The strength of the sliding zone soil determines the stability of reservoir landslides. Fluctuations in water levels cause a change in the seepage field, which serves as both the external hydrogeological environment and the internal component of a landslide. Therefore, considering the strength changes of the sliding zone with seepage effects, they correspond with the actual hydrogeological circumstances. To investigate the shear behavior of sliding zone soil under various seepage pressures, 24 samples were conducted by a self-developed apparatus to observe the shear strength and measure the permeability coefficients at different deformation stages. After seepage-shear tests, the composition of clay minerals and microscopic structure on the shear surface were analyzed through X-ray and scanning electron microscope (SEM) to understand the coupling effects of seepage on strength. The results revealed that the sliding zone soil exhibited strain-hardening without seepage pressure. However, the introduction of seepage caused a significant reduction in shear strength, resulting in strain-softening characterized by a three-stage process. Long-term seepage action softened clay particles and transported broken particles into effective seepage channels, causing continuous damage to the interior structure and reducing the permeability coefficient. Increased seepage pressure decreased the peak strength by disrupting occlusal and frictional forces between sliding zone soil particles, which carried away more clay particles, contributing to an overhead structure in the soil that raised the permeability coefficient and decreased residual strength. The internal friction angle was less sensitive to variations in seepage pressure than cohesion. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Numerous incidents and failures of bank slopes are caused by the creep behavior of sliding zone soil. During reservoir regulation, the pore water pressure in the sliding zone undergoes cyclic changes. Under such complex cyclic hydraulic conditions, the creep behavior may differ from that under the monotonic seepage condition, which is still poorly understood. In this paper, the Majiagou landslide in the Three Gorges Reservoir area is taken as a case study. Triaxial creep tests were first carried out to study the creep behavior of the sliding zone soil specimen under cyclic seepage pressure. Then, the nonlinear Burgers creep model was proposed to characterize the observed creep behavior of the sliding zone soil specimen, and the secondary development was performed based on FLAC3D software. Finally, the proposed model was applied to the Majiagou landslide to simulate its deformation under fluctuating reservoir water levels. The following results were obtained: (1) Under low deviatoric stress levels, cyclic seepage pressure causes the creep strain curve to fluctuate significantly. The decrease of seepage pressure leads to a reduction in pore pressure, resulting in a sharp increase in the strain rate of sliding zone soil. (2) The proposed model can well reflect the creep characteristics of sliding zone soil under cyclic seepage pressure. (3) During reservoir operation, the landslide deformation exhibits a step-like growth, and the proposed creep model can effectively simulate the retrogressive deformation characteristics of the Majiagou landslide. The research results provide the theoretical basis for the long-term stability of reservoir landslides under fluctuating water levels.

Surrounding rocks of underground engineering are subjected to long-term seepage pressure, which can deteriorate the mechanical properties and cause serious disasters. In order to understand the impact of seepage pressure on the mechanical property of sandstone, uniaxial compression tests, P-wave velocity measurements, and nuclear magnetic resonance (NMR) tests were conducted on saturated sandstone samples with varied seepage pressures (i.e. 0 MPa, 3 MPa, 4 MPa, 5 MPa, 6 MPa, 7 MPa). The results demonstrate that the mechanical parameters (uniaxial compressive strength, peak strain, elastic modulus, and brittleness index), total energy, elastic strain energy, as well as elastic strain energy ratio, decrease with increasing seepage pressure, while the dissipation energy and dissipation energy ratio increase. Moreover, as seepage pressure increases, the micro-pores gradually transform into meso-pores and macro-pores. This increases the cumulative porosity of sandstone and decreases P-wave velocity. The numerical results indicate that as seepage pressure rises, the number of tensile cracks increases progressively, the angle range of microcracks is basically from 50 degrees-120 degrees to 80 degrees-100 degrees, and as a result, the failure mode transforms to the tensile-shear mixed failure mode. Finally, the effects of seepage pressure on mechanical properties were discussed. The results show that decrease in the effective stress and cohesion under the action of seepage pressure could lead to deterioration of strength behaviors of sandstone. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).