Abstract:

[Objective] Since the impoundment of the Three Gorges Dam in 2003, many cataclastic bedding landslides in the reservoir area have been reactivated due to the influence of external factors such as reservoir water level fluctuations, variations in groundwater levels, and rainfall. These landslides are typically large in scale, exhibit complex deformation mechanisms, and pose significant challenges for early warning and disaster prevention. This paper attempts to establish an interaction model linking rainfall, reservoir water, and groundwater with groundwater as the main triggering factor of landslide deformation, and to further reveal the dynamic migration patterns of groundwater under the coupled effects of reservoir water level fluctuations and rainfall. [Methods] The study took the Muyubao Landslide, a cataclastic bedding landslide in the Three Gorges Reservoir area, as a case study. By integrating data statistics with field investigations, a statistical analysis was conducted on nearly seven years of manual and automated monitoring data, field investigation materials, and hydrometeorological information to investigate the quantitative relationship among reservoir water level, rainfall, and groundwater level, as well as the interaction between groundwater level and slope displacement and deformation. [Results] The research results indicated that: (1) A groundwater level of 175 meters at the front platform of the slope served as the critical threshold for the initiation of landslide deformation. When the groundwater level at the front platform approached 175 m, deformation began under the influence of buoyancy-induced weight reduction. When the groundwater level at the front platform exceeded 175 m, both buoyancy-induced reduction and dynamic water pressure acted on the slope, with the effect of dynamic water pressure intensifying as the groundwater level rose.(2) Statistical analysis of monitoring data revealed thresholds for rainfall-induced groundwater level rise. Ten consecutive days of rainfall totaling 150 mm increased the groundwater level at the front of the slope by 3.22 m to 6.88 m. In the case of 300 mm of cumulative rainfall within 30 days, the groundwater level at the front of the slope increased by approximately 10 m. A “lag effect” was observed in groundwater response to rainfall, typically lasting 3 to 13 days.(3) From October to December 2017, rainfall occurred on 27 out of first 32 days, totaling 310.6 mm. As a result, the groundwater level in borehole QSK1 at the front platform of the slope rose to 184.2 m, nearly 10 m above the highest reservoir water level (175 m). Over the 72-day period, the slope displacement totaled 88.5 mm. [Conclusion] (1) Groundwater level fluctuations significantly precede slope deformation. Given known reservoir water levels, it is possible to forecast groundwater level based on reservoir water level and rainfall data, and further predict slope deformation based on the groundwater level. This approach provides a strong basis for landslide early warning and prediction.(2) The effective contribution of rainfall to groundwater recharge varies with different types of rainfall. Compared to intense rainstorms, prolonged and continuous rainfall causes a more significant rise in groundwater levels, especially around periods of high reservoir water levels, posing greater risks to slope stability. Therefore, in landslide disaster prevention, the role of rainfall and groundwater should be carefully considered. It is crucial to optimize the layout of monitoring points, enhance real-time automated groundwater monitoring capacity and service quality, and better understand the impact of groundwater dynamics on slope deformation.

Key words: groundwater level, rainfall, reservoir water level, cataclastic bedding rock landslide, deformation mechanism