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A large number of laboratory investigations related to the permeability have been conducted on the sliding zones. Yet little attention has been paid to the particular sliding zones of the slide-prone Badong Formation. Here, we experimentally investigate the permeability nature and the mechanism of seepage in the viscous sliding zone of the Huangtupo Landslide. Saturated seepage tests have been performed first with consideration of six dry densities and thirteen hydraulic gradients, in conjunction with the mercury intrusion porosimetry test and scanning electron microscopy test for the microstructure analysis after seepage. The results show that seepage in the sliding zone soil does not follow Darcy's Law, since there is a threshold hydraulic gradient (i0) below which no flow is observed and a critical hydraulic gradient (icr) over which the hydraulic conductivity (K) tends to be stable. The percentage of bound water could be responsible for the occurrence of i0 and icr. Furthermore, pore size distributions (PSD) less than 0.6 µm and between 10 and 90 µm exhibit positive and negative correlations with the i0, respectively, indicating that the i0 is related to the PSD. The mechanism accounting for this result is that pore water pressure forces fine clay particles into the surrounding large pores and converts arranged particles to discretely distributed ones, thereby weakening the connectivity of pores. The seepages in the sliding zones behave differently from that in the sliding mass and sliding bed in response to the permeability. [ABSTRACT FROM AUTHOR]

The evaluation of landslide runout behavior plays a vital role in the risk assessment of landslides. In this study, the runout behavior of a landslide that occurred on December 10, 2019, in the Three Gorges Reservoir region, China, known as the Kamenziwan landslide, is studied with the discrete element method. A field survey is first conducted to study the geological characteristics and failure mechanism of this landslide. The survey results indicate that this landslide is possibly caused by the long-term fluctuation of the reservoir water level. Then, a three-dimensional (3-D) numerical model of this landslide is constructed on the basis of the terrain data obtained before and after the landslide occurrence. The critical input parameters to the numerical model, in terms of the contact micro-parameters among particles, are calibrated with the machine learning method and uniaxial compression tests. To depict the effectiveness of the built numerical model, the simulation results are compared with the video snapshots taken by a local resident. Finally, the runout behavior and the energy transformation of the landslide are analyzed based on the validated numerical model. [ABSTRACT FROM AUTHOR]

The in situ stress field in valley areas is an important factor to consider in the construction of hydropower projects in southwest China. Due to intense tectonic movement, the action of gravity, and rapid river erosion, the resulting stress has a complex distribution and a large magnitude. The availability of stress measurements is insufficient to provide adequate understanding of the regional stress state. In this research, inversion analysis for the in situ stress field of a valley in the upper reaches of the Lancang River is carried out by using stepwise regression analysis and the FLAC3D numerical simulation, and the temporal and spatial distribution characteristics of the in situ stress are obtained. Based on those results, the influence of stress unloading effects on slope stability is discussed. The results show that the formation of the modern in situ stress field is the comprehensive result of interactions among gravity, tectonic movement, and river erosion. The reliability of the inverse result can be increased by improving the mesh density of the calculated model and considering multistep river erosion. Under the influence of the unloading effects, tensile fractures were formed and the rock mass loosened near the slope surface. With the presence of bedding dipping into the slope, toppling failure was triggered by the unloading rebound that mainly occurred in the stress relaxation zone. [ABSTRACT FROM AUTHOR]

Red-bed mudstone is widely distributed throughout the Three Gorges Reservoir Area, China. This geological material easily disintegrates into fragments during engineering activities. The potential factors that influence the disintegration (grain composition, slake durability index, grain roundness, fractal dimension, and surface energy) and shear strength (cohesion and friction angle) of red-bed mudstone fragments are investigated via cyclic wetting and drying tests and direct shear tests. The results show that the disintegration process includes three stages and that the grains gradually become well graded. The roundness exhibits a linear relationship with the number of cycles. However, the variations in the slake durability index, fractal dimension (mass-size distribution based), surface energy, cohesion, and friction angle show three distinct stages. In addition, the fractal dimension (quantity-size distribution based) and friction angle are significantly affected by the assumption that the specimens are isotropic, and the grain behaviors involve compaction, rotation, and separation. The cohesion and friction angle display linear relationships with the slake durability index, roundness, fractal dimension (mass-size distribution based), and surface energy, indicating the potential to assess the shear strength of mudstone fragments from the aspect of disintegration. [ABSTRACT FROM AUTHOR]

Toppling deformations generally develop in interbedded anti-inclined rock slopes in the upper reaches of the Yalong River in southwest China. With the characteristics of large scales, strong deformation degrees and complex deformation modes, toppling bodies may eventually lead to instability failure in the form of landslides, which seriously threaten the construction and operation of hydropower stations in the basin. Taking three typical examples of toppling failures for interbedded anti-inclined rock slopes in the basin, the sensitivity analysis of 19 impact factors affecting the toppling deformation of this type of slope is obtained via the Grey correlation method and UDEC. The results show that the geometric parameters represented by slope angles are the most sensitive to toppling deformation, the mechanical parameters of joints are more sensitive to toppling deformation than the physical and mechanical parameters of intact rocks, and the sensitivities of the parameters of sandstone and slate gradually converge in the deformation process. [ABSTRACT FROM AUTHOR]

Accurate estimates of the three-dimensional (3-D) rock fracture orientation distribution are crucial for generating a reliable fracture system model. Although an earlier method by Fouché and Diebolt allows one to estimate such distributions from one-dimensional (1-D) samples, this method does not typically produce highly accurate estimates of the 3-D orientation distribution (HAE3DOD). In this study, the minimum scanline-to-fracture angle (minimum θ) and the minimum orientation sample size (minimum n) required to produce HAE3DOD are investigated. Firstly, the factors significantly influencing minimum θ and minimum n are identified, and the influence clarified. For minimum θ, the possible influencing factors include the orientation concentration parameter (к) and n, while for minimum n, the possible influencing factors include к and θ. Fractures from three selected sites in China provide sufficient data for this investigation. The investigation results reveal that minimum θ varies almost linearly with к and n. Moreover, minimum n varies linearly with к and θ. Variations in the minimum θ and minimum n values are strongly associated with discrepancies in sample density resulting from different values of these factors. To ease the estimation of minimum θ and minimum n, empirical relations that take into account the separate factors are proposed for these two variables. A practical example demonstrates that the proposed relations accurately and efficiently estimate minimum θ and minimum n and can provide sampling guidelines (i.e., the intervals of scanline direction and sample size) for producing HAE3DOD. The potential limitations of the proposed relations are also discussed. [ABSTRACT FROM AUTHOR]

This study presents a hybrid approach based on two-step cluster and decision tree C5.0 algorithms to establish a deformation forecasting model for a step-like landslide. The Zhujiadian landslide, a typical step-like landslide in the Three Gorges Reservoir area, was selected as a case study. Approximately , 6 years of historical records of landslide displacement, precipitation , and reservoir level were used to build the forecasting model. The forecasting model consisted of seven comprehensive rules governing hydrologic parameters and their magnitudes and was developed to predict landslide deformation. This model was applied to rapidly forecast the likelihood of step-like landslide deformation resulting from rainfall and water level fluctuations in the Three Gorges Reservoir area. Given the satisfactory accuracy of the trained model, the presented approach can be used to establish forecasting models for step-like landslides and to facilitate rapid decision making. [ABSTRACT FROM AUTHOR]

The Huangtupo landslide is the volumetrically largest, most complex, and economically most significant landslide in the Three Gorges Reservoir Region. Detailed information about the character and dynamics of this tractive landslide is available because of access provided by a unique, 908-meter in situ research tunnel, augmented by long-term data provided by an extensive monitoring system for ground water levels, rainfall, and in situ deformation. Three major rupture surfaces plus 12 interlayer sliding zones (weak layers) were recognized during tunnel excavation, which also provided access and samples that establish a complex, multistage deformation history. Data from the in situ monitoring system shows that the toe of the landslide is creeping at a steady annual rate of about 25-30 mm, but at a slower rate of about 12 mm/year near the crown. However, the monthly creep rates are variable, being highest during May-August when rainfall is heavy and reservoir levels are low. Displacement-depth curves in the HZ6 borehole inclinometer establish that the creep of Slump Mass I# is most rapid above a depth of 37 m, where a major rupture zone occurs immediately above bedrock. Engineered defense structures along the toe of Huangtupo landslide can improve its stability to some extent. The effect of rapid changes of the reservoir level on landslide stability should be considered in reservoir management. [ABSTRACT FROM AUTHOR]

Huangtupo landslide, volumetrically the largest, most complex landslide in the Three Gorges Reservoir region of China, is a dangerous mass on which the district of Badong has been inadvertently situated. Risk remediation efforts include the construction of a large observational tunnel and monitoring system that are unique in the world. This tunnel and its side branches permit detailed mapping of its 3D structure while providing samples for laboratory analysis. The new investigations validate that the Huangtupo landslide is a composite of several independent landslides and that movement occurs along the major rupture zones as well as on interlayer sliding zones in the underlying Badong Formation. Uranium-thorium disequilibrium dating establishes that the northern part of the landslide, called the Riverside Slump, underwent at least two periods of movement at about 100 and 40 ka (ka stands for a thousand years). These events were induced by the steep slope created by the downcutting of the Yangzte River. The results from in situ displacement monitoring over a 7-year period confirm that the central part of the landslide is creeping at a slow, relatively stable rate of about 15 mm/year rather than being in a stage of acceleration under the protection of anchored concrete beams and other defense structures at its toe. Available data suggest that engineering measures can control the independent landslides that together constitute the huge Huangtupo mass, which will avoid the need for costly relocation of thousands of people. [ABSTRACT FROM AUTHOR]

In recent years, a large number of bank destruction occur in the reservoir area under the effect of water fluctuation, which may be lead to reservoir accumulative landslide geological hazards finally. The paper conducted the bank destruction forecasting study for accumulative landslides in the Three Gorges Reservoir Region, China utilizing back-propagation (BP) neural network approach. A representative scenario of Jinle landslide is then taken for analysis purposes. On the basis of the existing data sets of bank destruction cases, the BP neural network forecasting model and the corresponding programs for bank destruction are both presented, whose forecasting result is validated by two independent approaches, namely empirical method and numerical modeling method. Furthermore, the BP neural network model had obvious advantages over the convention approaches in the aspects of the fast calculation speed and high convenience. According to the bank destruction forecasting scale presented above, the corresponding revetment measures can be proposed to prevent the occurring of the bank destruction, whose effectiveness has been further validated by the actual engineering practice. [ABSTRACT FROM AUTHOR]

Recent research has paid little attention to weakening of discontinuities with different joint wall material (DDJM) induced by wetting and drying cycles. Experimental investigations were conducted to examine the effects of wetting–drying cycles on weakening of three typical types of natural planar DDJM collected from a sliding-prone formation in the Three Gorges reservoir region of China. Deterioration of the corresponding four types of discontinuities with identical joint wall material (DIJM) were also been studied for comparison. The weakening laws of DDJM were revealed from macro- and microperspectives by testing on mineral compositions, microstructures and water absorption rate of joint walls as well as shear strength of discontinuities. The results indicate that the repeated wetting–drying cycles lead to obvious changes in microstructure and increases of macro water absorption rate for the studied joint walls with the changing degree closely related to their clay mineral contents; the peak shear strengths of DDJM gradually decreases with the wetting and drying cycles for a certain normal stress with the weakening rate between that of the DIJM with two joint wall materials of the DDJM, while closer to that of DIJM with the weak half materials of the DDJM; significant correlations have been found between the variation coefficient of rock microstructure of joint walls and the macrodeterioration coefficient of DDJM; the changing rates of multi-scale parameters before and after six wetting–drying cycles vary obviously, indicating the weakening is slowed down after six wetting–drying cycles for the studied discontinuities. Highlights: Effects of wetting-drying cycles on weakening discontinuities with different joint wall material were studied. Deterioration of the discontinuities with identical joint wall material were also been investigated for comparison. The weakening laws were revealed from macro and micro perspectives. Significant correlations have been found between the variation coefficient of microstructure of joint walls and the macro deterioration coefficient. The results could provide a basis to better understand the stability evolution of rock slopes with soft and hard rock interbedded. [ABSTRACT FROM AUTHOR]

Recent research has paid little attention to the shear damage of discontinuities with different joint wall material (DDJM). In this paper, we present an investigation on the evolution of the shear behaviour and the damage of three typical types of natural DDJM in a sliding-prone stratum of China. Experimental direct shear tests were performed on 14 pairs of natural DDJM specimens to examine the changes in the shear strengths and surface damages of the DDJM with increasing normal stresses and an increasing number of shear cycles by evaluating surface damages via damage zone distribution, damage area percentage, and variation of joint roughness coefficient (JRC). The results indicate that the differences in the shear damage between the two halves are closely related to the difference in strength of the two joint walls of the DDJM specimens with similar initial JRC values of the two joint surfaces. Simultaneously, parallel numerical direct shear tests were conducted in PFC3D. The performance of the numerical modeling was examined by comparing the parameters of shear strength, damage area and damage depth of DDJM specimens with those obtained in the experimental direct shear tests. Then the validated models were used to explore the evolution of the damage depth of DDJM specimens during the shearing process. The results demonstrate that the proposed numerical approach has the ability to reproduce the shear behavior and damage of DDJM reasonably and could be used to examine the internal damage of DDJM which are not easy to investigate via laboratory experiments. [ABSTRACT FROM AUTHOR]

Hydraulic parameters are key data for calculating groundwater level, which is critical for assessing reservoir landslide stability. However, the quantitative understanding the influence of hydraulic parameters on the groundwater level of a reservoir landslide remains limited. In this paper, we apply a novel global sensitivity analysis method, PAWN, to quantify the sensitivity of hydraulic properties. The Shuping landslide, which is a typical reservoir colluvial landslide located in the Three Gorges Reservoir area, China, is used as a study case. The hydraulic parameters are first sampled within their entire feasibility space by the Latin hypercube sampling method. These samples are then used as inputs into a nonintrusive finite element program to automatically compute the corresponding groundwater level outputs. Finally, sensitivity indices are calculated based on the input–output dataset via the PAWN method. The global sensitivity analysis results provide useful guidelines for site investigation and reservoir colluvial landslide model simplification and calibration. [ABSTRACT FROM AUTHOR]

Due to complex geological structures and strong crustal activity, geological disasters occur frequently in the upper reaches of the Yalong River, Southwest China. Toppling failure on anti-dip rock slopes is a common phenomenon; in particular, after the Wenchuan earthquake in 2008, numerous landslides caused by toppling deformation occurred in reservoir areas, seriously threatening the construction and operation of large-scale water conservancy projects and hydropower projects. Taking three typical toppling slopes in the basin as research objects, the dynamic behaviour, failure evolution, and failure plane location of toppling failure under seismic waves were investigated by shaking table tests and UDEC simulations. The test results indicated that slope angle and stratum dip angle were both inversely proportional to the stability of the slope in the tested ranges. The failure evolution of toppling on anti-dip rock slopes could be divided into four stages: (1) the development stage of tensile cracks in the strata; (2) the formative stage of tensile cracks at the crest of the slope; (3) the formative stage of toppling zones; and (4) the failure stage. Two equations were proposed to reveal the relationships between the surface peak ground acceleration and the horizontal depth of the failure plane and displacement of the slope surface. The propagation and coalescence of discontinuous cross joint in the slope and the entire failure evolution simulated by the UDEC damage model agreed well with the shaking table test results. [ABSTRACT FROM AUTHOR]

Landslides frequently disrupt roadway networks in mountainous regions worldwide. Because of the relatively long roadway extension and low roadway density in mountainous regions, the occurrence of a landslide hazard along a local road segment will cause traffic paralysis on the individual roadway and will further impact regional roadway network accessibility. This paper establishes an integrative risk assessment framework based on risk theory and complex network theory to combine the results of landslide susceptibility mapping along roadways and impact assessment on the roadway network. Through an analysis of the relationship between various geo-environmental conditioning factors and historical landslides along roadways, the support vector machine (SVM) model is used to assess landslide susceptibility across the regional roadway network. Both topological connectivity of the entire roadway network and transport accessibility between local residents are considered in the impact assessment on roadway networks. To illustrate the effectiveness of the proposed risk assessment framework, a case study of the roadway network in Fengjie County, China, which is prone to landslide occurrence, is conducted. The resulting landslide risk heatmap of Fengjie County's roadway network is generated using ArcGIS software. The most critical road segments are identified as being highly susceptible to landslides, and if they are disrupted, the entire roadway network will suffer significant performance loss. The results can support adaptive strategies for landslide mitigation, preparedness, and emergency response services, as well as improve roadway plans to reduce exposure and associated consequences by adding new links to the existing roadway network. [ABSTRACT FROM AUTHOR]

Monitoring data show that many landslides in the Three Gorges region, China, undergo step-like displacements in response to the managed, quasi-sinusoidal annual variations in reservoir level. This behavior is consistent with motion initiating when the reservoir water level falls below a critical level that is intrinsic to each landslide, with the subsequent displacement rate of the landslide being proportional to the water depth below that critical level. Most motion terminates when the water level rises back above the critical level, so the annual step size is the time integral of the instantaneous displacement rate. These responses are incorporated into a differential equation that is easily calibrated with monitoring data, allowing prediction of landslide movement from actual or anticipated reservoir level changes. Model successes include (1) initiation and termination of the annual sliding steps at the critical reservoir level, producing a series of steps; (2) prediction of variable step size, year to year; and (3) approximate prediction of the shape and size of each annual step. Annual rainfall correlates poorly with step size, probably because its effect on groundwater levels is dwarfed by the 30 m annual variations in the level of the Three Gorges Reservoir. Viscous landslide behavior is suggested. [ABSTRACT FROM AUTHOR]

Slip zone soils play a crucial role in the evolution of landslides. In this paper, slip zone soils of the Huangtupo Landslide in the Three Gorges Reservoir area, which is an ancient landslide affected by water level fluctuation, are taken as an example to research the influence of wetting–drying cycles on soil properties. The mercury intrusion porosimetry test is used to study the soil's microstructure, while the triaxial test is used to measure the mechanical parameters. The experimental results indicate that the wetting–drying cycles can change the micropore structures and macro-mechanical properties of the soils. Meanwhile, it is demonstrated that there is a certain relationship between the macro- and micro-properties. From the microcosmic perspective, the fractal structure unit formed in the long-term geological environment is the key factor affecting the mechanical properties of the slip zone soils. Then, based on fractal theory, a weakening model of slip zone soils during wetting–drying cycles is proposed. The correctness and reliability of the model are verified by the mechanical parameters from triaxial tests. Finally, it is indicated that the weakening model can be used to forecast the weakening coefficient of slip zone soils under wetting–drying action, which results from the periodic fluctuations of the reservoir level. [ABSTRACT FROM AUTHOR]

A field investigation of the Huangtupo landslide in the Three Gorges Reservoir area, China, indicated that the particle-size distribution—characterized by an index termed the fractal dimension—varied along the main sliding direction: the fractal dimension tended to increase in this direction. A preliminary study of the spatial variability of the slip zone based upon the fractal dimension was performed. Remolded soil samples with various fractal dimensions obtained from the Huangtupo landslide were prepared for laboratory testing that was carried out to determine the effect of the fractal dimension on the micro and macro behavior of the soils. The microstructure of each soil sample was determined using scanning electron microscopy, while its macromechanical properties were evaluated via consolidated undrained triaxial tests. The results showed that there were close relationships of the fractal dimension with the microstructure, stress-strain behavior, and shear strength. The test results were also consistent with the results of the field investigation of the Huangtupo landslide. Thus, the fractal dimension can be used as an indicator of the spatial variability of the slip zone of a landslide. [ABSTRACT FROM AUTHOR]

Complex block-flexure slope toppling is observed in interbedded sandstone and slate in the upper Yellow River, China. Block toppling is observed in the relatively hard sandstone and flexural toppling is observed in the relatively soft slate. The evolution of toppling slope deformation is characterized by long-term progress and spatial variability. In order to study these characteristics, field investigations, adit prospecting, borehole drilling, sonic tests and numerical simulation were carried out. In addition, the effect of the structure, the composite mode of the rock mass, the unloading fissures and geomorphology are discussed. Furthermore, we explored the toppling mechanisms and simulated it numerically; on this basis, the slope evolution is divided into four stages. Results obtained from the numerical simulation compared well with field investigation data (investigation data, rock sonic survey and strata dip statistics). Comparative studies have demonstrated that the analytical methods presented in this paper are appropriate for back analysis of the toppling evolution process. The result showed that this toppling slope has not yet entered the later progressivity failure stage and is currently limited to collapse at shallow levels, whereas the deep-seated rock mass will remain stable for a long time. This study might provide reference for a stability evaluation and hazard prevention analysis for rock toppling. [ABSTRACT FROM AUTHOR]