Research on Damage Mechanism and Mechanical Characteristics of Coal Rock under Water Immersion

Abstract

This study aims to reveal the impact of immersion duration on the internal structural damage and mechanical property degradation of coal rocks. Coal rocks from the post-mining area of Liangshuijing Coal Mine were selected as the research subject. Physical and mechanical tests were carried out on these with different immersion durations (0 d, 15 d, 30 d, 60 d, 120 d, and 240 d) using scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), acoustic emission (AE), digital image correlation (DIC), and compression testing, further elucidating the damage degradation mechanisms of water-immersed coal rocks. The research demonstrates that changes in the pore structure of coal rocks can be divided into two stages as the soaking time varies: the stage of water swelling (saturation process) and the stage of soaking damage (long-term immersion process). The water swelling stage of coal rock extends from surface drying and contraction to water swelling, and the soaking damage stage of coal rock extends from expansion to soaking damage. During the stage of soaking damage, the water showed dynamic changes from macropores to mesopores to micropores, with a gradual increase in the number of micropores. The AE count and cumulative count of coal rock decrease first and then increase, and the four stages’ acoustic characteristics and macroscopic characterization phenomena appear. The mechanical properties declined. After 240 d of immersion, the uniaxial compressive strength and elastic modulus decreased by 48.93% and 29.53%, respectively, and the plastic characteristics were enhanced. These research results provide a beneficial reference for understanding and predicting the instability and destruction of water-immersed coal rocks.

1. Introduction

As China’s main energy source, coal plays a vital role in ensuring national energy security [1,2,3]. To achieve the efficient and sustainable use of coal resources, green development strategies are necessary [4,5,6]. In particular, the sustainable use of water resources is a key factor in supporting the long-term viability of the coal industry [7,8,9]. Given the low utilization rate of mined areas after coal extraction, storing water in these areas could not only enhance their utilization efficiency, but also alleviate water scarcity in surrounding regions [10,11,12]. However, water can alter the physical and mechanical properties of coal rock, which can lead to further mine inrush accidents [13,14,15]. Therefore, it is of great significance for the storage, utilization, and safe operation of coal mine water resources to study the influence of water on the physical and mechanical properties of coal rock, and to reveal the mechanisms of water soaking time affecting coal rock damage.

The effect of water on the physical and mechanical properties of coal rocks has been shown in two main areas. One is the change in the conditions of the coal rock itself. At the micro level, changes in the microstructure, such as mineral composition and pore structure of coal rock are also affected by soaking time [16]. Song et al. [17] observed soaked coal rocks by SEM and showed the changes in pore characteristics of soaked coal. Moreover, different degrees of microscopic and macroscopic expansion were observed on the surface of coal samples containing clay minerals [18]. Further studies have shown that expansion caused by water absorption can significantly reduce the permeability of coal seams [19]. At the mesoscopic scale, Han et al. [20] and Zou et al. [21] compared the advantages and disadvantages of mercury intrusion porosimetry (MIP), CT, and NMR measurement of rock structure, and concluded that NMR is more suitable for measuring the pore structure of rock. Moreover, NMR can be used for pore classification [22]. Xu et al. [23] established and demonstrated the relationship between lignite kerosene low-field NMR signals and porosity. Long-term water invasion greatly damages the pore diameter and surface morphology of macropores and mesopores [24]. Xie et al. [25] compared the pore structures of different argillaceous meagre coal after drying and soaking, and found that the total pore volume, macropore ratio, and permeability of coal rocks increased before and after soaking. To further reveal the influence mechanism of water soaking on the microscopic pores of coal, subsequent research needs to adopt more accurate methods to monitor the changes in pore parameters of coal rocks before and after soaking.

Under another external test condition (uniaxial compression strength, acoustic feature and failure form), the results of the uniaxial compression test showed that the attenuation of the uniaxial compressive strength and elastic modulus was very sensitive to increases in the water content [26,27]. In terms of acoustic characteristics, Wu et al. [28], Chen et al. [29], and Ali et al. [30] investigated the stress–strain and acoustic characteristics of coal rocks with different water contents, and determined that the peak AE number and cumulative AE number of coal samples decreased with increasing water content, while the peak AE energy and cumulative AE energy of coal samples both tended to decrease with increasing water content. Gao et al. [31] studied the effects of temperature and immersion on the AE and electromagnetic radiation signals of damage under coal and rock loading. They revealed a variation in AE and electromagnetic radiation signals during coal and rock compression. Xu et al. [32] tested the ultrasonic response characteristics of coal during loading and obtained the P-Wave and S-Wave Velocity characteristics of different stages in the process of coal and rock compression. Tang et al. [33,34] further analyzed the crack extension process in coal samples with different water contents and drying–saturation conditions based on AE counts and cumulative AE counts. In terms of failure form, based on the uniaxial compression test and the DIC method, Gao et al. [35] determined the mechanical behavior of the coal samples, such as the evolution of the strength, surface deformation, crack propagation, and elastic strain energy of the coal under the various loading rates. Yao et al. [36,37,38] studied the influence of water content on the physical and mechanical properties and crack propagation of coal rocks, and found that water content impacts the strength, crack growth, and failure modes of coal rocks. The above studies mainly focus on discussing the difference between saturated and dry states, and most of the immersion studies concentrate on short-term coal seam water injection, with fewer studies focusing on the damage mechanism of immersed coal rocks in long-term coal mine storage water. However, it is crucial to understand the long-term effects of water immersion on coal rocks, especially when the water used for this purpose is sourced from coal mine storage. The long-term immersion of coal rocks in coal mine storage water may lead to significant changes in the mechanical and physical properties of the coal, which can have substantial implications for both the safety and efficiency of mining operations.

In the present work, SEM and NMR were used to measure the variation in the microstructure and different types of pores during dry coal rock soaking. Based on the above results, uniaxial compression experiments were carried out under different soaking times to study the relationship between the immersion times and mechanical properties. Accordingly, we captured the change characteristics of AE parameters and the 3D strain cloud when loaded via the AE system and non-contact strain measurement system. We analyzed the evolution of the failure process and the change in failure mode with soaking time. Finally, by linking the microstructure and pore phase composition with the macroscopic mechanical properties of water-immersed coal rock, the mechanism by which immersion modifies the mechanical behavior of water-immersed coal rock was determined.

2. Experimental

2.1. Coal Rock Sample Preparation

The coal block was obtained from the 4-2 coal seam of the Liangshuijing coal mine in Shenmu City, Shaanxi Province, China (Figure 1). To avoid influencing the variability of the coal samples, all coal samples were selected in parallel sections in the same primary structural coal rock. The upper aquifers of the 4-2 coal seam from top to bottom are the sand pore aquifer of the Salawusu Formation, the bedrock fissure aquifer of the Yan’an Formation, and the weathered bedrock fissure aquifer of the Yan’an Formation and the Zhi-luo Formation. Block samples were obtained from coal mines following the Chinese Standard Method GB/T19222-2003 [39], and were carefully packed and delivered to the laboratory for experiments. The coal rock was made into a standard sample of Φ 50 mm × 100 mm, as suggested by the ISRM, through a series of processes such as core drilling, cutting, and grinding, with the central axis of the sample perpendicular to each joint surface of the sample, the vertical deviation angle not greater than 0.25°, and the diameter error at both ends of the sample not greater than 0.05 mm. The processed sample is shown in Figure 2. The coal quality results of the coal rocks are shown in

Table 1. Analysis of elemental composition of coal rock.

Sample	Proximate Analysis (%)			Mineral Content (%)
	Mad	Aad	Vdaf	C	H	O	N	S	C/H	H/O
Liagnshuijing coal mine	5.65	5.23	32.65	78.21	4.77	15.32	1.65	0.43	1.37	0.226

.

To minimize the effects of the discrete nature of the coal rock on the test results, Samples with visible voids, cracks, and impurities were excluded from the study, and then the remaining samples were subjected to ultrasonic testing to screen out samples with similar longitudinal wave speeds for testing. Finally, a random selection of 36 coal rock samples was prepared for testing their physical and mechanical properties before and after water soaking. These samples were divided into two groups: The first group consisted of 12 samples used for testing physical properties related to water soaking, including porosity, SEM, NMR and microstructural analysis. The second group included 24 samples separated into 6 subgroups with 4 samples each, for conducting mechanical properties tests. The mechanical properties of the samples are shown in

Table 2. Numbering and mechanical parameters of the mechanical properties group at different immersion times.

Sample Number	Soaking Time (d)	Diameter (mm)		Height (mm)		Peak Stress (MPa)		Elastic Modulus (GPa)
		Tested	Average	Tested	Average	Tested	Average	Tested	Average
0-1	0	49.62	49.75	100.04	99.71	33.23	32.95	2.68	2.44
0-3		49.89		99.76		33.76		2.28
0-4		49.62		99.26		37.58		2.74
0-5		49.86		99.76		27.24		2.05
15-1	15	49.52	49.65	99.82	99.76	22.51	22.76	2.05	2.00
15-2		49.54		99.86		21.72		1.91
15-4		49.98		99.36		23.48		2.14
15-5		49.57		99.99		23.35		1.91
30-2	30	49.85	49.75	99.68	99.90	20.73	20.68	2.01	1.93
30-3		49.71		99.68		20.16		1.85
30-4		49.69		100.22		21.21		1.99
30-5		49.76		100.02		20.62		1.86
60-1	60	50.07	50.04	100.01	99.88	18.69	18.71	1.96	1.89
60-2		50.02		99.76		19.81		1.94
60-3		49.99		100.07		18.14		1.79
60-4		50.06		99.69		18.21		1.85
120-1	120	49.98	50.00	100.06	100.02	17.63	17.61	1.89	1.83
120-2		50.02		100.02		18.06		1.87
120-3		50.05		100.09		17.13		1.73
120-5		49.93		99.9		17.62		1.83
240-1	240	50.07	50.13	100.62	100.42	16.97	16.75	1. 63	1.69
240-2		50.28		100.55		17.01		1. 78
240-5		50.03		100.23		16.99		1.72
240-6		50.12		100.27		16.01		1.63

.

By testing the same primary structural properties of coal rock samples before and after controlled water soaking, changes in the physical parameters and mechanical performance could be analyzed to reveal the effects of water on the coal rocks. The integrated results provide insights into the microscale and mesoscopic structural evolutions and their correlations with macroscopic property variations of the soaked coal.

2.2. Experiment Procedure

In order to undertake this research on the effects of soaking time on the microscale and mesoscopic structure, acoustic characteristics, and mechanical properties of coal rocks, and to reveal the damage mechanisms of coal rocks under the action of soaking time and load, the following test steps were designed:

(1) The samples were kept flat in the oven at 110 °C for 24 h of continuous drying. When the change in weight before and after ceased to exceed 0.1%, it was considered to have reached the dry state;

(2) The dried samples were placed in a desiccator, cooled to room temperature, and weighed. They were then soaked according to the method recommended in GB/T 23561.5-2009 [40], titled “Method for Determination of Physical and Mechanical Properties of Coal and Rock” (In Chinese). Following this procedure, the physical properties group was subjected to an immersion test. The samples used for the immersion test were monitored for a continuous period of 30 d to calculate water saturation;

(3) SEM tests were carried out on the same coal rock samples under three conditions for the physical properties group—dry (0 d), naturally saturated (5 d), and after soaking for 240 d;

(4) For the physical properties group, NMR analysis was conducted on the same coal rock samples at different soaking durations—0 d, 15 d, 30 d, 60 d, 120 d, and 240 d;

(5) For the mechanical properties group, with 24 samples, mechanical testing was conducted using a DDL600 test machine. The coal rock samples were soaked for different durations—0 d, 15 d, 30 d, 60 d, 120 d, and 240 d—before mechanical testing. The tests measured the compressive strength, tensile strength, and elastic modulus at a loading rate of 0.005 mm/s until failure;

(6) During the loading tests, acoustic emissions from coal rock deformation were monitored in real-time and collected using acoustic instrumentation. Two CCD cameras were set up to record synchronized images, capturing the entire deformation process of the samples. A non-contact strain measurement system (VIC-3D) was utilized to obtain time-resolved strain data for the coal rocks throughout the loading process.

2.3. Water Saturation of Coal Rock

The water saturation of coal rock is the ratio of the volume occupied by water to the pore volume of the coal rock. Its value can be calculated based on the quality of coal rock samples with different immersion periods, as shown in Equation (1).

$$S=\left(\frac{V_t^{}-V_0^{}}{V_n^{}-V_0^{}}\right)\times 100\%$$

(1)

where S is the water saturation of the coal rock (%). V_t is the volume of the coal rock after t d (cm³). V_n is the volume of the coal rock after natural saturation and water absorption (cm³). V₀ is the volume of the coal rock after drying (cm³). V_t, V_n, and V₀ can be obtained by the relationship between density and mass.

$$V_t^{}-V_0^{}=\frac{M_t^{}-M_0^{}}{{\rho }_w}$$

(2)

where M_t is the mass of coal immersion for t d (g). M₀ is the mass of coal after drying, g. ρ_w is the density of water (g/cm³).

$$V_n^{}-V_0^{}=\frac{M_n-M_0}{{\rho }_w}$$

(3)

where M_n is the natural saturated mass of the soaked coal (g).

Substituting Equations (2) and (3) into Equation (1), we can obtain the following:

$$S=\left(\frac{M_t^{}-M_0^{}}{M_n^{}-M_0^{}}\right)\times 100\%$$

(4)

Considering the influence of coal rock dispersion on the test results, a group of coal rock samples from the physical properties group was selected to study the relationship between water content saturation and immersion duration. The soaking process was divided into the stages of water swelling (saturation process) and soaking damage (long-term immersion process) according to the variation of water content saturation, as shown in Figure 3. It can be concluded from Figure 3 that the water saturation of the coal rock gradually increased with increasing soaking time during the water absorption and swelling phases, with the growth rate first fast and then slow, and finally reaching saturation. After 1 d of immersion, the average water saturation of the coal rock reached about 70.49%, and then the growth rate gradually decreased. After 5 d of immersion, the water saturation of the sample reached 100% and reached a saturated state. After saturation, the coal enters the stages of soaking damage (long-term leaching process).

2.4. Scanning Electron Microscope of Soaking Coal

The SEM was used to scan the coal rocks in the dry state (0 d), the soaked with natural water absorption state (5 d), and the soaked for 240 d state, and the results are shown in Figure 4.

As shown in Figure 4, the soaking length significantly affects the microstructure of the coal sample. The SEM result of the dry state is shown in Figure 4a. The mineral particles and coal matrix in the coal rock are cemented together to form a massive and plate-like structure; the larger particles are composed of fine coal matrix particles, and the pores and micropores are relatively evenly distributed between the particles.

The SEM results of the saturated state are shown in Figure 4b. The sample was full of pores and cracks; a large amount of coal matrix and minerals absorb water and expand, as a result of which the particles squeeze each other, the connections between particles become closer, and a large number of microcracks are generated. The insoluble minerals in the coal rock are separated out.

The SEM results after 240 d of immersion are shown in Figure 4c. The initially dense coal rock gradually adopted a porous loose structure due to the effects of long-term water immersion, and the linkage between mineral particles and matrix became looser. At the same time, the matrix became distributed in a flaky or flocculent structure, and the flaky and flocculent structures were damaged by mutual extrusion and collision under the action of water and stress, resulting in the formation of many pores and a debris-like structure, causing mineral loss. The long-term immersion of coal rocks leads to changes in the matrix structure, affecting their internal structural distribution and macro-mechanical properties.

According to Table 1, the coal contained calcite (CaCO₃) and kaolinite (Al₂Si₂O₅(OH)₄), and the coal rock was soaked for a long time [41]. This causes changes in coal rock minerals, which will affect the internal element distribution and macro-mechanical properties. It is generally believed that the carboxyl and phenolic groups in coal rock can react with water to produce a small amount of acid, which then reacts with carbonate minerals such as calcite. In addition, weak acidity will cause the dissolution of very few carbonate minerals, and accelerate the damage of the sample. Kaolinite is a highly permeable granular material that can quickly absorb water into pores and cracks. The volume of kaolinite swells freely and increases by 28% with water soaking. Hydrates will form or absorb water to form gypsum, and the volume of gypsum will increase by 61%. This conclusion has been confirmed [24]. The dissolution and diffusion of minerals has an important influence on the pore structure during the soaking process.

2.5. Analysis of the Mesoscopic Structure of Soaked Coal Rocks

NMR measures the relaxation properties of water in rock pores mainly through the magnetic properties of atomic nuclei and their interactions with external magnetic fields. The protons in the sample resonate and absorb the energy generated by a certain radio frequency pulse in the magnetic field. After the radio frequency pulse passes, these protons release energy and generate NMR signals that are detected by a special coil. The energy release rate of samples with different properties is different, so the NMR signal can directly reflect the change in the pore structure.

According to the IUPAC classification, materials are divided into microporous (2.0–0.4 nm), mesoporous (50.0–2.0 nm) and macroporous (>50.0 nm). Combined with the relaxation times of 0~3 ms, 3~33 ms, and 33~10,000 ms, they correspond to micropores, mesopores, and macropores, respectively [22]. The T₂ spectrum of coal rock with different immersion times was obtained by NMR tests on the same sample with drying and soaking for 15 d, 30 d, 60 d, 120 d, and 240 d, as shown in Figure 5. It can be seen from the figure that the T₂ spectrum of the soaked coal rock presents a three-peaked shape, and the three peaks correspond to micropores, mesopores, and macropores (from left to right), changing dynamically with immersion time. Although the third peak of the T₂ spectrum of the dry coal rock is not apparent, it can be characterized by the fact that there are still hydrogen atoms in the pores after the coal rock is dried.

The peak area of NMR is proportional to porosity [23]. The larger the peak area is, the greater the number of pores. In Figure 6, the classifications of pores are based on the IUPAC definitions: microporous (pore diameters of 2.0–0.4 nm), mesoporous (pore diameters of 50.0–2.0 nm), and macroporous (pore diameters > 50.0 nm). The graph in Figure 6 indicates the distributions of these pore categories in our samples. After soaking saturation, the volumes of macropores and mesopores accounted for about 10% of the total pore volume. In comparison, the volume of micropores accounts for more than 78.67% of the total pore volume, which indicates that micropores dominate the pores of soaked coal rock. At 15 d of soaking, the peak area of the pore spectrum reached 8398.57; at 240 d of soaking, the peak area of the pore spectrum reached 9703.53; compared with 15 d of soaking, the amplitude of the third peak of the T₂ spectrum increased, and the peak area of the macropores spectrum gradually increased. With the increase in immersion time, the micropores of the coal rock gradually increased.

The increases in microcroporosity and macroporosity with prolonged soaking indicate that water immersion over time opens up smaller pores and microcracks (micropores), as well as larger fractures and channels (macropores) in the coal rock. The dominance of micropores (about 80% of pore volume) in soaked samples suggests that water primarily penetrates the coal matrix through microscopic pores rather than fractures.

3. Analysis of Macroscopic Mechanical Properties of Soaked Coal Rock

3.1. Acoustic Emission and Compression Test and Analysis of Soaked Coal Rock

The acoustic emission, non-contact strain measurement system, and compression system used in the tests are shown in Figure 7. AE activity correlates with the accumulation of damage in the sample. Changes in AE hit rate can indicate different stages of crack development. The non-contact strain measurement system measures axial and radial strains on the sample surface during compression. The servo-controlled compression machine or the loading frame provides axial loading on the coal rock samples. This system can simultaneously monitor the macro and meso failure characteristics of the coal rock compression process.

The strain and acoustic characteristics of coal rock are monitored by acoustic emission, non-contact strain measurement, and compression systems. The main amplifier of the acoustic emission system is set to 40 dB, the detection threshold is 40 dB, and the resonance frequency of the probe is 20~400 kHz. The results of the uniaxial compression and acoustic emission of soaked coal rock are shown in Figure 8.

It can be seen from Figure 8 that the stress–time curves of coal rock with different immersion times are divided into the following four stages, according to the acoustic emission ringing count and cumulative ringing count:

(1) Primary pore and fissure density stage—At the initial loading stage, the primary pores and cracks in coal rock gradually closed and compacted, and the acoustic characteristics were not obvious. The cumulative ring count curve of acoustic emission was close to the X axis, and the stress–time curve was concave. The time growth rate was greater than the stress growth rate, and the degree of concavity of the stress–time curve is closely related to the pores and cracks of coal rock. The lower the number of primary pores and fissures and the denser the structure, the smaller the temporal concavity of the stress, but the faster the slope of the stress curve increases, and vice versa;

(2) Elastic stage—As the load increases, the acoustic emission ringing count gradually increases, and the cumulative acoustic emission ringing count curve begins to rise as the coal rock enters the elastic deformation stage after the primary pore fissures of the coal rock are closed by the load. At this stage, the primary pore fractures within the coal rock are largely compacted, the stress increases approximately linearly with time, and the rate of increase in stress time is almost constant;

(3) Crack stable propagation stage—As the load continues to increase, the coal rock emits a distinct acoustic signature of internal extrusion friction, the acoustic emission ringing count continues to increase, and the slope of the cumulative ringing count curve continues to rise, signaling that the coal rock is entering the stage of stable crack expansion. During this stage, the coal rocks are internally squeezed and rubbed against each other, creating new cracks, and some of the strain energy stored in the coal rocks is released through fractures or acoustic features;

(4) Unstable crack propagation stage—Further loading increases the cumulative acoustic emission ringing count to a roughly exponential pattern, during which the acoustic response is violent, with occasional crisp popping sounds and unstable fracture expansion until the sample is damaged.

Significant differences are present in the evolution of acoustic emission ring counts and ring accumulation counts for dry coal rocks and for different immersion times, particularly in the stable and unstable crack extension stages. The frequent and concentrated distribution of acoustic emission activity in dry coal rocks indicates that the internal structure of dry coal rocks is relatively intact and the distribution of defects is concentrated. Compared with dry coal rocks, the acoustic emission signals of coal rocks after immersion for 15 d and 30 d gradually increased, and the cumulative growth range gradually expanded, indicating that the internal structure of coal rocks at this stage became loose and defects gradually increased. At 60 d, 120 d, and 240 d of immersion, compared to the coal rock before 60 d of immersion, not only did the acoustic emission signal gradually increase and the cumulative growth range gradually expand, but the duration after the peak stress point also gradually increased, indicating that the internal structure of the coal rock become looser and the defects further increased at this stage, and the deformation changed from brittle to plastic at the same time.

3.2. Non-Contact Strain Measurement Test and Analysis of Soaked Coal Rock

A typical strain cloud of soaked coal rock obtained from the acoustic emission non-contact strain measurement and compression system is shown in Figure 9 (the specimen depicted in Figure 9 was chosen for its representative nature, as it underwent stages consistent with other specimens in our study).

In the VIC-3D test system, positive strain was preset for tension and negative strain for compression. The colors represent the coal rock’s different strain states: the red area is the tensile strain zone, the green area is the zero strain zone, and the purple area is the compression strain zone. The strain and crack extension of the coal rock can be observed for each stage of loading from Figure 9, which also correspond to the four stages:

(1) Primary pore and fissure density stage (Figure 9a,b)—In the starting state, the surface of the coal rock, which is not subjected to loading, does not generate stresses and strains and appears green. After starting loading, the primary pore fissures compressively close, and the coal rock begins to show areas of tensile and compressive strain with corresponding stresses and strains;

(2) Elastic stage (Figure 9b)—As the load increases, the coal rock strain increases, and the color of the surface strain field changes, with red and purple areas appearing in various sections one after another, and areas of tensile and compressive strain changing, but no obvious macroscopic cracks or areas of stress concentration appear;

(3) Crack stable growth stage (Figure 9c,d)—At this stage, the coal rock strain cloud is generally purple, with only red areas appearing near the cracks, indicating a sustained stretching of the sample, with obvious macroscopic cracks beginning to appear on the surface of the sample and stress concentration around the cracks. As the load continues to increase, the overall state of compressive strain appears, but tensile strain occurs under the action of tensile stress around the crack, and the crack further expands and penetrates;

(4) Crack unstable growth stage (Figure 9e,f)—After the formation of the through crack, as the load increases further, the existing cracks on the surface of the coal rock expand unsteadily, forming new cracks and triggering brittle fractures and the occasional fly out of coal chips. The compressive strain energy is released after the coal chips fly out, and the strain cloud returns to green.

3.3. Compression Failure Form of Soaked Coal Rock

The typical forms of compression deformation and failure of coal rocks under different immersion durations are shown in Figure 10. It can be seen from the figure that under the action of axial load, the stress forms of coal rock are different, and the compression deformation and damage show different forms. In this test, the failure forms of soaking coal rocks are summarized into three types: tensile failure form, shear failure form, and conjugate shear failure form. The soaked coal rock damage model changed from tension to shear failure as the soaking time increased, ending with conjugate shear failure.

As shown in Figure 10a, due to the actual shear stress on the fracture surface of the dipping coal rock being more significant than the ultimate shear stress, shear cracks appear on the surface of the dipping coal rock under stress, tensile strain is generated near the cracks, and the through shear surface formed by the gradual development of the shear cracks causes shear damage.

Figure 10b shows that the Poisson effect is generated in the soaked coal rock under axial loading (the Poisson effect is defined as a material’s tendency to expand in a direction perpendicular to the compression direction). As the radial tensile strength limit is less than the radial tensile stress generated by the Poisson effect, fractures are generated under the tensile stress. Tensile strains are generated near the fractures, prompting the formation of a through tensile damage surface, resulting in tensile damage.

As shown in Figure 10c, the tensile microcracks and shear microcracks inside the soaked coal rock are not visible to the eye. Under the joint action of tensile stress and shear stress, the tensile micro cracks and shear micro cracks will expand and derive, together forming the X-destructive form.

3.4. Compressive Strength and Deformation Characteristics of Soaked Coal Rock

The stress–strain curve obtained from the soaked coal rock compression test is shown in Figure 11. The uniaxial compressive strength of the coal rock gradually decreased with increasing immersion time, from 33.23 MPa at 0 d dry coal rock to 16.97 MPa at 240 d immersion—a reduction rate of 48.93%. After 15 d of immersion, the internal defects of the coal rock evolved, and the corresponding peak strength decreased rapidly; after 60 d of immersion, the damage accumulation rate tended to be stable, and the newly generated pores effectively delayed the development of mesoscopic defects. After 240 d of immersion, the expanding space for hydraulic conductivity caused by prolonged immersion contributes to a further deterioration in the strength of the soaking damage. After 240 d of immersion, the expanding space for hydraulic conductivity caused by prolonged immersion contributed to further deterioration, causing strength loss.

The elastic modulus was reduced from 2.68 GPa of dry coal to 1.63 GPa after 240 d of soaking, and the reduction rate was 29.53%. The decrease in elastic modulus reflects that the ability of coal rock to resist deformation was weakened. At the same time, due to the influence of immersion time, the deformation characteristics were also accompanied by the development of internal defects in the rock, which aggravated the damage to rock samples.

With the increase in immersion time, the axial strain of coal rock increased gradually under the same stress, and the expansion and compression processes of internal pore cracks were frequent. The stress–strain characteristics of coal rock under different immersion times correspond to the characteristics of the divided curve primary pore and fissure density stage, elastic stage, crack stable growth stage, and crack unstable growth stage.

4. Discussions

4.1. Analysis of Sample Soaking Process

A combination of intermolecular forces (Including van der Waals forces and hydrogen bonding) and capillary forces provides the driving force for water adsorption onto coal rock. Specifically, water molecules and coal rock particles are attracted to each other through van der Waals forces. Additionally, the hydrogen molecules in water bond with polar adsorption sites on the surface of the coal rock particles, allowing water to penetrate into the interior of the coal rock. In addition, the capillary force generated between water and the internal pores acts as a driving force to enhance water penetration into the pores and facilitate migration through the pores under capillary action. Moreover, the greater the capillary force, the faster the absorption. As the capillaries gradually fill with water, the capillary force continues to weaken, and the water migration slows. Thus, the water absorption rate decreases over time, such that the growth rate of water saturation is initially rapid, but then slows and eventually stabilizes, as shown in Figure 3.

4.2. Analysis of Damage Mechanism of Coal Rock by Immersion Time

There are many pores and fissures inside the coal rock, which will cause damage to the internal structure of the coal rock as the soaking time increases. Coal rock has multi-scale pore structure characteristics, and multi-level pores in coal are connected in series and permeable [42]. During the soaking process, the coal rock experienced drying shrinkage of the rock surface as well as swelling caused by water absorption, and finally, soaking damage after being saturated with water, as shown in Figure 12. In the dry state, the surface of the coal rock was rough and prone to drying and shrinkage, and the microscopic features were manifested with the drying of the internal mineral microstructure, as shown in Figure 12b. After immersion and absorption, the matrix in the coal rock became swollen with water, and the microcracks in the coal rock were squeezed with the swelling of the minerals and matrix during immersion [18,19,24], as shown in Figure 12c. The surface became raised, the original structure became more compact, the contact surface became smoother, and the gloss was improved. As the soaking time continued to increase, the cement gradually diluted, the internal structures extruded and destroyed each other, the pores increased, and new cracks were created, as shown in Figure 12d. Water damage to the coal rock accumulated.

The difference in NMR signal amplitude for different immersion times of the coal rock shows a positive correlation with the water content in the sample, and the different responses relate to a change in the number of flow channels into which water is soaked. As the soaking time increases, the matrix and minerals continue to swell, the matrices continue to squeeze each other causing an increase in fractures and pores, the pores in the coal rock are filled with intrusion water, and the intensity of the NMR signal increases, as shown in Figure 6. During the long-term immersion of coal rock, matrix particles and mineral particles are soaked by water and undergo different degrees of volumetric deformation. Uneven deformation between particles with different expansion coefficients produces significant tensile stress at the cementation, and the damage pattern formed by loading is shown in Figure 10a. With the increase in immersion time, the formation of a coal rock structure containing apparent layers and weak surface structure is prone to shear slip, and the damage mode formed by loading is shown in Figure 10b. The long-term immersion of coal’s internal structure is loose; the damage pattern caused by loading is shown in Figure 10c. The distribution range of acoustic characteristics during compression gradually expands, thus weakening the cohesion and compressive capacity of the coal rock matrix and minerals, as shown in Figure 11.

4.3. Limitations

Despite the significant findings of this study, it is important to acknowledge its limitations. First, the research samples primarily came from coal mines in the Shenmu area, which might not fully represent coal rock properties worldwide. As a result, the findings may not be directly applicable to coal rocks in other regions, especially those with significantly different geological characteristics. Secondly, the experiments were conducted under laboratory conditions, which might differ from actual underground environments. For instance, the laboratory cannot fully replicate the high pressure and temperature conditions underground, nor can it reproduce other chemical or physical factors in the underground environment. Therefore, the experimental results might be somewhat oversimplified to a certain extent. Given these limitations, future studies could collect and analyze a broader range of coal rock samples, simulate conditions closer to actual underground environments, and further identify more general patterns. Despite its limitations, the study provides a further understanding of the microstructure, mesostructure and macromechanical properties of coal rock under long-term immersion.

5. Conclusions

This study investigates the effects of soaking time on the structural, pore, and mechanical properties of coal rocks based on a series of tests (immersion tests, wave velocity tests, SEM, NMR, AE, DIC, and uniaxial compression tests). The mechanism of water damage to coal rocks during long-term immersion was discussed. The conclusions are as follows:

(1) The water saturation increases with increasing immersion time, and the growth rate is initially faster and then slower. After 5 d of immersion, the coal rock reaches saturation and water content stabilization. In the early stage of water swelling, the stronger capillary force enables a faster absorption rate and a rapid increase in water saturation. As the soaking time continues to increase, the weaker internal capillary force slows water absorption and decreases the water saturation growth rate;

(2) The coal rock immersion process undergoes drying shrinkage, water swelling, and soaking damage. As the soaking time continues to increase, the minerals continue to absorb water and swell before saturating the coal rock and squeezing each other, and after saturation, as the soaking time continues to increase, the minerals and matrix swell differently, and the damage continues to accrue. The NMR T₂ spectrum of the coal rock shows a three-peak distribution, with micropores dominating within the soaked coal rock. With the increase in immersion time, micropores, mesopores, and macropores are transformed into each other. The generation and collapse of pores of different sizes within the coal rock occur together;

(3) According to the AE and VIC-3D observation data, the stress–strain process in soaked coal rock can be divided into primary pore and fissure density stages, elastic stages, the crack stable growth stage, and the crack unstable growth stage. The degree of concavity in the stress–strain curve during the pore–fissure compression and closure phases is determined by the pore–fissure distribution in the coal rock. The cumulative AE ringing count in the elastic phase undergoes an exponential increase. Cracks are produced on the surface during the stable crack extension phase. Crack evolution in the unstable crack extension phase increases, and the AE ringing count undergoes an exponential increase;

(4) The compressive strength and elastic modulus of the Liangshuijing coal rock tend to decrease with increasing immersion time. Immersion has a significant effect on the mechanical properties of coal rocks. The strength of soaked coal rocks is not only related to the water content. It is also influenced by the duration of immersion, which further affects the mechanical properties under long-term immersion in water. As the soaking time increases, the matrices swell and crush each other, and the different swelling coefficients of the matrix and minerals exacerbate the damage.