Shear Strength Reduction Research Articles

Experimental investigation on the temperature-dependent shear strength of frozen coarse-grained soil-rock interfaces by considering three-dimensional roughness

Climate warming has led to landslides, especially the slopes containing soil-rock interfaces. A series of shear tests were conducted on the coarse-grained soil (CGS) rock interface at low temperatures. The interface shear strength comprises the pre-peak bonding strength produced by ice and post-peak residual strength created by friction between the CGS and rock surface. As the temperature rose from −15 to −1 °C, the bonded ice strength at different rough interfaces rapidly decreased by a maximum of 82.99 %, because more than 21.29 % of the pore ice had thawed within the CGS. However, the average interface residual strength had decreased by only 18.56 %, which is consistent with the variation of the internal friction angle of the frozen CGS. The interfacial shear strength was much lower than the shear strength of the frozen CGS below −1 °C and that the shear slide typically occurred at the interface. In addition, as the freezing temperature increased, the impact of the three-dimensional roughness to the interfacial shear strength decreased, because the warm frozen CGS could easily shear rupture and adhere to the interface, reducing the influence of the roughness. Finally, it was demonstrated that the Mohr-Coulomb failure criterion could be employed to predict the interface peak shear strength for different normal stresses. This study aims to determine the mechanism responsible for shear strength reduction at a frozen CGS-rock interface during warming and to provide some references for preventing landslides caused by rising temperatures.

Numerical Investigation of the Slope Stability in the Waste Dumps of Romanian Lignite Open-Pit Mines Using the Shear Strength Reduction Method

Open-pit mining generates significant amounts of waste material, leading to the formation of large waste dumps that pose environmental risks such as land degradation and potential slope failures. The paper presents a stability analysis of waste dump slopes in open-pit mining, focusing on the Motru coalfield in Romania. To assess the stability of these dumps, the study employs the Shear Strength Reduction Method (SSRM) implemented in the COMSOL Multiphysics version 6 software, considering both associative and non-associative plasticity models. (1) Various slope angles were analyzed, and the Factor of Safety (FoS) was calculated, showing that the FoS decreases as the slope angle increases. (2) The study also demonstrates that the use of non-associative plasticity leads to lower FoS values compared to associative plasticity. (3) The results are visualized through 2D and 3D models, highlighting failure surfaces and displacement patterns, which offer insight into the rock mass behavior prior to failure. (4) The research also emphasizes the effectiveness of numerical modeling in geotechnical assessments of stability. (5) The results suggest that a non-associative flow rule should be adopted for slope stability analysis. (7) Quantitative results are obtained, with small variations compared to those obtained by LEM. (6) Dilatation angle, soil moduli, or domain changes cause differences of just a few percent and are not critical for the use of the SSRM in engineering.

Application of limit equilibrium and shear strength reduction techniques for stability assessment of slope cuts- A case study of Khalid-Dijo dam project, Southern Ethiopia

Dam failure can occur due to foundation instability, downstream and upstream slopes instabilities. This study assesses the stability of upstream and downstream slope cuts at Khalid-Dijo irrigation dam project, which is located in Southern Ethiopia, 3km south of Werabe town. Limit equilibrium and finite element shear strength reduction methods are adopted. Validation of results and comparisons between those methods are carried out. The analysis considers anticipated site conditions, including static dry, static saturated, dynamic dry and dynamic saturated conditions. Slope material properties are measured from insitu, laboratory tests and used as input parameters for the analysis to obtain factor of safety and critical strength reduction factors. The properties considered in the analysis include unit weight, cohesion, angle of internal friction, poison’s ratio, dilation angle and Young’s modulus. The analysis indicates that the factor of safety values for limit equilibrium methods and the critical strength reduction factor for finite element method are very similar across the three slope cuts under all anticipated conditions. The lowest factor of safety and critical strength reduction factor is 1.56 and 2.07 respectively. Generally, the proposed dam project is safe against upstream and downstream slope failures. These studies suggest that maintained the average safety factor values of both methods during the design stage are crucial to avoid unnecessary risk.

Seismic stability charts for three-dimensional rock slopes using Hoek–Brown criterion

Seismic excitation serves as a crucial trigger for slope collapse in seismically active regions. This study employs the non-linear shear strength reduction (SSR) technique in conjunction with the pseudo-static method to analyse the seismic stability of three-dimensional (3D) rock slopes following the generalised Hoek–Brown (GHB) criterion. Considering the non-linear characteristics of the GHB envelope, the instantaneous Mohr–Coulomb (MC) parameters for rock masses are identified by locating the tangent at the intersection of the GHB envelope with the shear and normal stress planes. These parameters are subsequently incorporated into the non-linear SSR analysis for seismic stability based on the MC criterion. The non-linear SSR technique is then validated by comparing the calculated factor of safety (FOS) with those from other methods and used to assess the impact of geometric and GHB parameters on the FOS of 3D rock slopes. Finally, a set of seismic stability charts for 3D rock slopes in GHB media has been developed and applied to evaluate the seismic stability of two practical rock slope cases following the GHB failure envelope.

Geophysical and numerical approaches to solving the mechanisms of landslides triggered by earthquakes: A case study of Kahramanmaraş (6 February, 2023)

Many landslides occurred in the 11 Turkish provinces affected by the earthquakes centered in Kahramanmaraş in February 2023, and many people lost their lives due to those landslides. To reduce the risks and hazards brought on by landslides induced by earthquakes, it is essential to identify the mechanisms causing landslides to arise in such situations. In this study, the geological strength index and both seismic refraction tomography and electrical resistivity tomography (ERT) geophysical methods were used to determine the failure mechanism of layered sedimentary units defined as flysch (claystone-siltstone-sandstone-marl). The shear strength reduction finite element method (SSR-FEM) was used with the RS2 computer program for model solutions. One of the geophysical methods, ERT, facilitated an interpretation of the cyclic failure mechanism that occurs in flysch, and it was found to be congruent with numerical models. Landslide slip circle depth, water-saturated layers, and resistivity values were determined with the ERT method. Seismic methods were employed to determine the velocity values (Vp, Vs) of the landslip mass. These velocity data, along with the elasticity modulus and Poisson ratios derived from them, were utilized in our numerical model. In the maximum shear strength and horizontal displacement analyses performed on the section line determined before the landslide, the natural state and the situation under the influence of groundwater were examined and the strength reduction factor (SRF) was determined as 1.35 and 1.24, respectively. In an evaluation conducted under the influence of the maximum horizontal ground acceleration (0.22 g) determined by taking into account the seismicity of the study area, it was concluded that instability occurred in the flysches and the SRF value was 0.72. An important finding of this study is that the results of geophysical methods used to determine the lithological properties of sedimentary rock masses and understand the causes of landslides align closely with those of numerical analysis methods, providing high accuracy. These methods also support new approaches, particularly in identifying landslip boundaries and detecting areas with high water saturation. It has been discovered that earthquakes actively contribute to the instability of these kinds of soils.

Numerical modelling of the pit wall stability while optimizing its boundaries to ensure the ore mining completeness

Purpose is to assess changes in the stress-strain state of walls along the whole periphery of a super-deep open pit while optimizing its current and final boundaries for the complete ore excavation. Methods. Finite element 3D analysis of stress-strain state (SSS) of the soil and rock mass relies upon the models varying in their scales. Macrolevel model includes the full pit helping perform initial evaluation of its stability depending upon changes in the general wall slope along the pit periphery. Then, the macromodel is separated into sectoral models with smaller scales oriented radially in such a way to include potentially unstable wall areas. The sectoral models make it possible to show the complex bench line in more detail after the peripheries were optimized in terms of economic factor and simulate layered structure of the rock mass. Elastoplastic model of the medium as well as Mohr-Coulomb strength criterion has been implemented using RS3 (Rocscience) program codes. Findings. An indicator of wall strength (safety factor) distribution along the pit periphery has been identified; potential sliding surfaces within each of the separated open pit sectors have been localized based upon the shear strength reduction (SSR) procedure. Influence by the general wall slope as well as by the indicator of the ore excavation completeness on the stripping ratio has been demonstrated. Originality. For the first time, two-level modelling has shown difference in a safety factor depending upon a model scale and a reflection degree of the soil-rock mass structure. In the context of the actual mining and geological conditions of Kacharsky open pit, changes in the safety factor along the pit periphery have been identified depending on the general slope of the wall. Practical implications. Based upon the pit wall stability along the whole periphery, the possibility has been substantiated to optimize its design boundaries for the excavation of those amenable ore reserves, occurred near them, inclusive of ore, occurring in a bottom, which mining is impossible due to inaccessibility.

Stability assessment of jointed rock capturing roughness degradation under cyclic loading with special reference to railway formation

Repeated train loading on a jointed rock subgrade can cause excessive displacements of certain discontinuities leading to instability. Repeated shearing of discontinuities leads to a gradual reduction in joint roughness (i.e. wearing of asperities), which is quantified by the change in the joint roughness coefficient (JRC). This process reduces the joint shear strength over time. In this study, the classical shear strength criterion proposed by Barton and Choubey (1977) is extended to capture the influence of cyclic loading on joint degradation and the corresponding shear strength reduction, also considering the scale effect. This modified cyclic shear strength is implemented in FLAC-3D and validated with conventional cyclic triaxial data available from selected past studies. The model is applied to a simulated real-life track operating over a jointed sandstone formation commonly found towards the eastern coast of NSW. A modified limit equilibrium approach based on an Equivalent Factor of Safety (EFOS) is introduced and adopted to quantify the extent of instability, whereby an increase in the number of loading cycles affects a decrease in the EFOS of an unstable block. For a specific joint strike inclined to the track, the potential adversities are exacerbated when the joint dip angle is greater and when the initial JRC is smaller. In this paper, alternative geometrical combinations and different initial joint properties are considered to determine the worst combination of JRC and joint orientation upon cyclic train loading. As most past studies adhere to traditional static load analyses, the extended shear strength criterion described in this study is novel, and it offers significant practical benefit for railways that are subjected to prolonged cyclic loading.

Three-Dimensional and Two-Dimensional Stability Analysis of Bentonite Slurry Trenches Using a Shear Strength Reduction Technique and Limit Equilibrium Methods

Bentonite slurry trenches are becoming increasingly popular in the excavation of trenches, especially for diaphragm wall construction. The problem that needs to be addressed is the stability of bentonite slurry trenches. This paper presents a stability analysis of trenches with temporary support from bentonite slurry, with unit weights ranging from 10.5 to 12.0 kN/m3 in the realistic stratum, and C3 in the Hue city area. Our analysis employs the Shear Strength Reduction Technique (SSR) with Mohr–Coulomb materials to numerically evaluate the factor of safety (FS). The finite element method (FEM) software program (RS2 v. 121 and RS3 v. 4.0) and the finite difference method (FDM) FLAC v. 7.0 software were used. Additionally, the limit equilibrium method (LEM) of Bell–Washbourne and three-dimensional (3D) Bishop were used to calculate trench stability. The results of the analysis show a good agreement between RS2 and FLAC2D, and between RS3 and FLAC3D. Secondly, upon comparison, it was noted that the factor of safety of the 3D software programs (RS3 v. 4.0, FLAC v. 7.0) was higher than that of the 2D software programs (RS2 v. 11.0, FLAC v. 7.0), ranging from 52.3 to 63.0% for trench lengths of 6 m. However, for trench lengths of 54 m, the factor of safety values in 2D and 3D configurations were nearly equal. Thirdly, the factor of safety of the Bell–Washbourne method (LEM) was lower than that of the numerical analysis methods (FLAC and RS programs). Using the three-dimensional numerical method appears to be effective for estimating stability.

Analisis Stabilitas Tempat Pembuangan Akhir (TPA) Sampah Akibat Pengaruh Perubahan Sifat Mekanis Material Sampah

The stability of landfill is a crucial aspect of waste management. Numerous factors influence the stability of landfill, one of which is the alteration of the mechanical properties of the waste material. This research aims to analyze landfill stability by considering the effects of changes in the mechanical properties of waste material. A construction stage analysis was conducted to model the process of altering the mechanical properties of waste material. The analysis was divided into 13 stages corresponding to the number of landfill steps. The initial five stages were modeled as waste with an age of < 5 years, stages 6-10 were modeled as waste with an age of 5-10 years, and the last three stages were modeled as waste with an age of > 10 years. Stability analysis using the finite element method was conducted using the concept of shear strength reduction. The analysis results indicated changes in the landfill's safety factor throughout its operational period. Stages 1-5 experienced changes in the safety factor, although these changes were not significant. Stages 6-10 witnessed a considerable decrease in the safety factor before experiencing an increase in stage 11, followed by a continuous decline until the end of stage 13. Despite changes in the safety factor during the operational process, the overall safety factor values obtained still met the minimum safety factor criteria required.

Rock slope stability analysis under Hoek–Brown failure criterion with different flow rules

The stability analysis of homogeneous rock slope following the Hoek–Brown failure criterion under the hypothesis of different flow rules is performed based on limit equilibrium and finite element methods. The applied failure criterion is the generalized Hoek–Brown that can be introduced as a shear/normal function in analysis applying different flow rules. The results are compared with those obtained by the application of equivalent shear strength parameters of the Mohr–Coulomb criterion, considering that this is still the most widely used criterion in rock slope stability analysis and is still the base for the shear strength reduction method applied in finite element modelling. Different proposals for estimating the equivalent strength parameters based on confining stress level are evaluated. The limitation of stress-dependent linear Mohr–Coulomb parameters is emphasized by analysing the vertical cut problem, for which, depending on the chosen stress level, different critical heights are obtained for the same material. Sensitivity analysis of geotechnical parameters used as input for failure criterion is performed to determine their influence on slope stability. Probabilistic analysis is conducted to determine the probability of failure when different flow rules are applied. If slope stability analysis is performed with an assumption of associative flow rule, the probability of failure is within the acceptable limits for the considered case study, while employing non-associative flow rule, the probability of failure is rather high. The chart is presented that could be readily used to estimate the combination of σci, GSI, and mi values that produce failure for the analysed case study.

A New Shear Strength Model with Structural Damage for Red Clay in the Qinghai-Tibetan Plateau

Under the background of climate warming in the Qinghai-Tibetan Plateau (QTP), frequent freeze–thaw cycling (FTC) brings about great geological disasters such as subgrade failure, landslides, and mudslides, which is closely related to the strength reduction caused by the structural damage of soils. In this study, to explore the association between macro shear strength and microstructure evolution of soils subjected to FTC, the red clay distributed widely in the QTP was chosen and used to conduct a series of triaxial shear and nuclear magnetic resonance (NMR) tests in the range of 1 to 7 FTCs. Triaxial shear test results reveal that the shear strength reduction of specimens mainly occurs within five FTCs, and the trend of peak deviator stress with increasing FTCs can be described in three stages: rapid descent (FTCs less than three), slow descent (FTCs between three and five), and stabilization (FTCs greater than five). NMR tests show that the T2 spectrum curves exhibit a distinct bimodal distribution characteristic, corresponding to macropores and micropores. Part of the micropores gradually develop into macropores with increasing FTCs, especially within five FTCs. The increase in macropores proportion leads to a loose soil structure, which is consistent with the deterioration of the shear strength of specimens. Finally, based on the experimental results and classical Mohr–Coulomb theory, a new shear strength model with structural damage for red clay has been proposed by introducing a damage factor expressed by T2 spectral area.

Mechanical properties and energy evolution of drilling shaft lining concrete under hydro-mechanical coupling

As the drilling shafts in the western mines of China are going to cross the deep water-rich and weakly cemented rock layer, investigating the mechanical properties and energy evolution of shaft lining concrete within the context of hydro-mechanical coupling is crucial. In this study, concrete compression tests were conducted using the TAW-2000 rock testing machine. The concrete damage and rupture mechanisms were analyzed using a combination of mercury intrusion porosimetry and scanning electron microscopy. Moreover, a thorough investigation of the energy evolution throughout the concrete stress-strain process was conducted. The key results include the following: (1) The formulation of the normalized decay equation for the characteristic strength of concrete under hydraulic coupling was derived by computing the ratio of pore water pressure to confining pressure as the characteristic parameter. (2) The effective shear strength reduction factor characterizing the impact of water pressure on the effective shear stress with a correlation coefficient greater than 0.999. (3) The pore water pressure adversely affected the pore size distribution within the concrete, enhancing the connectivity between pores. (4) The total input energy, elastic energy, and dissipated energy were positively related to the confining pressure and negatively related to the pore water pressure. The energy consumption ratio served as a tentative criterion for assessing the strength failure of the shaft lining concrete, which is considered to be in a critically stable state when the energy consumption ratio is 1. These results provided valuable insights into the damage laws and energy-driven mechanisms of shaft-lining concrete under hydraulic coupling conditions.

Estimating weakening on hillslopes caused by strong earthquakes

The weakening of hillslopes during strong earthquakes increases landsliding rates in post-seismic periods. However, very few studies have addressed the amount of coseismic reduction in shear strength of hillslope materials. This makes estimation of post-seismic landslide susceptibility challenging. Here we propose a method to quantify the maximum shear-strength reduction expected on seismically disturbed hillslopes. We focus on a subset of the area affected by the 2008 Mw 7.9 Wenchuan, China earthquake. We combine physical and data-driven modeling approaches. First, we back-analyze shear-strength reduction at locations where post-seismic landslides occurred. Second, we regress the estimated shear-strength reduction against peak ground acceleration, local relief, and topographic position index to extrapolate the shear-strength reduction over the entire study area. Our results show a maximum of 60%–75% reduction in near-surface shear strength over a peak ground acceleration range of 0.5–0.9 g. Reduction percentages can be generalized using a data-driven model.

Investigation of Shear Strength Reduction Method in Slope Stability of Reinforced Slopes by Anchor and Nail

Since the stability of slopes in infrastructures such as road and railroad embankments, excavations, and, in general, earthwork is important, analyzing the stability of these slopes has been one of the main focuses of geotechnical engineers. Although analyzing both reinforced and unreinforced slopes is needed, reinforced slopes require special attention as the reinforcement elements significantly affect the calculations. Hence, the current study’s aim is to find out the differences between obtained safety factors using the Limit Equilibrium Method (LEM) and Shear Strength Reduction Method (SSRM). For this purpose, first, the origin differences in terms of Safety Factor (SF) are theoretically determined according to basic formulas for the aforementioned techniques. Then, to verify the formula, several numerical modelings are carried out using in situ measured geotechnical data to better understand the differences in terms of safety factors. The results indicate that for the reinforced slope with an SF value of higher than 1, the SSRM provides a higher SF in comparison with the other techniques, and the origin of this difference is the definitions of the SF in the different methods.

Slope Stability Analysis of East Ring Road Construction at Sadawarna Dam With Shear Strength Reduction Method

Slope stability has been a problem that studied on geotechnical works for the uncertainties such as varieties of soil behaviours to unpredicted failure of measurements and samplings. The Limit Equilibrium Method (LEM) has been popular for decades for its convenience but cannot determine displacement thus the result could be uncertain. As computational calculations have been developed, the Finite Element Method (FEM) began to use as a tool to not only determine factor of safety, but also determine displacement and forces that affect slope’s stability. This research was done to analysed slope stability using Shear Strength Reduction (SSR) and compared it to general method such as Limit Equilibrium Method (LEM). The research was conducted on a section in Sadawarna DAM ring road area, Subang, West Java. Slope on research area was divided into two layers with the bottom layer relatively non-cohesive compared to the top of layer. Both soil however dominantly consist with fine grain soil such as silt and clay. The upper layer of slope can contain more water with liquid limit of 73,46% compared to the lower layer with 68,27% liquid limit. Simulation result showed slope that analysed using SSR method has Factor of Safety (FoS) 0,12 lower than LEM method. SSR method could be used to analysed pessimistic value at worst scenario and could predicted deformation of slope.

Stable Slope Design Based On Limit Equlibrium Method (Lem) And Finite Element Method (Fem) At Pit X, Lahat, South Sumatra

Coal mining process using open pit mining method is closely related to slope stability. A slope whose stability is disturbed will have a higher potential for landslides. The slope stability analysis in this research is conducted by Limit Equilibrium Method (LEM) with the calculation of Morgenstern-pice slice method and Finite Element Method (FEM) with the calculation of Shear Strength Reduction. The highwall simulation was modeled with a Bench height of 10 meters, Bench width of 6.5 meters and bench tilt angle varying between 30°, 45°, 60° and groundwater condition using steady state FEA. From the results of the LEM analysis on the highwall with a bench tilt angle of 30 °, 45 °, 60 ° has a safety factor value of 1.005; 0.76; 0.584. While the results of the FEM analysis on the highwall with a bench tilt angle of 30 °, 45 °, 60 ° have a Strength Reduction Factor value of 0.98; 0.72; 0.57. Comparison of the safety factor values of the two methods has an average difference of 1-2%. This is because the FEM takes into account the stress-strain in the material which describes how the material behaves. The stable slope design based on LEM is a highwall slope with a Bench slope angle of 22° which has a safety factor value of 1.478 and based on FEM is a highwall slope with a Bench slope angle of 22° which has a Strength Reduction Factor value of 1.42.

Stability Assessment of the Dam of a Tailings Pond Using Computer Modeling—Case Study: Coroiești, Romania

Anthropogenic activities related to mining generate both progress and a vast amount of waste that is responsible for environmental degradation. The Jiu Valley is one of the areas of Romania where mining has affected large areas of land, used to build mines and tailings ponds. The former Coroiesti coal processing plant (CCPP) is such a location with a total area of 25 ha containing approximately 5.5 million tons of tailings. The assessment of the stability of tailings dams is extremely important from safety and environmental aspects. This study proposes a solution based on numerical methods for determining the stability of a section of the dam of a tailings pond. The model of tailings pond no. 1, compartment B, from the Coroieşti Coal Preparation was built using COMSOL Multiphysics. Two scenarios of stability analysis were conducted on a section of the tailings dam: the FOS was determined using the shear strength reduction (SSR) method for both the initial and the current state of this TP. This method is a modern alternative to the limit equilibrium method, and its implementation by COMSOL is new to our country, thus aligning this methodology with current worldwide trends and developments in the field. The results obtained proved to be in line with those calculated in the past with traditional analytical methods, proving that the safety criteria of the studied TP/TD are being met.

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.

Shear strength reduction factor used in critical state models with hardening

The slope stability analyses are important to predict environmental, financial, and human life impacts. The Limit Equilibrium Method (LEM) is commonly used to estimate a slope’s Safety Factor (SF). However, the Finite Element Method (FEM) is increasingly applied to slope stability analyses, using different approaches, among which the technique of Shear Strength Reduction (SSR) is commonly used in perfectly plastic elastic models. The objective of this study is to present a discussion about these two methodologies, using critical state models with or without hardening will be used to model the stress-strain behavior of the soil mass. The results obtained in the case study presented, using LEM and FEM considering critical state models with and without hardening are consistent and allowed verifying the stability condition of the slope. Also, the reduction factors are smaller when compared to the results using perfectly plastic elastic models.

Effects of extreme drought–rainfall on slope failure mechanisms: centrifuge modelling

The extreme drought–rainfall seesaw is projected to occur with an increasing frequency. However, there still lacks a thorough understanding of its impacts on slope behaviour, in which desiccation crack plays a key role. To address this issue, a centrifuge test was conducted to investigate the effects of drought-induced desiccation crack on slope instability under extreme rainfall. During the test, the non-cracked slope was firstly subjected to extreme rainfall with 100-year return period. Subsequently, a long-term drying was applied to induce desiccation crack, and hence forming a cracked slope. The cracked slope is then subjected to an identical extreme rainfall. The non-cracked slope only exhibits swelling deformation, whereas for the cracked slope, a slip surface (2 m in depth) is clearly observed to initiate from one deep crack at crest. The global sliding failure of the cracked slope is mainly related to preferential flow, which could result in soil shear strength reduction.