Deformation and Failure Mechanism and Control of Water-Rich Sandstone Roadways in the Huaibei Mining Area

Abstract

The sandstone roof rock in the Huaibei mining area contains abundant water at depths of 2–3 m. Water–rock interactions in the rock-surrounding roadway can cause significant deformation, seriously threatening the safety of mine operations. Investigating the deformation and failure mechanisms of water-rich sandstone is therefore of critical importance. In this study, X-ray diffraction and scanning electron microscopy were used to analyze the composition and microstructure of water-rich sandstone. Based on the stress state during the roadway excavation, a true triaxial loading scheme with four different stress paths was designed to study the effects of different moisture contents and loading methods on the mechanical properties of the sandstone. The results show that the deviatoric stress decreased for all stress paths. Acoustic emission (AE) characteristics during the deformation and failure processes were also studied, which indicated that the AE b-value decreased, increased, and then decreased again corresponding to the primary compaction, elastic deformation, and plastic deformation evolutionary processes in the internal microstructure of the rock. The variation in the b-value reflected the development and expansion of internal fractures. These findings provide useful insights for controlling the stability of the surrounding rock in water-rich roadways in coal mines.

1. Introduction

In complex environments in the field of coal mining, surrounding rock control on roadways is a critical technical challenge that needs to be addressed urgently, particularly under the influence of excavation-induced disturbances [1,2,3,4,5]. The infiltration of pores and fracture water in the roof during roadway construction can cause the collapse, swelling, and softening of immature rock layers rich in clay minerals, leading to significant strength degradation. This significantly increases the risk of roof instability [6,7,8,9,10]. Studying the mechanical response characteristics of rocks under different moisture conditions is essential for addressing engineering problems related to the support design and stability assessment of rocks surrounding water-rich roadways, thus ensuring the safe operation of coal mines [11,12,13,14,15].

Recently, significant progress has been made in both domestic and international studies of the mechanical properties of rocks under water–rock interactions, particularly through uniaxial experiments that reveal the deformation and failure behavior of rocks under water-saturated conditions [16,17]. Previous studies indicate that water infiltration significantly reduces the strength and compressive resistance of rocks, thereby accelerating their failure. There have been many studies that have systematically investigated the changes in rock characteristics under moist conditions using several different methods, such as uniaxial compression tests and acoustic emission (AE) monitoring.

For example, Chen et al. [18] investigated the mechanical properties of coal–rock combinations under different soaking times and found that both the compressive strength and elastic modulus of the coal–rock decreased gradually with increasing soaking time, accompanied by a significant reduction in cumulative AEs, indicating a noticeable degradation effect. Further experiments by Xia et al. [19] highlighted that the compressive strength and elastic modulus of water-saturated rocks decreased significantly over time, with AE characteristics showing a clearer trend of crack propagation during deformation and failure. Li et al. [20] examined the effect of the moisture content on weakly cemented sandstone and found that increasing the moisture content led to a significant reduction in both the compressive strength and elastic modulus. This pointed to enhanced plasticity and a greater tendency for large deformations. In studies on the direct shear characteristics of sandy mudstone, Yao et al. [21] found that an increased moisture content significantly reduced the shear strength, cohesion, and internal friction angle of sandy mudstone, further weakening rock stability. Verstrynge et al. [22] conducted a multiscale analysis of iron-rich sandstone and observed a significant decrease in strength and stiffness under saturated conditions, showing degradation effects, particularly in low-quality iron-rich sandstone, where softening was more pronounced. For water-saturated sandstone, Yao et al. [23] investigated the instability and failure characteristics of roof rocks in the Ningdong coalfield and proposed a failure mechanism and classification method for water-rich roadways, providing a theoretical basis for roadway support in coal mines. Zhou et al. [24] revealed the evolution of the pore structure in red-bed soft rocks under water–rock interactions, noting that water infiltration significantly affected the pore diameter and porosity, leading to structural changes in the rock. Regarding the tensile properties of rocks, Wong et al. [25] pointed out through Brazilian splitting tests that gypsum showed a significant decrease in tensile strength after saturation, and the crack propagation rate was substantially slowed, indicating that the moisture content significantly influences crack propagation in rocks. Overall, previous uniaxial experiments have demonstrated that water infiltration typically results in a weakening of rock mechanical properties. However, the specific effects are variable and depend on the rock type. For sedimentary rocks, such as siltstones and sandstones, water saturation frequently leads to a substantial decrease in the compressive strength, elastic modulus, shear strength, and tensile strength, accompanied by a significant increase in crack development and failure severity. In contrast, metamorphic rocks, such as granites, may exhibit different responses. The impact of water saturation on their strength and deformation parameters is more complex and is influenced by factors such as mineralogical and petrographic composition, particularly the content of clay particles.

Using true triaxial loading experiments can effectively replicate the actual stress conditions of rock masses in the complex underground environment found in mines. The impact of water–rock chemical interactions on rock degradation under true triaxial conditions has gained increasing attention. Wang et al. [26] conducted true triaxial experiments on sandstone using different chemical solutions and found that the chemical composition of water significantly influenced the strength and deformation characteristics of rocks. Acidic and alkaline solutions accelerate the transition of rocks from brittle to ductile behavior, significantly degrading their mechanical parameters. Similarly, Li et al. [27] studied the effect of the moisture content on siltstone and pointed out that an increased moisture content significantly weakened the local cohesion of the rock, leading to a decrease in bearing capacity, with the failure mode exhibiting enhanced plasticity. Through true triaxial experiments, Feng et al. [28] revealed the failure modes of rocks under multiaxial stress conditions and their mechanical properties under the influence of water. Their study found that the failure modes of red sandstone in the natural and water-saturated states were significantly different. Under water-saturated conditions, the rock exhibited tensile failure, whereas under natural conditions, a combined tensile–shear failure occurred, indicating that the presence of water changed the mode of rock failure. Shen et al. [29] used true triaxial Hopkinson pressure bar tests to study the dynamic characteristics of coal samples under different pre-stress conditions. They found that static pre-stress under true triaxial conditions significantly increased the dynamic strength of the coal samples and reduced the extent of failure, whereas water infiltration further exacerbated the degradation process. Additionally, true triaxial experiments during the unloading process provide a clearer understanding of the effects of water–rock interactions on the rock energy evolution. Rong et al. [30] conducted unloading disturbance experiments to study the failure behavior of deep rocks under different stress paths. They found that the rocks exhibited strong dilation in the unloading direction and that the presence of water further exacerbated crack propagation and reduced rock mass stability. Wang et al. [31] developed a shear deformation band prediction model for sandstone under unloading conditions in true triaxial experiments and revealed the impact of the intermediate principal stress on the rock failure mode. Their study provided a theoretical basis for understanding the failure behavior of rock masses under complex hydrogeological conditions. In studies on the impact of water–rock interactions on rock permeability, Tian et al. [32] used true triaxial hydraulic fracturing experiments to compare the enhancement of permeability in sandstone roofs and soft coal seams. Their results showed that under hydraulic fracturing, the fracture network in the sandstone developed significantly, leading to a noticeable increase in permeability, whereas the coal seam exhibited compaction and fracture closure, indicating that water infiltration not only affected the mechanical properties of the rocks but also directly influenced the fracture propagation and permeability.

Although significant progress has been made recently in the study of rock mass mechanical behavior under water–rock interactions, there are still several shortcomings in previous studies. Many studies have focused on determining the mechanical properties and weakening mechanisms of rocks under saturated conditions through uniaxial and true triaxial experiments. These studies primarily target different types of rocks, such as siltstone, mudstone, and coal rock, and are mostly broad-based laboratory investigations that lack in-depth explorations of rock behavior in specific regional and conditional contexts. On the other hand, while previous studies have demonstrated the significant effects of water on rock mechanical properties and failure behavior, studies on sandstone in shallow burial and water-rich conditions, particularly in large coal mining areas like the Huaibei mining region, remain insufficient. The deformation and failure mechanisms of sandstone under unique water–rock interactions in such regions have yet to be fully explored. It should be noted that the Carboniferous sandstone in such scenarios is highly susceptible to water influence [33]. Especially in regions like the Huaibei mining area with extensive coal seams, the research on the deformation and failure mechanisms under the special water–rock interactions is still far from satisfactory. Therefore, this study focuses on sandstone from the Huaibei mining region, aiming to investigate the deformation and failure mechanisms of water-rich sandstone roadways under complex stress states and to propose effective strategies for controlling the surrounding rock stability.

2. Engineering Geological Background

The Renlou Coal Mine is situated at the junction of Suixi and Mengcheng counties in Huainan City, Anhui Province, on the southeastern edge of the North China Platform. The primary coal-bearing strata are Carboniferous–Permian formations. The immediate roadway roof consists of mudstone, with an average thickness of 1.87 m, while the overlying stratum is water-bearing fine sandstone, with an average thickness of 5.68 m. The sandstone layers within the roadway contain aquifer zones, with a normal water inflow ranging from 0 to 11.8 m³/h and a maximum inflow of up to 20 m³/h. The presence of water has weakened the mechanical properties of the surrounding rock, leading to a decrease in the self-bearing capacity of the anchoring system and a reduction in roof strength. Under actual conditions, the hazards of mine water can be identified by monitoring the chemical indicators of mine water, such as the pH value, TDS (Total Dissolved Solids) concentration, dissolved oxygen level, and flow velocity, to observe the corrosion status of anchoring materials. In addition, the geological conditions can be used to analyze the reactions of minerals with water and oxygen [34]. The roof wetting conditions at the site, along with a comprehensive column chart of the roadway’s roof and floor, are illustrated in Figure 1.

The weakening mechanism of the roof rock strength under water-rich conditions is illustrated in Figure 2.

As Figure 2 shows, the roof of the roadway comprises a sandstone layer overlaid by a water-bearing aquifer. The moisture in this aquifer gradually infiltrates downward into the underlying sandstone layer through percolation, causing significant weakening of the surrounding rock. As the roadway construction progresses, the stress state within the surrounding rock undergoes a marked change [35,36], which is clearly depicted in the stress variation curve shown in the figure. Within the area affected by the roadways, stress concentration zones have developed in the roof and sidewall regions. To enhance the roadway stability, rock and cable bolts were installed on the roof for support. However, owing to the presence of a water-bearing aquifer in the sandstone layer of the roof, water has infiltrated the boreholes of the bolts, forming new water-conducting pathways that have further intensified the water–rock interactions.

The formation of new water-conducting pathways has resulted in the expansion of internal fractures, significant strength degradation, and a marked reduction in the shear resistance of the rock. Continuous water infiltration has caused the sandstone layer to gradually transition from the elastic deformation phase to the plastic deformation phase, accompanied by rapid fracture development and expansion. During this process, the sandstone layer of the roof has undergone substantial deformation, which has manifested as a noticeable subsidence of the roadway roof, with inward displacement of both sidewalls and bulging of the floor. The subsidence of the roof and displacement of the sidewalls have exacerbated the damage to the surrounding rock, resulting in a significant decline in the overall stability of the roadway. To further understand the rock properties of the sandstone in this area, a thin-section analysis of the water-bearing sandstone was conducted, as shown in Figure 3.

According to the identification results provided by the Hubei Geological Laboratory (Figure 4), the water-bearing sandstone is a fine-grained rock; the structural type is fine sandstone, with the main components being quartz, feldspar, lithic fragments, and a matrix content of 20%. Microscopic observation revealed a homogenous microstructure influenced by compaction and recrystallization processes. This indicates that the rock underwent certain diagenetic processes after deposition, resulting in a low content of pore-filling materials, weak water absorption, limited expansion, and a relatively hard rock structure.

In the Huaibei region, most sandstone roof strata exhibit high stability when exposed to water. However, because of the unique depositional environment and geological structure of the region, the sandstones in the No. 7 coal seam display distinct properties that underscore the need to study water-bearing sandstones in the Huaibei mining area. Under water–rock interactions, these sandstones demonstrate unique mechanical behavior, and their low water absorption and expansion properties make it difficult to directly apply research findings from traditional mudstone studies. Therefore, in this study field, observations and experimental analysis were combined to systematically investigate this specific type of sandstone, focusing on its composition, microstructure, and physical-mechanical properties. Additionally, by controlling the triaxial loading–unloading path to alter stress environments, the degradation and instability mechanisms of the sandstone under water–rock interactions were examined, providing reliable theoretical and engineering guidance for roadway support design and surrounding rock control.

3. Materials and Methods

3.1. Experimental Scheme

Considering the characteristics of this specific type of sandstone, the experimental plan was designed from both micro- and macro-perspectives, as shown in Figure 5.

To reveal the intrinsic structural changes in the sandstone under water–rock interactions, a series of microscopic experiments, including X-ray diffraction (XRD) and scanning electron microscopy (SEM), were designed for this study. These experiments aimed to investigate changes in the microstructure and mineral composition of the water-saturated sandstone. XRD analysis was employed to identify the mineral composition and crystal structure of the sandstone, clarifying the types and concentrations of the internal minerals. SEM was used to observe the microfractures, pore structure, and the effects of water on these structures at the surface of the sandstone.

To simulate the actual stress state of the surrounding rock in a complex stress environment and investigate the deformation and failure behavior of the sandstone, this study designed a true triaxial loading experimental plan. Through multi-path stress loading, the deformation and failure characteristics of the surrounding rock under water-saturated conditions were systematically investigated [37]. Based on in situ geostress measurements from the No. 6 mining district of the Renlou Coal Mine, the experiment included three moisture content states (dry, medium saturation at 30%, and fully saturated) and four true triaxial loading–unloading paths, as shown in Figure 6. The specific loading–unloading paths and experimental procedures are outlined below.

1. Loading–Unloading Path I and Procedure: The minimum horizontal stress was maintained constant, while the maximum horizontal stress was unloaded, and vertical stress was applied. The specific steps were as follows: ① Axial stress σ₁ was applied in the z-direction, and horizontal stresses σ₂ and σ₃ were applied in the x- and y-directions simultaneously to the design values (σ₂ > σ₁ > σ₃), and this initial stress level was maintained for 180 s. ② Horizontal stress σ₃ was kept constant, horizontal stress σ₂ was unloaded, and axial stress σ₁ was applied until the specimen failed. The loading and unloading rates were set at 0.1 MPa/s. The specific loading–unloading path is illustrated in Figure 6a.

2. Loading–Unloading Path II and Procedure: The maximum horizontal stress was maintained constant, while the minimum horizontal stress was unloaded, and vertical stress was applied. The specific steps were as follows: ① Axial stress σ₁ was applied in the z-direction, and horizontal stresses σ₂ and σ₃ were applied in the x- and y-directions simultaneously to the design values (σ₂ > σ₁ > σ₃), and this initial stress level was maintained for 180 s. ② Horizontal stress σ₂ was kept constant, horizontal stress σ₃ was unloaded, and axial stress σ₁ was applied until the specimen failed. The loading rate was set at 0.1 MPa/s, and the unloading rate was set at 0.05 MPa/s. The specific loading–unloading path is illustrated in Figure 6b.

3. Loading–Unloading Path III and Procedure: Both the maximum and minimum horizontal stresses (σ₂ and σ₃) were unloaded while applying the vertical stress (σ₁). The specific steps were as follows: (i) Axial stress σ₁ was applied in the z-direction, and horizontal stresses σ₂ and σ₃ were applied in the x- and y-directions simultaneously to the design values (σ₂ > σ₁ > σ₃), and this initial stress level was maintained for 180 s. (ii) Both horizontal stresses σ₂ and σ₃ were unloaded without change, and axial stress σ₁ was applied until the specimen failed. The loading–unloading rate for σ₁ and σ₂ was 0.1 MPa/s, and for σ₃, the unloading rate was 0.05 MPa/s. The specific loading–unloading path is shown in Figure 6c.

4. Loading Path IV and Procedure: This was the conventional true triaxial loading stress path. The specific steps were as follows: (i) Axial stress σ₁ was applied in the z-direction, and horizontal stresses σ₂ and σ₃ were applied in the x- and y-directions simultaneously to the design values (σ₂ > σ₁ > σ₃), and this initial stress level was maintained for 180 s. (ii) Horizontal stresses σ₂ and σ₃ were kept unchanged, and axial stress σ₁ was applied until the specimen failed. The triaxial compressive strength σ_k of the sandstone sample under this stress condition was obtained. The loading rates of the specimen were 0.1 MPa/s. The specific stress paths are shown in Figure 6d.

While uncovering the macroscopic failure mechanisms of the surrounding rock, this study used AE technology to monitor the failure process with the aim of investigating the microscopic failure characteristics of the sandstone under different moisture contents and stress conditions.

3.2. Sample Preparation

To investigate the mechanical characteristics of the water-saturated sandstone in the roof under true triaxial stress conditions, core samples were drilled onsite, sealed in a plastic film to preserve their original structure and moisture, and transported back to the laboratory in foam-padded wooden boxes, as shown in Figure 7. Following the national standard “Standard Test Methods for Engineering Rock” (GB/T 50266-2013), the sandstone core samples were processed into standard cubic specimens with dimensions of 50 mm × 50 mm × 100 mm (length × width × height). The end face of the sample was perpendicular to the sample axis (with an error within 0.001 radians), and the unevenness did not exceed 0.02 mm.

The sandstone samples possessed gentle wavy bedding, were calcareously cemented, and had a compact internal structure. To minimize experimental variability, samples were selected and cut to exclude areas with visible fractures. During sample preparation, the surface flatness was maintained within 0.04 mm. Additionally, preliminary testing was conducted using an ultrasonic velocity tester to identify significant deviations, with outliers excluded. The samples were then grouped according to their wave number, as shown in Figure 8, with the grouping results presented in Table 1.

3.3. Water Immersing Characteristics

The samples were prepared using a REY-CC-101-00B drying oven and a custom-made vacuum saturation device. The sandstone samples were placed in the drying oven, where the temperature was set to 108 °C. After drying for 24 h, the samples were removed and cooled to room temperature. Two samples were then selected and placed in the vacuum saturation device for water absorption testing using the manual weighing method. During the first six hours, the samples were removed every half hour, and the surface water droplets were wiped off before weighing with an electronic balance. When the weight increment difference between measurements was less than 0.01 g, the samples were considered to have reached water saturation. The water content of each sample was calculated using the following equation:

(1)$W_t={\displaystyle \frac{R_t-R_0}{R_0}}\times 100\%$

where W_t is the water content of the sample at time t (%), R₀ is the mass of the sample when it was removed from the drying oven (g), and R_t is the mass of the sample at time t (g). The average water content of the sample as a function of immersion time is shown in Figure 9.

As shown in Figure 9, the water absorption process of the top-coal sandstone can be divided into four stages: a rapid absorption stage (0–6 h and 0–3 h), during which the absorption curve rose sharply as water was quickly absorbed through a large number of highly permeable pores and fractures; a slow absorption stage (6–14 h and 3–10 h), when the absorption curve became less steep as the initial pores were filled and the absorption rate gradually decreased as the water spread into the rock through the smaller pores and microfractures; a saturation absorption stage (14–19 h and 10–20 h), when the absorption curve flattened out as the rock approached saturation, indicating that the internal pores and fractures were nearly filled with water and the absorption reached its maximum; and a long-term absorption stage (20 h and 19 h onward), when the curve remained flat, suggesting the presence of tiny pores or the need for pressure equilibrium within the pores, which caused a prolonged period of minimal water absorption. The average saturated water content of the sandstone was 1.54%.

3.4. Experimental System

The true triaxial testing machine used in this experiment primarily consisted of a three-directional electrohydraulic servo loading system, a true triaxial loading frame with a pressure chamber, a data acquisition system, and an operation platform. The maximum loading pressures in the three directions were 2000, 500, and 500 kN, respectively, with a load control accuracy and data acquisition precision of 1% and a frequency of 50 Hz. The stress loading precision of the test system was 0.01 KN, and the displacement measurement precision was 0.002 mm. Different specimen requirements for true triaxial tests can be satisfied by replacing the spacers inside the pressure chamber. The spacers were equipped with holes to allow the installation of AE probes, enabling the monitoring of the mechanical behavior and failure characteristics of the specimen during the test.

The loading system comprised three independent hydraulic cylinders, each responsible for applying the triaxial stresses (σ₁, σ₂, and σ₃). Two horizontal cylinders were housed within the pressure chamber, and a vertical cylinder was fixed at the bottom of the main loading system. The oil pump pressurized the cylinders to apply triaxial stresses to the specimen, and pressure sensors were connected to the pump valves to control and monitor the applied pressures. Horizontal loading was facilitated by the reaction frame.

To ensure a tight fit between the loading spacers and the specimen and to prevent slippage or misalignment during loading—both of which could cause stress concentration at the specimen’s corners and lead to premature failure—interlocking spacers were used. Additionally, provisions for AE probe holes were included to connect external monitoring equipment. Interlocking spacers were used to ensure a tight fit between the loading spacers and the specimen and to prevent slippage or misalignment during loading, both of which could cause stress concentration at the corners of the specimen and lead to premature failure.

4. Results

4.1. Microscopic Characteristics of Sandstone

1. Composition Analysis of Sandstone

Small samples of the obtained top-layer water-saturated sandstone were crushed to a fine powder by passing through a 325-mesh sieve for XRD analysis. The analysis was compared with the powder diffraction file standard card database. The XRD spectrum of the water-saturated sandstone from the top layer of the roadway is shown in Figure 10, and the component analysis results are presented in Table 2 and Table 3.

According to the XRD analysis, the sandstone primarily consisted of quartz (54.1%), sodium feldspar (15.8%), potassium feldspar (8.8%), and clay minerals (21.3%). Among the clay minerals, kaolinite (57.26%) and illite (42.74%) were dominant. The water absorption and weathering characteristics of these minerals significantly influence the mechanical properties of sandstones. Kaolinite exhibits strong water absorption, whereas illite expands upon water absorption, further affecting the rock stability. Moreover, after water penetration, illite weathers into montmorillonite, which exhibits strong swelling and high water-absorption properties. This results in volumetric expansion and strength reduction of the rock, leading to the deformation and strength weakening of the top-layer sandstone of the roadway.

• 2.. Microscopic Structural Analysis of Sandstone

In this experiment, SEM was conducted using a VEGA COMPACT device to examine the samples and reveal the microstructural characteristics of the water-saturated top sandstone layer in the roadway. As shown in Figure 11, at 50× magnification, the surface of the top-layer sandstone appeared dense with no obvious cracks. At 2000× magnification, irregularly developed cracks were observed with no clear directional patterns. At 3000× magnification, the pore channels were clearly visible, and the spaces between the particles were relatively large, facilitating the movement of water.

Water absorption by rock triggers a series of physical and chemical changes. From a physical perspective, water molecules fill micropores and cracks on the rock surface. Chemically, water interacts with the mineral components within rock, leading to dissolution, ion exchange, and other reactions. These processes alter the chemical composition and structure of the rock, resulting in a reduction in its strength and a significant impact on its mechanical properties.

4.2. Effect of Moisture Content on Mechanical Properties

4.2.1. Axial Compression and Unloading Confining Pressure (σ₂) Stress Path

Figure 12 shows the stress–strain relationships of the sandstone specimens under axial compression and confining pressure unloading conditions with different moisture contents. As the water content increased, the deviatoric stress of the sandstone specimens gradually decreased. The deviatoric stresses of the dry, 30% saturated, and saturated specimens were 107.6, 105.1, and 102.1 MPa, respectively, corresponding to a maximum reduction of 5.11%. During this loading path, the expansion deformation primarily occurred on the confining pressure unloading side (ε₂ > ε₃). As the water content increased, the strain in the direction of the minimum horizontal stress increased, and plastic deformation became more pronounced. This was because during the unloading process, the crack propagation tended to follow the path of least resistance, where stress release or energy dissipation was minimized.

Figure 13 shows that under the loading and unloading stress path, the failure surface of the specimen primarily exhibited lateral expansion along the σ₂ direction, with fractures appearing in the σ₁-σ₂ plane. This phenomenon is consistent with the maximum shear stress theory, which suggests that fractures propagate along planes oriented at an angle of 45° to the directions of the maximum and minimum principal stresses. During the unloading process, the stress concentration facilitated the propagation of microdefects, leading to the deformation and failure of the specimen.

4.2.2. Axial Compression and Unloading Confining Pressure (σ₃) Stress Path

Figure 14 shows the stress–strain relationship of the sandstone specimens under axial compression and unloading of the minimum horizontal stress at different water content levels. As the water content increased, the deviatoric stress of sandstone gradually decreased. The deviatoric stresses for the dry, 30% saturated, and saturated specimens were 124.4, 113.4, and 97.4 MPa, respectively. The deviatoric stress decreased by 21.7% from the dry state to the saturated state. Under this loading path, the specimen primarily failed along the direction of the minimum horizontal stress (σ₃). As the stress was unloaded, the internal cracks in the rock gradually propagated, forming new tensile fractures along the vertical plane in the unloading direction. The number of cracks increased and extended in parallel at certain angles. As the cracks continued to expand, they eventually coalesced into a macroscopic failure surface, resulting in the fracture of the rock specimen (Figure 15). With an increase in the water content, the number of cracks steadily increased, and the crack propagation process became more complex. Ultimately, the cracks merged into larger fracture planes, indicating that water accelerated the complexity of crack extension and failure.

4.2.3. Axial Compression and Unloading Confining Pressures on σ₂ and σ₃ Stress Paths

Figure 16 shows the stress–strain relationships of the sandstone specimens under axial compression and unloading of the confining pressure at different water content levels. As the water content increased, the strength and elastic modulus of the sandstone specimens decreased significantly. The specimens exhibited a ductile failure trend, characterized by a slow failure process after the peak stress and a longer softening phase in the total stress–strain curve, indicating noticeable plastic deformation. From the dry state to the saturated state, the deviatoric stress decreased by 27.4%, which was much larger than that observed under the other stress paths.

As Figure 17 shows, under this stress path, the sandstone specimens primarily failed along the σ₂ direction, with the fracture plane perpendicular to the direction of the maximum horizontal stress. The rapid unloading of the maximum horizontal stress induced stress concentrations along this direction, causing cracks to propagate in the stress-concentrated zones. In the dry state, the sandstone exhibited evenly distributed throughgoing cracks, dividing the specimen into three major blocks. At a moderate water content, the cracks displayed a combination of brittle and ductile characteristics, with more branched cracks and significant localized fragmentation. In the saturated state, the crack network became more complex, exhibiting ductile failure characteristics with a widespread crack distribution but without full penetration.

4.2.4. Conventional True Triaxial Stress Path

Figure 18 presents the curves of the σ₁, σ₂, and σ₃ deviatoric stresses as a function of axial strain (ε₁), lateral strains (ε₂, ε₃), and volumetric strain (ε_v) for sandstone with different moisture contents in conventional triaxial tests. It is evident that under stress Path I, as the water content increased, the stress–strain curves of the sandstone specimens exhibited distinct nonlinear characteristics, particularly during the crack propagation and plastic deformation stages. The higher the moisture content, the more pronounced the swelling phenomenon in the sandstone, resulting in a significant decline in its mechanical properties. Specifically, the maximum deviatoric stress values for the dry, 30% saturated, and saturated states were 135.2, 133.5, and 124.8 MPa, respectively. This indicates that the moisture caused a significant deterioration in the mechanical properties of the sandstone, with the maximum deviatoric stress decreasing by 7.7% under saturated conditions.

The experimental results indicated that under varying moisture contents, the initiation stress, closure stress, and damage stress of the sandstone decreased with an increase in the water content. This effect was particularly pronounced at higher moisture contents, where the damage stress was significantly reduced. During the true triaxial loading process, the presence of moisture accelerated the expansion and through-cracking of existing fractures, leading to a substantial decrease in the strength and stability of the sandstone. Consequently, the failure modes became more complex. As shown in Figure 19, the macroscopic failure patterns of the sandstone specimens under different moisture conditions in the conventional triaxial tests exhibited distinct differences. In the dry state, failure primarily occurred owing to shear stress, with fractures along the main shear plane. In contrast, at 30% saturated and saturated moisture contents, the fractures became more intricate, forming a three-dimensional crack network characterized by dense and interwoven fracture patterns.

4.2.5. Overall Influence of Different Water Contents

This experiment investigated the mechanical behavior of sandstone specimens with different moisture contents and found that the water content significantly affected the mechanical properties of the rock. As shown in Table 4 and Figure 20, with increasing moisture content from the dry to saturation states, the initiation, closure, and damage stresses of the sandstone decreased substantially. Specifically, for Path I, the initiation, closure, and damage stresses for the dry specimens were 9.58, 52.69, and 95.75 MPa, respectively. In contrast, with a saturated moisture content, the initiation, closure, and damage stresses dropped to 8.05, 39.06, and 88.79 MPa, respectively, indicating a noticeable overall reduction.

As the moisture content increased, the mechanical properties of the rock gradually deteriorated. This was primarily due to the penetration of water into the rock fissures and pores, which weakened the cementation and friction between particles, thereby reducing the stiffness and strength of the rock. This effect varied under the different loading paths. In Path I, the influence of the moisture content on the mechanical parameters of the rock was most pronounced, with the largest decreases in the initiation, closure, and damage stresses, where the damage stress decreased by 26.81%. This suggests that under complex stress perturbation conditions, water plays a more significant role in promoting the expansion of microcracks and deterioration of the rock structure.

To better explain the mechanical behavior and crack evolution mechanisms of water-saturated sandstone under different moisture conditions, this study combined a damage mechanics model using the damage stress point as a characteristic value to conduct a detailed analysis of the experimental results. The rock damage variable D is defined as the reduction in the elastic modulus during loading, given by:

(2)$D=1-{\displaystyle \frac{E_r}{E_0}},$

where E₀ is the initial elastic modulus of the rock, and E_r is the instantaneous elastic modulus of the rock at damage stress. The initial elastic modulus refers to the modulus value during the elastic phase of the rock before any damage or crack initiation occurs and is calculated from the initial linear segment of the stress–strain curve. The equation used is as follows:

(3)$E_0={\displaystyle \frac{△\sigma }{△\epsilon }},$

where $△\sigma$ is the increment in stress, and $△\epsilon$ is the increment in strain. Based on Equations (2) and (3), the damage variables of the sandstone samples with different water contents were calculated. The calculated damage variables are shown in Figure 21.

Through the calculation of the damage variable, it was found that as the water content increased, the damage variable exhibited a significant increasing trend. The damage variable increased by 53.33%, 36.37%, 47.37%, and 14.09% under the four loading stress Paths I–IV, respectively. This indicates that under high-water-content conditions, the initiation and development of cracks became more intense. The elastic modulus of the rock decreased significantly, leading to a considerable reduction in its overall stiffness and strength. The results from the damage mechanics model aligned with the experimental data, further confirming the role of increased water content in weakening the rock structure. As water infiltrates, the microcracks inside the rock become more active, promoting their expansion and propagation, which, in turn, reduces the overall load-bearing capacity of the rock.

4.3. Microscopic Fracture Evolution and Macroscopic Failure Characteristics

In this experiment, a high-precision AE monitoring system was used to capture the AE signals of sandstone during the loading process in real time. Through the analysis of the AE counts and b-values, the mechanisms by which the water content influences the initiation, development, and ultimate failure of microcracks in rocks was clarified.

4.3.1. Variation Law of AE Count

As Figure 22 shows, the stress–strain behavior and AE activity of the rock samples exhibited different characteristics under different stress paths. For Path I, as the sample transitioned from a dry to a saturated state, water infiltration reduced the friction between the particles, thereby enhancing the strain capacity of the sample. In the saturated state, the time required for failure was shorter than that in the dry state, and the AE activity was more dispersed with fewer peak events. For Path II, as the water content increased, the bond strength within the rock decreased, resulting in a reduced overall strength. This is reflected in the larger axial strain and shift in the failure mode from brittle to plastic or ductile fracture. The dry samples exhibited more frequent ringing counts, reflecting rapid crack propagation, whereas the brittleness of the saturated samples was reduced, resulting in fewer AE events during crack propagation and significantly shorter failure times. For Path III, with the simultaneous unloading of the horizontal stress, the rate of crack development and propagation in the rock slowed, and the ringing count of the low-water-content samples decreased. As the water content increased, the cumulative ringing count gradually increased, reflecting the slow expansion and increase in cracks during the loading–unloading process. However, the cumulative ringing count of the saturated samples was lower than that of the dry samples (approximately 50% of the dry sample count). For Path IV, the dry samples exhibited rapid crack propagation during the early stages of loading, with a sharp increase in AE counts. As the sample approached the peak strength, the speed and number of crack extensions increased significantly, causing a dramatic increase in the AE counts. Under the same axial loading conditions, the AE event frequency in the saturated samples was lower, indicating that the increase in moisture reduced the crack propagation rate through the pore pressure and lubrication effects.

4.3.2. Characteristic Analysis of AE b-Value

The b-value of AE is an important indicator for analyzing material fracture and microcrack activity. A high b-value indicates a higher proportion of small-scale microfracture events, suggesting that the rock material predominantly experienced the development and expansion of small-scale cracks with relatively low damage to the sample. A low b-value, on the other hand, reflects a higher proportion of large-scale microfracture events, indicating that small-scale cracks further coalesce into microscopic or even macroscopic fractures, leading to a higher level of damage in the rock sample [38]. The relationship for the b-value is given by:

(4)${\log }_{10}N=a-bM$

where N is the number of events observed within a specific time and region; M is the amplitude or energy of the AE events; and a and b are constants.

Based on Equation (4), the evolution of the b-value during the loading and unloading processes was calculated. To avoid large errors due to the limited number of AE events, a 50 s time interval was chosen, with a 10 s time step for sliding sampling. The amplitude interval was set to 5 dB.

As Figure 23 shows, although the b-value fluctuated with increasing deviatoric stress owing to inherent natural defects in the samples, the overall evolution trend exhibited a high degree of similarity. By combining the triaxial test results from the four loading paths, it was observed that the decreasing trends of the initiation, closure, and damage stresses were closely related to changes in the AE counts and b-value. As the water content increased, the initiation and damage stresses of the rock decreased significantly, indicating that microcracks began to initiate and propagate at lower stress levels. In particular, under high-water-content conditions, the AE counts increased significantly, indicating that microcracks formed and expanded more frequently at earlier stages. This is consistent with the rapid decrease in the b-value, which indicates that not only did the number of cracks increase, but their scale also progressively increased, ultimately leading to a significant reduction in rock strength. The sharp decline in the b-value reveals the intensity and instability of crack propagation, especially under the complex stress conditions of Paths III and IV, where this change was more pronounced.

Macroscale failure images further validated these findings. Under high-water-content conditions, the failure mode was characterized by more pervasive cracks, leading to the eventual disintegration of the rock. The increase in the AE counts and decrease in the b-value, as indicated by the AE results, reflect the continuous initiation and propagation of microcracks, which eventually coalesced into macrocracks. Once these cracks reached a certain scale, they ultimately led to macroscale instability and rock failure. Combining the stress–strain curve, AE counts, cumulative energy, and evolution of the b-value with the macroscopic failure mode showed that under high-water-content conditions, the rock exhibited lower initiation stress, more frequent crack activity (higher AE counts), more intense crack propagation (rapid decline in b-value), and faster energy accumulation, ultimately resulting in a failure mode characterized by pervasive cracks.

5. Industrial-Scale Test

The evolution of the internal microstructural behaviors in rock samples from initial compaction and elastic deformation to plastic deformation, ultimately leading to macroscopic failure, is of significant engineering importance for controlling the stability of coal mine roadways. Under water-rich conditions, the strength and stiffness of the surrounding rock decrease significantly, and crack propagation becomes more active. Consequently, more stringent support measures are required to ensure the stability of the surrounding rock. To address this issue, a support scheme that combines advance grouting reinforcement and full-length anchor-grout support is proposed in this study. Two key technical measures, the encapsulation of the surrounding rock and grouting modification, were introduced to enhance the stability and support effectiveness and ensure construction safety under complex geological conditions.

5.1. Rock Mass Stability Control Plan

Grouting modification technology aims to improve the physical properties of the surrounding rock under water-rich conditions, thereby enhancing its overall stiffness and strength. Under high-moisture-content conditions, the surrounding rock is prone to developing microcracks, thereby weakening its compressive and shear strengths. Grouting materials can fill these cracks, restore the integrity of the surrounding rock, mitigate the deterioration caused by water, and enhance the shear and compressive resistances of the rock. Research findings have indicated that grouting effectively slows the rate of crack propagation and significantly enhances the stability and impermeability of the surrounding rock [39]. Combined with full-length anchor-grouting support technology, grouting materials are evenly distributed within the surrounding rock, strengthening the bond between the rock and support structure, thereby enhancing the overall stability of the support system. Through grouting modification, the compactness of the surrounding rock was enhanced, the expansion of microcracks was effectively suppressed, and the likelihood of macroscopic failure was significantly reduced.

The core objective of encapsulating the surrounding rock is to prevent direct exposure to air, thereby effectively inhibiting weathering. Weathering causes a significant reduction in the surface strength of the surrounding rock, particularly with the decrease in the intermediate principal stress (σ₂), which may induce stress imbalance and lead to instability. Studies have shown that under high-moisture-content conditions, the crack initiation and damage stresses of the surrounding rock decrease significantly, making cracks more likely to form and propagate rapidly, thus accelerating macroscopic failure. Encapsulating the surrounding rock can effectively maintain its stress state, prevent stress redistribution caused by weathering, delay the formation and propagation of microcracks, and reduce macroscopic failures, thereby extending the service life of the roadway.

The advanced grouting reinforcement scheme aims to strengthen the rock surrounding the roadway by injecting grout in advance, thereby enhancing support effectiveness. Owing to the high clay mineral content in the sandstones of the Huaibei mining area and the water leakage phenomenon associated with conventional grouting materials (cement slurry), they are inadequate for reinforcing water-rich roadways. Therefore, grouting materials with high specific surface areas are required. This study improves the original grouting scheme by replacing conventional grouting materials with ultrafine cement-based grouts.

A full-length anchor-grouting support scheme was designed to enhance the stability of a support structure under special geological conditions. Based on the original support scheme, the timing of the support installation and the parameters of the anchor rods and cables were optimized. Owing to the uncertainty in determining the position of the roof water and the possibility that the roof anchor rods may not penetrate the aquifer, it was necessary to promptly grout the surrounding rock after installing the anchor rods and then proceed with the installation of the anchor cables. Additionally, grouted anchor cables were used for full-length anchoring to seal the roof water.

5.2. Manifestation Law of Mine Pressure

To comprehensively assess the effectiveness of the experimental scheme in controlling the stability of the surrounding rock, this study monitored a 200 m section of the 7₂64 ventilation roadway, with monitoring stations placed every 50 m. The monitoring included data from five measurement points, covering the surrounding rock deformation, anchor rod (cable) stress, and roof delamination, among other parameters (Figure 24). An in-depth analysis of the interrelationships between these data was conducted, and the results are presented in Figure 25.

When the surrounding rock deformation increases, it is typically accompanied by an increase in the surrounding rock stress, which in turn leads to higher anchor (cable) loads. In the monitoring data from the 7₂64 ventilation roadway, following optimization of the support scheme, the surrounding rock deformation significantly decreased. Specifically, the maximum relative displacement between the two sides decreased from 59–30 mm, and the relative displacements of the roof and floor were reduced by 50%. This change indicated that the surrounding rock stress was effectively controlled, thereby alleviating the pressure on the anchor (cable) and enhancing the stability of the support system.

The optimized support scheme ensured that the readings from the anchor (cable) load cells remained within a relatively stable range. For example, at Measurement Station II, when the roadway reached 30 m, the anchor (cable) load remained stable, indicating that the support system effectively adapted to the variations in the surrounding rock stress, thereby preventing excessive stress concentration. This further demonstrates that the reduction in the surrounding rock deformation directly alleviates the pressure on the anchor (cable).

The variation in the roof delamination reflects the distribution of the roof stress, particularly the interaction between the tensile and compressive stress zones. In the monitoring data from the 7₂64 ventilation roadway, the roof delamination values exhibited corresponding trends as the surrounding rock deformation and anchor (cable) stress changed. The data indicated that after optimizing the support scheme, the roof delamination values significantly decreased, and the delamination changes became more stable. This suggests that the support system effectively controlled the roof stress distribution, preventing the excessive expansion of the tensile stress zone. For instance, the monitoring results from Delamination Instrument I showed significant changes in the roof delamination along the roadway, especially within the 10–25 m range, where the delamination changes at the four measurement points were 13, 11, 3, and 2 mm, respectively. As the roadway construction continued, the delamination gradually stabilized, indicating that the roof stress state approached equilibrium. Similarly, the monitoring results from Delamination Instrument II confirmed that the optimized support scheme significantly reduced the roof delamination changes, indicating effective control over the tensile stress zone in the roof.

The joint variation in the surrounding rock deformation and roof delamination provides an intuitive basis for evaluating the roadway’s stability. Increased surrounding rock deformation is typically accompanied by an increase in roof delamination, reflecting the uneven roof stress caused by surrounding rock deformation, which generates larger tensile stresses. In the original support scheme, the roof delamination in the 7₂64 ventilation roadway increased rapidly during the initial phase of the roadway, particularly within the 0–30 m range, indicating excessive roof stress and poor roadway stability.

However, after the optimization of the support scheme, the roof delamination changes became more stable, indicating a significant improvement in the surrounding rock stability and effective control of the tensile stress zone in the roof. Through data analysis, it was found that the reduction in surrounding rock deformation directly led to a decrease in roof delamination values, suggesting that the support system successfully mitigated the water–rock interaction in the surrounding rock, thereby ensuring the roadway’s safety.

6. Conclusions

In this study, the weakening mechanisms of the roof sandstone in the Renlou Coal Mine under water-rich conditions and the corresponding roadway’s surrounding rock control measures were systematically investigated. By combining multi-path true triaxial tests, microstructural analysis, and industrial trials, the significant impact of water–rock interactions on the deformation and failure characteristics of water-saturated sandstone was revealed, and effective support strategies were proposed. The main conclusions are as follows.

1. The results of the microstructural analysis indicated that under water-rich conditions, the expansion and degradation of clay minerals led to a significant weakening of the sandstone structure. XRD and SEM analyses revealed that the main mineral components of the sandstone included quartz, feldspar, and clay minerals such as kaolinite and illite. The expansion and degradation of these minerals are key factors contributing to the reduction in rock strength.

2. The true triaxial experiments revealed the failure modes of water-saturated sandstone under complex stress conditions. As the water content increased, the failure mode of the sandstone gradually shifted from brittle to ductile, and crack propagation became more intense, leading to the formation of numerous associated cracks and complex crack networks. This significantly reduced the stiffness and strength of the rock. The increase in the water content caused a marked decrease in the initiation, closure, and damage stresses of the sandstone, accelerating the initiation and development of cracks. As the moisture content ranged from 0 to 100%, the damage stresses of the samples under the four paths decreased by 26.81%, 7.27%, 6.67% and 21.94%, respectively. Water infiltration weakened the load-bearing capacity of the rock, making the surrounding rock more prone to failure under lower-stress conditions, which, in turn, led to a significant decline in the roadway’s stability.

3. The AE monitoring results demonstrated the significant evolution of cracks in the surrounding rock under water-rich conditions. The variations in the AE count and b-value revealed the entire process of crack initiation, propagation, and coalescence. Under a high water content, the AE activity increased, and the b-value decreased significantly, indicating more intense crack propagation and an increased risk of instability and failure. This behavior is consistent with the observed macroscopic failure patterns.

4. The industrial trials validated the effectiveness of the proposed support and grouting modification measures. The stability of the surrounding rock of the roadway was significantly enhanced by integrating closure of the surrounding rock and grouting modification techniques. The displacements of the roof, floor, and sidewalls were substantially reduced, and the roof delamination was effectively controlled. The supporting loads of bolts and anchor cables decreased by an average of 22.5% and 31.07%, respectively, and the overall bearing capacity of the support system was significantly improved, ensuring the safety and service life of the roadway.

The conclusions of this study have significant engineering implications for the stability control of coal mine roadways under water-rich conditions. They also provide scientific guidance for the design and construction of support systems in coal mines with similar geological conditions.

Author Contributions

Z.Z.: writing—original draft, formal analysis, data curation, conceptualization. Y.F.: methodology, funding acquisition, conceptualization. Q.X.: visualization, data curation, funding acquisition. K.L.: methodology, conceptualization. M.H.: visualization, conceptualization. L.F.: data curation, conceptualization. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to laboratory regulations.

Conflicts of Interest

Author Kai Li was employed by the company CHN Energy Shendong Coal Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Water inflow into the roadway roof.

Figure 2. Schematic diagram of roadway roof degradation under water-rich conditions.

Figure 3. Thin section of sandstone and a physical sample.

Figure 4. Identification of the mineral composition of thin rock sections.

Figure 5. Design of experimental scheme.

Figure 6. True triaxial stress loading and unloading paths for sandstone samples with different water contents: (a) Path I; (b) Path II; (c) Path III; (d) Path IV.

Figure 7. Sampling location.

Figure 8. Pre-treatment of samples: (a) parent rock sample—part; (b) wave velocity test; (c) wave velocity results.

Figure 9. Water content vs. time curve.

Figure 10. X-ray diffraction (XRD) pattern.

Figure 11. Scanning electron microscopy (SEM) results for sandstone.

Figure 12. Three-dimensional deviatoric stress–strain relationship diagrams: (a) dry; (b) 30% saturated; (c) saturated.

Figure 13. Failure modes of sandstone with different water contents: (a) dry; (b) 30% saturated; (c) saturated.

Figure 14. Three-dimensional deviatoric stress–strain relationship diagrams: (a) dry; (b) 30% saturated; (c) saturated.

Figure 15. Failure modes of sandstone with different water contents: (a) dry; (b) 30% saturated; (c) saturated.

Figure 16. Three-dimensional deviatoric stress–strain relationship diagrams: (a) dry; (b) 30% saturated; (c) saturated.

Figure 17. Failure modes of sandstone with different water contents: (a) dry; (b) 30% saturated; (c) saturated.

Figure 18. Three-dimensional deviatoric stress–strain relationship diagrams: (a) dry; (b) 30% saturated; (c) saturated.

Figure 19. Failure modes of sandstone with different water contents: (a) dry; (b) 30% saturated; (c) saturated.

Figure 20. Influence of water content on mechanical behavior of sandstone.

Figure 21. Damage variable at characteristic points with different water contents.

Figure 22. Relationship between acoustic emission (AE) count, cumulative count, and strain.

Figure 23. Relationship between AE b-value and offsetting force.

Figure 24. Sensor arrangement.

Figure 25. Industrial-scale test results: (a) comparison of rock mass deformations; (b) comparison of bolt (cable) loading conditions; (c) comparison of roof separation conditions.

Table 1

Sample grouping.

Group	Wave Speeds (m/s)
Group 1	3788, 3861, 4049, 4065, 4098
Group 2	3906, 3984, 4016, 4032, 4082
Group 3	3906, 3984, 4000, 4032, 4096
Group 4	3861, 4032, 4032, 4082, 4098

Table 2

Rock composition.

Serial Number	Rock Lithology	Mineral Composition Proportions %
		SiO2	NaAlSi3O8	KAlSi3O8	Clay
1	Fine-grained Sandstone	54.1	15.8	8.8	21.3

Table 3

Clay mineral composition.

Serial Number	Rock Lithology	Mineral Composition Proportions %
		Al2(Si2O5)(OH)4	KAl2(Si3Al)O10(OH)2
1	Fine-grained Sandstone	57.26	42.74

Table 4

Variation in crack initiation, closure, and damage stresses in sandstone with different water contents.

Path	Moisture Content (%)	Initiation Stress (MPa)	Closure Stress (MPa)	Damage Stress (MPa)
Path I	0	9.54	48.37	87.17
	30%	9.57	45.06	80.47
	100%	9.61	36.75	63.8
Path II	0	9.58	52.69	95.75
	30%	9.57	50.61	91.59
	100%	8.05	39.06	88.79
Path III	0	12.32	44.96	77.81
	30%	11.85	44.64	77.8
	100%	9.52	41.1	72.62
Path IV	0	9.55	49.44	89.29
	30%	9.54	45.33	81.1
	100%	9.69	39.69	69.7

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