With the continued development of water resources in Southwest China, fluctuations in water levels and rainfall have triggered numerous landslides. The potential hazards posed by these events have garnered considerable attention from the academic community, making it imperative to elucidate the landslide mechanisms under the combined influence of multiple factors. This study integrates laboratory tests and numerical simulations to explore the instability mechanisms of landslides under the combined effects of rainfall and fluctuating water levels, as well as to compare the impacts of different factors. Results indicate that the sensitivity of landslide deformation decreases as the number of water level fluctuations increases, exhibiting a gradually stabilizing tendency. However, the occurrence of a heavy rainfall event can reactivate previously stabilized landslides by increasing pore water pressure and establishing a positive feedback loop with rainfall infiltration. This process reduces boundary constraints at the toe of the slope, promotes the development of an overhanging surface, and ultimately leads to overall instability and landslide disaster. Under the same rainfall intensities, the presence of water level fluctuations prior to rainfall significantly shortens the time for the landslide to reach a critical state. The key mechanisms contributing to landslide failure include terrain modification, fine particle erosion, and outward water pressure, all of which generates substantial destabilizing forces. This research offers valuable insights for the monitoring, early warning, and risk mitigation of landslides that have already experienced some degree of deformation in hydropower reservoir areas.

Tensile cracks play a pivotal role in the formation and evolution of reservoir landslides. To investigate how tensile cracks affect the deformation and failure mechanism of reservoir landslides, a novel artificial tension cracking device based on magnetic suction was designed to establish a physical model of landslides and record the process of landslide deformation and damage by multifield monitoring. Two scenarios were analyzed: crack closure and crack development. The results indicate that under crack closure, secondary cracks still form, leading to retrogressive damage. In contrast, under crack development conditions, the failure mode changes to composite failure with overall displacement. The release of tensile stresses and compression of the rear soil are the main driving forces for this movement. Hydraulic erosion also plays a secondary role in changing landslide morphology. The results of multifield monitoring reveal the effects of tensile cracking on reservoir landslides from multiple perspectives and provide new insights into the mechanism of landslide tensile-shear coupled damage.

Direct shear creep tests have scarcely been used for long-term creep behavior studies of landslides in the Three Gorges reservoir area. In this study, based on field investigations and monitoring of the Huangtupo Landslide, direct shear creep tests were performed on the sliding zone soil of Riverside Slump #1, and the creep characteristics of sliding zone soil after varying cycles of reservoir water level fluctuation were studied. Using the creep results, the Mohr-Coulomb parameters were obtained by numerical simulation, and the deformation pattern of the reservoir landslide was analyzed. The results show that the direct shear creep of sliding zone soil can mainly be divided into stages of attenuation creep and steady-state creep. Under the same shear stress, with the increase of loading-unloading cycles N, the soil's strain and shear strain rate in the sliding zone decreased accordingly, and the long-term strength gradually improved. As the shear stress increases, the shear strain rate increases and the creep of the soil in the sliding zone has an obvious time effect. Our numerical simulation results showed good agreement with both the landslide deformation monitoring data and direct shear testing data. The Burgers model is suitable for describing creep deformation of landslides under fluctuating reservoir water levels. Under high shear stress, the fitted curve showcased both attenuation and constant velocity characteristics. Numerical simulation and burger model can reflect the direct shear creep test characteristics well. These research findings can provide an important reference on the creep characteristics of landslides, potentially aiding geotechnical engineering applications.

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.

The construction of the Three Gorges Reservoir dam in China has led to an increase in reservoir landslide events. To mitigate these geohazards, multiple rows of stabilizing piles (MRSP) have been employed to stabilize massive reservoir landslides. This study utilizes centrifuge and numerical modeling to investigate the behavior of unreinforced landslides and MRSP-reinforced landslides in reservoir areas. The failure mechanisms of unreinforced landslides, as well as the mechanical behavior and stabilizing mechanisms of MRSP under reservoir water level (RWL) fluctuations, are examined. The results indicate that elevated downward seepage forces contribute to prefailure sliding, but are not the sole cause of catastrophic failure. Instead, rapid pre-failure sliding leads to soil particle compression and crushing in the saturated sliding zone, resulting in excess pore water pressure and accelerated overall failure. This excess pore water pressure-dependent mechanism explains the observed steplike deformation pattern and rapid failure pattern in reservoir landslides. Furthermore, the study reveals the formation of soil arches between adjacent MRSP groups, causing stress concentration on boundary columns and necessitating reinforcement. The finding challenges traditional one-dimensional load transfer ratios, advocating for a two-dimensional approach that accounts for variations across rows and columns. Notably, the study also highlights significant variations in load transfer laws within MRSP under different RWL operations, emphasizing the need for a more nuanced understanding of MRSP behavior.

For reservoir landslide, in addition to hydrological conditions, creep properties of soils play an important role in explaining the mechanisms behind landslide movement. When studying the creep characteristics of geotechnical materials in reservoir area, most of them only consider the influence of a single factor (i.e. confining pressure, water content or osmotic pressure) on the creep behavior at present. In order to better describe the creep behavior of clay-gravel composite, this paper considers the effects of the initial water content and the saturation-undersaturation cycle together on the creep properties, and introduces the time-damage function to establish a novel creep-damage constitutive model. Compared with the classical Burgers model, this model can well characterize the creep experimental data with high accuracy, especially in the accelerated creep stage. Then, FLAC3D software is used as a platform to realize the proposed model applied to the long-term stability analysis of Woshaxi landslide by using C + + language. Comparing the Burgers model by the calculation, the proposed model shows a better reflection of the effect of initial water content and the number of saturation-undersaturation cycles on creep as well as being able to describe the accelerated creep phase. It is hoped that this research can provide scientific and engineering application value for the mitigation of the disasters on the reservoir bank slope.

Reservoir water fluctuation is the key factor affecting the stability of reservoir landslides. Existing research on the evolution of landslides under cyclic reservoir water fluctuations is limited. However, further research is needed focusing on the evolution of the first-order natural frequency of reservoir landslides. In this study, model tests were conducted to investigate the evolution of the stress, displacement, inclination angle and first-order natural frequency of reservoir landslides under different rates of water level fluctuations during cyclic reservoir water fluctuations. The tests demonstrated that cyclic fluctuations in the reservoir water level resulted in oscillatory increases in the pore water pressure and soil pressure; while, the effective stress exhibited an oscillatory decrease, leading to a reduction in the landslide stability. The landslide displacement and inclination angle exhibited periodic increases, without distinct stages of initial deformation, uniform deformation, or accelerated deformation. Regarding landslide failure below the water surface, the inclination angle was more sensitive than the displacement. Changes in the inclination angle preceded changes in the displacement, making this approach highly suitable for early warning of reservoir landslide instability. Before the occurrence of landslide failure, the development and connection of cracks led to fragmentation of the sliding mass into multiple smaller blocks with reduced masses, resulting in a drastic increase in the first-order natural frequency of the landslide. Changes in the first-order natural frequency preceded changes in the inclination angle and displacement, rendering this approach very suitable for early warning of reservoir landslides.

With the construction of the Three Gorges Reservoir dam, frequent reservoir landslide events have been recorded. In recent years, multi-row stabilizing piles (MRSPs) have been used to stabilize massive reservoir landslides in China. In this study, two centrifuge model tests were carried out to study the unreinforced and MRSP-reinforced slopes subjected to reservoir water level (RWL) operation, using the Taping landslide as a prototype. The results indicate that the RWL rising can provide lateral support within the submerged zone and then produce the inward seepage force, eventually strengthening the slope stability. However, a rapid RWL drawdown may induce outward seepage forces and a sudden debuttressing effect, consequently reducing the effective soil normal stress and triggering partial pre- failure within the RWL fluctuation zone. Furthermore, partial deformation and subsequent soil structure damage generate excess pore water pressures, ultimately leading to the overall failure of the reservoir landslide. This study also reveals that a rapid increase in the downslope driving force due to RWL drawdown significantly intensifies the lateral earth pressures exerted on the MRSPs. Conversely, the MRSPs possess a considerable reinforcement effect on the reservoir landslide, transforming the overall failure into a partial deformation and failure situated above and in front of the MRSPs. The mechanical transfer behavior observed in the MRSPs demonstrates a progressive alteration in relation to RWL fluctuations. As the RWL rises, the mechanical states among MRSPs exhibit a growing imbalance. The shear force transfer factor (i.e. the ratio of shear forces on pile of the nth row to that of the first row) increases significantly with the RWL drawdown. This indicates that the mechanical states among MRSPs tend toward an enhanced equilibrium. The insights gained from this study contribute to a more comprehensive understanding of the failure mechanisms of reservoir landslides and the mechanical behavior of MRSPs in reservoir banks. (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/).

Temporal variability in the macro-mechanics and microstructure induced by periodic water fluctuations during reservoir operation is widespread but adverse for slip zone soils. Herein, taking the slip zone soils of Huangtupo No. 1 landslide in the Three Gorges Reservoir area as a research case, the consolidation undrained (CU) triaxial tests coupled with wetting-drying cycles are organized to address macroscopic temporal variability of shear strength parameters. Then, quantitative microscopic characterizations are performed based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) combined with mercury compression test (MIT). Eventually, the macro and micro connections are identified via gray rational analysis (GRA) and dynamic time warping (DTW) to be thus mathematized. Moreover, the weakened constitutive model is constructed. The test results show that the temporal variability of macroscopic shear strength parameters can be quantified as negative exponential decay. The wetting-drying cycles prominently contribute to the generation of intra-agglomerate pores (0.9-35 mu m). Besides, the inter-granular pores (0.007-0.9 mu m) and porosity are the connections to bridge microstructural parameters and macroscopic shear strength parameters. Furthermore, empirical equations for macro and micro connections are tentatively derived; the temporal variability of slip zone soils is invited to appropriately model the weakening laws of stress-strain. This study is expected to provide ingenious perspectives and promising references in stability evaluation and even disaster prevention of reservoir landslides.