and oxygen ions on the coal sample. It alters the skeletal fabric and microscopic composition of the coal sample and then weakens the machine capability of the coal samples [33]. As the moisture content rises, the elastic deformation phase and the steady development phase of micro fissures are relatively shortened. While the unstable damage stage extends gradually by the growth of moisture content, the development of inner cracks in the coal samples slows down by the action of water. This shows the trend from unstable failure to stable failure.

The changes in summit pressure and summit strain of coal samples as a function of moisture content are given in Figure 7b,c. There is a negative linear relationship between the maximum pressure and strain of the coal samples and the moisture content. During the increase of water content from 0% to 12.32%, the peak stress attenuates from 18.24 MPa to 10.65 MPa, which is 41.6%. The peak strain decreased from 0.0205 to 0.0185, with an attenuation of 9.76%. The major cause is that water enters a lattice of coal samples and changes the internal microstructure of coal via the complicated interaction of physics and chemistry, resulting in instability of the internal bearing structure dominated by the coal matrix. Macroscopically, under the effect of outer load and pore moisture stress, the expansion of pore space causes the decrease of compression strength of the coal sample, which makes it more prone to failure [34].

The changes in the modulus of elasticity of the coal sample according to the moisture content are presented in Figure 7d. As can be seen, the impact on the deterioration of the modulus to the elasticity of the coal samples by water can be significant, and the elastic modulus and water content satisfy the exponential relationship. In the process of increasing water content from 0 to 4.35%, the elastic modulus decreased from 11.01 MPa to 7.43 MPa, which decreased by 32.5%. This is because, in the internal lattice structure of moist coal samples, the layer of water film is formed between the particles, which weakens the bonding force between the particles and reduces the elastic modulus.

The tension–strain profiles of coal samples with distinct wet and dry circles are presented in Figure 8a. The crack compaction phase of the coal samples is progressively shortened as the number of drying–wetting circulation rises, and the elastic and plastic properties are stronger before the failure peak, while the stable development stage and unstable failure stage of coal samples are not obvious. This is because of the particle reaction between coal micro-molecules and water molecules as the frequency of drying–wetting cycles increases [35]. After repeated immersion and drying, the moldability of coal progressively improves.

Changes in compression strength and maximum strain of the coal samples dependent on the number of the drying–wetting cycle are presented in Figure 8b,c. The compressive intensity of the coal samples along with a frequency of wet and dry circulation show an index-decreasing trend. It is not difficult to see that the degree of change in compression intensity of coal samples is more obvious than the strain. With the increase of cycle times, coal samples show stronger plasticity.

According to Figure 8d, the quotient of elasticity presents an exponential decreasing trend as the frequency of drying–wetting circulation rises. The dry coal sample has the largest elastic modulus and strong ability to resist elastic deformation. When the drying–wetting cyclic frequency is one, the average quotient of elasticity of the coal sample is changed to 8.02 MPa, which is reduced by 2.99 MPa, and the reduction amplitude is 27.16%. When the dry–wet cycles were applied twice, the elastic modulus was reduced to 7.35 MPa by 33.24%. When the dry–wet cycles are repeated three times, the elastic modulus decreases sharply to 3.66 MPa, with a decrease of 66.76%. In the process of elastic modulus from dry state to continuous flooding, micro-cracks gradually appear in the interior of the coal sample. When this number of wet and dry circulation reaches three times, internal fractures infiltrate into the whole coal sample, which makes the coal samples unable to have their own integrity and stability. In addition, the water-immersed cyclic failure process of coal samples has a certain buffer time. From the dry state to one or two cycles, the reduction of elastic modulus is only about 27–33%, and the coal samples currently still have a certain integrity and stability. At the end of the third cycle, the overall instability failure of the coal samples occurs.

3.2. Failure Modes and Acoustic Emission Characteristics

3.2.1. Failure Modes

Figure 9 shows the fracture development as well as damage characteristics of coal samples at various moisture contents. With increasing moisture content, the main type of fracture is a tensile crack, and there are few shear cracks. The position of crack development is transferred from the edge of the specimen to the inside of the specimen, and the length of the crack decreases, the degree of failure decreases, and the amount of cinder debris decreases. From the failure characteristic diagram, the final form of failure in the saturated coal sample was observed to be a tensile failure, only a small amount of cinder debris peels off, and the coal sample still has high integrity when it is destroyed. This is mainly because, under the action of water tension, more solid–liquid coupling weakening surfaces are formed in the coal sample, the proportion of solid medium carrier structure is reduced, and the ability to resist external force is reduced [36].

Figure 10 is a diagrammatic sketch of failure cracks of coal samples with different drying and watering circles. There is an accumulation of small-formed tension cracks, as the frequency of drying–wetting circles increases, and most of them are in the form of penetration. In addition, the shear crack decreases, the tension crack is progressively added, and the damage pattern of coal sample shifts from shear-dominated tension–shear mixed damage to tension-dominated mixed failure.

3.2.2. Acoustic Emission Characteristics

The total stress–strain curve can directly reflect the macro-scale invalidation in coal samples, but the internal damage development of coal needs to be revealed indirectly with the help of the characteristics of acoustic–thermal effect [37,38,39]. Therefore, the internal damage evolution characteristics of coal samples are analyzed by acoustic emission counting, cumulative counting, and other eigenvalues.

According to

Table 2. Statistical table of acoustic emission peak count and cumulative count of coal samples in different water states.

	w = 0	w = 4.35%	w = 8.12%	w = 12.32%
Peak value of AE count (×103)	32.73	31.46	28.78	11.94
AE cumulative count (×105)	51.39	20.43	11.62	8.40
	N = 1	N = 2	N = 3
Peak value of AE count (×103)	27.07	27.04	26.94
AE cumulative count (×105)	24.56	22.83	11.37

and Figure 11, as the moisture content rises, the AE count summit position is advanced, and structural failure occurs in coal samples in advance. The peak value of the AE count gradually decreases, the brittleness of the coal sample is weakened, and the energy generated by failure is reduced. As the moisture content rises, the slope of the AE cumulative counting curve of the coal sample before peak stress gradually decreases, that is, the growth rate of AE counting slows down and the internal damage frequency of coal sample decreases, indicating that under the action of solid–liquid coupling weakening surface, the overall force of the coal sample becomes more uniform, and the failure rate decreases [40].

According to Table 2 and Figure 12, the drying–wetting circles promote the AE summit position of coal samples ahead of time, and the number of peaks decreases. The AE phenomenon is not strong for the three groups of coal samples in various dry and wet circles compared to the dry coal samples. The AE cumulative count profile of the flexible phase and crack stable development stage rises slowly. As the frequency of drying and wetting cycles increases, the extent of water damage to coal samples also increased, while the accumulated count of AE decreased throughout the entire process of loading. The water–rock interplay weakens the cohesion within coal and causes clay minerals to expand and fill the pore space and crack space in the coal sample, which makes it easy for plastic deformation to occur instead of brittle fracture, and the stress distribution under loading is more uniform. When the sample reaches the peak stress, it will not be destroyed but continue the function of bearing capacity and gradually fail.

During the invalidation of coal samples, internal surrounding rupture is accepted by the acoustic emission instrument as an elastic wave, thereby forming the acoustic emission AE signal [41]. From the above analysis, it is observed that AE counting can reflect a definite degree of inner micro-damage of water-bearing coal samples. However, only analyzing the destruction features of coal samples at various moisture contents and cycles of drying and wetting through AE parameters has limitations. Previous studies by scholars have shown that the RA-AF evaluation scheme offers a better foundation to the crack types or their progression [42].

Reference RA value definition [43]:

$$RA=\frac{RiseTime\left(RT\right)}{Amplitude\left(Amp\right)}$$

(2)

In the AE wave shape, RT refers to its rise time in us. Amp is the vibration amplitude, and its unit is dB. In the case where the vibration amplitude is presented in a log scale (e.g., dB), the conversion to raw voltage ought to be performed. AE solves the following equation for V so that the result is expressed in units of volts.

$$A\left(dB\right)=20\times \lg \left(\frac{V}{V_{ref}}\right)-G$$

(3)

In the equation, G represents the AE magnification tone-up (threshold), in the experiment, G = 40 dB, and V_ref is the reference value applied in the AE, which is 1 uv. For Equation (3), after deformation, AE can be expressed as original voltage V:

$$V=V_{ref}\times {10}^{\frac{G+A\left(\mathrm{dB}\right)}{20}}$$

(4)

AF value is defined as:

$$AF=\frac{AE Count}{Duration\left(us\right)}$$

(5)

At the same time, Ohno and Ohtsu analyzed the corresponding relationship between rock acoustic emission parameter RA-AF value and crack attribute from the perspective of probability statistics and concluded that the RA value corresponding to the tensile crack is lower and the AF value is higher; on the contrary, the value of RA corresponding to the shear crack is usually higher and the value of AF is lower [44].

The probability density profiles of RA-AF for coal samples at various moisture contents and in wetting and drying circles are produced using MATLAB 2022b to reflect the placing features of RA-AF more intuitively, which is illustrated in Figure 13. The shift of the blue region to the red region indicates a change in placing density from minimal to maximal. The diagonal line in the RA-AF density distribution map represents the judgment line of the damaged form of the coal sample. The left top on the diagonal line takes into account the main tensile failure, and the lower left is correspondingly the main shear failure. The slope k is the maximum AF/RA [45].

Figure 13a–d show the RA-AF probability density diagrams of coal samples of various moisture contents. The density distribution is concentrated in the area with high AF value and low RA value, and the damage pattern in coal samples primarily consists of stretching failure. According to the coal sample failure diagram and fracture characteristic diagram in Figure 8, there is a good correspondence between the RA-AF density distribution and the failure diagram. It indicates that tensile cracks caused the macroscopic tensile damage of the water-bearing coal sample, during the damage of the water-bearing coal sample, and its overall failure mode is tension–shear composite failure.

Figure 13e–g are the RA-AF probability density diagrams for coal samples treated by various times of drying and wetting circles, which indicates that the density distribution is concentrated in the areas with upper AF levels and lower RA levels. Combined with the coal sample failure diagram of Figure 9, the tensile crack contributes to the macro-damages of coal samples, indicating that the main failure mode for coal samples following drying and wetting circles is tensile failure, which increases with the times of wet and dry circles, and the RA-AF density distribution gradually shifts to the areas with high AF value and low RA value.

3.3. Energy Dissipation Properties of the Aqueous Coal Samples

According to the first law of thermodynamics, the aqueous coal sample is deformed under the action of external forces. To keep the study object simple, we hypothesized the environment around the coal body is a closed system, i.e., there is no heat exchange with the outside world, and the total work performed via the outer forces on the water-bearing coal sample is U. By the theorem of conservation of energy [46], it is obtained that:

$$U=U^e+U^d$$

(6)

where U^e is the releasable flexible energy in coal bulk, J; U^d is the depletion energy of the coal body, J.

The overall energy U taken up from the coal sample under uniaxial compression conditions can be expressed as [47]:

$$U=\int {\sigma }_{1}d{\epsilon }_1={\displaystyle \sum }_{i=0}^n\frac{1}{2}\left({\epsilon }_{1i+1}-{\epsilon }_{1i}\right)\left({\epsilon }_{1i}+{\epsilon }_{1i+1}\right)$$

(7)

The body of coal can release elastic energy U^e which can be expressed as:

$$U^e=\frac{1}{2}{\sigma }_1{\epsilon }_1\approx \frac{{\epsilon }_1^2}{2E_0}$$

(8)

where ${\sigma }_1$ and ${\epsilon }_1$ stand for the main stress and strain in uniaxial compression, respectively; ${\sigma }_{1i}$ and ${\epsilon }_{1i}$ stand for the main stress and strain at the individual points in the tension–strain profile, respectively; i and n are the data point numbers and the total number of data points, respectively; $E_0$ are the modulus of elasticity in uniaxial compression.

The overall energy U taken up from the aqueous coal sample can be found by integrating under the pressure–strain profile, and the dissipation energy U^d can be found by combining Equations (6)–(8).

$$U^d=U-U^e=\int {\sigma }_{1}d{\epsilon }_1-\frac{{\epsilon }_1^2}{2E_0}$$

(9)

Based on the experimental data, the above energy calculation method was used to plot the characteristic curves of uniaxial compression stress–strain and energy change in coal samples at various moisture content states, as illustrated in Figure 14 as well as in Figure 15.

During a deformation and destruction process in the coal samples, energy consumption takes place primarily for elementary fracture sealing and new fracture sprouting and expansion in the coal. Comparing the energy dissipation curves of coal samples at various moisture contents, their energy composition changes accordingly at different phases of this curve for coal samples of varying moisture content. In the fracture compression phase, the dissipation energy curve develops gently and approximately horizontally, while the elastic energy curve rises steadily, at which time the energy taken up from the coal sample can be primarily stored in the form of flexible energy, few of them being used for energy dissipation. In the elastic phase, the flexible energy of the coal sample continues to increase. In addition, after entering the microfracture development stage, the elastic energy curve increases as the axial displacement rises, and the dissipated energy increases rapidly at this time owing to the dilatation of the coal sample cracks. The elastic energy drops sharply with the loss of brittleness from the pressure profile, and the flexible energy saved inside the coal sample becomes instantaneously liberated and transformed into dissipative energy to produce the main fracture of destruction, and nearly the entirety of the energy taken in during that time is converted to dissipative energy expenditure.

With increasing moisture content, there is a slowdown in the entire energy taken up by the sample as the strain increases, and the greater the moisture content, the weaker the overall energy taken up by the sample. The timing of the release of dissipative energy varies for coal samples containing water. The greater the moisture content, the sooner the instantaneous release point of dissipated energy which is owing to erosion of the coal sample by water, which reduces the agglomeration of the adhering granules and destroys the inner mechanical structure, resulting in lower energy storage and increased energy dissipation of the sample. According to Figure 15, the quantity of dry–moisture cycles has an obvious effect on the total and dissipated energy of the coal sample. This suggests that the wet and dry cycle handling of coal samples leads to the weakening of the sample properties, and this in return causes a reduction in energy storage. Moreover, the higher the number of drying–wetting cycles, the flatter the dissipated energy curve in the fracture-compacting and elastic stages, and the earlier the energy is released.

4. Discussion

In this paper, the strength damage mechanism of coal samples with different water contents and number of dry and wet cycles is revealed by means of acoustic emission monitoring techniques and energy dissipation analysis under uniaxial compression experiments.

Through uniaxial compression experiments on coal samples with water content, we obtained the strength damage pattern of coal samples under the influence of water content and the number of dry and wet cycles. The mechanical parameters such as compressive strength and modulus of elasticity of the coal samples show different degrees of decay as the water content and the number of dry and wet cycles increase. In engineering applications, this helps us to monitor the stress values of coal pillar dams under long-term water immersion conditions by means of hydraulic sensors, thereby obtaining the development pattern of plastic and damage zones of coal pillar dams on a large scale and providing theoretical references for evaluating the stability of coal pillar dams after water erosion. The siting of underground coal mine reservoirs and the design of dam widths are also controlled by the strength damage law of water-bearing coal samples [48].

In this paper, camera and acoustic emission monitoring techniques were used to monitor the coal samples in real time during the experiments. Acoustic emission counts and cumulative counts show a decreasing trend with increasing water content and number of wet and dry cycles. In addition, the damage pattern of the samples was analyzed by acoustic emission RA-AF fracture determination and camera. The damage of the coal samples was in the form of tension damage in the water content condition and mixed tension–shear damage in the dry and wet cycles. The coal pillar dams under real working conditions are in a stressed state, and under the long-term action of mine water and high stresses in the top and bottom slabs of rock, their internal fracture extensions will be produced to varying degrees. However, internal crack extensions may form a significant safety hazard at this working face if not detected in time. Acoustic emission monitoring data can provide an indication of the development of cracks within the coal pillar dam, which in turn can provide timely information on possible instability formation within the coal pillar, which can then be reinforced [49]. This helps to avoid sudden hazards in the coal pillar and reduces the likelihood of coal pillar dam failure.

The destabilization of coal samples with different water contents is often accompanied by an energy transformation during the process of destabilization. The analysis of the dissipation energy shows that the water content and the number of wet and dry cycles affect the location of the dissipation energy release point of the coal sample. The higher the moisture content and the number of wet and dry cycles, the earlier the point of dissipation energy release. The findings explain the damage and destruction characteristics of the coal samples from an energy perspective and explain the internal mechanism of damage and destruction of coal samples. This is an important reference for the interpretation of the damage evolution characteristics during the long-term destabilization damage of coal pillar dams.

5. Conclusions

In this paper, based on the background that a coal column dam in a subsurface coal mine reservoir is immersed and washed repeatedly by mine water for a long time, two experimental methods of non-destructive soaking and dry–wet cycling are innovatively proposed to simulate the dynamic change in the moisture content in the coal column dam, the strength damage law of coal column dam samples under various water distribution terms is discussed, and the conclusions below are presented.

(1)

The water content of the coal sample satisfies a logarithmic function with the soaking time. The water absorption process was separated into four phases based on the characteristics of this curve: accelerated growth stage (0–2 h), uniform growth phase (2–4 h), accelerated growth stage (4–9 h), and water absorption saturation stage (9–51 h). This provided the basis to produce coal samples at various moisture contents required for subsequent experiments.

(2)

The mechanical performance of the coal samples consists of the moisture content and the frequency of dry–wet cycles. The summit pressure and strain and modulus of elasticity of the coal samples are negatively related with the water content. As the number of dry and wet circles increases, the summit stress and modulus of elasticity of the coal samples decay exponentially, the brittleness decreases and the plasticity increases, and the dry and wet cycle process influences the destruction of coal samples in time.

(3)

As the moisture content rises and the dry–wet cycle times increase, three features are shown: first, the breakage pattern of coal samples is dominated by stretching fracture; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot is more consistent with macro-damage characteristics of coal samples.

(4)

The material–energy exchange during the destruction of the coal sample is indirectly reflected by energy development. The lower the moisture content of the coal sample, the more total energy is absorbed, the higher the moisture content, the earlier the instantaneous release point of dissipative energy, and the more the frequency of dry and wet circles, the flatter the dissipative energy curve is in the fracture compression and elastic stages, and the earlier the energy is released.

This paper investigates the strength damage characteristics of coal pillar dams under the long-term influence of mine water. However, since the experimental conditions are limited, this paper only conducts the strength damage research under uniaxial conditions, which is not matched with the stresses of the coal column dam body under actual circumstance, such as confining pressure, hydraulic pressure, and long-term water weakening, etc. Therefore, the experimental conditions should be gradually improved in the future to give more information for the steadiness assessment of dams in the subsurface reservoir in the coal mine.