Shear Strength Reduction Research Articles

Empirical shear strength criteria for filled jointed of metasedimentary sandstone

Rock joint shear failure criteria constitute numerical simulation that significantly governs the calculation for rock mass stability analysis. The presence of joint infilling potentially reduces the estimation accuracy for the deformation of rock joint. This study discovers the role of infilling thickness in governing the empirical calculation of linear Mohr-Coulomb failure criterion and non-linear Barton-Bandis failure criterion. A series of direct shear tests with constant surface roughness and controlled infilling material composition facilitates the joint shear strength with various infilling thicknesses. The results indicate that the joint shear strength decreases primarily with infilling material within the joint aperture. Although all the friction angles are closely similar, different cohesion values show the influence of infilling material thickness on shear strength characteristics. The joint shear strength values indicate significant differences where the filled joint shear strength reduction depends on associated infilling thicknesses and the adopted failure criterion. Multiplication to the Filled Joint Factor (FJF ) normalized from the τfilled joint/τcleaned joint ratio will precisely evaluate the filled joint shear strength. Hence, the shear strength estimation from Mohr-Coulomb and Barton-Bandis failure criteria to the various thicknesses of joint infilling will provide sufficient filled joint deformability characteristics.

DEM-Based Numerical Investigation of Seismic Stability of Rock Slope in Terms of Rigid Block Displacement

Abstract For the seismic behavior of a rock slope or more specifically a rock wedge, the limit equilibrium method is frequently utilized to determine the static and pseudo-static safety factor, and the Newmark method is usually applied to estimate the permanent dynamic displacement. However, the two methods are relatively complex, which negatively affects their usage in engineering practice. A new idea to analyze the static and dynamic stability of a complex rock slope was provided in this paper by combing the rigid discrete element method (DEM), shear strength reduction (SSR) method, and Newmark method. The research results show that (1) by setting the joint contact stiffness to a very large magnitude, the joint displacement caused by the reduction of joint contact stiffness during the SSR calculation process can be considered ignorable, making the contact behavior between the respective two joints approximately satisfy the rigid-plastic assumption. In this manner, it is feasible to calculate the static and pseudo-static safety factor (SF) and the permanent dynamic displacement of a rock wedge with the rigid DEM. (2) The SF and sliding forces of wedges can be directly calculated by this new method without any auxiliary assumption of motion direction or sliding type. Additionally, the new method is suitable for complex wedges, which is important to practical design. (3) The rigid DEM can be realized in 3DEC, which is testified in this paper by 5 classic static examples and 3 typical dynamic examples. (4) The effect of some important parameters, such as the excitation acceleration amplitude and dip of intersection line between two joints, on the dynamic displacement and SF under dynamic excitation was analyzed. The parameter analysis revealed that evaluating the seismic performance of a rock slope only with the pseudo-static SF would potentially underestimate the seismic displacement, under some special conditions. (5) An example case was presented in this paper that the seismic stability assessment of a block was performed with the proposed approach in terms of seismic displacement. And the slope reinforcement scheme was suggested with respect to exceedance probability. It is noted that the reinforcement scheme obtained from the rigid DEM has a better cost-saving benefit under the same SF. Overall, the knowledge gained in this paper may offer a new idea for the static and dynamic analysis to the rock slope stability.

Predicting short beam shear strength reduction in carbon/epoxy laminates containing voids

The presence of voids in composite materials reduces the material’s strength, particularly matrix-dominated properties such as short beam shear (SBS) strength. Until now there has been no satisfactory way to reliably predict the effect of voids on SBS strength. Instead, there has been a reliance on using average void content combined with extensive testing to assess the linear reduction of strength with increasing void content to determine a 'knock-down' factor. Here, the reduced cross-sectional area is used to predict SBS strength of laminates containing voids. A detailed characterisation of the internal void location and morphology for numerous samples was obtained through X-ray CT scanning, prior to mechanical testing. The ‘critical plane’ in each sample, i.e. the length–width plane with the maximum cross-sectional area of voids was located and used to predict SBS strength. To additionally account for non-uniformity in the shear stress distribution, a stress scaling factor (SSF) is introduced.

System reliability analysis of soil slopes using shear strength reduction and active-learning surrogate models

System reliability analysis of soil slopes using shear strength reduction and active-learning surrogate models

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.

3D stability analysis method for toppling failure on rock slopes

In this paper, an optimized solution method is proposed for the 3D stability analysis of rock slopes subject to toppling failure, based on their geometric and mechanical properties. It was verified by a 3D block system that focused on the geometric properties of toppling slopes, considering the force and its action point on the interface of the block system as unknown variables, as well as introducing the definition of a safety factor considering both tension and shear strength reduction. The proposed method implied setting constraints, such as the balance equation corresponding to block force and moment, as well as non-violation of the yield criterion, considering the minimum value of the safety factor as the objective function. It was applied to the analysis of two typical 3D models simulating toppling failure on slopes. The example of a 3D spherical toppling slope was reconstructed and corroborated by calculation. The experimental results demonstrated that the proposed method could appropriately reflect the mechanical properties and stability behaviour of a 3D toppling slope, thereby facilitating the 3D stability analysis of toppling rock slopes.

Typhoon, rainfall and trees jointly cause landslides in coastal regions

Typhoon-induced landslides are the primary natural hazard in hilly coastal regions of South East Asia and can potentially devastate coastal communities and infrastructure. They occur at approximately the same time as typhoon strikes, showing complex failure mechanisms under the coupled action of wind, rainfall and trees. Therefore, special assessments should be performed for such geohazards. Here we elucidated the general features of typhoon-induced landslides in southeast coastal China in terms of their geological characteristics, spatial and temporal distribution, and vegetation cover characteristics. We further established a framework to investigate their possible failure mechanisms coupled to wind, rainfall and trees. Specifically, the mechanical response of trees in a typhoon was estimated through a combination of the autoregressive method (AR) and a multi-degree-of-freedom tree swaying model, while the trees and rainfall contributions were determined by calculating the cohesion attributable to roots and the soil shear strength reduction caused by the water content variation. Our results suggest that the root reinforcement disappears when the sliding surface is below the root embedded depth, and the impact of wind load is significant during the slope stability analysis. The Force 14 wind (mean wind velocity up to 45 m/s) reduces the factor of safety (Fs) of the slope by 20% when blowing downhill, and its combined contribution with rainwater infiltration will lead to a higher risk of slope failures (Fs reduction of 45% at the base of the root-soil composite). Additionally, typhoon-induced shallow slope failures are prone to develop at the base of the root-soil composite or the rock-soil interface. Our model results suggest that the impacts of wind and vegetation on the generation of landslides during a typhoon can be significant and deserve more attention in hazard assessment studies.

Study on Stability Reduction Characteristics of Earth and Rockfill Dams under Rapid Drawdown Using Fully Coupled Seepage-Stress Analysis

It can be a great expense to examine individually the stability of earth and rockfill dams on rapid drawdown in civil engineering practice. The aim of this present work is to clarify the safe type on the rapid drawdown among the most common types of earth and rockfill dams and to introduce cheaply the types in dam design. First, a transient analysis of saturated-unsaturated seepage coupled with stress is carried out in the cross sections of typical earth and rockfill dams the during rapid drawdown, and the safety factors of the upstream slopes are determined by the shear strength reduction method. Then, the typical dams are compared for the stability characteristics so that designers can select the safe type of earth and rockfill dams on rapid drawdown. The obtained results show that the decreasing rate of safety factor in a central core dam is 0.72–0.85 times than one of the homogeneous dams and 0.17–0.40 times than one of the sloping upstream core dams so that it is more stable than other earth and rockfill dams during rapid drawdown.

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.

Wedge method for landfill stability analysis considering temperature increase.

The increase in landfill temperature often results in shear strength reduction of both the solid waste and the liner, which leads to slope instability. However, very few landfill slope analysis methods can simultaneously consider the effect of temperature on the shear strength of the waste solid and the liner. In this study, based on the strength parameters of the liner and waste with temperature, a wedge method for translational failure analysis of landfills considering temperature increase was established. The results showed that rising temperatures caused by biochemical degradation at the bottom and middle of the landfill reduced the anti-slide force of back slope more than that of bottom slope. With the leachate level increasing, the effect of temperature rise on landfill stability became obvious. The feasibility of the proposed wedge method was verified by the engineering case study of Xiaping Landfill, Shenzhen, China. This study probably provides important guidance for the design, operation and management of municipal solid waste landfills.

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.

Propose a viable stabilization method for slope in weak rock mass environment using numerical modelling: A case study from the cut slopes

The design of a stable slope in a rock mass environment is a quite complicated job due to the anisotropic behaviour of the rock mass. In this research work, the cut slopes at the Swat motorway in the weakest schist rock is numerically analyzed by the shear strength reduction (SSR) approach using the Finite Element-based 2D RS2 software. The slope is divided into two cases according to the nature of the rock. Each case of the cut slope is analyzed by two stabilization methods: 1) changing the characteristics of the slope 2) support system installation based on the Mohr-Coulomb (MCC) and Generalized Hoek and Brown (GHB) failure criteria in order to propose the most appropriate method for slope stabilization. The results obtained reveal that the Critical Strength Reduction Factor (CSRF) before applying the stabilization methods is 1.34 (MCC) and 1.04 (GHB) for Case-I and 1.21 (MCC) and 0.53 (GHB) for Case-II. CSRF for Case-I after changing the characteristics of the slope is observed to be 2.43 (MCC) and 2.33 (GHB), while for Case-II is 1.82 (MCC) and 1.26 (GHB), respectively. CSRF for Case-I after the support installation criteria is 1.59 (MCC) and 1.07 (GHB), while for Case-II is 1.65 (MCC) and 0.5 (GHB), respectively. Based on the comparative analysis, it is revealed that changing the characteristics of the slope method shows prominent results in both cases; therefore, this method can be effectively used in order to stabilize the slope in the weakest rock mass environment.

Optimization and variational principles for the shear strength reduction method

AbstractIn this paper, a modified shear strength reduction method (MSSR) and its optimization variant (OPT‐MSSR) are suggested. The idea of MSSR is to approximate the standard shear strength reduction to be more stable and rigorous from the numerical point of view. The MSSR method consists of a simplified associated elasto‐plastic model completed by the strength reduction depending on the dilatancy angle. Three Davis' modifications suggested by Tschuchnigg et al. (2015) are interpreted as special cases of MSSR and their factors of safety are compared. The OPT‐MSSR method is derived from MSSR on the basis of rigid plastic assumption, similarly as in limit analysis. Using the variational approach, the duality between the static and kinematic principles of OPT‐MSSR is shown. The numerical solution of OPT‐MSRR is obtained by performing a regularization method in combination with the finite element method, mesh adaptivity and a damped Newton method. In‐house codes (Matlab) are used for the implementation of this solution concept. Finally, two slope stability problems are considered, one of which follows from analysis of a real slope. The softwares packages Plaxis and Comsol Multiphysics are used for comparison of the results.

Optimisation of a Slope-Stabilisation System Combining Gabion-Faced Geogrid-Reinforced Retaining Wall with Embedded Piles

Slope instability is a common geotechnical issue in South Gippsland, Victoria, Australia. The network of rural roads constructed and maintained by the local authority are greatly affected by the instability. An integrated slope-stabilisation system that combines two well-developed slope stabilisation methods has been used widely in this district. These two methods are gabion-faced geogrid-reinforced retaining wall and pile retaining structures. These two methods are connected by a steel rail that is welded at the top of the piles to buttress the lowest row of the gabion basket wall. In order to reflect the site conditions accurately, a three-dimensional non-linear finite element approach is adopted in this study. The elastic-perfectly plastic constitutive model with the Mohr-Coulomb yield criterion is used to describe the behaviour of the soil and the gabion basket. With the assistance of the shear strength reduction technique, the effectiveness of the integrated system is demonstrated through the comparison of the representative indicators of the slope stability among various slope configurations. A series of parametric studies related to the ratio of the embedded length of geogrid to the height of the slope, the ratio of the embedded length of the pile to the thickness of the unstable soil layer, and the ratio of the spacing to the diameter of the pile have also been conducted for the purpose of optimising this slope stabilisation infrastructure. The results from the parametric studies indicate that the optimised and improved integrated infrastructure can stabilise the road and slope economically without the loss of the safety margin.

Finite element analysis of a slope stability by incrementally increasing the mobilised principal stress deviator

ABSTRACT The assessment of slope stability factor by a FE analysis using the shear strength reduction method (SRM) enjoys several advantages and owns many shortcomings. As an alternative, a new FE approach called stress deviator increasing method (SDIM) is proposed. By means of a factor termed Mohr’s circle expansion factor, the new approach assesses the slope stability by incrementally increasing the mobilised principal stress deviator until the soil failure is reached. The numerical procedure is based then on a rigorous formulation as it preserves the definition of the safety factor consistent with that of LEM and maintains the progressive development of the shear stress on same plane on which the shear strength will occur at failure. The proposed method deals with the actual material by using the real strength parameters and rather than those reduced by a factor. The results of SDIM encoded in a computer code called were thoroughly assessed against those of both SRM and LEM using several slope examples involving and purely cohesive soils. Results of comparison encompassing factor of safety along with plastic strain distributions showed the reliability and the strength of the present method.

The role of joint spacing on the stability analysis of wedge failures

Kinematic analyses of wedge failures in rock slopes are usually carried out based on stereographic techniques. Nevertheless, this methodology presents several limitations that could lead to poorly accurate conclusions. In line with this idea, the main objective of this work is to evaluate the effect of joint spacing in the estimation of the factor of safety of rock slopes affected by potential wedge failures. For this purpose, a 3D numerical distinct-element code (3DEC) was selected to carry out a good number of simulations in which the factor of safety a slope, affected by different discontinuity sets, was studied by using the Shear Strength Reduction (SSR) method. Different values of joint spacing, cohesion and friction angles were considered, combined with two angles of the slope face under study. The joint spacing has been found to relevantly affect the values of the factor of safety, which showed variations of up to 40% in comparison with those obtained from limit-equilibrium methods for rock slopes with similar structural features. This work provides an insight into a more realistic interpretation of rock slope analyses against wedge failures, and particularly to more accurate estimations of the factor of safety.

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.

The Setting of Strength Parameters in Stability Analysis of Open-Pit Slope Using the Random Set Method in the Bełchatów Lignite Mine, Central Poland

The slopes of open-pit mines are often at risk of failure. To identify this hazard, stability analyses are performed. An important element of these stability analyses is the reliable selection of input parameter values for the calculations. This selection is difficult because the slopes of the open pit are disturbed by mining activities. In such conditions, rheological processes, intensified by weathering, develop in open-pit slopes. This study is aimed at setting the strength parameters for the stability analysis of open-pit slopes with a developed slide process, using the random set method. The study was performed on the example of the open pit of the Bełchatów lignite mine, central Poland. A four-stage methodology, according to the random set method, was proposed. The methodology covered the following: site investigation, sensitivity analyses, shear strength reduction (SSR) analyses using numerical calculations, and probability analyses of the factor of safety (FoS) calculation results. The setting of the input parameters took into account the peak and residual strength parameters for each lithological unit in the physical model of the open-pit slope. Samples for laboratory tests were taken from the cores of nine test boreholes. The sensitivity analysis included all peak and residual strength parameters for each lithological unit in the body. As a result of the sensitivity analysis, specific strength parameters were adopted that would have a great impact upon the results of the calculations. Selected sets of parameter values were then used for the FoS calculations. The resultant FoS values revealed the probable slide planes. The positions of the slide planes were consistent with the interpreted slide surfaces based on the control boreholes and terrain observations. Knowledge of the slide planes positions and the values of the strength parameters enabled the designing of a securing approach for this landslide, and the taking of preventive measures to reduce this risk.

Probability Methods for Stability Design of Open Pit Rock Slopes: An Overview

The rock slope stability analysis can be performed using deterministic and probabilistic approaches. The deterministic analysis based on the safety concept factor uses fixed representative values for each input parameter involved without considering the variability and uncertainty of the rock mass properties. Probabilistic analysis with the calculation of probability of failure instead of the factor of safety against failure is emerging in practice. Such analyses offer a more rational approach to quantify risk by incorporating uncertainty in the input variables and evaluating the probability of the failure of a system. In rock slope engineering, uncertainty and variability involve a large scatter of geo-structural data and varied geomechanical test results. There has been extensive reliability analysis of rock slope stability in the literature, and different methods of reliability are being employed for assessment of the probability of failure and the reliability of a slope. Probabilistic approaches include Monte Carlo simulation (MCS), the point estimate method (PEM), the response surface method (RSM), first- and second-order reliability methods (FORMs and SORMs), and the first-order second-moment method (FOSM). Although these methods may be complicated, they provide a more complete definition of risk. Probabilistic slope stability analysis is an option in most commercial software; however, the use of this method is not common in practice. This paper provides an overview of the literature on some of the main probabilistic reliability-based methods available for the design of the rock slope in open pit mining. To demonstrate its applicability, the paper investigates the stability of a rock slope in an open pit mine in the Goldfields region, Western Australia. Two different approaches were adopted: deterministic stability analysis using two-dimensional limit equilibrium and finite element shear strength reduction methods using SLIDE and RS2 software, respectively, and probabilistic analysis by applying the MCS and RSM methods in the limit equilibrium method. In this example, the slope stability analysis was performed using the Spencer method with Cuckoo search optimization to locate the critical slip surface. The results obtained were compared and commented on.