Shear Strength Reduction Technique Research Articles

Slope stability of reclaimed coal mines through a new water filling index

A common reclamation practice for closed coal surface mines is filling them with water to form pit lakes. The creation and sustainability of these lakes are significantly affected by the stability of the corresponding slopes. The present study provides a general framework for analyzing the water filling's effect on slope stability based on a new water filling index, which can indirectly consider the factors affecting the process and efficiently quantify the filling speed's influence. The assumptions of the proposed approach are thoroughly discussed, and the range of the water filling index is identified. Furthermore, the safety factor is calculated using the finite element method with the shear strength reduction technique during the filling process for various conditions (soil properties, slope geometry, hydraulic conditions, and water filling speed). Results are presented as normalized stability charts for practical use. During the water filling, the stability gradually decreases until the reservoir reaches a critical level of 10%–40% of the total height; it then increases to even more stable conditions than the initial one. Overall, the present analysis allows for the preliminary stability evaluation of a coal mine during the formation of a pit lake and the appropriate quantification of the water filling's effect.

Soil Reinforcement Methods for Environmental Protection of Slope Failure Against Landslides

Slopes are not stable due to the variations of slope gradient combined with the driving forces including the groundwater action, producing shear stresses in the slopes, which are contradicted by the soil shear strength. The slopes instability may cause the displacement of the mass of soil downstream recognised as landslide. Consequently, landslides are one of the most significant catastrophic phenomena observed on the earth surface. This phenomenon can be considered as a major concern on the social and economic point of view and for the quality of life of a local population, including also the environmental impact on the ground water level, blockage of rivers, road destruction, forests and ecosystems destruction, etc. Therefore, soil reinforcement has become one of the best forms of embankment protection against slopes failure. This work used soil reinforcement methods by using retaining structures, slope design with low gradient and vegetation cover to stabilize the embankment. The usage of the shear strength reduction technique was carried out to evaluate the stability of embankment slope. This process has tendency to reduce shear strength as rarer as failure is produced. This study is achieved along with pseudo-static and static methodologies. Varied parameters influence on the stability of slopes are revealed by accomplishment of many studies. To end, the Static method findings are acceded to comparison with the ones found by Pseudo-Static method. The results showed that the stability of reinforced slopes was reached and the slope can resist against landslides.

A combined shear strength reduction and surrogate model method for efficient reliability analysis of slopes

Surrogate models are often used to alleviate extensive computational burden for slope reliability analysis. How to efficiently train a surrogate model with high precision is always a nontrivial task. This paper proposes a novel method, which combines the shear strength reduction (SSR) technique, the surrogate model and the adaptive pool-based sampling strategy for efficient slope reliability analysis. The surrogate model is used to replace the time-consuming slope stability model in simulation-based reliability analysis, while the SSR technique coupled with an adaptive pool-based sampling strategy is innovatively adopted to search for the most informative samples to train the surrogate model, which can improve the computational efficiency significantly. Further, an expanded sampling method is suggested to improve the local prediction of surrogate model in other design space, enabling an efficient evaluation of the slope failure probability corresponding to different threshold safety factors. The main feature of the proposed method is that it takes advantage of SSR technique to efficiently generate training samples in the region of interest. Several slope examples are analyzed to validate the accuracy, computational efficiency as well as the applicability of the proposed method. The results show that proposed method outperforms other approaches in computational efficiency and can give accurate estimation of slope failure probability as well.

Numerical Analysis of an Explicit Smoothed Particle Finite Element Method on Shallow Vegetated Slope Stability with Different Root Architectures

Planting vegetation is an environmentally friendly method for reducing landslides. Current vegetated slope analysis fails to consider the influence of different root architectures, and the accuracy and effectiveness of the numerical simulations need to be improved. In this study, an explicit smoothed particle finite element method (eSPFEM) was used to evaluate slope stability under the influence of vegetation roots. The Mohr–Coulomb constitutive model was extended by incorporating apparent root cohesion into the shear strength of the soil. The slope factors of safety (FOS) of four root architectures (uniform, triangular, parabolic, and exponential) for various planting distances, root depths, slope angles, and planting locations were calculated using the shear strength reduction technique with a kinetic energy-based criterion. The results indicated that the higher the planting density, the stronger the reinforcement effect of the roots on the slope. With increasing root depth, the FOS value first decreased and then increased. The FOS value decreased with an increase in slope angle. Planting on the entire ground surface had the best improvement effect on the slope stability, followed by planting vegetation with a uniform root architecture in the upper slope region or planting vegetation with triangular or exponential root architecture on the slope’s toe. Our findings are expected to deepen our understanding of the contributions of different root architectures to vegetated slope protection and guide the selection of vegetation species and planting locations.

Estimating the Optimal Overall Slope Angle of Open-Pit Mines with Probabilistic Analysis

Slope instability of open-pit mines has adverse impacts on the overall mine profitability, safety and environment. The slope of an open-pit mine is crucially influenced by the slope geometry, quality of rock mass and presence of geological features and their properties. The objective of this study is to demonstrate a method to select the optimal overall slope angle of open-pit mines according to three design parameters, namely, safety (e.g., probability of instability), productivity (e.g., profit) and mining costs (e.g., cost of removal of overburden). Therefore, this study attempts a hybrid approach in which numerical modelling is integrated with probabilistic analysis to evaluate the stability of an open-pit mine at various overall slope angles. Two-dimensional elasto-plastic finite-element, RS2D, has been used to develop a series of models at different ultimate slope angles employing shear strength reduction technique (SSRT). Li’s point-estimate method of n3 has been invoked in deterministic analysis to tackle the inherent uncertainty associated with host rock mass properties. The results reveal that the mine profitability increases and the cost of overburden removal decreases as overall slope angle becomes steeper. However, the slope stability deteriorates. Therefore, it is highly advisable to combine these three design parameters (e.g., safety, productivity, and cost) together when selecting overall slope angle of open-pit mines.

A new method for defining the local factor of safety based on displacement isosurfaces to assess slope stability

Slope stability is mainly assessed by the factor of safety (FS) parameter. As an improvement to the integral FS derived from the conventional limit equilibrium method (LEM), the local factor of safety (local FS) is used to describe the FS of each material element. However, although current slope stability algorithms with LEM rely on only strength criteria, some areas of a slope show almost no displacement, even though plastic yielding is occurring within the slope. For these reasons, we developed a new approach for defining the local FS based on the displacement isosurface. Adopting the Mohr–Coulomb (M–C) strength criterion, this new algorithm calculates the local FS in the potential sliding zone with a change in the apparent displacement. Based on the shear strength reduction technique, the distributions of the local FS within 2D homogeneous slopes with different shear strengths and within a real 3D slope with a complex topography are analyzed. Firstly, the applicability of the new approach was examined and we found that the local FS decreases significantly only when the direction of the yielding surface approaches the displacement direction, which also reveals that the LEM may underestimate the FS. In addition, the areas of a slope where the local FS strongly changes depend on the slope geometry and also control the instability patterns. Further, the results of a local FS in a 3D slope analysis show that complex topography and internal structure can cause the alternation of stable and unstable blocks in the slope and that the mobilities of the blocks induced by the instability differ in different areas. Finally, some local optimized retaining countermeasures were successfully implemented for supporting a real engineering slope guided by the local FS. Our results indicate that this improved algorithm can provide a more suitable and accurate local FS with which to assess slope stability and reveal more realistic instability patterns than approaches using the integral FS.

Environmental Protection Measures Against Landslides Impacts Caused by Slope Instability

Each slope is affected by the variations of slope gradient combined with the driving forces including the groundwater action, producing shear stresses in the slopes, which are contradicted by the soil shear strength. The slopes instability may cause the displacement of the mass of soil downstream recognised as landslide. Consequently, landslides are one of the most significant catastrophic phenomena observed on the earth surface. This phenomenon can be considered as a major concern on the social and economic point of view and for the quality of life of a local population, including also the environmental impact on the ground water level, blockage of rivers, road destruction, forests and ecosystems destruction, etc. The current study makes the usage of the shear strength reduction technique to evaluate the stability of slopes. This process has tendency to reduce shear strength as rarer as failure is produced. This study is achieved along with pseudostatic and static methodologies. Varied parameters influence on the stability of slopes are revealed by accomplishment of many studies. To end, the Static method findings are acceded to comparison with the ones found by Pseudo-Static method.

Estimating the Basal Heave Stability of Narrow Braced Excavations

Engineering practices indicate that narrow braced excavation exhibits a clear size effect. However, the slip circle method in the design codes fails to consider the effect of excavation width on basal heave stability, causing waste for narrow excavation. In this paper, numerical simulation for basal heave failure of excavation with different widths was performed by FEM with SSRT (shear strength reduction technique). The results revealed that the failure mechanism of narrow excavation is different from the complete slip circle mode. In addition, the safety factor decreases increasingly slowly as the excavation widens and stabilizes when approaching the critical width. Subsequently, the corresponding computation model was presented, and an improved SCM (slip circle method) was further developed. Finally, the engineering case illustrated that it can effectively optimize the design, which exhibits clear superiority.

Correlations of geotechnical monitoring data in open pit slope back-analysis -A mine case study

Geotechnical monitoring plays an important role in the detection of operational safety issues in the slopes of open pits. Currently, monitoring companies offer several solutions involving robust technologies that boast highly reliable data and the ability to control risky conditions. The monitoring data must be processed and analysed so as to allow the results to be used for several purposes, thereby providing information that can be used to manage operational actions and optimize mining plans or engineering projects. In this work we analysed monitoring data (pore pressure and displacement) and its correlation with the tension and displacement of the mass of an established failure slope calculated using the finite element method. To optimize the back-analysis, a Python language routine was developed using input data (point coordinates, parameter matrix, and critical section) to use software with the rock mass parameters (cohesion, friction angle, Young's modulus, and Poisson's ratio). For the back-analysis, the Mohr-Coulomb criterion was applied with the shear strength reduction technique to obtain the strength reduction factor. The results were consistent with both the measured displacements and the maximum deformation contours, revealing the possible failure mechanism, allowing the strength parameters to be calibrated according to the slope failure conditions, and providing information about the contribution of each variable (parameter) to the slope failure in the study area.

Computational tools for the analysis of circular failure of rock slopes

Among other types of failure in slopes created by excavation or filling, circular (also referred to as rotational) type of failure plays an important role in the design and stability analysis of high rock slopes, such as those encountered in deep open pit mines. When a rock slope of significant height involves predominantly one type of jointed rock mass, the scale of the problem is frequently such that the rock mass (intact rock and rock discontinuities) can be regarded as a homogeneous isotropic material. In such case, if the rock slope is unstable and collapse occurs, circular (or rotational) failure as in failed slopes in soils is also observed for the rock slope. One main goal in practical design of deep open pit mines is to determine safe inclination angles for the pit faces to be excavated to avoid failure. Computational tools for quick assessment of the stability conditions, in particular determination of factor of safety and location of the critical circular failure surface, are useful during the preliminary stages of open pit design. Among others, such tools allow the engineer to gain insight in the role that the different variables controlling the problem have in defining the stability conditions for the rock slope. This paper provides first an overview of typical numerical techniques used to assess the stability of high rock slopes. For the case of slopes in rock masses that are assumed to obey the Mohr-Coulomb or the Hoek-Brown failure criteria, the paper discusses how application of transformation rules leads the criteria to be re-casted in dimensionless form, so when these are applied to the solution of slope problems, the results (factor of safety and coordinates that define the location of the critical circular failure surface) can be summarized in dimensionless compact forms. The paper presents then computational tools in the form of dimensionless charts and computer spreadsheets for stability analysis. These are developed from numerical limit equilibrium models that use the mentioned transformation rules. Among others, the computational tools allow the concept of mechanical similarity of rock slopes to be naturally introduced. Finally, the paper presents various rock slope examples that illustrate the use of the computational tools. In these examples, results from limit equilibrium models are also compared with results obtained with full numerical analysis, in particular with finite difference models implementing the shear strength reduction technique.

Slope stability of deep surface coal mines in the presence of a weak zone

Coal and lignite (brown coal) are geo-resources that govern energy production for decades. Geomechanical challenges, particularly slope stability, related to surface coal and lignite mines are critical during operation and determine the post-coal era. Several failure incidents have been reported in mining areas, typically associated with a sub-horizontal failure surface on a weak—clay or marl—layer or an interface of low strength. This weak zone controls the soil profile in terms of stability and is common in several mines globally. In this work, the finite element method with the shear strength reduction technique is primarily employed to evaluate the slope stability of this profile. Three geotechnical software are initially compared, and results are identified as practically identical. Moreover, slopes with benches, as typically in mines, and without benches, as typically in slope stability analysis, are compared, with the no-benches analysis being consistently more conservative. The crucial parameters' effect is then examined: the height and the inclination of the slope, the inclination, thickness, and strength of the weak zone, and the strength of the overburden soil. Their effect on slope stability is quantified by combining the probabilistic point estimate method with the finite element method. It is concluded that the inclination and strength of the weak zone and the water conditions are the most critical parameters and control the stability. This work can support a preliminary slope stability analysis and expands the knowledge and understanding of slope stability of a weak zone profile.

Reliability-based prediction of landslide using efficient Monte Carlo Simulation

Several studies have attempted to develop thresholds for prediction of landslide initiation due to antecedent rainfall by using the physical-based approach. The approach offers a better understanding of the underlying process of the landslide mechanism. In the previous studies, however, determinate values of hydro-mechanical soil parameters are commonly considered, neglecting the uncertainties of the variables. Thus, this study aims to propose a relatively computationally, inexpensive reliability approach to address the uncertainties of the multivariate. The application of the proposed model is demonstrated by taking a case study from Sabah, Malaysia. The Latin Hypercube Sampling was adopted for efficient sampling, while the dependencies of the multivariate were modelled using a higher-order copula. The infiltration process and slope stability were assessed based on a fully-coupled seepage-deformation and shear strength reduction technique, respectively in a finite element programme. The probabilities of failure for various antecedent rainfall condition were computed by using Monte Carlo Simulation. In order to reduce the computational cost of the simulation, the Multilayer Perceptron regressor was applied to form the non-linear surrogate model for an implicit limit state function.

Maximum safe freight train speed on railway embankments under rapid drawdown conditions based on coupled stress-seepage slope stability analysis

The rapid drawdown of the water level adjacent to a railway embankment can reduce its stability, which could compromise the safety of train movements. To investigate the effect drawdown on railway embankments as a function of train speed, a coupled finite element-based model was developed accounting for simultaneous deformations, stresses and seepage. To determine maximum safe train speeds, a comprehensive study was carried out that included slope parameters (e.g. railway embankment height, natural slope height, slope ratio), rapid drawdown ratios, and external freight train loads as a function of speed. The models used the shear strength reduction technique, in which the factor of safety for each model was obtained. Generally, steeper natural slopes with lower railway embankments built upon them are more prone to failure, thus severely limiting train movements when a drawdown condition occurs. However, for a large number of cases, mainly for flatter slopes, a safe passage can be achieved by limiting train speed, while tall railway embankments appear to be able to withstand drawdown conditions without a considerable reduction in train operating speeds. The results of findings are summarized in a table and it is hoped that the study will be useful to increase the safety of freight train operations, particularly in cases when there is a sudden change or an unusual event in the waterways adjacent to a railway embankment.

Nonlinear Shear-Strength Reduction Technique for Stability Analysis of Uniform Cohesive Slopes with a General Nonlinear Failure Criterion

Abstract This paper develops a nonlinear shear-strength reduction technique for analyzing the stability of uniform slopes with the general nonlinear failure criterion. To incorporate the nonlinear ...

Evaluation of valley topography effects on the seismic stability of earth-rockfill dams via a modified valley topography coefficient

This paper presents the impact of valley topography on the stability of an earth-rockfill dam slope during earthquakes. The valley site is simplified as an ideal trapezium. A series of three-dimensional numerical models with different valley floor widths and valley wall slopes are used to represent different geometries of the valley topography. A modified dimensionless valley topography coefficient ς is proposed to describe the shape characteristics of the valley topography quantitatively. Considering the most commonly used parameter, the length-height ratio of the dam (L/H), ς replaces L with the length of the median line, Lm, and introduces a shape index, S, as a correction factor on the basis of Lm/H. Comparing to L/H, data from previous studies indicate that ς has a better correlation with the increment ratio of FoS3D to FoS2D. The shear strength reduction technique and the pseudo-static approach are adopted to calculate the factor of safety and the associated critical slip surface of the dam slopes. The results show that the valley topography plays a positive role in the seismic stability of the dam in the cases investigated in this study, especially for narrow valleys and that the valley topography effect will decrease with increasing earthquake intensity.

Upper bound stability analysis of slurry-supported trenches in layered soils

Abstract A stability analysis of slurry-supported trenches in horizontally layered cohesive-frictional soils by using the kinematical approach of limit analysis is presented. To ensure the kinematical admissibility, the failure surface in the formulated multi-block rotational mechanism was composed of several segments of logarithmic spirals with a same rotation center. To account for varying soil properties among different layers, both toe and above-toe failure patterns were considered. The associated factor of safety was determined by the shear strength reduction technique and was verified by comparison with the shear strength reduction finite element method (SSRFEM). It was found that the results obtained from the simplified approach by using weighted average soil parameters were not reliable and sometimes led to significantly overestimated safety factors on trench stability in layered soils, which deserves more attention in practice.

Dynamic modeling of large deformation slope failure using smoothed particle finite element method

In this paper, a novel node-based explicit smoothed particle finite element method (SPFEM), on the basis of the particle finite element method (PFEM) framework, is utilized to evaluate the stability of slopes and to simulate the post-failure behavior of soil. The main advantage of SPFEM in slope stability analysis lies in its capabilities to consider the whole dynamic failure process of slope and to simulate large deformation and post-failure of soils. For the stability analysis of a cohesive soil slope, the shear strength reduction technique with a kinetic energy-based criterion for distinguishing slope failure is adopted to obtain the factor of safety (FOS) of a slope, and the FOS is compared with that obtained by the classical FEM and LEM approaches for further validation. Then, the dynamic failure process of a non-cohesive granular material slope is simulated using Drucker-Prager constitutive model. The influence of friction resistance of granular material, as well as the repose angle of slope after failure, is discussed. Finally, the progressive failure behavior of a long clayey slope is modeled using SPFEM in conjunction with a strain-softening Tresca constitutive model. The retrogressive failure behavior of a long clayey slope is analyzed.

Comparative analyses of finite element and limit-equilibrium methods for heavily fractured rock slopes

Limit-equilibrium method (LEM) and finite element method (FEM) with shear strength reduction (SSR) technique are the most widely used analysis tools in slope stability assessment. Recently, researchers have reported that both factor of safety (FOS) values and failure surfaces obtained from LEM and FEM are generally in good agreement except in some particular cases. On the other hand, the consistency between two methods has not been adequately discussed for heavily fractured rock mass models by employing Generalized Hoek–Brown Criterion (GHBC). In this study, the FOS values and failure surfaces derived from LEM and FE-SSR based on GHBC were compared concerning static and pseudo-static conditions, various overall slope angles, geological strength index (GSI) values, and various water table levels. In this context, three homogeneous, highly fractured rock slope models with irregular geometry and different slope heights were generated by two-dimensional Slide and Phase2 software. Limit-equilibrium (LE) analyses were performed by Bishop, Fellenius, Morgenstern–Price, and Spencer techniques. The comparisons of global minimum FOS values for 431 cases and the effects of variables on two methods were investigated by statistical analyses. Consequently, it was determined that the difference between the FOS values are statistically significant. However, if the seismic coefficient is higher than 0.1 g, slope angle is higher than 34°, and the slope is assumed to be fully saturated, Morgenstern–Price is the most well-matched technique with FE-SSR than the others. For the same cases, the failure surfaces detected by Fellenius is more similar to the ones detected by FE-SSR.

Rock slope stability analysis using shear strength reduction technique (SSRT) – case histories

The authors acknowledge the support of Rocscience Inc. for getting a free full educational version of RS2D (Rock-Soil two-dimensional finite-element analysis program). The authors are grateful for their support.

An improved numerical manifold method with multiple layers of mathematical cover systems for the stability analysis of soil-rock-mixture slopes

In order to carry out stability analysis of soil-rock-mixture(SRM) slopes, an improved numerical manifold method (NMM) with multiple layers of mathematical cover systems is proposed. For SRM problems, the computational cost of the improved NMM is smaller than that of the traditional NMM. Besides, an improved shear strength reduction technique(ISSRT) which can eliminate spurious plastic deformation of the slopes is implemented. Based on the improved NMM with multiple layers of mathematical cover systems, the stability of three slopes, namely, a homogeneous slope and two SRM slopes with different contents of rock blocks is analyzed. The numerical results about the three slopes indicate that: 1) the proposed numerical model can obtain the FOS of a slope with high accuracy; 2) the bigger the content of rock blocks, the larger the FOS of the SRM slope will be; 3) due to the existence of rock blocks, the failure mode of a SRM slope is different from that of a homogeneous soil slope.