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Variations of reservoir water level and seasonal precipitation have reactivated or accelerated many reservoir landslides in the Three Gorges Reservoir area since impoundment in 2003. Updated daily monitoring data since 2017 reveals details about the step-like pattern of annual movement of the Baijiabao landslide, a large creeping landslide with a maximal cumulative surface displacement of 0.2 m over this 2-year period. The spatiotemporal deformation characteristics show that mass movement was greatest in 2017 and boundary cracks exhibit more frequent steps and are more sensitive to hydraulic factors than surface displacement. Acceleration periods are triggered when the reservoir water level falls below 153 m above mean sea level, with most annual movement occurring before the reservoir rises back to that critical level. The rate of daily surface movement is controlled by the daily variation of reservoir water level and by cumulative rainfall during the previous 1 to 7 days. The movement responses vary from different years and movement periods, but the one-factor critical-level model can effectively predict surface movement. The correlation between surface displacement rate and reservoir water level also comprehensively indicates the association between landslide movement and hydraulic factors including rainfall and variation of reservoir water level. Low reservoir level and its drawdown, and heavy or continuous rainfall, increase hydraulic gradients and change stress conditions, which destabilize the Baijiabao landslide and accelerate its surface movement. [ABSTRACT FROM AUTHOR]

Zigui Basin is a major landslide-prone region in the Three Gorges Reservoir region of China, and the stabilizing pile is an effective and widely employed countermeasure to reinforce landslides in this region. However, stabilizing piles are mostly designed using deterministic and stability-oriented methods, which generally ignore the system performance and cost-effectiveness. Using the Majiagou landslide reinforced with stabilizing piles as a case study, a probabilistic multi-objective optimization framework for the design of stabilizing piles is proposed and illustrated. Specifically, performance objectives related to failure probability, system robustness and life-cycle cost of the landslide-stabilizing pile system with feasible designs are evaluated, then the best compromised design is obtained by means of Pareto optimality. Expert knowledge and professional judgment are required to set necessary restrictions and finally determine the optimal design. The results show that there is a better design of stabilizing piles than the existing one, with which acceptable reinforcement effectiveness, compromised life-cycle cost and robust system performance can be realized. The optimal design will also vary with the concerned performance objectives and knowledge-based judgment. Further relationships and interpretations between design parameters and system responses are discussed through parametric analyses. [ABSTRACT FROM AUTHOR]

The Green-Ampt (GA) model is one of the most widely used analytical methods of slope stability under rainfall. However, it may overestimate the soil water content above the wetting front. In this study, a novel approach to evaluate the time-dependent slope stability during intense rainfall based on a modified GA model is presented, and is known as the stratified Green-Ampt (SGA) model. By considering the stratified soil water content above the wetting front, the soil above the wetting front can be divided into saturated and transitional layers, and the SGA model is used to analyze the infiltration process of intense rainfall into slopes. Thereafter, safety factors (Fs) of infinite and finite slopes are derived using the SGA model. In the analysis of an infinite slope, the conventional limit equilibrium method is adopted to calculate the safety factor; as for a finite slope, the residual thrust method is introduced to obtain the safety factor with sliding mass divided into multiple soil slices. The performance of the SGA model is illustrated in two cases: an infinite slope and the Majiagou landslide as a finite slope. The results indicate that compared to the GA model, the calculated wetting front based upon the SGA model moves faster, and the wetting front depth shows a positive correlation with the slope surface angle and rainfall intensity. The evolution of the safety factor above the sliding surface can be divided into three phases, while the evolution of the safety factor above the wetting front can be divided into two phases. The critical time of the slope reaching a less stable state (safety factor is 1.05) or unstable state (safety factor is 1.00) decreases exponentially with an increase in rainfall intensity. In addition, the rainfall has a significant influence on the design of stabilizing piles for the Majiagou landslide. The presented SGA model appears to be accurate to investigate slope stability during intense rainfall events. [ABSTRACT FROM AUTHOR]

Stabilizing piles are commonly used to effectively reinforce landslides and unstable slopes. However, the reinforcement mechanism of stabilizing piles, especially portal double-row (PDR) piles, remains poorly understood. The deformation characteristics and stabilizing mechanism of PDR piles were examined through physical model tests and numerical modeling by considering various embedded depths and interbedded weak and hard bedrock combinations. First, physical model tests were performed to study the effects of interbedded bedrock, orthogonal joints in the upper hard stratum, and embedded depths on the stabilizing mechanism. Second, six groups of numerical models based on the physical models were developed using the distinct element method. The results show that the bending moment and horizontal displacement of PDR piles increase in the presence of orthogonal joints and decrease with increasing embedded depth. The effects of crossbeam and hard bedrock percentage on the PDR pile deformation mechanism were then assessed. The results show that the crossbeam significantly affects the stabilizing PDR pile mechanism, while the effect of hard bedrock percentage is relevant to the embedded depth of PDR piles. The study provides some theoretical information for landslide prevention in areas with interbedded weak and hard bedrock. [ABSTRACT FROM AUTHOR]

The construction of the huge Three Gorges reservoir affected a large region, and the resultant geological and environmental impacts have caused global concern. The remarkable, 30-m annual fluctuation in the reservoir water level poses a significant threat to slope stability in this area. Four hundred sixty-two landslides were identified in the Zigui basin using historical records, satellite images, field investigations and unmanned aerial vehicle (UAV) observations, enabling the construction of a complete landslide database and distribution map. Three failure modes of landslides in the Zigui basin are used to illustrate the major factors that govern reservoir-induced landslides. The results show that (1) > 99% of identified landslides occur on slopes angle <47°, while >80% occur at elevations below 600 m; (2) Jurassic Niejiashan Formation is highly prone to landslides; (3) Low reservoir levels of 145 m to 155 m greatly reduce slope stability. Based on the information entropy method that use the conditional probability of different influencing factors and principal component analysis as inputs, the relative contributions of various influencing factors are quantified and a landslide susceptibility map was drawn. This susceptibility map helps define the countermeasures that will best reduce fatalities and property losses for areas having different landslide conditions and susceptibilities. • Database of 462 reservoir-induced landslides in Zigui basin, TGR was created. • Identifying failure modes and quantifying contributions of influential factors. • Conditional probability and information entropy model yields landslide susceptibility. • Susceptibility map, failure modes, and strategies suppressing the slope instability. [ABSTRACT FROM AUTHOR]