With the growing energy demand and the depletion of shallow coal resources, coal mining operations are advancing to greater depths underground (Han et al., 2024). The increase in seepage pressure, a characteristic feature of deep earth environment, which can aggravate roadway instability and elevate the risk of water inrush accidents in coal mines (Ding and Tang, 2024; Ping et al., 2024). This issue is especially critical for the safe mining of deep coal mines, where high seepage pressures and complex seepage fields must be considered in the stability analysis of surrounding rock in both coalface and excavation roadways (Luo et al., 2025; Zhu et al., 2022).

The coal mine roadways are excavated within the underground rock mass, which possesses inherent characteristics of inhomogeneity, anisotropy, and discontinuity (Xie et al., 2024). These characteristics make the coal mine roadway surrounding rock highly susceptible to the influence of seepage fields. Groundwater infiltrates surrounding rock through seepage channels formed by pores and cracks, altering the internal stress distribution. In turn, dynamic changes in the stress field also affect the permeability characteristics of surrounding rock (Ma et al., 2022b; Yan et al., 2022; Zan et al., 2024; Zhao et al., 2021). Under seepage–stress coupling, surrounding rock undergoes hydrolysis, dissolution, deformation, and fracture, ultimately leading to instability and failure of coal mine roadways. Therefore, understanding the mechanical behaviour and permeability characteristics of rock under varying seepage field conditions is critical. Investigating this interaction helps to clarify the fracture mechanisms of surrounding rock, offering valuable insights for the design and stability analysis of deep coal mine roadways.

Extensive research has been conducted on the physical and mechanical properties of surrounding rock under seepage–stress coupling, particularly under unilateral seepage pressure (Geng et al., 2023; Li and Zhou, 2023; Zhang et al., 2024). Dong et al. investigated the damage of surrounding rock during roadway blasting excavation and found that the degree of damage was influenced by the internal water flow velocity. Their study revealed a strong coupling effect between failure and seepage pressure, which intensified as both seepage pressure and rock damage increased (Dong et al., 2023). Zhao et al. demonstrated that seepage pressure significantly affects the displacement of surrounding rock and that the use of anchor rods can effectively mitigate deformation (Zhao et al., 2023). Cao et al. performed triaxial seepage tests on split sandstone with varying particle sizes to examine differences in fracture surface morphology and permeability characteristics (Cao et al., 2024). Their findings indicated that sandstone with larger particles exhibited a flatter fracture surface. Ma et al. analysed the permeability and porosity of rock under one-dimensional particle migration, concluding that both parameters increased over time. Their study identified four distinct phases in the infiltration process: rapid increase, slow increase, gradual rise, and stabilisation (Ma et al., 2022a).

In the unilateral seepage pressure condition, one end of the rock specimen is subjected to seepage pressure, while the other end is exposed to the atmosphere as a fluid outlet. To investigate the effects of complex seepage field conditions on the mechanical and permeability characteristics of rock, researchers have applied seepage pressure on both sides of rock specimens while varying the pressure at the top and bottom (Wang et al., 2021a). Wang et al. conducted an experimental study on displacement changes and failure mechanisms of roadway surrounding rock during excavation. Their findings indicate that the stress field of surrounding rock undergoes three distinct phases: stress redistribution, stress adjustment, and stress rebalancing (Wang et al., 2021b). They also observed that the stable loading seepage pressure, pore pressure, and the distance from the measuring point to the roadway edge follow an exponential decay function. Xu et al. employed nuclear magnetic resonance (NMR) technology to analyse permeability characteristics and microscopic pore structure changes in fractured rock masses. Their study revealed that higher fluid velocity results in greater core displacement and a more concentrated fluid flow, leading to the rapid formation of primary seepage channels (Xu et al., 2022). Chen et al. developed a construction strength analysis model for a two-lane shield roadway under anisotropic seepage conditions and conducted numerical simulations to evaluate the stability of roadway surrounding rock in an underwater roadway project (Chen L. H. et al., 2023). These studies contribute valuable insights into the permeability characteristics of surrounding rock and the fracture processes induced by water infiltration.

When the seepage field interacts with surrounding rock, water weakens the mechanical behaviour of the crystal and cementing material through softening, lubrication, and erosion (Burns et al., 2023; Heap et al., 2023; Ji et al., 2024; Yang et al., 2024). Yang et al. investigated the scale effect on damaged rock masses under seepage pressure (Yang et al., 2017). By analysing the progressive failure process and applying fluid-structure coupling theory, they explored the permeability variations in damaged rock masses of different sizes. Wasantha et al. examined crack development patterns in water-saturated specimens, revealing that under low confining pressure, microcrack initiation occurred almost simultaneously with failure stress, resulting in brittle sudden failure (Wasantha and Ranjith, 2014). In contrast, under high confining pressure, microcrack initiation occurred earlier than the failure stress, displaying more quasi-brittle characteristics. Chen et al. investigated the fracture mechanism of rock in deep environment subjected to high seepage pressure and rapid unloading. Their findings indicated that as seepage pressure increased, granite exhibited an earlier onset of nonlinear permeability growth, accompanied by a progressive decline in both compressive strength and energy storage capacity (Chen Z. Q. et al., 2022). Additionally, the permeability, energy hardening, and brittle failure mechanisms of granite were found to be amplified under these conditions.

The conventional triaxial compression tests on surrounding rock in deep coal mine under bilateral seepage pressure were conducted to investigate its fracture mechanism under seepage–stress coupling. The axial stress–strain curves and permeability evolution curves over time were obtained, providing insights into the mechanical behaviour and permeability characteristics of surrounding rock. The failure mode and fracture mechanism were analysed from both macroscopic and microscopic perspectives. The findings offer theoretical support for assessing the stability of surrounding rock and optimising support systems in deep coal mine affected by groundwater.

During triaxial compression testing, the mechanical behaviours, permeability characteristics, and failure processes of sandstone specimens vary significantly with changes in seepage field conditions. Macroscopic fractures and microscopic crack surface morphologies of the damaged sandstone specimens are analysed to summarize the fracture mechanisms under seepage–stress coupling. Figure 9 presents photos of the macroscopic fractures on the specimen surface. Under triaxial loading with seepage pressure, the sandstone specimen exhibits a single inclined shear fracture, with a primary fracture plane passing through the specimen. However, when the top seepage pressure is 8.0 MPa, small tensile cracks appear beside the primary fracture, indicating a shift from linear to nonlinear fracture morphology with increasing unilateral seepage pressure. The application of bottom seepage pressure increases the pore pressure within the specimen, causing the crack propagation direction to deviate. Vertical tensile cracks emerge at both ends of the specimen, and the specimen exhibits compound failure, resulting from the combined action of tensile and shear fractures. As the internal pore pressure of the sandstone rises, its failure mechanism gradually evolves from predominantly shear failure to a mixed mode of tension–shear failure.

Scanning electron microscope (SEM) analysis is conducted to examine the microscopic morphology of the sandstone fracture surface, as shown in Figure 10. Under low top seepage pressure, the cementing material and crystals in the sandstone are significantly damaged (Zhou et al., 2017). The low seepage pressure is insufficient to wash away the detrital minerals produced during the failure of the sandstone, resulting in a large accumulation of these minerals on the fracture surface. As the top seepage pressure increases, the number of detrital minerals on the fracture surface decreases significantly, and the crystal fracture surfaces become cleaner. A reduction in peak stress results in less crystal damage but a marked increase in the occurrence of intergranular fractures. Under bottom seepage pressure, the bearing capacity of the sandstone continuously decreases, and the cementing material is hydrolysed and destroyed. The low stress levels do not cause complete crystal fracture, resulting in only a small amount of transgranular fractures (Wang et al., 2024). The crystal cracks are orderly, and their structure remains intact. The cementing material in the sandstone is completely destroyed, with no residual material attached to the fracture surface.

Based on the macroscopic fracture analysis of sandstone and the microscopic morphology of the fracture surface, the fracture mechanism of sandstone under seepage–stress coupling is summarized in Figure 11. From a macroscopic perspective, the fractures on the specimen surface exhibit an obvious nonlinear behaviour under the conditions of C9P5-0 and C9P8-0. The presence of seepage pressure causes the fracture surface to deviate significantly from a planar shape. The primary fracture extends from one edge of the specimen to the other, suggesting that the accumulation of bottom seepage pressure leads to stress concentration at the specimen’s edge.

From a microscopic perspective, the cementing material between crystals is more susceptible to destruction under high top seepage pressure, whereas quartz crystals are less affected by erosion but more influenced by high confining pressure and axial stress, resulting in rapid degradation of the crystal structure (Zhang et al., 2023). Additionally, the high pore pressure inside the sandstone specimen removes detrital minerals from the fracture surface after macro-fracture formation, resulting in a relatively clean fracture surface microtopography. The effect of seepage pressure on the microtopography of the sandstone fracture surface is chiefly reflected in the abundance of detrital minerals and cementing materials resulting from fracture development.

When unilateral seepage pressure is applied to a sandstone specimen, the cementing materials within the sandstone deform and break, with cracks initially forming in the cementing material between mineral particles. This results in an increase in the porosity of the sandstone. The energy required for crack tip propagation reaches a balanced state; as the surface free energy of the mineral particles decreases, the external work required for crack propagation also diminishes (Zhao et al., 2024). Consequently, the axial stress required for the fracture of mineral particles is reduced under high pore pressure. Additionally, the crack tip, influenced by pore pressure, partially offsets the constraining effect of confining pressure, increasing the work done by the fluid on crack tip expansion and accelerating the crack development rate. The primary fracture mode of sandstone under unilateral seepage pressure is a mixed fracture, consisting of both transgranular and intergranular fractures.

When the energy at the crack tip exceeds the energy required for propagation, the crack begins to expand and extend, causing adjacent micro-cracks to nucleate and form cracks in the cementing material. Due to the loose structure of the cementing material, seepage can occur, and cracks within the cementing material can be filled with fluid, generating pore pressure. This pore pressure further reduces the positive pressure on the crack, intensifies stress concentration, and facilitates the continuous development of the crack. Under triaxial loading, the loose mineral particles move and compress the cementing material. Seepage pressure also promotes fluid flow within the sandstone, carrying away part of the cementing material due to water erosion and abrasion, thereby creating space for the movement of mineral particles. At high pore pressures, the sandstone fails more quickly, its bearing capacity is reduced, and the degree of crystal destruction is lower. In this state, the primary failure mode of the sandstone shifts to intergranular fracture.

To explore the influence of seepage fields on the mechanical behaviour and fracture mechanisms of surrounding rock in deep coal mine roadways, this study conducted conventional triaxial compression tests on sandstone specimens under seepage–stress coupling conditions. By systematically analysing the stress–strain responses, macroscopic fracture patterns, and microscopic fracture surface morphologies, the evolution and failure characteristics of sandstone under complex hydro-mechanical environments were comprehensively revealed. This research not only deepens the understanding of water–rock interaction in deep mining environments, but also proposes a novel experimental framework for investigating fracture mechanisms under seepage–stress coupling, thereby providing a scientific foundation for the design of more effective support systems and long-term stability assessment of groundwater-bearing surrounding rock in deep coal mine roadways. The specific conclusions are as follows.

(1) Using a transparent triaxial compression servo-control test system, axial stress–strain curves and time-dependent permeability evolution data for sandstone were obtained. The results demonstrate that seepage pressure significantly impacts the mechanical behaviours of sandstone, particularly reducing its bearing capacity. Under unilateral seepage pressure, the peak stress decreases in a linear fashion, with a gradient of 2.20 MPa/MPa. In contrast, the peak stress decreases nonlinearly under bilateral seepage pressure condition, with the reduction diminishing as the bottom seepage pressure decreases. Moreover, increasing the bottom seepage pressure reduces the pressure differential between the ends of the specimen and simultaneously modifies the permeability characteristics of the sandstone.