Expansive Soil Slope Stability Research Articles

Stability of Expansive Soil Slopes under Wetting–Drying Cycles Based on the Discrete Element Method

The swelling-shrinkage behavior of expansive soils under climate changes will cause the crack development, which can be destructive of expansive soil slopes. This study investigated the effect of drying/wetting cycles on the stability of an expansive soil slope using the discrete element method (DEM), in consideration of the crack development induced by climate changes. The strength reduction method was adopted in the DEM calculations, which was coupled with the unsaturated seepage analysis given by the finite element method. The slope stability and the failure model of the slope after different times of wetting–drying cycles were analyzed, and the results were compared with those calculated by the limit equilibrium method and the finite element method. The results indicated that the failure pattern of the expansive soil slope was strongly influenced by the wetting–drying cycles. A shallow sliding surface of the expansive soil slope occurred after several wetting–drying cycles. Similarly, the safety factor of the expansive soil slope decreased gradually with the wetting–drying cycles. Considering the cracks’ evolution inside the expansive soil slope from the drying/wetting cycles, a shallower sliding surface with a smaller safety factor was obtained from the strength reduction method of the DEM, in comparison with the two conventional methods of the Limit equilibrium method and finite element method. Therefore, cracks play an essential role in the expansive soil slope stability. The strength reduction method of the DEM, which considers the cracks’ evolution during drying/wetting cycles, is more reliable.

The Impact of Vegetation Roots on Shallow Stability of Expansive Soil Slope under Rainfall Conditions

The impact of reinforcing vegetation roots on the stability of expansive soil slopes with moisture absorption and expansion was investigated. Then, poinsettia is selected as the slope protection plant, and ABAQUS software (version 2022) with secondary development is used to simulate the moisture absorption and expansion of the expansive soil slope. After that, the strength reduction method is employed to study the effects on the displacement and plastic zone, and on the shallow layer of the expansive soil slope at different rainfall conditions. The following points are revealed: (1) The roots of the poinsettia can reduce the displacement of the slope. But, when the rainfall intensity exceeds the soil permeability coefficient, the soil reinforcement effect decreases. (2) The poinsettia root system can alleviate the concentration of plastic strain, disperse the plastic zone, and increase slope stability along the distribution of the roots. (3) The poinsettia roots can improve the shallow stability of the slope. But when the rainfall intensity exceeds the surface permeability coefficient, the magnitude of the reinforcement decreases. The results demonstrate that the poinsettia roots can enhance shallow slope stability. However, with increasing rainfall intensity, the ability of the poinsettia roots to enhance shallow slope stability gradually weakens.

Influence of reinforcement method on the crack characteristic parameters of expansive soil experimental study

The expansive soil slope is mainly characterized by the decline of slope integrity caused by shallow expansive soil cracking and the destruction of internal soil structure, which seriously affects the overall stability of expansive soil slope. To study the effect of the combination of geogrid reinforcement and slope vegetation on inhibiting the development of expansive soil cracks, six groups of test models were made. The natural dry–wet cycle was simulated, and the crack image was binarized by using image processing technology. The crack characteristic parameters such as crack ratio, crack width, and crack length were extracted, and the effect of various reinforcement methods on inhibiting the development of cracks was comprehensively evaluated. The basic situation of the development of crack indexes in each group with the development of multiple dry–wet cycles was obtained, and the fluctuation changes of crack indexes in different stages were different under different reinforcement methods and dry–wet cycles. At the same time, the influence of different reinforcement methods on the crack development of expansive soil is obtained. It is considered that planting vetiver grass + geogrid backpacking has a good effect on inhibiting the crack development of expansive soil.

Study on Strength Attenuation Characteristics of Residual Expansive Soil under Wetting-Drying Cycles and Low Stress and Its Relationship with Shallow Landslide

The rapid development of expansive soil fissures and the attenuation of strength under the action of repeated atmospheric wetting-drying cycles have a very adverse impact on the shallow stability of expansive soil slope. In this study, shear strength test and fissure observation test were performed on typical residual expansive soil specimens at various wetting-drying cycles and low stress. The results show that the shear strength index of residual expansive soil should be obtained according to the high and low stress sections, respectively. The cohesion of the three residual expansive soils decreased to a similar low value range (0~5 kN) at multiple wetting-drying cycles and low stress; the attenuation of cohesion is the main cause of shallow slope collapse. The fissure rate of the specimen surface increases with the increasing number of wetting-drying cycles, and increasing the dry density can significantly inhibit the development of fissures. The repeated wetting-drying cycles leads to the rapid development of slope fissures from the surface to the inside, destroys the internal natural structure of undisturbed expansive soil, reduces the cementation between soil particles, and leads to the decrease of soil cohesion. For slope stability analysis of residual expansive soil, the strength parameters of the specimens with multiple wetting-drying cycles and internal fissure network should be selected according to the overburden pressure of the slope sliding surface. The research results provide a theoretical basis for the stability analysis of shallow landslide of expansive soil slope and slope protection design.

Algorithm Implementation of Equivalent Expansive Force in Strength Reduction FEM and Its Application in the Stability of Expansive Soil Slope

Loss of matric suction during rain has an important effect on the instability of expansive soil slope. A new device was designed for testing the expansive force in order to propose and determine the equivalent expansive force corresponding to the matric suction. First, the internal relation between the matric suction and the corresponding equivalent expansive force of unsaturated expansive soil was analyzed. Then, numerical algorithm implementation of the equivalent expansive force was discussed, and the equivalent expansive force was introduced into the strength reduction finite element method (FEM). Finally, the equivalent effects of the matric suction and the equivalent expansive force were compared and analyzed by evaluating the stability of an unsaturated expansive soil slope. The results show that the new testing device significantly improves the accuracy of the expansive force test and shortens the testing time. The relation between the matric suction and the equivalent expansive force with the change in initial water content is obvious. The equivalent expansive force can reflect the macro contribution of matric suction to unsaturated expansive soil, and the developed strength reduction FEM based on the equivalent expansive force can be used to evaluate the rainfall-induced instability of an expansive soil slope caused by the decrease in matric suction resulting from the rainfall infiltration.

Reliability Analysis of Expansive Soil Slope Stability Based on the Three-Broken Line Model

Based on the characteristics of an expansive soil slope, the slip mass can be simplified to a simpler model with three-broken line rigid bodies. A solution was formulated to calculate the safety factors of the slope, and the results are similar to those based on the strength reduction method. However, similar to conventional methods to analyze the stability of slopes, the deterministic method to obtain the safety factors only calculates the safety factor using deterministic values without considering the randomness of soil parameters, which leads to unstable results. To improve the rationality of the calculated results, this paper aims to construct a reliability analysis method based on the simplified three-broken line model of a landslide. The reliability is calculated with the response surface method in a spreadsheet with efficiency and convenience. The designed program considers the changes in the strength of the shallow soil and the depth of the strongly weathered layer for different stages of the wetting-drying cycles and solves for the probability of failure of the sliding surface at the interface between the strong and weak weathered layers. Considering an expansive soil slope as an example, the reliability of the slope was analyzed based on laboratory test data and the proposed formula. The results show that multiple wetting-drying cycles significantly increase the probability of failure of an expansive soil slope and that the slope typically becomes unstable after six wetting-drying cycles. Slope cutting helps alleviate the adverse effects of wetting-drying cycles.

A study on the method of stability calculation of soil nailing expansive soil slope

In order to analyze the stability and optimize the project of soil nailing engineering for expansive soil slope, the calculation formulas for unsaturated soils strength, soil nailing surface bond strength, and nail withdraw strength was deduced based on the Mohr-Coulomb strength formula and national regulations. The sliding resistance of the soil nailing was taken into consideration based upon the Bishop slice method; Calculating models on soil nailing expansive soil slope stability analysis was designed; The influence of fissure, rain infiltration, and soil nailing on the expansive soil slope stability were also taken into account. Based on engineering example, fixed slope soil parameter and according soil nailing parameter, the influential factors were analyzed by changing soil nails inclination, installation position, and length. As it was demonstrated by engineering example, the methods and corresponding calculating procedure were reasonable for stability analysis and design optimization for soil nailing expansive soil slope. It is of certain engineering application value.

Surface Crack Development Rules and Shear Strength of Compacted Expansive Soil Due to Dry–Wet Cycles

Expansive soil is a typical multi-crack soil, and the stability of expansive soil slope is significantly affected by cracks. In this study, pores (particles) and crack image analysis system (PCAS), a crack image processing software was used for the dynamic and quantitative measurement of the development of surface cracks in expansive soils with different compaction degree under the condition of dry–wet cycles. Furthermore, the shear strength of expansive soil with different degree of crack development was also tested. The test results showed that the greater the compaction degree, the smaller the cracking degree. The cohesion decreased with the increase of dry–wet cycles; however, the internal friction angle was less affected. For the same number of cycles, cohesion increased with the increase of the compaction degree. The relationship between the cohesion and the crack parameters including the number of crack joints, number of cracks, total length of cracks, and average width of cracks exhibited similar trends. This indicates that cohesion has some relevance with these crack parameters. The crack rate and fractal dimension exhibited linear relationship with cohesion. The research results provide an important basis for the application of crack parameters to calculate the strength parameter of expansive soil in engineering field.

Risk assessment model of expansive soil slope stability based on Fuzzy-AHP method and its engineering application

ABSTRACTTo systematically evaluate the risk grade of stability of expansive soil slope during construction, the recognition model is established by using the fuzzy recognition theory and the analytic hierarchy process (AHP). Based on the characteristics of shallow slide of 22 expansive soil road sections constructed in China, the condition of slope body, swell–shrink grade, hydraulic and meteorological characteristics, supporting and improvement measures and other factors such as the construction technology and management level, vegetation coverage, soil layer, and drying–wetting cycles are selected as the evaluation indices, and the corresponding criteria of risk assessment system of expansive soil slope stability are also established. The weights of evaluation indices are obtained by using AHP method, and the measure functions are established to assess single index measurement and synthetic measurement. The maximum membership degree criterion is adopted to discern the risk grade of expansive soil slope stability. Based on this, the stability of expansive soil slope with different condition located in Nanyou highway and Beijing west 6th ring highway of China are analysed to verify the reliability of the present method. For practical purposes, the present risk assessment methodology can be used to systematically assess the slope stability in expansive soil zones.

Influence of swelling behavior on the stability of an infinite unsaturated expansive soil slope

The influence of the stress regime change and associated softening can be significant on unsaturated expansive soil slope stability due to soil swelling upon wetting, which cannot be considered in conventional hydrological models. In this paper, the shallow expansive soil slope failure mechanism is addressed in the framework of an infinite slope formulation. The unsaturated soil elasto-plastic constitutive relationship is utilized for interpretation of stress regime evolution induced by expansive soil swelling during infiltration. The extended Mohr–Coulomb failure criterion for unsaturated soils is used as the yield surface, under which the nonlinear elastic behavior is considered by quantifying the effect of two stress state variables (net stress and suction) on elasticity parameters. The strain softening behavior in unsaturated soils is accounted for via reducing the material parameters of the yield surface with respect to plastic deviatoric strain. A numerical exercise is performed on a relatively gentle slope in Regina, Canada with highly expansive soil properties, using the developed computer program that implements the constitutive model into infinite slope formulation. The results suggest that neglecting the swelling-induced stress change and associated softening behavior can significantly overestimate the stability of expansive soil shallow layer under infiltration, in terms of both failure occurrence and failure time. Additional parametric study shows that all the considered parameters (including initial stress condition, softening rate and slope angle) have a considerable effect on the failure time and failure depth of the shallow deposit, which have important implications for the engineering design of expansive soil slopes.

Stability Analysis of an Expansive Clay Slope: A Case Study of Infiltration-Induced Shallow Failure of an Embankment in Regina, Canada

An expansive clay slope in Regina, Canada that failed in the spring of 1973 is reanalyzed using a comprehensive numerical procedure that involves the coupled hydro-mechanical finite element analysis. This procedure reliably simulates the behavior of the slope for computing its factor of safety (FS) under snow melting infiltration condition. The results of the study show that a bimodal permeability function with an increased saturated coefficient of permeability by 2-3 of magnitudes in comparison to the laboratory measured value can be used to reasonably describe the hydraulic properties of unsaturated cracked expansive soils and the effect of cracks on slope stability. An increase in saturated coefficient of permeability (up to the rate of snow melting) and use of residual shear strength parameters contribute to early failure of this slope under snow melting condition. The study highlights the use of field saturated coefficient of permeability and bimodal permeability function such that expansive soil slope stability can be reliably estimated.

Study on Shear Strength of Expansive Soil Considered Softening by Triaxial Tests

Swell-shrinking, crack development and over-consolidation are the characteristics of expansive soil, and it is an over-consolidation soil undergone dry-wet cycles. So there may be some faults to analyze expansive soil slope stability adopting the traditional strength criterion and calculation methods. In this paper, triaxial tests were carried out to obtain the relations between the parameters of shear strength and generalized plastic shear strain. The soil shear strength increases and then decreases along with the increasing of shear deformation due to soil over-consolidation. Moreover, the residual strength is achieved finally. The functions are applied in simulating the relations between the parameters and generalized plastic shear strain in order to analyze expansive soil slope stability.

Research progress on expansive soil cracks under changing environment.

Engineering problems shunned previously rise to the surface gradually with the activities of reforming the natural world in depth, the problem of expansive soil crack under the changing environment becoming a control factor of expansive soil slope stability. The problem of expansive soil crack has gradually become a research hotspot, elaborates the occurrence and development of cracks from the basic properties of expansive soil, and points out the role of controlling the crack of expansive soil strength. We summarize the existing research methods and results of expansive soil crack characteristics. Improving crack measurement and calculation method and researching the crack depth measurement, statistical analysis method, crack depth and surface feature relationship will be the future direction.

Analysis of Fissured Expansive Soil Slope Stability and its Reinforcement with Anti-Slippery Piles

Fissure is one of the most important characteristics in expansive soil. It’s an important factor affecting slope stability in the expansive soil area. With the study of expansive soils typical channel slope fissured characters in the South-to-North Water Diversion Project, different impaction on slope stability such as location and inclination of the fissure are analyzed. By using the limit equilibrium method, the stability of expansive soil slope is analyzed; the most adverse condition of expansive soil slope is concluded. Then anti-slippery pile reinforcement in expansive soil channel slope is studied by using finite difference method. The research is of great significance to the construction and reinforcement of expansive soil slopes in South-to-North Water Diversion Project.