Shear Strength Reduction Research Articles

Rigidity Strengthening of Landslide Materials Measured by Seismic Interferometry

Landslides have caused extensive infrastructure damage and caused human fatalities for centuries. Intense precipitation and large earthquakes are considered to be two major landslide triggers, particularly in the case of catastrophic landslides. The most widely accepted mechanistic explanation for landslides is the effective-stress dependent shear strength reduction due to increases in pore water pressure. The Chashan landslide site, selected for the present study, has been intensively studied from geological, geophysical, geodetic, geotechnical, hydrological, and seismological perspectives. Our seismic monitoring of daily relative velocity changes (dv/v) indicated that landslide material decreases coincided with the first half of the rainy period and increased during the latter half of the rainy period. The geodetic surveys before and after the rainy period identified vertical subsidence without horizontal movement. The results from the multidisciplinary investigation enabled us to draw a conceptual model of the landslide recovery process induced by water loading. Where all sliding materials were stable (safety factor > 1.0), unconsolidated landslide colluvium and impermeable sliding surfaces trapped the seepage water to form a water tank, provided that compact forces were acting on the materials below the sliding boundary. The vertical force of compaction facilitates an increase in the cohesion and strength of landslide materials, thereby increasing the landslide materials’ stability. We demonstrated that the recovery process periodically occurs only under the combined conditions of prolonged and intense precipitation and the related stability conditions.

Shear behaviour and design of cold-formed ferritic stainless steel channels with circular web openings

Web openings are commonly used for ease of maintenance, electrical and plumbing services. Placing openings in web, however, affects the shear strength of cold-formed stainless steel channels accordingly. This study aims to examine the effects of such web openings on the shear strength of cold-formed ferritic stainless steel channels without straps. The results of 21 shear tests on pair channels loaded at mid-length with span aspect ratio of 1.0 are presented. In terms of channels with web openings, the openings are placed either centred at mid-span or offset to the applied load. Quasi-static Finite Element (FE) model, considering material nonlinearity and initial geometric imperfection, is then developed and validated against experimental test results. In addition, The FE models are used for a parametric study to extend the test results for different channel thicknesses and circular web opening sizes. While no stainless steel standard provides shear strength reduction factors, and only the North American specification AISI S100:2016 and Australian/New Zealand Standard AS/NZS 4600:2018 for carbon steel members provide two equations for channels with centred openings, it is found that such equations are unreliable and un-conservative for ferritic stainless steel channels by as much as 20%. The results also demonstrated that the current equations in the literature for carbon steel channels are unreliable, and are either unconservative or too conservative to apply for ferritic stainless steel channels by up to 44%. Based on both experimental and numerical results, new reliable design equations in the form of shear strength reduction factors are proposed.

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.

Stress–strain behavior of marine calcareous soil-tire mixtures

The increase in the volume of scrap tires has led to environmental concerns in different locations of the world, particularly in the coastlines. Managing this solid waste, including reuse in civil engineering applications (e.g. geotechnical projects) can be an effective solution to solve the problem. A series of drained triaxial tests was performed on Qeshm calcareous soil obtained from Qeshm Island. Specimens were prepared at loose and dense relative densities and mixed with different percentages of tire crumbs (TCs), including 0%, 10%, 20%, 30%, and 50%. The variation ratio parameter was introduced to evaluate the effect of TCs on the engineering properties of the soil-tire mixture. Based on the results, the stress–strain behavior of the calcareous soil was strongly influenced by TCs contents. As the tire content (TC) increases, the stress transfer mechanism changes from particle-particle to tire-tire, which results in a shear strength reduction. For an example, the addition of 50% of TCs to the calcareous soil resulted in a 64% reduction in the maximum deviatoric stress under a confining pressure of 600 kPa. In addition, the secant modulus at maximum deviatoric strength of the soil-tire mixture with 50% of TCs decreased about 80%.

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.

Shear strength reduction of aluminium lipped channel beams due to web openings

Aluminium members are recently being used as main structural elements in residential, commercial and industrial buildings. Aluminium Lipped Channel Beam (LCB) is one of the popular thin-walled sections used in different applications in construction. Web openings are incorporated in these sections to accommodate building services when they are used as beams. Even though the bending, shear and web crippling behaviours of these sections were investigated in the past, their structural behaviour with web openings have not been investigated. Hence, experimental and numerical studies were conducted in this research to investigate the shear behaviour of these aluminium LCBs with web openings. An aspect ratio of one was considered in this study. A total of 17 tests were conducted first to investigate the shear behaviour of aluminium LCBs with circular web openings using three-point loading arrangement. Finite element models were developed with appropriate loading and boundary conditions and validated using the experimental results. A detailed parametric study was then undertaken using the currently available roll-formed aluminium LCBs with different thicknesses, grades and web opening sizes. The ultimate shear capacities and the shear capacity reduction factors obtained from the tests and finite element analyses were compared with the currently available design rules. This paper presents the details and results of the experimental and numerical studies on the shear behaviour of aluminium LCBs with circular web openings and the comparison with current and recently proposed design rules.

Effect of EPS Beads on the Behavior of Sand–EPS and Slope Stability Using Triaxial and Centrifuge Tests

The present paper is a study on the effect of expanded polystyrene (EPS) bead size on the mechanical behavior and strength parameters of Firouzkooh sand using a triaxial static apparatus. 27 triaxial (CD) tests were conducted to evaluate the effect of different factors such as EPS volumetric percentage (5%, 10%, 20%, 30%), EPS bead size, and confining pressure (100, 200, 300 kPa) on the strength parameters of the soil EPS mixture. While increasing the mixing percentage of EPS beads reduces the specific weight, the internal friction angle, the shear strength, and the initial elasticity modulus of the mixture, it increases the specimen cohesion. Specimens containing small EPS beads displayed higher internal friction angle, cohesion, shear strength, and initial elastic modulus compared with specimens containing large EPS beads. According to the results, compared with the specimens without EPS beads, a 30% addition of small and large EPS beads to the mixture increased the specimen cohesion from 0.78 to 13.8 kPa and 11.8 kPa, respectively, and reduced the friction angle by about 32% and 37%. The centrifuge tests were conducted on samples containing small EPS beads to compare the effects of the specific weight reduction and shear strength reduction parameters on the stability of the embankments. The results showed that increasing the EPS bead content increases the slip surface of failure and reduces its stability.

Stability of the foundation trench of the immersed tunnel subjected to combined wave and current loading

The determination of stability for the submarine artificial slope is important in the construction of offshore structures such as immersed tunnels and embedded pipelines. Aimed at this issue, a two-dimensional (2D) integrated model is proposed to investigate the slope stability of the foundation trench for the immersed tunnel under the combined wave and current loading, as in the real marine environment. The Reynolds-Averaged Navier-Stokes (RANS) equations are solved to simulate the wave field, meanwhile the current is realized by setting boundary inlet and outlet velocity at both sides. The interface between water and air is tracked by conservative Level Set Method (LSM). The wave-current induced pore pressure in seabed is calculated by Darcy's law, and the shear strength reduction method is adopted to judge the damage of the elastic-plastic seabed slope which is described through Mohr-Coulomb constitutive model. The wave-current model is validated through comparing with experimental data and analytical solution. The consolidation status of foundation trench after excavation is obtained and set as the initial condition for analysis. The factor of stability (FOS) for the slope is demonstrated to depend on the relative distance between wave crest and slope. The minimum of FOS corresponding to the most dangerous situation is regarded as the stability index for the slope with specific slope ratio in the whole process of dynamic wave-current loading. Through the parametric analysis based on this new evaluation method, the effects of soil strength parameters, slope ratio and current direction on the slope stability are discussed in detail.

Physical and Mechanical Characteristics of Shallow Expansive Soil due to Freeze-Thaw Effect with Water Supplement

Shear strength of shallow expansive soil varies along with the depth under the freeze-thaw effect. This work investigates shear strength characteristics of shallow expansive soil by simulating the actual freeze boundary conditions of seasonal frozen areas with water supplement. An integrated approach incorporating the freeze-thaw test and direct shear test was adopted. Firstly, unidirectional freezing tests for expansive soil columns under three different freezing temperature gradients were carried out. Secondly, direct shear tests under low vertical stress were performed on the standard samples, which were prepared by using cutting rings cut the thawed expansive soil columns into nine segments along with the depth. Temperature, water content, and dry density at different depths were also investigated after the freeze-thaw process. The test results showed that, after the freeze-thaw process, the shear strength of expansive soil columns showed significant differences along with the depth and highly correlated with water content, specifically the higher water content and the lower shear strength. The minimum shear strength in the expansive soil columns occurred at the soil layer below the frozen and unfrozen zones interface. The expansive soil column’s shear strength changed most under the moderate freezing temperature gradient corresponding to the most considerable shear strength reduction. Moreover, the significant decrease in cohesion was the main reason for the shear strength reduction of expansive soil after the freeze-thaw process. These results indicate significant depth variability in shear strength of expansive soil under the freeze-thaw effect.

Shear strength reduction of trapezoidal corrugated steel plates with artificial corrosion pits

To investigate the ultimate shear strength reduction of trapezoidal corrugated steel plates (TCSPs) under corrosive condition, TCSPs with artificial corrosion pits (ACP) were taken as the research objects, and nonlinear shear buckling analysis was conducted by numerical simulation and experimental verification based on finite element method (FEM). The formula of the ultimate shear strength reduction coefficient η is proposed using the typical parameters influencing ACP, including ACP range Rr, ACP depth Rd, and equivalent corrosion rate γ. Fifteen specimens were designed with different ACP parameters, including Rr, Rd, and ACP diameter dp, and the shear buckling load test of TCSPs was performed. Results show that the distribution pattern of ACP has no effect on the shear strength of TCSPs, and the error between random distribution and array distribution is less than 1.5%. The ultimate shear strength reduction of TCSPs is related to Rr, Rd, and γ. The influence of ACP parameters on the shear strength of TCSPs is similar in different initial geometric defect modes. The proposed formula for calculating the strength reduction of TCSPs is in good agreement with the FEM and experiment results.

A multiparameter non-proportional shear strength reduction method for slope stability analysis based on energy evolution theory

A multiparameter non-proportional shear strength reduction method for slope stability analysis based on energy evolution theory

Reliability Assessment of an Open Pit Slope on Bong Iron Ore Mine

Slope stability on Bong mine is crucial for securing a balance in the gross national product of Liberia. The mine is being operated using conventional open pit methods with slope angles optimally designed to maximize ore stripping quantity. However, a working slope has displayed signs of uncertainty. The overall inclination of the slope is 56°, and the proposed maximal excavation is 315.42 m, but current depth in ore is approximately 50%. Based on the study of slope geological characteristics, the physical and mechanical parameters of rock mass and the geometrical calculation of stoping, the study is tailored on the mechanism of inspecting stress-strain behavior in response to seasonal variation of rock moisturization as a more suitable means of stope slope stability analysis in this case. This study took full account of local rainfall and other meteorological conditions. Slope stability is investigated via stereographic projections and stability assessment using the Shear Strength Reduction (SSR) method based on FLAC (Fast Lagrangian Analysis of Continua) numerical modeling technique in three dimensions to predict the stress-strain behavior of the open-pit slope and evaluate its stability state. Global stability has been analyzed under natural and saturated conditions and it is found that the slope is critically stable and needs proper attention.

Screening-level analyses for the evaluation of the seismic performance of a zoned earth dam

Many existing earth dams have been designed and built worldwide before the establishment of a seismic code, so that it is of relevant interest to evaluate their seismic performance and post-seismic operational conditions. This requires an accurate geotechnical characterisation of the dam and foundation soils, a proper definition of the seismic scenarios at the site of the dam, the use of simplified procedures for screening-level seismic analyses and advanced non-linear dynamic analyses to study the most critical seismic scenarios.This process has been used for the evaluation of the seismic performance of a zoned earth dam located in a high seismic hazard area of Southern Italy.In this paper the available data of historical seismicity at the site of the dam and the results of a probabilistic seismic hazard analysis are first discussed and input ground motions are selected using compatibility criteria with the energy and frequency content of the expected target motion in a range of vibration period relevant for the non-linear response of the dam. Seismic performance of the dam is then evaluated through procedures based on Newmark-type computations, in which permanent displacements are related to ground motion characteristics and to the seismic resistance of the dam, the latter evaluated detecting the earthquake-induced plastic mechanisms and the corresponding critical accelerations. Also, an original improvement of the well-known Makdisi & Seed procedure, was proposed to better capture the actual influence of non-linear soil behaviour in the evaluation of horizontal acceleration and permanent displacements of the crest of the dam. The analysis results pointed out the relevant role of the earthquake-induced shear strength reduction on the dam permanent displacements.

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.

Analysis of Clay Slopes with Piles Using 2D and 3D FEM

The aim of this paper is to compare between results of the 2D finite element method (FEM) and results of the 3D (FEM) when they are used to assess the factor of safety of clay slopes stabilized with vertical piles. It is expected that this comparison will aid engineers to judge and convert the results of the simplified 2D analysis to the more realistic results of the sophisticated 3D analysis that simulates the real behavior of slopes having infinite length. Extensive numerical modelling of homogeneous clay slopes stabilized with a row of piles and underlined by a bed of rock are carried out using the PLAXIS 2D program (version 8.2) and the PLAXIS 3D program (version 2010), which are both based on the shear strength reduction method to evaluate the factor of safety of the slope. The increase in factor of safety (FSI) of the stabilized clay slope is investigated considering many parameters, which include, the slope width, location of the piles, piles diameter, piles spacing and shear strength of the clay. Results of both PLAXIS programs are presented and compared in terms of increase in the FSI % relative to cases of slopes without stabilizing piles.

Factors influencing the critical piles length in reinforced slope

In this research, the performance of a homogeneous slope reinforced with one row of piles is studied using coupled numerical analysis and the method of shear strength reduction as implemented in the finite element analysis software, ABAQUS. Parametric studies are performed by changing geometric and strength parameters such as piles head conditions, piles location and the soil cohesion to assess their influence on the slope stability. For each of these parameters, the safety factor of the slope and the piles critical length was assessed. The results indicates the performance level of piles with different head conditions will not be similar; so that hinged and fixed heads are the most effective and cause the piles critical length to reduce, whilst free heads are less effective. The optimum location of the piles depends on their length, but regardless of the piles head type, the maximum increase of the safety factor is achieved when the piles are located in the mid-section of the slope. In addition to this, various levels of soil cohesion do not affect the piles optimum location, whilst cause significant changes in their critical length and increase rate of safety factor.

Basal stability analysis of braced excavations in anisotropic and non-homogeneous undrained clay using streamline velocity fields

A basal heave stability analysis in anisotropic and non-homogeneous undrained clay by using the kinematic approach of limit analysis is presented. The proposed failure mechanism consists of four translational rigid blocks and a streamline-based shear zone. The velocity field for the shear zone, formulated by a streamline solution in a polar coordinate system, is a supplement to the previous streamline solution in rectangular coordinates. The present study is first verified by comparison with other existing solutions in homogeneous and isotropic soils. It is found that the streamline solution can produce sufficiently accurate upper-bounds compared with the multi-block mechanism, the finite element limit analysis and the shear strength reduction finite element method. Subsequently, validation of the streamline solution on basal stability in anisotropic and non-homogeneous clay is carried out by discussion on a well-documented case study.

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 geometrical parameters affecting on the optimal slope design of Shadan Gold-Copper Open Pit Mine

The main objective of this research is to present a stable and optimal design of the ultimate slope of the eastern wall of Shadan Gold-Copper open pit mine based on the calculation of minimum factor of safety (1.20) by numerical modeling method under different conditions of slope geometry. Numerical modeling was performed using finite difference method (FDM) and FLAC3D software which is capable of calculating the factor of safety (FOS) by shear strength reduction method. Therefore, a set of 3D numerical models in different scenarios were investigated by varying the geometrical properties of the pit slope (width, height and face angle of bench, pit depth and overall slope angle). Based on the analysis carried out during the different scenarios, finally the fifth scenario with minimum safety factor of 1.20 was introduced as the proposed pattern. According to the initial design (first scenario) with a safety factor of 1.39, in the fifth scenario by decreasing the benches width from 6.8 m to 5 m and increasing their height from 10 m to 12 m which increases the slope angle of the bench face (from 70 degrees to 73 degrees) and overall slope angle (from 43 degrees to 52 degrees), safety factor of the ultimate slope was reduced to a minimum of 1.20. The maximum impact of the change in the safety factor is related to shear strain and horizontal displacement in the model such that the maximum shear strain (1.12 e-2 m) and the maximum horizontal displacement (-25.75 cm) are related to the fifth scenario with minimum safety factor (1.20). Whereas, the maximum vertical displacement (29.63 cm) is related to the first scenario model (FOS = 1.39) with the maximum slope height (pit height) of about 219 m, which shows the least influence on the value of the safety factor.

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.