Limit Equilibrium Slope Stability Analysis Research Articles

Slope stability analysis of unsaturated colluvial slopes based on case studies of rainfall-induced landslides

Landslides in colluvial soils under rainfall have been identified as a significant problem due to their loose, heterogeneous nature and low shear strength. Evaluation of the stability of colluvial slopes under rainfall conditions is challenging. This study investigated two landslide failure case studies of colluvial soils to understand the failure patterns using finite element (FE) and limit equilibrium (LE) slope stability analysis methods under unsaturated conditions. Transient seepage conditions due to rainfall infiltration and failure were analysed using hydromechanical models. Here, a FE fully coupled hydromechanical model and a sequential coupling of a FE hydrological and LE mechanical model were used to evaluate the failure of variably saturated slopes. Results from the case studies revealed that the failure occurred due to the rise in the groundwater table in both cases. It was evident that there can be significant disparities in the pore water pressure profiles with the fully coupled and sequentially coupled analysis. The dynamic capability of the two models can also affect the interplay between the hydrological and mechanical aspects. When the thickness of the colluvium layer is large, the failure could potentially occur as a deep-seated failure along the boundary of overburden and the bedrock surface due to the large driving force. However, when the thickness is small, failure can occur along the colluvium-weathered rock surface. The outcomes from the study will contribute to mitigate the uncertainty of failure prediction of landslides in colluvial soils.

Analysis of Earthquake-Triggered Landslides through an Integrated Unmanned Aerial Vehicle-Based Approach: A Case Study from Central Italy

Landslides are one of the most hazardous(危险) secondary(二次) effects of earthquakes(地震) due(由于) to the potential(潜在) for large-scale(规模) damage(损伤) and long-term(学期) alterations to landscapes. During the 2016–2017 seismic sequence(序列) in Central Italy, many earthquake(地震)-triggered landslides (EQTLs) affected the road network(网络) and mountain trails. In this study, a methodological approach(方法) for analysing EQTLs, based on data(数据) derived from Unmanned Aerial Vehicle (UAV) surveys, is shown. The approach(方法) is applied to investigate(探讨) the geometric, structural(结构), geomechanical, and kinematic features of the Foce rockslide, which is introduced in the back analysis(分析). The investigation(调查) involved three main(主要) steps: (i) set up of UAV-based Virtual Outcrop Models (VOMs) of the slope(边坡), (ii) a geomechanical characterisation of the rock mass(大众) through the VOM interpretation(解释) and conventional(常规) field data(数据), and (iii) 3D Limit Equilibrium (LE) slope(边坡) stability(稳定性) analyses(分析). This study highlights the potential(潜在) of UAV surveys for providing valuable(宝贵) data(数据) for stability(稳定性) analyses(分析), especially in emergency(紧急) conditions such(这样) as in the aftermath of seismic events.

Limit equilibrium slope stability analysis of column-supported embankment on weak ground

PurposeThe study aims to analyse the stability of embankments over the improved ground with stone column (SC) and pervious concrete column (PCC) inclusions using limit equilibrium method. The short-term stability of PCC-supported embankment system is rarely addressed. Therefore, the factor of safety (FOS) of column-supported embankment system is calculated using individual column and equivalent area models.Design/methodology/approachThe stability analysis of column-supported embankment system is conducted using PLAXIS LE 2D. The various geometrical and shear strength parameters influencing the FOS of these embankment systems such as diameter of columns, spacing between columns, embankment height, friction angle of column material, undrained cohesion of weak ground and cohesion of PCC are considered.FindingsThe critical failure envelope of PCC-supported embankment system is observed to be of toe failure, whereas the failure envelope of stone column-supported embankment system is generally of deep-seated nature.Originality/valueIt is found that for PCC embankment system, FOS and failure envelope are not influenced by the geometrical/shear strength parameters other than height of embankment. However, for stone column-supported embankment system, FOS and failure envelope are dependent on all the shear strength and geometrical parameters considered in this study.

Effect of slope geometry on stability of slope in Almaty

Landslides bring destruction to buildings, and nearby located structures and mostly occur in rural areas. Such hazard commonly takes place in mountainous areas in Central Asia. Kazakhstan region has highly vulnerable areas to rainfall-induced landslides in South-Eastern parts due to presence of many mountains. The purpose of the research is to demonstrate the effect of slope geometry on slope stabilityunder heavy rainfall in Almaty, Kazakhstan. Transient seepage analyses were conducted using Seep/W while limit equilibrium slope stability analyses were performed using Slope/W. 15 sets of numerical analyses were carried out on different slope angle and slope height incorporating the soil-water characteristic curve and unsaturated permeability and unsaturated shear strength of soil from Almaty. According to the obtained simulation results in GeoStudio software for seepage analysis, the pore-water pressure is increased almost for 80 kPa at the mid slope of each simulation. The change of FoS for the gentlest slope with 27 degrees slope angle is the lowest for 10 m slope height and the highest for 30 m slope height, whereas the change of FoS for slope with 45 degrees of slope angle (9%) is almost the same for all slope height.

Three-dimensional limit equilibrium slope stability analysis on real slope failures caused by heavy rain and earthquake

Three-dimensional limit equilibrium slope stability analysis on real slope failures caused by heavy rain and earthquake

Examining the Effects of Suction and Nonlinear Strength Envelopes on the Stability of a High Plasticity Clay Slope

Slope failures in high plasticity clay deposits are common occurrences in many parts of the world. In western and central Alabama, expansive Prairie clays are commonly found, and shallow slope failures have occurred in both fill and cut slopes containing these high plasticity clays. The objective of this study was to examine the effects of suction and the use of nonlinear strength envelopes on the embankment stability of a section of highway AL-5. The testing program consisted of fifteen ring shear tests performed using a Bromhead Ring Shear Device. The results of the tests were used to develop both linear and nonlinear fully softened and residual strength envelopes. The saturated strength envelopes are then used in a limit equilibrium slope stability analysis with and without the effects of suction. The results show stability (factor of safety >1) for all cases except the residual friction angle without suction. Given these results, large slope failures are unlikely to occur in this area, but surficial failures and deformations due to creep may be possible. These results demonstrate the importance of considering the effects of suction and nonlinear strength envelopes when examining the potential for shallow slope failures in high plasticity clays.

Slope Stability Analysis to Correlate Shear Strength with Slope Angle and Shear Stress by Considering Saturated and Unsaturated Seismic Conditions

Assessment and analysis of soil slope stability is an important part of geotechnical engineering at all times. This paper examines the assessment of soil slope stability in fine-grained soils. The effect of change in shear strength (τ), shear stress (σ) and slope angle (β) on the factor of safety has been studied. It correlates shear strength with slope angle and shear stress by considering the horizontal seismic coefficients in both saturated and unsaturated conditions. The slope failure surface was considered a circular slip surface. Statistical package for social sciences (SPSS) and Slide, numerical modeling software and limit equilibrium slope stability analysis software, respectively, are used to find out the correlations between the three basic parameters. The slope angle varied from 70 to 88 degrees, which are the most critical values for slope angles, and a total of 200 analyses were performed. τ, β and σ are correlated, and the correlations are provided in the results section. The results indicate that the correlations developed between the parameters have a very close relationship. The applicability of the developed equations is above 99%. These correlations are applicable in any type of soil slope stability analysis, where the value of shear strength and factor of safety is required with the variation of slope angle and shear stress.

3D analysis of 2014 Oso landslide

The United States witnessed its deadliest landslide on March 22, 2014 in Oso, Washington. Forty-three people were killed and the direct financial damage exceeded 170 million dollars. The topography of the slope prior to this landslide was complex due to the occurrence of several historical landslides on the same slope and three-dimensional (3D) features of the slope. This paper utilizes an inverse 3D limit equilibrium analysis to propose a 3D failure mechanism and sequence for the 2014 Oso Landslide. The 3D mechanism consists of two main phases (Phase I and II) with three retrogressive slides that were identified during field reconnaissance and confirmed via the 3D limit equilibrium slope stability analyses. The 3D analysis suggests that Phase II of the Stark et al. (2017) mechanism consists of two separate retrogressive slides instead of a single slide. The two slides of Phase II are referred to as Phase IIa and Phase IIb herein for comparison with Stark et al. (2017) . Photographs herein show the geometry and runout of Phases IIa and IIb. After Phase IIa, sloughing of the steep scarp in the upper plateau onto the Phase IIb slide mass continued for months. The 3D analysis includes the complexities of the ancient landslide bench along with surrounding slope geometry and features, which help explain the initiation and direction of slide movement. • An inverse 3D limit equilibrium analysis was performed to reveal the 3D failure mechanism of 2014 Oso Landslide. • Future LiDAR-based hazard mappings should identify areas in which prior landslides removed the support of the upper plateau. • The Phase I failure surface starts at the crest of the eastern and narrowest end of ancient landslide bench. • The 3D analysis suggests that the second phase of the landslide consists of two separate retrogressive slides. • The results of the 3D stability analyses agree with field observation, flow trajectories, and ground motion records.

Levee Fragility Behavior under Projected Future Flooding in a Warming Climate

Adaptation to climate change requires careful evaluation of infrastructure performance under future climatic extremes. This study demonstrates how a multidisciplinary approach integrating geotechnical engineering, hydrology, and climate science can be employed to quantify site-specific impacts of climate change on geotechnical infrastructure. Specifically, this paper quantifies the effects of changes in future streamflow on the performance of an earthen levee in Sacramento, California, considering multiple modes of failure. The streamflows for historical (1950–2000) and projected (2049–2099) scenarios with different recurrence intervals were derived from routed hydrological simulations driven by bias-corrected global climate models. The historical and future flood levels were then applied in a set of transient coupled finite-element seepage and limit equilibrium slope stability analyses to simulate the levee subjected to extreme streamflow. Variability in hydraulic and mechanical properties of soils was addressed using a Monte Carlo sampling method to evaluate and compare the probability of failure of the levee under different historical and future climate scenarios. Three individual modes (underseepage, uplift, and slope stability) along with lower and upper bounds for the combined mode of failure were examined. The results showed that incorporating future floods into levee failure analysis led to considerable reductions in the mean factor of safety and increases in the levee’s probability of failure, suggesting that risk assessment based on historical records can significantly underestimate the levee’s failure probability in a warming climate. Despite inherent uncertainties in future projections and substantial variability across climate models, evaluating infrastructure against projected extremes offers insights into their likely performance for the future.

Design and construction of soil nail reinforced soldier pile wall as a temporary shoring system for deep excavation – a case study

ABSTRACT This paper discusses design and construction of an innovative shoring system known as soil nail reinforced soldier pile wall to support a temporary excavation at an actual construction site. The excavation works consisted of an existing wall of 7 m high, followed by a 5-m-high proposed wall located below the existing wall which were further complicated by other site constraints. Results showed that the soil nail reinforcement could effectively restrain the deformation of the soldier pile wall and cut-off potential deep-seated slip planes. The system also enabled more working space in front of the wall as no waler beam and anchorage head were required. Both finite element stress-strain analysis and limit equilibrium slope stability analysis showed reasonably good agreement with the field monitoring results. The wall deformed 6–65 mm over a 8-week period with the deformations generally stabilized 10 days after the wall completion.

Surface altering optimisation in slope stability analysis with non-circular failure for random limit equilibrium method

ABSTRACT In limit equilibrium slope stability analysis, surface altering optimisation (SAO) is a novel approach to minimise the factor of safety for a given slip surface using spline curves in 2D. It is a local search algorithm that when combined with a global search method, can form a powerful hybrid optimisation technique used in slope stability analysis. Probabilistic analysis of a slope with spatial variability is a computationally intensive example that would demonstrate the accuracy and speed of optimisation techniques. In this paper, the probabilistic analysis results of three different slopes with both complicated and straightforward geometries are presented, and the application of SAO in spatial variability analysis using random limit equilibrium method (RLEM) is demonstrated. It was found that SAO combined with a global search method provides fairly accurate results and yields curtailed computational effort.

Design procedure for landslide stabilization using sheet pile ribs

A design procedure was developed for a relatively unknown slope stabilization technique consisting of a series of parallel sheet piles installed parallel to the direction of slope movement. This technique was introduced in Alberta by R.M. Hardy in the 1970s and is locally referred to as “Hardy Ribs.” A case study is discussed where Canadian National (CN) Rail installed Hardy Ribs to stabilize a landslide affecting its rail line in western Manitoba. A proposed design procedure is discussed that consists of a de-coupled approach with a separate limit equilibrium slope stability analysis and laterally loaded pile analysis using p–y curves, where p is the soil reaction per unit length and y is the lateral deflection of the pile, to model the soil–pile interaction. Example calculations are provided for the proposed design procedure for the CN case study site to illustrate its use and to estimate the stabilizing effect from the Hardy Ribs at this site.

Probabilistic stability evaluation for wildlife-damaged earth dams: a Bayesian approach

ABSTRACTThere is a growing body of evidence confirming the detrimental effects of invasive wildlife activities on the performance and – ultimately – the safety of earth dams and levee systems. Modelling cavities and burrows dug by animals in earth structures is rather cumbersome due to their geometrical complexity and randomness. This study proposes the use of the probabilistic approach to estimate the risk associated with the presence of wildlife chambers in an arbitrary dam section. Two dimensional limit equilibrium slope stability analyses were conducted for a homogeneous silty sand dam. The animal chambers were modelled as idealised highly permeable circles with a near-zero strength. Uncertainty in the dam material was introduced using the Monte-Carlo technique to randomise the input parameters. The conditional probability of failure, P(F|C), was spatially evaluated by manual perturbations of chamber locations within the dam section. Using the Bayesian approach, the section was accordingly mapped into three stability zones: “safe”, “marginally safe” and “likely to fail” based on P(F|C) values associated with the chamber location. For this very complex problem, the probabilistic handling seems to provide a mathematically normalised basis for risk assessment in lieu of the classical belief-based single-value factor of safety (FoS).

The inclination of the interslice resultant force in the limit equilibrium slope stability analysis

Currently, all of the semi-precise slice methods are developed based on the assumed relationships between interslice shear and normal forces (or interslice force directions). These assumed relationships lack theoretical basis, and reduce the accuracy of determination of the thrust force of sliding mass in the reinforced structure design. Therefore, a precise relationship of interslice forces is required. A functional relationship between the interslice force direction and influencing factors including the gradient of the sliding surface, the gradient of the slope surface and the shear strength of the sliding surface is deduced based on the Mohr-Coulomb failure criterion. Then, the deduced equation is employed to determine the interslice force directions and assess the stability of the Houzishi landslide, which is a case in the Three Gorges Reservoir area in China. The comparison among the inclinations of the interslice resultant force obtained by the numerical simulation, our derived equation and assumed values of other slice methods shows that our derived equation provides the best match with the simulation results. In addition, the safety factor of our method that incorporates the newly derived equation, is consistent with the precise slice method, further demonstrating the effectiveness of the equation. Therefore, the semi-precise slice method that incorporates the derived relationships of interslice forces provides a more reliable approach for estimating the thrust force in the design of reinforced constructions.

Design and implementation of site-specific rainfall-induced landslide early warning and monitoring system: a case study at Nam Dan landslide (Vietnam)

ABSTRACTThis paper proposes and implements an early warning and monitoring system for rainfall-induced landslide (named as EWMRIL) with a case study at the Nam Dan landslide (northern Vietnam). The proposed system consists of six sensor nodes and one rainfall station that are used to sense large amounts of data in real-time such as soil moisture, pore-water pressure (PWP), movement status, and rainfall. In addition, a new flexible configuration for the wireless communication system is proposed that is capable not only to save the energy consuming but also to ensure the reliability of the system. Using wireless communication system, the sensed data were sent to the computer station for analyzing and predicting the instability of the landslide in terms of factor of safety (FoS) using the finite element seepage analysis and the limit equilibrium slope stability analysis methods. These methods are available in the SEEP/W and SLOPE/W modules of the GeoStudio software. Based on the analyzing results, the system proposed three warning levels for the landslide Early, Intermediate, and Imminent. Experiment result in the rainy season from August to September 2016 has proven the validity of the EWMRIL system. The result of this study is useful for landslide risk prevention and management in landslide prone-areas.

Limit state design framework for geosynthetic-reinforced soil structures

Conventional design of geosynthetic-reinforced soil structures is divided into two categories, walls and slopes, based on the batter of its facing system. Internal stability, characterized as sufficient reinforcement anchoring and strength, is performed using earth pressure-based design criteria for reinforced walls while reinforced slopes are founded on limit equilibrium (LE) slope stability analyses. LE analyses are also used to assess the global or compound stability of both types of structures, accounting for the geometry of the reinforced, retained and foundation soils. The application of LE-based methods typically results in determination of a slip surface corresponding to the lowest attained Safety Factor (SF), known as the Factor of Safety (Fs); however, it yields little information about reinforcement loading or connection load. In this study, use of the analyzed spatial distribution of SF known as a Safety Map, is modified to attain a prescribed constant Fs at any location in the reinforced soil mass. This modified framework, implemented through an iterative, top-down procedure of LE slope stability analyses originating from the crest of a reinforced structure and exiting at progressively lower elevations on the facing, enables the determination of a Tension Map that illustrates the required distribution of reinforcement tension to attain a prescribed limit state of equilibrium. This tension map is directly constrained by a pullout capacity envelope at both the rear and front of each reinforcement layer, providing a unified, LE-based approach towards assessing an optimal selection of mutually dependent strength and layout of the reinforcement. To illustrate the utility of the Limit State framework, a series of instructive examples are presented. The results demonstrate the effects of facing elements, closely-spaced reinforcements, secondary reinforcement layers, and is compared to conventional design approaches.

Probabilistic stability evaluation and seismic triggering scenarios of submerged slopes in Lake Zurich (Switzerland)

Subaqueous landslides and their consequences, such as tsunamis, can cause serious damage to offshore infrastructure and coastal communities. Stability analyses of submerged slopes are therefore crucial, yet complex steps for hazard assessment, as many geotechnical and morphological factors need to be considered. Typically, deterministic models with data from a few sampling locations are used for the evaluation of slope stabilities, as high efforts are required to ensure high spatial data coverage. This study presents a simple but flexible approach for the probabilistic stability assessment of subaqueous slopes that takes into account the spatial variability of geotechnical data. The study area (~2 km2) in Lake Zurich (northern Switzerland) shows three distinct subaquatic landslides with well-defined headscarps, translation areas (i.e. the zone where translational sliding occurred) and mass transport deposits. The ages of the landslides are known (~2,210 and ~640 cal. yr BP, and 1918 AD), and their triggers have been assigned to different mechanisms by previous studies. A combination of geophysical, geotechnical, and sedimentological methods served to analyse the subaquatic slope in great spatial detail: 3.5 kHz pinger seismic reflection data and a 300 kHz multibeam bathymetric dataset (1 m grid) were used for the detection of landslide features and for the layout of a coring and an in situ cone penetration testing campaign. The assignment of geotechnical data to lithological units enabled the construction of a sediment-mechanical stratigraphy that consists of four units, each with characteristic profiles of bulk density and shear strength. The thickness of each mechanical unit can be flexibly adapted to the local lithological unit thicknesses identified from sediment cores and seismic reflection profiles correlated to sediment cores. The sediment-mechanical stratigraphy was used as input for a Monte Carlo simulated limit-equilibrium model on an infinite slope for the assessment of the present slope stability and for a back analysis of past landslides in the study area, both for static and earthquake-triggered scenarios. The results show that the location of failure initiation in the model is consistent with stratigraphic analysis and failure-plane identification from sediment cores. Furthermore, today’s sediment-charged slopes are failure-prone, even for a static case. This approach of including an adaptable sediment-mechanical stratigraphy into a limit-equilibrium slope stability analysis may be applied as well to the marine realm.

A Case Study of a Cut Slope Failure Influenced by Snowmelt and Rainfall

In cold regions, frequent incidents of soil slope failures occur due to snowmelt and rainfall. These failures are triggered by an increase in the degree of saturation of soil due to infiltration of water derived from rainfall and snowmelt. This study investigates a soil slope failure occurred on November 27, 2015 at a cut slope of an expressway in Hokkaido, Japan. The soil slope failure is investigated using a recommended stability assessment approach considering non-isothermal coupled seepage flow simulation followed by a limit equilibrium slope stability analysis. It is concluded that at the thawing period, the slope stability is affected by rainfall and snowmelt water infiltration and later in November the failure happened due to overflow of water from a drainage ditch.

Influence of Saturation on Rainfall Generated Landslides in Shale along Murree-Kohala Road, Pakistan

Abstract: On August 10, 2010 a series of landslides of more than 90,000 m3 occurred along the Muree-Kohala road in the northern area of Pakistan. A study was undertaken to evaluate the likely impacts of percent saturation and bulk density on mobilized shear strength along the basal rupture surface of the landslide. A series of unconfined compression test and unconsolidated undrained triaxial tests were performed on remolded samples of different densities with varied percentages of saturation. The results of these tests suggest that soil cohesion and friction decreases with increasing saturation. The tests also showed that the shear strength parameters tend to increase with increasing dry density; however, all the samples exhibited a noticeable loss of shear strength with increasing degree of saturation, independent of soil density. Limit equilibrium slope stability analyses were performed along the most probable failure planes, based on shear strength parameters corresponding to degrees of saturation, which varied between 30% to about 100%. This resulted in drop of factor of safety from FS = 1.64 down to 0.51 as the degree of saturation approaches unity. These results suggest that the causative factor in triggering the landslide along Murree-Kohala was the partial saturation of the zone that developed the basal rupture plane. As rain infiltrated the slope, the bulk unit weight of the soil increased, while the shear strength along the developing plane of rupture decreased sufficiently to concentrate shear strain when the material became more than 60% saturation (FS < 1.0).

Optimising deep mixed soil zones in land reclamation projects

The use of deep soil mixing technologies has become increasingly common in North America following its widespread development and adoption in Japan and Scandinavia. This research project considered how design of soft soils stabilised with deep soil mixing may be optimised to satisfy global slope stability requirements to assist the construction of large reclaimed land projects. The land reclamation case that was studied in the present paper deals with raising the elevation of a terrestrial area above a tidal or flood zone, where the water table is close or at the original grade. In this work, the effect of several key design parameters on the global factor of safety against slope failure for a land reclamation project was studied using limit equilibrium slope stability analyses. The parametric analyses considered design parameters, including the thickness of the soft soil layer, as well as the strength, width and location of a stabilised soil zone. The purpose of this work is to provide guidelines for the preliminary design of ground improvements for future land reclamation projects using deep soil mixing methods by identifying design parameters that have the most significant impact on the global factor of safety.