Slope Instability Mechanisms Research Articles

Undulating sediments of the Cape Fear submarine landslide system, offshore U.S. Atlantic margin: Sediment waves versus creep deformation

We interpret a region of undulatory sediments adjacent to a major headwall of the Cape Fear submarine landslide system offshore of North Carolina, USA, as sediment waves rather than creep or fault-related deformation. The wave package extends 19 km upslope from the S4 landslide headwall and thickens upslope from approximately 250 to 450 m. The field of undulating sediments displays the continuity of seismic horizons, upslope-migrating crests, downslope thinning, and wave heights and lengths of approximately 26 m and approximately 1 km, respectively, which are consistent with sediment wavefields. The Western Boundary Undercurrent formed contourites on the nearby Blake Ridge and it is possible that these sediment undulations were deposited via similar mechanisms. Ocean Drilling Program (ODP) cores near the sediment undulation field suggest that the turbidity currents also may have played a role in wave formation. Although most of the 19 km long field comprises unaltered sediment waves, we observe an approximately 5 km long zone adjacent to the landslide scarp that expresses evidence of faults that offset and deform the sediment wave strata. We interpret this deformation as the result of reduction in stress following the removal of the landslide mass. Given that the Cape Fear system has generated several episodes of potentially tsunamigenic slope failure, the future stability of the system is pertinent. Redefining these undulatory sediments as sediment waves eliminates a major slope instability mechanism of the system and is important for understanding the future slope stability hazards of Cape Fear. Our analysis highlights the importance of understanding sediment waves in hybrid submarine landslide-sediment wave systems. Geological feature: Cape Fear submarine landslide sediment waves Seismic appearance: Continuous undulating horizons Alternative interpretations: Downslope creep or faults Features with similar appearance: Extensional slope failure Formation: Alongslope and downslope currents Age: Quaternary Location: Cape Fear submarine landslide complex, offshore North Carolina Seismic data: High resolution multichannel seismic data Analysis tools: Multichannel seismic data, multibeam bathymetry, sub-bottom Chirp

Slope Instability Analysis in Permafrost Regions by Shear Strength Parameters and Numerical Simulation

With the onset of global warming, the environment has profoundly changed. The melting of ice and slope instability were widely observed in the permafrost regions of the Qinghai–Tibet Plateau, which has destroyed infrastructure and threatened people’s safety. For this study, we conducted a direct shear test, monitored field temperature, and performed a numerical simulation to explore the mechanism of slope instability in the permafrost regions of the Qinghai. The results show that the shear strength of the permafrost soil decreased with the increases in water content and thawing condition. Moreover, the thawing depth of the slopes increased with the rising temperature. From the temperature monitoring data and field observations, cracks and slope instability phenomena were observed in the Qinghai and occurred with the thawing of the permafrost soil. The safety of slopes in the permafrost regions decreased with increases in the temperature, slope gradient, and pore water pressure and with decreases in the shear strength parameters. In addition, the sliding interface had a direct correlation with the freeze–thaw interface, which is a migrated interface of water in liquid form. Therefore, the thawing of the permafrost soil causes the shear strength to decrease and pore water pressure to increase, which leads to slope instability in the permafrost regions of the Qinghai.

The Instability of High-Steep Road Cutting Slope: A 3D Continuum-Discrete Coupling Method

In this paper, the 3D continuum-discrete coupling analysis method combined with the strength reduction theory is used to study the macrodeformation, stress law, and microfracture mechanism of a high and steep cutting slope of Zhongkai Expressway in Guangdong Province. The 3D continuum-discrete coupling slope model is established based on the finite difference software FLAC and the discrete element program PFC. Then, based on the correlations between stress tensor and force chain, displacement and shear strain increment, plastic deformation, and microfracture, the mechanism of slope instability is analyzed from macro- and microscales. The research shows the following: (1) the continuum-discrete coupling model is reasonable and effective in the three-dimensional slope stability analysis. (2) The coupling domain was penetrated by the shear band, and the upper part displacement of the coupling domain was large, whose direction was basically consistent with the shear band direction; thus, the slope slip surface was formed. (3) The microfracture can represent the macroscopic failure phenomenon, and the slope landslide can be interpreted as the result of shear fracture in slip direction, accompanied by the extension and penetration of tension crack.

Seismic behaviour of granular slope under railway embankment in large-scale shaking table test

This study investigates shear zone development in a coarse-grained soil slope beneath a railway embankment under seismic action. Clarifying the dynamic stress and residual strain responses under different seismic excitations reveals the slope instability mechanism. A typical coarse-grained soil slope under construction of the Sichuan-Tibet Railway Railway was selected for large-scale shaking table model testing. The evolution of slope response parameters (dynamic displacement, acceleration, dynamic strain and residual strain, and dynamic earth pressure) was monitored. The seismic damage modes and resulting deformations of the embankment and slope were analysed. The evolution characteristics of a shear zone were described; the relationship among acceleration, dynamic stress, and dynamic strain was clarified; and the critical state of earthquake-induced slope instability was defined. The main conclusions are as follows: under seismic excitation, the slope slip surface forms mostly near the interface between the sand and breccia layers, and a weakness zone exists between the subgrade and slope surface. Under high-energy seismic excitation (i.e., PGA ≥ 0.4 g), positive peak acceleration is more suitable for assessing the elevation amplification effect. Considering the cumulative strain distribution contour maps, the growth characteristics in the dynamic stress peak area can describe the development of a shear slip zone. Acceleration amplification factors can reflect slope responsiveness, but the energy contained in the seismic waves determines deformation. When the ratio of dynamic to shear stress exceeds 2, the dynamic strain in the slope slip zone increases considerably with shear zone extension and slope instability. With increasing dynamic stress, the growth characteristics of residual strain exhibit stability, transition and failure states.

Influence of Swelling on Shear Strength of Expansive Soil and Slope Stability

Rainfall is the main factor leading to the landslide of expansive soil slope. To deeply understand the instability mechanism of expansive soil slope under rainfall conditions, it is necessary to clarify the strength attenuation mechanism of expansive soil caused by water absorption and swelling. Therefore, the improved constant volume direct shear test was used in this study to compare with the conventional test, focusing on the variation of effective shear strength parameters with dry density (volume). The micromechanism of expansive soil strength attenuation is further studied, and the influence of expansive soil on slope stability is analyzed by numerical calculation. The results show that the improved direct shear apparatus and method can keep the volume of expansive soil specimens unchanged before and after shear and obtain the real effective shear strength parameter of soil. Under the same conditions, the shear strength parameter of the improved direct shear test is lower than that of the conventional direct shear test. The rapid thickening of the lamellar structure of montmorillonite in expansive soil will lead to the fragmentation and dispersion of clay particles, which is one of the fundamental reasons for the strength difference between expansive soil and ordinary clay. In slope stability analysis, the effect of swelling on the shear strength of slope cannot be ignored and should be considered in engineering design.

The mechanism of slope instability due to rainfall-induced structural decay of earthquake-damaged loess

Natural loess slopes are characterized by a strong geological structure, which is an important factor in maintaining slope stability. The magnitude and duration of the earthquake may disturb the soil structure at different levels degrees, locally changing the arrangement between soil particles. The process of rainfall humidification weakens the cementation between soil particles, and the disturbance and humidification change the structural state of the soil, which in turn causes sliding of the slope along with the decay of soil mechanical properties. As slope instability is often the result of a series of post-earthquake ripple effects, it is of great scientific significance to study the mechanism of slope instability due to the structural decay of earthquake-damaged loess exacerbated by rainfall. In this paper, the impact of structural decay of loess on slope stability is simulated by GEOSTUDIO software under three conditions: pre-earthquake rainfall, post-earthquake rainfall and earthquake, taking the landslide in Buzi Village, Min County, Gansu Province as an example. The comparative analysis of the calculation results shows that the structural properties of the slope without earthquake disturbance are influenced by infiltration amount. When it is fully saturated, the structural properties are similar to those of saturated soil, and the safety factor is reduced by 12.9%. In addition, the earthquake intensity and duration have different degrees of structural damage to the soil. When the structure is fully damaged, it is similar to that of remodelled soil, and the safety factor is reduced by 45.84%. Notably, the process of the earthquake and the following humidification generates the most serious damage to the loess structure, with a reduction in the safety factor of up to 56.15%. The quantitative analysis above obviously illustrates that the post-earthquake rainfall causes the most severe damage to structural loess slopes, and the resulting landslide hazard should not be underestimated.

Deformation instability mechanism of slope in Fa'er Town, Shuicheng County, Guizhou, China

This paper mainly explores the deformation instability mechanism and influencing factors of landslide in a town called Fa'er, in southwestern China’s Guizhou Province. Drawing on the theories of engineering geological mechanics, the exploration was carried out through the analysis of geological data, field investigation, and drone survey. The results show that: The mining of underground coal seams led to multiple tensile cracks on the trailing edge; the fragmented rock and soil mass, as well as the free face on the front edge create a favorable landform for initiating the landslide; the landslide is mainly induced by the heavy rainfall for several consecutive days; under the continuous heavy rainfall, the creep shear zone was penetrated by cracks, and the landslide was pulled by gravity to slide; the deformation mode shifted from creeping-tensile cracking to plastic flow-tensile cracking.

Landslide mechanisms along carbonate rock cliffs and their impact on sustainable development: a case study, Egypt

Recently, Egypt has faced unprecedented development with rapid urban and infrastructure expansion. Vast areas of Egypt are underlain by karst rocks (carbonate and evaporite). Highways and roads constructed along these karst areas are susceptible to slope stability problems. Various factors could trigger different landslides along the carbonate rock cliffs and slopes. Hence, urgent actions need to be considered to understand the landslide mechanisms that could occur along these cliffs and slopes. Carbonate rock cliffs and slopes along with some highways in Sohag-Assiut area, Egypt, were chosen as a case study in this work. Field and laboratory investigations were carried out as tools in understanding and identifying these failure mechanisms. In addition, remote sensing high-resolution images were applied to help in identifying different features, causing slope instability. Our finding indicated that five landslide mechanisms in carbonate rock cliffs and slopes were identified and categorized. These five slope instability mechanisms are including collapses of weak materials from filled caves, the breakdown of caves, instability due to differential erosions, dissolution along discontinuities, and sliding along weak surfaces. This study could help the decision-makers and planners to understand the causes of slope instability problems, to maintain the sustainability of the infrastructure, and to design appropriate prevention and remediation measures to avoid future problems.

Influence of acid rain on slope instability mechanism—a case study in Sichuan provincial highway, China

Acid rain has important influences on the physical and mechanical properties of rock as a result of mineral conversion and dissolution of cement. There are clear spatial correlations between the occurrence of slope failure and rainfall pH value in acid-rain areas. Although some fatal landslides have been ascribed to acid rain, those landslides all consisted of sedimentary rocks that generally contained carbonate minerals, which are particularly susceptible to dissolution in acidic conditions. This paper describes the mechanical deterioration of the structural plane based on acid-rain simulation experiments on a slope cut in hard rock along Sichuan provincial highway S211, China. In the experiments, the cohesion and internal friction angle of the structural plane fell to 35.4% and 6.8% of their original values, respectively, and the strength deterioration exhibited a power-law relationship with the exposed area of the rock bridges. Mineralogical and hydrochemical analyses including polarising microscopy, X-ray diffraction, scanning electron microscopy, and inductively coupled plasma optical emission spectrometry were employed to describe the micromechanisms of weakening. Mechanical weakening of the structural plane is attributed to deterioration of both the composition and the structure. Transformation from hard essential minerals to clay minerals through pyroxene chloritisation, plagioclase illitisation, illite smectitisation, and chlorite dissolution reduced the strength properties of rock bridges. In addition, abundant secondary micro-cracks were formed by propagation of worm-like dissolution pits, thus disintegrating rock bridges and further reducing the strength of the structural plane. This paper provides a better understanding of slope failure mechanisms in areas affected by acid rain.

Simulation of the Q2 loess slope with seepage fissure failure and seismic response via discrete element method

The Loess Plateau has a history of frequent landslides. Considering the Xiangning landslide that occurred on March 15, 2019, in Shanxi Province, China, a numerical slope model was constructed using the PFC3D discrete element software. A tension fissure and building load were set on the top of the slope to elucidate the slope instability mechanism and failure evolution process under the condition of surface water infiltration. The failure process and dynamic response of the loess slope in this area were analyzed by a seismic wave load input into the model. The results show that the instability process of the loess slope under water infiltration into the fissure can be divided into four stages: subsidence, extrusion, failure, and stability. Under earthquake motion, the slope began to suffer damage from the shoulder and toe of the slope, with a final sliding crack surface occurring near the fissure. Additionally, the failure surface of the slope top changed at 25 m from the free surface to 22 m, while the accumulation body at the toe of the slope changed from a length of 80 m in the direction of the gully to a length of 280 m. The peak ground acceleration (PGA) amplification factors (PAF) of the loess slope are as follows: free face (range: 1.73–4.5), elevation (range: 1.01–1.72), and fissure face (range: 1.40–1.55). The results provide a theoretical basis for reducing the instability risk of loess slope in this area.

Instability analysis of jointed rock slope subject to rainfall using DEM strength reduction technique

Uncertain climatic factors like rainfall can induce the complex hydro-chemo mechanical processes within the rock mass, and thus substantially increase the failure probability of rock slope. This paper presents a numerical investigation into the rainfall-induced instability mechanism of jointed rock slope with two joint inclinations of dipping 45° and 60° and the corresponded safety factor using the two-dimensional Distinct Element Method (DEM), where the non-uniform strength reduction approach is employed. For this purpose, some crucial aspects were mainly discussed, i.e. the quantifications of water softening and chemical weathering effects in the micro bond contact model, the necessary fitted functional relationships used in the safety factor, and determinations on the intensities of water softening and chemical weathering. The results show that the proposed DEM strength reduction provides a feasible approach to analyze the rainfall-triggered instability of jointed rock slope. The significant rise of bond breakage and the accompanied large displacement of sliding block indicate the achieved critical infiltration depth of rainfall and the consequent destabilization of jointed rock slope. The jointed rock slope with steeper dipping joints has lower safety factor, and thus is more prone to instability under the same rainfall effect.

Shaking Table Test on Dynamic Response of Bedding Rock Slopes With Weak Structural Plane Under Earthquake

Taking a bedding rock slope with weak structural plane as the prototype, a shaking table test with a similarity ratio of 1:10 is designed and carried out. By analyzing the acceleration and displacement responses at different positions of the slope, the seismic response and instability mechanism of rock bedding slope under different seismic amplitudes, frequencies, and durations are studied. Before the failure of the slope, the rock bedding slope shows an obvious “elevation effect” and “surface effect” under the action of Wenchuan Wolong earthquake wave with different amplitudes. With the increase of the amplitude of the input seismic wave, the elevation effect and the surface effect gradually weaken. When the amplitude of the seismic wave reaches 0.9 g, the rock bedding slope begins to show damage, which demonstrates that the difference of PGA amplification coefficients on both sides of the weak structural plane increases significantly. Compared with the Kobe seismic wave and Wenchuan Wolong seismic wave, the excellent frequency of EL Centro seismic wave is closer to the first-order natural frequency of slope model and produces resonance phenomenon, which leads to the elevation effect of PGA amplification coefficient more significantly. Through the analysis of the instability process of rock bedding slope, it can be found that the failure mechanism of the slope can be divided into two stages: the formation of sliding shear plane and the overall instability of the slope.

Time-dependent reliability analysis of rainfall-induced shallow landslides considering spatial variability of soil permeability

The rainfall-induced earth slope instability is time-dependent as a result of the infiltration process. In order to cater for the uncertainty of geo-material parameters, we conduct a time-dependent reliability analysis of the rainfall-induced landslide, which incorporates discretizing the stochastic process of rainfall infiltration, assigning instantaneous landslide limit state functions at discrete time instants, and calculating the time-dependent slope reliability index within the forecast time as a series system reliability problem. Via the widely used numerical package Python, we adopt a simplified Hasofer-Lind-Rackwitz-Fiessler (HLRF) algorithm to calculate the first-order reliability indices, and the numerical calculations extended to slope reliability analysis are easily implemented. In addition, we account for the spatial variability of soil’s permeability in the water infiltration model, which for simplicity is based on the Karhunen–Loève (KL) expansion technique. At last, we present extensive parametric studies of an example earth slope using the proposed method, revealing the potential influences of various sources of uncertainties and spatial variability when probabilistically modelling the water infiltration process and the slope instability mechanism, which may be used further for rainfall-induced landslide forecast.

Mechanism and future risk of slope instability induced by extreme rainfall event in Izu Oshima Island, Japan

A volcanic slope in Izu Oshima Island in Japan experienced a profound rain-induced disaster in October 2013. Since this slope had been stable for centuries except for minor failures, a special investigation was carried out on the cause. Because of its volcanic origin, the failed slope consisted of layers of ash, sand and lava. While the investigation concerned many disciplines, the present paper addresses one part of the geotechnical studies. The particular emphasis was put on the reason why some part of the slope “did not” fail because the post-disaster construction of infrastructures in the affected area relied on the future stability of the affected mountain slope. In line with this, another focus of the study was on the geohydrological feature of the underlying lava layer that possibly controlled the instability of the volcanic slope. It was concluded finally that the lava layer is pervious and allows drainage of infiltrated rainwater and that, only during extremely heavy rain, the subsurface lava topography triggered slope failure only in its “valley” parts. For 7 years after the disaster, the remaining parts of the slope have been stable as judged and many local infrastructures have been reconstructed.

Dynamic Response Characteristics and Failure Mechanism of Coal Slopes with Weak Intercalated Layers under Blasting Loads

Rock slopes with weak intercalated layers could experience disturbance from various deep mining activities; however, their dynamic stability has not been thoroughly investigated. In this paper, the dynamic response characteristics and failure mechanism of the coal slopes with weak intercalated layers under blasting loads were studied by means of numerical analysis, shaking table tests, and field tests. The effects of dynamic loads with different frequencies on the dynamic response of the slope were analyzed, and the natural frequency of the slope was also determined. The results show that the dynamic amplification effect of the slope is smaller than that of the homogeneous slope, and weak layers weaken the wave propagation in the rock mass. Both experimental and field investigation results show that the slope’s natural frequency was approximately 35 Hz. The slope deformation decreased with the distance of the blasting source. Cracks appear along the weak interlayer firstly under the action of horizontal vibration; then, longitudinal cracks occur at the slope crest. With the increase of dynamic loads, cracks continue expanding, deepening, and penetrating in the main controlled weak interlayer; then, the sliding body presents tensile shear failure along the sliding surface. This study could provide insights into the understanding of the coal slope instability and failure mechanism; this could benefit the blasting operation of the coal slope in fields.

Forecasting slope deformation field using correlated grey model updated with time correction factor and background value optimization

Developing precise models to characterize the slope instability mechanism quantitatively and predict slope movement behavior accurately remains a challenge in engineering geology. This paper updates the traditional GM (1,1) grey model to the TGM (1,1,p,q) grey model by adding a nonlinear time correction factor and optimizing the weighting coefficients of the background values using Genetic Algorithm. Furthermore, the TGM (1,1,p,q) has been updated to the TGM (1,n,p,q) correlated grey model by using a multivariate model of multiple monitoring boreholes to characterize the integrated deformation field. Taking the Majiagou reservoir bank landslide as an example, the time-series data of five monitoring boreholes were compared with their predicted values. The results show that the prediction accuracy of the updated grey model TGM (1,1,p,q) is acceptable, and the accuracy of the correlated updated grey model TGM (1,n,p,q) is much better. TGM (1,n,p,q) holds the potential of attaining a satisfactory prediction accuracy for the landslide deformation field forecasting.

Investigation and modeling of direct toppling using a three-dimensional distinct element approach with incorporation of point cloud geometry

Block toppling instability can be a common problem in natural rock masses, especially in mining environments where excavation activity may trigger discontinuity-controlled instability by modifying the natural slope geometry. Traditional investigations of block toppling failure consider classic kinematic analyses and simplified two-dimensional limit equilibrium methods. This approach is still the most commonly adopted, but the simple two-dimensional conceptual model may often oversimplify the instability mechanisms, ignoring potential critical factors specifically related to the three-dimensional geometry. This paper uses a three-dimensional distinct element method approach applied to an example case study, identifying the critical parameters that influence direct toppling instability in an open pit environment. Terrestrial laser scanning was used to obtain detailed three-dimensional geometrical information of the slope face geometry for subsequent stability analyses. A series of sensitivity analyses on critical parameters such as friction angle, discontinuity shear and normal stiffness, discontinuity spacing, and orientation was performed, using simple conceptual three-dimensional numerical modeling. Results of the analyses revealed the importance of undertaking three-dimensional analyses for direct toppling investigations that allow identification of critical parameters. A three-dimensional distinct element analysis was then performed using a more realistic complex volumetric mesh model of the case study slope which confirmed the previous modeling results but also identified unstable blocks in high slope angle areas, providing useful information for life of mine design. The paper highlights the importance of slope geometry and fracture network orientation on potential slope instability mechanisms.

Stability analysis of submarine slopes in the area of the test production of gas hydrate in the South China Sea

ABSTRACT In this paper, the mechanical properties of gas hydrate-bearing sediments (GHBS) were summarized and the instability mechanism of submarine hydrate-bearing slope (SHBS) was analyzed under the background of the test production of gas hydrate in the northern part of the South China Sea. The strength reduction finite element method (SRFEM) was introduced to the stability analysis of submarine slopes for the safety of the test production. Two schemes were designed to determine the physical and mechanical parameters of four target wells. Through the division of the hydrate dissociation region and the design of four working conditions, the range and degree of hydrate dissociation at different stages during the test production were simulated. Based on the software ABAQUS, 37 FEM models of SHBS were set up to analyze and assess the stability of the submarine slopes in the area of the test production. Necessary information such as safety factors, deformation, and displacement were obtained at different stages and under different working conditions. According to the calculation results, the submarine slope area is stable before the test production, and the safety factors almost remains the same during and after the test production. All these indicate that the test production has no obvious influence on the area of the test production and the submarine slopes in the area are stable during and after the test production.

The indoor model test of loess landslide instability induced by artificial rainfall in Tianshui area

The loess slope stability is influenced by rainfall and other factors. In order to find out the mechanism of loess slope instability, especially the influence of rainfall intensity and slope, the indoor model test was performed to study rainfall-induced loess landslide in Tianshui area, Gansu Province. Slope gradient and rainfall intensity are considered as variables, and their influence on slope stability are analyzed based on monitoring of soil suction and water content, and slope deformation process. The results show that the higher the rainfall intensity, the faster the infiltration rate. The volumetric moisture rate under heavy rainfall is more than 10% under small rainfall intensity. The steeper the slope, the lower the infiltration rate for the slope model. The loess slope is prone to overall sliding from bottom to top under the heavy rainfall, and easily lead to down-top retrogressive landslide under light rain.

Impact of fluid turbulent shear stress on failure surface of reservoir bank landslide

Based on geological investigation, in situ monitoring data analysis, and numerical simulation, a new approach to explain the effect of the reservoir water storage on activation of the sliding phenomenon is discussed in this paper. The paper proposed the method of groundwater level distribution function in case of different rate of water level rise in reservoir by using Boussinesq equation solved by Laplace change and Laplace inverse transformation. On comparing the in situ deformation monitoring data and fluid flow property near sliding surface, the mechanism of reservoir water-induced slope instability was understood. Fluid dynamic property affecting the slope deformation can be described as turbulent bottom shear stress at the failure surface for the phreatic water type landslide. The relationship between the stability coefficient, the deformation of the slope, and the fluid flow was established by using modified Boussinesq equation. Finally, with due consideration to the stability of the reservoir bank landslide, the method of calculation for rate of critical reservoir water storage was provided.