Limit Equilibrium Stability Analysis Research Articles

From Chingford to Carsington: impact of soil mechanics on dam construction, 1937–1987

In 1937 the investigation of the failure during construction of Chingford number 2 embankment dam brought the concepts of modern soil mechanics to bear on the construction of British embankment dams. Effective stress theory highlighted the crucial significance of pore-water pressure in understanding embankment stability and it became standard practice to monitor pore pressures during embankment construction using twin-tube hydraulic piezometers. By the 1980s, embankment stability might have seemed assured, with embankment design utilising limit equilibrium stability analyses that incorporated strength parameters of fill and foundation materials determined by laboratory testing. However, in 1984 the upstream slope of Carsington dam suffered a major slip in the final stage of construction. Recently developed advanced finite-element analyses were able to demonstrate that progressive failure was a major factor in the slope instability, but the failure was also a reminder of the continuing need for experience-based judgement in design.

Digital tools used on the teaching-learning process in geotechnical engineering

This article presents the incorporation of information and communication technologies on the teaching-learning process of geotechnical engineering to improve the quality of education and provide practical knowledge to civil engineering students. The content of this paper is divided into three main modules, which are: Treatment of laboratory tests results, Slope stability and Rock mass stability. For this, some software were used, including 2D limit equilibrium stability analysis, stability analysis of rock wedges and a dynamic mathematics, to obtain and analyze soil shear strength parameters, among others. Furthermore, a digital tool was developed to analyze the results gathered in compaction, one-dimensional consolidation, and direct soil shear tests, in order to clarify the relationship between theoretical concepts and practical results of the tests and analyses and to help students on doubts, in addition to increase their interest and motivation to perform the complete interpretation of the collected data. The activities were conducted in classes of geotechnical disciplines of the undergraduate course in Civil Engineering at the Federal University of Viçosa, aiming to promote active learning and improve teaching quality. Based on the results of an applied feedback questionnaire, it was observed that most students were satisfied with the resources used in the classroom, demonstrating that the implemented digital tools work as a didactic instrument that facilitates learning and comprehension of practical problems, in addition to enabling the resolution of several geotechnical engineering problems much more quickly and efficiently.

3D Limit Equilibrium Stability Analysis of Concave and Convex Slopes Considering Kinematic Constraints

In the previous limit equilibrium stability analyses of concave and convex slopes, the kinematic constraints are not considered in the generation of slip surfaces. To tackle this problem, this technical note proposes a method to compute safety factors of concave and convex slopes, combining the simplified Bishop method with an adaptive “point-by-point” technique. Through the adaptive “point-by-point” technique, the failure surfaces of slopes are linked by numerous lines that connect two neighboring discretized points, at which the velocity compatibilities are strictly satisfied. Stress analyses are made for the vertical discretized slices where the lateral pressure on the interface between soil slices is represented by the Rankine active earth pressure. Based on the simplified Bishop method and the strength reduction method, the safety factor and failure surfaces of concave and convex slopes are derived, which are verified by numerical simulations. Comparative outcomes show that the results would be closer to those of numerical simulations if the strength reduction is made for the Rankine active earth pressure on the interface between soil slices. And the proposed discretized slip surface considering kinematic constraints is more consistent with the shear bands by numerical simulation, as compared with the circular arc slip surface. Under homogeneous soil conditions, the proposed discretized slip surface can degenerate into a logarithmic spiral.

Rock Landslides Induced by Earthquakes: A Study on Influence of Strength Criterion on Limit Equilibrium Stability Analysis

Rock Landslides Induced by Earthquakes: A Study on Influence of Strength Criterion on Limit Equilibrium Stability Analysis

Behaviour of geosynthetic in anchored geosynthetic-reinforced slopes subjected to seepage

The anchored geosynthetic system (AGS) is a technique to improve the stability of unstable or quasi-stable slopes. In this study, the behaviour of geosynthetic in slopes reinforced with AGS subjected to seepage is investigated. Centrifuge model tests were carried out at 50g centrifuge acceleration on silty sand slopes with an inclination of 63·4° (2V:1H) and reinforced with AGS. Seepage was generated by gradually raising the water table within the reinforced slope models. By means of a digital image analysis technique, the behaviour of the surface geosynthetic during the seepage was studied and the compressive stresses produced beneath the geosynthetic were determined. Limit equilibrium stability analyses were conducted with the application of AGS loads measured by digital image analyses. The results indicate that the maximum tension mobilised in the geosynthetic during seepage depends on the tension induced in the geosynthetic due to the anchorage prior to introducing seepage. Furthermore, including the compressive stresses produced by the tensioned geosynthetic in the analyses gives a more realistic value for the safety factors.

Redundancy and coherence of multi-method displacement monitoring data as key issues for the analysis of extremely slow landslides (Isarco valley, Eastern Alps, Italy)

The paper describes the study of two extremely slow, active-continuous, deep-seated landslides located in the Isarco Valley (Eastern Alps, Italy): a Multiple Rotational Rock Slide (MRRS) and partial reactivation of a Deep-Seated Gravitational Slope Deformation (DSGSD). Both landslides interact with viaducts on the E45 highway. Extensive multi-method field investigation, monitoring of surface and subsurface displacements and limit equilibrium stability analyses were adopted to fully characterize slope dynamics. In order to overcome the limitations due to the systematic errors affecting each single monitoring system and analysis method, an operative framework based on redundancy and coherence tests was introduced to check the reliability of the data and results. In this framework, the geological and geotechnical models of the investigated slopes were progressively refined. This allowed assessment of the type of interaction between the slopes and the highway viaducts.

Effect of repeated rise and fall of water level on seepage-induced deformation and related stability analysis of Princeville levee

The Princeville levee, and flooding associated with Hurricanes Floyd and Matthew, is used as a case study in which the analyses are focused on the effect of repeated rise and fall of water levels (representing severe storm cycles) on the stability of the levee and the risk of failure. The analyses included strain-based and strength reduction approaches and are conducted using the finite element program Plaxis 2D. The limit equilibrium stability software “Slope/W” was also used for comparative study. The strain-based limit state approach considers the uncertainty of soil properties and is used to characterize the levee performance under repeated storm loading in terms of damage levels (or limit states). The strain-based analyses results show a progressive development of plastic shear strain zone within the levee as the number of storm cycles is increased. The accumulation of such shear strain leads to increasing the probability of exceeding a given performance limit state. As more flooding cycles are introduced, the shear strain values increase by a factor of 3.5 from cycle 1 to 6, and therefore reflect the increasing level of failure risk. In parallel, the deterministic stability factor of safety obtained from limit equilibrium method remains unchanged and slightly changes for strength reduction method with an increased number of rises and falls of the water level. The consideration of “rapid” drawdown conventionally used in limit equilibrium stability analyses (where no consideration for time is included), instead of more realistic rate based on drawdown hydrograph leads to conservative estimate of factor of safety. The analyses results demonstrate the increase in risk with repeated hydraulic loading.

ENGINEERING GEOLOGICAL INVESTIGATION ON KARAMBUNAI-LOK BUNUQ LANDSLIDES, KOTA KINABALU, SABAH

This paper describes landslide occurrences in debris materials, together with its engineering geological and geotechnical setting. The predictions from conventional geotechnical slope stability analyses, taking into account topography, hydrological, geotechnical and engineering geological effects, are compared with the observed pattern of instability. Physical and mechanical properties of eight (8) soil samples indicated that the failure materials mainly consist of poorly graded materials of sandy clay soils and characterized by low to intermediate plasticity, containing of normal clay (0.42 to 0.95), very high degree of swelling (5.63 to 10.35), variable low to high water content (11.95 % to 19.92 %), specific gravity ranges from 2.60 to 2.68, low permeability (6.68 X 10-4 to 1.52 X 10-4 cm/s), friction angle ( ) ranges from 18.50˚ to 34.20˚ and cohesion (C) ranges from 3.36 kN/m2 to 19.50 kN/m2 with very soft to soft of undrained shear strength (9.47 kN/m2 to 32.30 kN/m2). Geotechnical limit equilibrium stability analyses of entire slopes are rarely able to predict the smaller-scale initiation events leading to landslide occurrences, because the se are controlled by local topography, water runoff and groundwater conditions, weathered materials and engineering geological setting. Slope stability analysis shows that the factor of safety value is ranges from 0.805 to 0.817 (unstable). It is concluded that the failures was debris flow and resulted from a combination of factors. Engineering geological evaluation should be prioritized and take into consideration in the initial step in all infrastructure program. Development planning has to consider the geohazard and geoenvironmental management program. This engineering geological study may play a vital role in slope stability assessment to ensure public safety.

Investigation of landslide failure mechanisms adjacent to lignite mining operations in North Bohemia (Czech Republic) through a limit equilibrium/finite element modelling approach

Understanding the impact of data uncertainty is a fundamental part of ensuring safe design of manmade excavations. Although good levels of knowledge are achievable from field investigations and experience, a natural geological environment is subject to intrinsic variability that may compromise the correct prediction of the system response to the perturbations caused by mining, with direct consequences for the stability and safety of the operations.Different types of geoscientific evidence, including geological, geomorphic, geotechnical, geomatics, and geophysical data have been used to develop and perform two-dimensional Limit Equilibrium and Finite Element Method stability analyses of a lignite open-pit mine in North Bohemia (Czech Republic) affected by recent landslides. A deterministic-probabilistic approach was adopted to investigate the effect of uncertainty of the input parameters on model response. The key factors affecting the system response were identified by specific Limit Equilibrium sensitivity analyses and studied in further detail by Finite Element probabilistic analyses and the results were compared. The work highlights that complementary use of both approaches can be recommended for routine checks of model response and interpretation of the associated results. Such an approach allows a reduction of system uncertainty and provides an improved understanding of the landslides under study. Importantly, two separate failure mechanisms have been identified from the analyses performed and verified through comparisons with inclinometer data and field observations. The results confirm that the water table level and material input parameters have the greatest influence on the stability of the slope.

Geotechnical investigations for landslide hazard and risk analysis, a case study: the landslide in Kojour Region, North of Iran

In this article, the occurred landslide on the 27th kilometer of Kojour pathway, in the west of the Gat-e Kash village was surveyed and studied using geological maps, aerial photography and satellite images of the region. According to field investigations, this landslide is a transitional slide that occurred on Cretaceous marl successions near the Khachak over thrust fault and the Zanou’s river. Seven factors including lithology, topography, distance from fault, climate, land use, hydrology and hydrogeology and engineering geological conditions of the sliding mass were recognized and analyzed as effective parameters in the landslide. Based on engineering geological studies, the soil of the landslide is laid in SM-SC classes of the Unified Soil Classification System and has low swelling potential. The obtained safety factor by limit equilibrium stability analyses method indicated that the slope is stable in the present condition, but by changing in effective factors it may take place to unstable conditions again. The results of regression analyses showed the safety factor of the landslide is more controlled by specific gravity and degree of geomaterial saturation. During the field investigation, some sections with high potential of sliding were recognized; therefore, the most effective method including vertical and horizontal drainage and Gabion wall were offered for stabilization of the slope. Environmental assessments of the landslide were analyzed by the clustering technique, that negative impact was seen on soil erosion, surface and ground water quality, agriculture, road and communication.

Three-Dimensional Limit-Equilibrium Stability Analyses of Slopes and Effect of Inclusion of Soil Nails

Three-Dimensional Limit-Equilibrium Stability Analyses of Slopes and Effect of Inclusion of Soil Nails

Limit equilibrium stability analysis of slopes under external loads

Two calculation modes for the effect of external load on slope stability, i.e., mode I in which the external load is thought to act on slope surface, and mode II in which the external load is thought to act on slip surface along the force action line, were considered. Meanwhile, four basic distribution patterns of external load were used, of which complex external loads could be composed. In analysis process, several limit equilibrium methods, such as Swedish method, simplified Bishop method, simplified Janbu method, Spencer method, Morgenstern-Price (M-P) method, Sarma method, and unbalanced thrust method, were also adopted to contrast their differences in slope stability under the external load. According to parametric analysis, some conclusions can be obtained as follows: (1) The external load, with the large magnitude, small inclination angle, and acting position close to the slope toe, has more positive effect on slope stability; (2) The results calculated using modes I and II of external load are similar, indicating that the calculation mode of external load has little influence on slope stability; (3) If different patterns of external loads are equivalent to each other, their slope stability under these external loads are the same, and if not, the external load leads to the better slope stability, as action position of the resultant force for external load is closer to the lower sliding point of slip surface.

Grouping discontinuities in representative sets: influence on the stability analysis of slope cuts

Based on their orientation, different methods of grouping discontinuities in sets can be used; thus, three grouping methods and their influence on subsequent stability analyses are compared in this study. The first method combines the approaches of Priest, Shanley, Mathab and Yegulalp; the second method consists of grouping discontinuities manually based on an examination of density contour plots; and the third method is based on the spectral method. Unlike the others, the first method allows for the automatic determination of the number of discontinuity sets. The first and third methods were programmed in the Mathematica software environment, while DIPS software was used for the second method. Depending on the grouping process used, each discontinuity or only a portion of all discontinuities is assigned to a set. Therefore, the results in terms of the number of sets; the mean and dispersion of the sets; and the spacing of a given set differ with different methods. The consequences of a statistical analysis of orientation on the stability analysis were studied with simulations using RESOBLOK software, which couples the construction of 3D geometric block systems and a quick, iterative, limit-equilibrium stability analysis. Different fracture networks were generated with variable input parameters that were derived from previous statistical analyses of orientation and spacing. The software provides statistical outputs, such as the number and volume of unstable blocks, that can be compared across different cases. An application in a cutting slope near Ax-les-Thermes is presented; the influence of the grouping methods and of the uncertainty of slope orientation on stability indicators is presented. A synthetic indicator of global stability is also proposed.

Limit equilibrium slope stability analysis using the nonlinear strength failure criterion

The limit equilibrium stability analysis of two- and three-dimensional slopes with the nonlinear failure strength criterion uses a number of variables to determine the normal and shear stress on the slip surface. The equation for the nonlinear strength failure criterion is expressed using a Taylor series after analyzing the stress of an elemental slice or column. Multivariate linear equations are then derived to determine these variables based on the force and moment equilibrium conditions the sliding body is subject to. The stress on the slip surface can also be obtained to calculate the slope safety factor. The validity of the current method was verified by comparing it with established examples. Charts were produced for slope stability analysis with the nonlinear strength failure criterion under general conditions using the current method. The results of this study show that the slope safety factor decreases with an increase in the geotechnical material parameter m in the nonlinear strength failure criterion. The results of the current method are in close correspondence with other traditional limit equilibrium methods and are more reliable than the Swedish method. The charts can be used to determine slope design parameters that meet specific requirements.

Utilization of the limit equilibrium and finite element methods for the stability analysis of the slope debris: An example of the Kalebasi District (NE Turkey)

The stability of the slope debris in residential area of the Kalebasi District (Ozkurtun–Gumushane) was investigated using the Limit Equilibrium (LE) and Finite Element Shear-Strength Reduction (FE-SSR) methods. Along the survey lines, four trial pits were dug and fourteen boreholes having a total length of 345m were drilled. Also, seismic refraction studies were conducted along the five lines. According to the field studies, thickness of the slope debris covering the 98ha of the study area varies between 1 and 36m. To determine the physical and shear strength properties of the slope debris, undisturbed samples were taken from the trial pits. As a result of the laboratory tests, soil categories of the debris were found to be as Clayey Sand (SC), Silty Sand (SM) and Low Plasticity Clay (CL). The deformation-controlled shear box tests were carried out to determine the shear strength parameters of the slope debris. According to these tests it was found that the peak cohesion and peak friction angle varies between 2.63–16.35kN/m2 and 20–27°, respectively. Stability analyses were performed using the obtained data from field and laboratory investigations in the Slide v5.0 and Phase2 v6.0 software programs and results were compared. In LE stability analyses, the factor of safety (FOS) of survey lines were found to be as 1.44, 1.80, 1.96, and 1.72; however for the FE-SSR method they were determined as 1.39, 1.72, 1.59, and 1.58, respectively.

Ultimate bearing capacity analysis of strip footings on reinforced soil foundation

Reinforced soil foundations (RSFs) have been employed in engineering practice to increase the soil bearing capacity and to reduce the potential footing settlement. The aim of this study is to develop analytical solutions for estimating the ultimate bearing capacity of strip footings on RSFs. A general failure mode for RSFs was first proposed based on previous studies conducted by the authors and test results from literature study. A limit equilibrium stability analysis of RSFs was performed based on the proposed failure mechanism. New bearing capacity formulas, which consider both the confinement and the membrane effects of reinforcements on the increase in ultimate bearing capacity, were then developed for strip footings on RSFs. Several special cases of RSFs were presented and discussed. The proposed model was verified by the experimental data reported in the published literature. The predicted ultimate bearing capacity was in good agreement with the results of model tests reported in the literature. The study showed that the depth of the punching shear failure zone (DP) depends on the relative strength of the reinforced soil layer and the underlying unreinforced soil layer, and is directly related to the reinforced ratio (Rr).

Geomechanics of a snowmelt-induced slope failure in glacial till

An integrated approach involving field, laboratory and numerical investigations was undertaken to study the progressive deformation mechanism of a slope in glacial till above the Town of Alta, Utah that experienced catastrophic failure due to rapid snowmelt in June 2010. Detailed geometry of the slope surface and of the exposed sliding surface obtained from global positioning system surveying together with strength and stress–strain parameters derived from laboratory triaxial tests on undisturbed samples of glacial till collected from the landslide site were employed with finite-element modeling to examine the effects of an increase in snowmelt-induced perched water table on the yielding behavior of the slope prior to catastrophic failure. The numerical results indicate gradual development of a plastic yield zone along the sliding surface with progressive rise in the perched water table above the toe of the slide mass, which was spreading out throughout the slide mass at the stage corresponding to the onset of catastrophic slope failure. The evolution of the slope safety factor obtained from limit-equilibrium stability analyses in relation to the development of the finite-element computed yield zone along the sliding surface is also discussed.

A study on locating critical slip surface of slopes

The various methods of optimization or random search have been developed for locating the critical slip surface of a slope and the related minimum safety factor in the limit equilibrium stability analysis of slope. However, all these methods are based on a two-dimensional (2D) method and not one had been adapted for a search of the three-dimensional (3D) critical slip surface. In this paper, a new Monte Carlo random simulation method has been proposed to identify the 3D critical slip surface, in which assuming the initial slip to be the lower part of an ellipsoid, the 3D critical slip surface in the 3D slope stability analysis is located by minimizing the 3D safety factor of limit equilibrium approach. Based on the column-based 3D limit equilibrium slope stability analysis models, new geographic information systems (GIS) grid-based 3D deterministic limit equilibrium models are developed to calculate the 3D safety factors. Several practical examples, of obtained minimum safety factor and its critical slip surface by a 2D optimization or random technique, are extended to 3D slope problems to locate the 3D critical slip surface and to compare with the 2D results. The results shows that, comparing with the 2D results, the resulted 3D critical slip surface has no apparent difference only from a cross-section, but the associated 3D safety factor is definitely higher.

Centrifuge study of anchored geosynthetic slopes

ABSTRACT: This paper presents the results of centrifuge model testing on slopes reinforced by anchored geosynthetics and subjected to seepage conditions. The tests were conducted on 2V:1H slope models under steady seepage condition at 50g. Surface settlements and pore water pressures at different locations were recorded during the test. The influence of anchor stiffness, anchor length, and anchor pretensioning were examined. With an increase in anchor strength or stiffness, a considerable decrease in crest settlements and face movements was observed. Further, pretensioning of strands was found to be effective in enhancing the efficiency of anchored geosynthetic systems. Limit equilibrium stability analyses were in good agreement with the experimental results of slope models at failure.

A Comparative Study on Locating Critical Slip Surface in 2D and 3D Limit Equilibrium Stability Analysis of Slopes

The Various methods of optimization or random search have been developed for locating the critical slip surface of a slope and the related minimum safety factor in the limit equilibrium stability analysis of slope. But all these methods are based on a two-dimensional (2D) method and no one had been adapted for a search of the three-dimensional (3D) critical slip surface. In this paper, a new Monte Carlo random simulating method has been proposed to identify the 3D critical slip surface, in which assuming the initial slip to be the lower part of an ellipsoid, the 3D critical slip surface in the 3D slope stability analysis is located by minimizing the 3D safety factor of limit equilibrium approach. Based on the column-based three-dimensional limit equilibrium slope stability analysis models, new Geographic Information Systems (GIS) grid-based 3D deterministic limit equilibrium models are developed to calculate the 3D safety factors. Several practical examples, of obtained minimum safety factor and its critical slip surface by a 2D optimization or random technique, are extended to 3D slope problems to locate the 3D critical slip surface and to compare with the 2D results. The results shows that, comparing with the 2D results, the resulting 3D critical slip surface has no apparent difference only from a cross section, but the associated 3D safety factor is definitely higher.