Progressive Slope Failure Research Articles

Investigation of retrogressive and progressive slope failure mechanisms using the material point method

Retrogressive and progressive slope failures are a dynamic process, in the sense that they involve a progressively changing scenario. This paper uses the contemporary material point method (MPM), to provide a view of how such failures develop. Two main scenarios are presented: (a) a relatively small slope, which, when subjected to an initial failure, is steepened, leading to the initiation of further failures retrogressing backwards; and (b) a long slope, where an initial perturbation (e.g. an excavation) triggers a series of failures that can retrogressively move up-slope.

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Even if the various data analyzing methods were suggested to examine the measured slope behaviors, it is difficult to find methods or procedures for connecting the analyzed results of slope stability and measured slope data. This research suggests the analyzing methods combing the stability analysis and measured data based on progressive failure of slope. Slope failure analysis by time degradation were calculated by strength parameters composed of strength reduction coefficients, also which were compared to the measured data according to the variations of safety factor and displacement of slopes. The accumulated displacement curve were shown as 3rd degree polynomials by suggested procedures, which was the same as before researches. The reverse displacement velocity curves were shown as linear function for prediction of brittle slope failures, also they were shown as 3rd degree polynomials for ductile slope failures, which were the same as the suggested equation by Fukuzono (1985) and they were very similar behaviors to the in-situ failure cases.

Numerical Investigation of Seismically Induced Rock Mass Fatigue as a Mechanism Contributing to the Progressive Failure of Deep-Seated Landslides

The importance of earthquakes in triggering catastrophic failure of deep-seated landslides has long been recognized and is well documented in the literature. However, seismic waves do not only act as a trigger mechanism. They also contribute to the progressive failure of large rock slopes as a fatigue process that is highly efficient in deforming and damaging rock slopes. Given the typically long recurrence time and unpredictability of earthquakes, field-based investigations of co-seismic rock slope deformations are difficult. We present here a conceptual numerical study that demonstrates how repeated earthquake activity over time can destabilize a relatively strong rock slope by creating and propagating new fractures until the rock mass is sufficiently weakened to initiate catastrophic failure. Our results further show that the damage and displacement induced by a certain earthquake strongly depends on pre-existing damage. In fact, the damage history of the slope influences the earthquake-induced displacement as much as earthquake ground motion characteristics such as the peak ground acceleration. Because seismically induced fatigue is: (1) characterized by low repeat frequency, (2) represents a large amplitude damage event, and (3) weakens the entire rock mass, it differs from other fatigue processes. Hydro-mechanical cycles, for instance, occur at higher repeat frequencies (i.e., annual cycles), lower amplitude, and only affect limited parts of the rock mass. Thus, we also compare seismically induced fatigue to seasonal hydro-mechanical fatigue. While earthquakes can progressively weaken even a strong, competent rock mass, hydro-mechanical fatigue requires a higher degree of pre-existing damage to be effective. We conclude that displacement rates induced by hydro-mechanical cycling are indicative of the degree of pre-existing damage in the rock mass. Another indicator of pre-existing damage is the seismic amplification pattern of a slope; frequency-dependent amplification factors are highly sensitive to changes in the fracture network within the slope. Our study demonstrates the importance of including fatigue-related damage history—in particular, seismically induced fatigue—into landslide stability and hazard assessments.

Yumokjeong Landslide: an investigation of progressive failure of a hillslope using the finite element method

The presented case study describes the progressive movement of a shallow landslide in Central South Korea after excavation of its toe and subsequent, sustained above-average rainfall. Measured soil data as well as real-time measurements of slope movement and precipitation enabled calibration of a numerical model based on the finite element method (FEM) to capture the destabilization and associated movement of the slope. The numerical model employed the Bishop’s effective stress approach to define the state of stresses in unsaturated soils, as well as transient seepage analyses to introduce the non-uniform effects of actual rainfall data into the analysis. The resulting model provided insight into the complex behavior of a progressively failing slope. It demonstrates that use of numerical methods like FEM that allow for coupled transient unsaturated seepage-stress analyses presents a means of evaluating the progressive failure of slopes under rainfall. Furthermore, it demonstrated that for certain slopes under partially saturated conditions, sustained above-average rainfall may cause shallow landsliding when driven by stimuli like excavation. Additionally, the kinematics of failing slopes may not always follow classical slope stability approaches (sliding block method, circular method of slices, log-spiral, etc.) prior and during slope failure. The given kinematics of a slope failure may change with movement, redistribution of principal stresses, and dissipation of suction stress. Therefore, analyses like FEM present a unique tool to evaluate the deformation behavior of failing slopes on a case-by-case basis.

Debris flow activity and gully propagation: Glen Docherty, Wester Ross

Investigation into debris flow activity was undertaken in Glen Docherty. Both the historical and recent records of debris flow activity in the glen are outlined. While the glen’s psammites have a low debris flow spatial density relative to that recorded in areas underlain by other rock types, the volumes involved are high. Debris flow gully propagation, as it occurs on both vegetated and non-vegetated talus, is examined. Evidence from recent events calls into question whether debris flows are effective agents of flow channel scour. Progressive slope failure is proposed as a possible mechanism in gully propagation. Fluvial activity was also found to play an important role in debris flow processes including gully development.

Multistage, progressive slope failure in the Pleistocene pro-deltaic slope of the West Nile Delta (Eastern Mediterranean)

Abstract The vast majority of submarine mass transport deposits (MTDs) have been attributed to retrogressive slope failure processes whereby an initial rupture zone migrates in a generally upslope direction towards the position of the ultimate headwall. Here we use 3D seismic data from the prodelta slope of the Western Nile delta to describe a series of volumetrically extensive MTDs (termed MTDs A, B and C) that are part of a linked sequence that failed progressively but with retrogressive modifications of the lateral and headwall margins. The MTDs have c. 500–1000 ms of relief on their marginal scarps with a minimum total volume of remobilised sediments of 750 km3. By comparing their motion histories, and by correlating their basal surfaces, we demonstrate that MTDs B and C are remnants of a single original body that was later cut by MTD A. This sequence is confirmed by cross-cutting relationships at the lateral boundaries between the three MTDs and the absence of any significant infill and burial of residual topography at the tops of MTDs B and C prior to the incision of MTD A. This implies that these two major submarine failures (MTD B/C, and then MTD A) were closely grouped in time. We suggest a mechanistic model whereby rapid load redistribution resulting from the initial failure led to localization of a deeper cutting failure, and the unloading in the headwall region then led to expansion of the deeper failure in an upslope direction until a new merged headwall was formed. Correlation with MTDs identified downslope in previous studies suggests a date between 117 and 105 ka for this giant slope failure. This was a period of relative sea level fall in the Nile region which may have contributed to increased pore pressure. Slope failure on this scale was probably preconditioned by high sedimentation rates and under-compaction in a mud-rich succession, leading to local increases of pore pressure.

Slope creep behavior: observations and simulations

Rock slopes undergoing long–term effects of weathering and gravity may gradually deform or creep downslope leading to geological structures such as bending, bucking, fracturing, or even progressive failure. This study uses geomechanics-based numerical modeling to qualitatively explain the cause and evolution of slope creep behavior. Constitutive models used include the creep, Mohr–Coulomb, and anisotropic models. The last two models are used with the strength reduction in calculation. First, the results of field investigation around a landslide site occurring in slate are present. The causes and modes of creep structures observed on slopes and underground are studied. Second, the study investigates the influences of slope topography and anisotropy orientations on slope creep behavior. Finally, progressive failure of slopes with different shapes is examined. The simulated results show that the bending type of structures develops near slope surfaces, and the buckling type of structures is associated with the deformation or slides of a slope. The creep pattern varies with the orientation and position of an original planar structure. The shear zone involves a joint or fracture along which displacement has taken place. Moreover, creep behavior is more significant on slopes with greater height and inclination as well as on steeper portions whether on concave or convex slopes. In addition, with the same topographic conditions, consequent slopes with coinciding cleavage and obsequent slopes with steep cleavage result in greater creep behavior. Without the effects of anisotropic cleavage, concave and straight slopes develop failure surfaces from the crowns downwards, whereas convex slopes develop failure surfaces from the toes upwards.

Modeling the Progressive Failure of Jointed Rock Slope Using Fracture Mechanics and the Strength Reduction Method

The fracturing process during the progressive failure of a jointed rock slope is numerically investigated by using fracture mechanics and the strength reduction method (SRM). A displacement discontinuity method containing frictional elements is developed for the calculation of the stress intensity factor (SIF). The failure initiation of the jointed rock slope is analyzed by evaluating the SIF. A new joint model is proposed by combining solid elements with interface elements in the commercial software FLAC3D. These represent the discontinuous planes in a rock mass on which sliding or separation can occur. The progressive failure process is simulated by reducing the shear strength of the rock mass, which includes the process of stress concentration, crack initiation, crack propagation, slip weakening, and coalescence of failure surfaces. The factor of safety (FS) and location of the critical failure surface are determined by the SRM. The influence of the joint inclination is investigated using the FS and the SIF. Laboratory experiments on specimens containing an inclined flaw under compression-shear stress are also conducted to investigate the effect of the angle between the shear direction and the flaw inclination, which provides an experimental explanation for the shear behavior of jointed rock. The results show that the joint inclination dominates the failure behavior of jointed rock slope, and two failure patterns have been classified.

Seasonal slope surface deformation measured with TLS

In temperate European climates, soil water removal due to vegetation transpiration peaks in summer and soil rewetting from higher levels of precipitation occurs in winter. In clays of high plasticity, the seasonal cycles of drying and wetting cause the soil to experience a volumetric change, resulting in seasonal shrinking and swelling. For a clay slope exhibiting volume change, such behaviour can lead to excessive deformation and could contribute to strain-softening and progressive slope failure. This can in turn cause traffic disruption and loss of life if roads and railways are founded on or surrounded by such slopes. This paper discusses the driving forces of seasonal surface movement, in particular the role of vegetation, and presents the use of Terrestrial Laser Scanning (TLS) to measure the surface movement of a lightly vegetated London Clay slope near Newbury, UK. Two TLS scans were carried out in early and late summer respectively, representing relative wet and dry conditions of the slope. Continuous field measurements of soil water content in upper layers of the slope were obtained from TDR ThetaProbes already installed at the site. The water content data are used to support the results obtained from TLS by indicating the likely volumetric change in the soil due to loss of water.

Research on the Mechanism of Progressive Slope Evolution and its Failure Criterion

The unstable failure of rock and soil slope was a progressive evolution process from local failure to total failure. The no-uniformity of rock and soil materials and differences of boundary conditions caused the discrepancy of stress field, together with the elastic-plastic characteristics of the sliding zone which controls the progressive unstable failure process. In this paper, any point of slop sliding zone was taken for the research object. On basis of limit slice method, the mechanical condition of slice was considered as a plane strain problem. According to Mohr-Coulomb linear failure criterion the stress state of a point is obtained with selecting stress function to establish the mechanical criterion of progressive slope failure. The analysis of engineering example shows that the mechanical criterion is reasonable to estimate the state of progressive slope evolution.

Wetting-induced softening behavior of an unsaturated expansive clay

Studying the wetting-induced softening behavior of unsaturated soils is important for understanding rain-induced slope failures in unsaturated soils. However, experimental studies on expansive soils are limited. This study investigates the wetting-induced softening characteristics of an unsaturated expansive clay via suction-controlled triaxial tests on recompacted and natural specimens. It was found that the wetting-induced swelling of the soil included a significant component of plastic strain and its magnitude decreased with the net confining stress. When a soil specimen was subjected to wetting at a constant deviator stress, a threshold value of soil suction was identified after which the axial strain increased greatly. This value tended to increase with the applied net stress ratio. After the threshold suction, the wetting process for recompacted specimens could be divided into a yielding process associated with volumetric strain hardening/softening and a failure process in which the volumetric behavior depended on the applied net stress ratio. The natural specimens exhibited a brittle failure like a heavily overconsolidated soil during wetting at a constant deviator stress and continuous volumetric dilation occurred after the threshold suction. The wetting-induced softening soil behavior revealed from the present study could interpret the rain-induced progressive slope failure in unsaturated expansive soils.

Modeling progressive failures in rock slopes with non‐persistent joints using the numerical manifold method

SUMMARYRock slope failure is a complex process that usually involves both opening/sliding along pre‐existing discontinuities as well as fracturing of intact rock bridges. Discontinuity persistence is an important factor governing rock slope instabilities. However, traditional slope failure analysis assumes persistent discontinuities, and rock slope fails along a predefined persistent continuous potential failure surface because of the limitations of the analysis tools. This paper proposes the numerical manifold method (NMM) incorporated with a Mohr–Coulomb criterion‐based fracturing algorithm to simulate the progressive failure of rock slopes with non‐persistent joints. Detailed fracturing algorithm is first presented. Then, the NMM enabling fracturing is calibrated through simulating an edge‐cracked plate and the Brazilian test. Lastly, the developed code is applied to investigate the failure process of rock slopes involving non‐persistent joints. Numerical results indicate that the proposed method can capture the opening/sliding along existing discontinuities, the fracturing in intact rock bridges and the final kinematic release. Progressive slope failure is well exhibited. Copyright © 2013 John Wiley & Sons, Ltd.

Progressive modelling of the gravity-induced landslide using the local dynamic strength reduction method

Abstract The failure of slope is a progressive process, and the whole sliding surface is caused by the gradual softening of soil strength of the potential sliding surface. From this viewpoint, a local dynamic strength reduction method is proposed to capture the progressive failure of slope. This method can calculate the warning deformation of landslide in this study. Only strength parameters of the yielded zone of landslide will be reduced by using the method. Through continuous local reduction of the strength parameters of the yielded zone, the potential sliding surface developed gradually and evolved to breakthrough finally. The result shows that the proposed method can simulate the progressive failure of slope truly. The yielded zone and deformation of landslide obtained by the method are smaller than those of overall strength reduction method. The warning deformation of landslide can be obtained by using the local dynamic strength reduction method which is based on the softening characteristics of the sliding surface.

Analysis of a progressive slope failure in the Xiangjiaba reservoir area, Southwest China

The rehabilitation construction in the reservoir area of Xiangjiaba hydropower station in Southwest China has caused many landslides. A shallow progressive failure that occurred on the resettlement site of Xin’an Town of Pingshan County in Sichuan Province was selected as a case study. This landslide occurred in a long and gently inclined area by slope excavation under rainfall conditions. It is about 3.0 m deep with a total length of 35 m and is composed of some subfailures. Undisturbed samples were retrieved, on which the basic properties, shear strength, expansive potential, mineral compositions, and microstructures were tested and analyzed. The results show (1) the landsliding materials belong to medium expansive soil, consisting of the clay minerals of illite–smectite (I/S) and chlorite–smectite (C/S); (2) shear strength of the soil is sensitive to water, which greatly decreases once saturated; and (3) many fractures and relatively large pores are developed in the soils. Back analysis of the landslide shows that the shear strength at failure is less than the residual shear strength obtained from lab tests, indicating that some processes contributing to the slope failure could not be reflected by the shear box test. Based on the above analysis, the progressive process of the slope failure was interpreted, and it is inferred that the rainfall entered into the slope mainly through fractures and relatively large pores in the soil. It caused not only the great decrease in soil strength but also the swelling trend. The latter one would lead to growth, interaction, and coalescence of the fractures. Soon after, these fractures formed the shear planes (zones), which further decreased the resistance of the landslide. Under these favorable conditions, the slope excavation directly triggered the failure.

A large deformation analysis for the assessment of failure induced deformations of slopes in strain softening materials

This paper presents a numerical method that can be used to evaluate the post-failure deformation of slopes and embankments. The method is based on a large deformation finite element analysis employing the updated Lagrangian formulation. It can simulate the changing geometry of slopes during failure. An extended Mohr–Coulomb constitutive model represents the strain softening behaviour of slope material. The results of a series of analyses show that the post-failure deformation of slopes is a function of the rate of strength reduction and also the stiffness of the slope material. These two factors influence the initiation of progressive failure of slopes. The validity of the method is evaluated by simulation of a test embankment failure and comparison of the predicted results with the observed record of the failure.

Investigation on infrastructural damages by rainfall-induced landslides during November 2009 in Nilgiris, India

India is among the top ten countries with the highest percentage of landslide fatalities for the past few years. Intense rainfall during the 2009 monsoon in the hilly district of Nilgiris, in the state of Tamilnadu in India, triggered landslides at more than 300 locations which affected road and rail traffic, destroyed buildings, caused the death of more than 40 people and left hundreds homeless. In this paper, three case histories from Nilgiris district are investigated: the slope failure of a railway track at Aravankadu, failure of retaining walls supporting buildings at Coonoor, failure of the slope and retaining wall along national highway (NH67) at Chinnabikatty. Laboratory investigations are carried out on soil samples collected at these sites. Soils at all the three locations have high fine content and low values of coefficient of permeability. Finite element analyses of all the three case histories were carried out using PLAXIS2D software in order to understand the failure mechanism and contributing factors. Slope stability analysis using strength reduction technique is carried out for the slope at Aravankadu to determine the critical slip surface and factor of safety. Results reveal that the increase in pore pressures led to a reduction in shear strength of the soil and consequently resulted in progressive failure of slope at Aravankadu site. Displacement analysis is carried out for Coonoor and Chinnabikatty sites. The results show that combined effect of surcharge load of building and high pore pressure led to intense shearing behind the retaining walls at Coonoor site. Results indicate occurrence of large displacements along the face and at toe of the slope at Chinnabikatty site.

3D modeling of stratified and irregularly jointed rock slope and its progressive failure

Little has been published on the three-dimensional (3D) simulation of the progressive failure of rock slopes, possibly because the process of failure involves a complex, nonlinear evolution from initiation, through propagation and crack. In addition, rock is typically anisotropic, which makes it difficult to identify and describe the slope constituents and failure processes accurately. Despite such difficulties, further study of the fracture process is just as important as analyzing stress fields in 3D rock slope failures. In this paper, the 3D realistic failure process analysis code using finite element programming, and an extended version of numerical centrifugal method, is used to simulate slopes failure with different dip angles. The numerical centrifugal analysis results in this paper are found that the critical failure surface develops along the weak structural surface when the slope dip angle β is below 30°; conversely, the failure surface is formed along the toe of circular sliding when β is above 30°. In addition, it is also found that whether or not including the irregularity of joint into modeling to analyze the 3D slope stability problem will lead to a significant difference in factors of safety, it can reach 8.41 % at the same slope angle. Furthermore, the acoustic emission analyzing reveals deformed location characters of rock slope during the failure processes. With such capabilities, the approach contributes significantly to the in-depth study of the mechanisms of rock slope instability process.

Slope Stability Analysis of Jiu-Fen Using Finite Element Method and Limit Equilibrium Method

This study simulates the slope stability at Jiu-Fen, using the limit equilibrium method and the finite element method. The limit equilibrium method can find potential slide surfaces and safety factors rapidly. The advantage of the finite element method is that it utilizes more reasonable soil behavior and boundary conditions. A critical slide surface obtained by the limit equilibrium method can be compared with the displacement vector, stress field and location of plastic zone obtained by the finite element method. Furthermore, we can compare the safety factors produced by the two methods. This study shows that the safety factor from the limit equilibrium method is higher than that from the finite element method. The displacement analysis by the finite element method agrees well with the progressive slope failure. So, if we combine the two methods using monitored data in the field, we can analyze slope stability clearly.

Factors controlling the seasonal variation in soil water content and pore water pressures within a lightly vegetated clay slope

Seasonal cycles of soil water content cause shrinking and swelling in clay soils, which can in turn contribute to strain-softening and progressive slope failure. This paper presents and analyses six years of field measurements of soil water content and pore water pressures in the upper layers of a lightly vegetated London Clay slope near Newbury, UK, and shows how they can be related quantitatively to the climate using a water balance model. The field observations are set in the context of a 40-year run of rainfall data for the site. Moderately extreme rainfall and drought events were experienced over the period 2003–2008, allowing almost the full variation in likely pore water pressures to be characterised. Pore water pressures were found to return to near hydrostatic during most winters. Variations in summer rainfall, particularly during June–August, are shown to have a large influence on the magnitude of the cycles of pore water pressure and effective stress. The 40-year rainfall dataset is used to calculate approximate return periods for the observed soil conditions, and provides a benchmark for calculating the impacts of expected climate change on similar sites.

Progressive failure of sheeted rock slopes: the 2009–2010 Rhombus Wall rock falls in Yosemite Valley, California, USA

ABSTRACTProgressive rock‐fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high‐resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 15‐month‐long sequence of at least 14 rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock‐fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 736 m3 progressed upward along a sheeting joint behind the overhanging slab. Over the next 3 weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m3, radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions accompanying propagation of sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how sheeting joints can propagate to lengths many times greater than their depths behind cliff faces. We posit that as a region of failure spreads across a cliff face, stress concentrations along its margin will spread with it, promoting further crack propagation and rock falls. Published in 2012. This article is a US Government work and is in the public domain in the USA.