Stability of tunnels in water saturated shale under cyclic stresses: investigation on the coupling between mechanical and seepage characteristics

Under the coupled effect of water and cyclic stress, the mechanical properties of mine surrounding rock deteriorate, and its permeability changes, posing challenges to tunnel excavation support and mine stability control. The interaction between water infiltration into the rock mass and externally applied cyclic stress may lead to significant degradation of the rock’s mechanical properties, further affecting its deformation, strength, and permeability. Based on this, this study investigates a water-rich tunnel in a phosphate mine in Yunnan, employing in-situ experiments, laboratory tests, model construction, numerical simulations, and field monitoring. In-situ hydraulic pressure and geostress tests were conducted, and shale samples were retrieved and processed into standard cylindrical and cubic specimens in the laboratory. Considering the stress and hydrogeological conditions during tunnel excavation, the study systematically examined the damage characteristics of water-saturated shale under cyclic stress disturbances and its mechanical-seepage coupling properties. The effects of water saturation, hydraulic pressure, and cyclic stress on the mechanical properties and permeability of the specimens were analyzed, leading to an understanding of the degradation mechanism of mechanical properties and the evolution of permeability under water-mechanical coupling. Furthermore, FLAC3D software was used to simulate the excavation process of a water-saturated tunnel under cyclic stress disturbances. The deformation and pore water pressure evolution of the excavated tunnel under different support schemes after drainage were analyzed, and field monitoring and verification were carried out. Main Research Conclusions: 1) Engineering Geological Characteristics of the Water-Saturated Tunnel Surrounding Rock. Based on field geological surveys, core sampling, and laboratory tests, it was determined that the rock mass in the mining area mainly consists of Class IV–V hard to moderately hard rock layers, with interbedded soluble salt rocks contributing to a medium complexity hydrogeological environment. Transient electromagnetic detection revealed an anomalous water-rich zone within 0–8 m ahead of the working face and two sets of water-conducting fractures between 25–40 m, providing insight into the spatial distribution of groundwater migration pathways. Geostress testing results indicated that the maximum principal stress at the tunnel location is σ₁ = 15.68 MPa, the intermediate principal stress is σ₂ = 7.32 MPa, and the minimum principal stress is σ₃ = 4.98 MPa, confirming a geostress regime dominated by tectonic stress. Pore water pressure monitoring showed that water pressure stabilized at 1.42 MPa (fluctuation <4.2%) on the 14th day after tunnel excavation. These geostress and water pressure test results provided boundary conditions for subsequent cyclic stress and hydraulic pressure loading experiments. 2) Damage Mechanism of Water-Saturated Shale under Cyclic Stress Disturbances. Conventional triaxial compression and cyclic loading tests revealed the regulatory mechanism of water saturation duration on shale’s mechanical properties. The experiments showed that the compressive strength of the specimens gradually decreased with increasing saturation time. Cyclic stress altered the internal crack distribution (closure of pre-existing cracks and initiation of new cracks), thereby changing the mechanical properties of the specimens. Water infiltration weakened the specimen’s strength through both physical softening and chemical corrosion, resulting in multiple post-peak fluctuations in the stress-strain curve under cyclic loading and saturated conditions, with significantly enhanced plasticity. Acoustic emission (AE) monitoring indicated that water molecules suppressed the rapid expansion of newly formed cracks, leading to a gradual decrease in AE response during the loading process of saturated specimens. A complex-plane fatigue damage model was established, correlating AE parameters with plastic strain rate to predict mechanical parameter degradation, and a critical saturation threshold of tw = 24 h was determined, providing a quantitative basis for engineering damage assessment. 3) Mechanical-Seepage Coupling Characteristics and Model Construction of Water-Saturated Shale under Cyclic Stress Disturbances. Through cyclic loading and seepage experiments, the reconstruction mechanism of seepage pathways under the synergistic action of hydraulic pressure and stress was elucidated. The results showed that water-saturated specimens exhibited enhanced stress-strain curve fluctuations due to local expansion effects, while post-failure compressive conditions restricted the instantaneous increase in water flow. Increased hydraulic pressure elevated the effective stress in the σ₁ direction, promoting the main crack’s expansion along the maximum principal stress direction while reducing crack density in the σ₂ direction. The permeability critical point gradually disappeared with prolonged saturation, indicating that water infiltration altered the fracture network’s flow-conducting characteristics. A complex-plane damage model (real part representing fatigue damage and imaginary part representing post-disturbance mechanical properties) was developed, integrating a neural network algorithm to dynamically predict permeability and strength parameters under different hydraulic pressure conditions. A visualization software tool was also incorporated to support engineering support design. 4) Surrounding Rock Control and Stability Analysis of Water-Saturated Tunnel under Cyclic Stress Disturbances. Through numerical simulation and parameter sensitivity analysis, an optimized "long-anchor + dense support" scheme was proposed for water-saturated tunnels. Simulation results showed that this scheme significantly reduced roof subsidence and substantially decreased the extent of the plastic zone. The pre-drainage measures reduced the pore water pressure from 1.5 MPa to 0.5 MPa (a 66.7% reduction), and when combined with dynamic support sequencing (pre-drainage → long-anchor support), effectively mitigated secondary damage induced by hydraulic pressure redistribution. During excavation, pore water pressure evolution exhibited a "convergence to the excavation face → gradient diffusion" pattern, while roof subsidence followed a three-stage dynamic response: an initial slow increase phase, a rapid increase phase during excavation, and a stress adjustment stabilization phase. The central area of the second excavation stage experienced the largest proportion of subsidence, necessitating targeted monitoring and reinforcement. 5) Mine Pressure Monitoring and Engineering Application in the Water-Saturated Tunnel. After implementing the new support scheme in a tunnel of the Yunnan phosphate mine, joint monitoring using multipoint displacement meters, roof separation instruments, and pore water pressure gauges verified the engineering effectiveness of the support system. The monitoring data showed that the deformation of the tunnel sidewalls decreased with depth in a gradient manner (from 0.173 mm at 1 m depth to 0.157 mm at 1.5 m depth), and the deformation rate tended to converge after the 10th day of monitoring. Roof subsidence stabilized at 0.45 mm within 16 days, entering a stable phase after the 14th day. The coordinated effect of support and drainage significantly alleviated pore water pressure concentration, providing theoretical references for the excavation and support of deep water-rich tunnels.