Unsaturated Soil Slope Research Articles

Reliability analysis of unsaturated soil slope stability under infiltration considering hydraulic and shear strength parameters

In general, soil properties, including shear strength and hydraulic parameters, are characterised as a spatial variability. This paper aims to investigate the effect of spatial variability of the soil properties on slope stability during rainfall infiltration. The effective friction angle, saturated hydraulic conductivity, and soil water characteristic curve parameters of sand are simulated using random field theory. A seepage analysis is conducted using the random finite element method to obtain pore water pressure distribution. A stability analysis is performed to show the variation of safety factors and failure probability. The results show that the random field of the soil-water characteristic curve produces a significant variation of pore water pressure, while the random field of the effective friction angle is the most important parameter for probabilistic stability analysis.

Effect of Slope Angle on the Stability of a Slope Under Rainfall Infiltration

The most devastating factor for slope failures is rainfall infiltration. Surficial slope failures in residual soil slopes are a common sight. In general, slope angles vary considerably from being flat to steep in natural conditions, which govern the hydrological regime of the slope. Further, different types of soils behave differently under rainfall condition in terms of rainfall infiltration and the associated changes in their mechanical behaviour, particularly under different slope inclinations. This paper presents a study on the effect of slope inclination on stability of different soil slopes under rainfall conditions. Two-dimensional numerical models were prepared with three different soils having different hydraulic and mechanical properties to investigate the stability of the unsaturated soil slopes. Seepage analysis is performed in the finite element framework, and stability analysis for the slopes is performed in limit equilibrium framework using numerical package SLIDE. The water table in the slope rises high for flat slopes, which is contrary to levels observed in steep slopes. From the results, it was observed that flat slopes allowed more infiltration leading to the rapid reduction in negative pore pressure thereby decreasing the safety factor. Both the initial factor of safety and minimum factor of safety decreased nonlinearly with slope angle. The critical slip surface becomes shallower (shifts towards the slope surface) with increase in angle of the slope. There was very little influence of slope inclination on the stability for slopes higher than 55°.

Stability of Unsaturated Soil Slopes with Cracks under Steady-Infiltration Conditions

Most classical approaches for evaluating the stability of soil slopes with tension cracks are performed under dry and/or saturated conditions. The suction-induced effect of unsaturated soils is generally neglected in assessing slope stability. This article presents an analytical framework for assessing the stability of unsaturated soil slopes with tension cracks under one-dimensional (1D) steady-infiltration conditions using the limit-analysis method. The boundary-value problem was formulated based on the log-spiral failure mechanism by taking the effects of suction stress, effective unit weight of soil, and tension cracks into account. A simplified method was developed to calculate the external work rate of unsaturated soil weight. Detailed investigations of the effects of suction, Poisson’s ratio, steady infiltration, and water in tension cracks on the slope stability are presented.

The model tests of rainfall infiltration in two-layer unsaturated soil slopes

Rainfall infiltration experiments were conducted on an unsaturated slope consisting of two-layer soils. The water content and the pore-water pressure in the slope were monitored. The monitored data are analysed and discussed in the paper. The analysis indicates that the water content and the pore-water pressure in the model slope change with the depth and that their change conditions are related with the physical properties of soils. These characteristics are different from single-layer slope significantly. The response time of the water content and the pore-water pressure increases with the slope depth. The water content and the pore-water pressure of the lower layer decrease faster than those of the upper layer. Under the same rainfall intensity, the water content and the pore-water pressure develop with the duration of rainfall until the steady seepage. The response time is quicker when the rainfall intensity is higher. However, the time effect is not significant near the slope surface. The pore-water pressure and water content in the upper layer of the model slope show more marked changes than those in the lower layer. Antecedent rainfall greatly affects the pore-water pressure and water content profiles before the steady seepage state, especially at early stage of rainfall infiltration in unsaturated regions.

Three-dimensional seismic stability of unsaturated soil slopes using a semi-analytical method

Seismic stability analyses of soil slopes are routinely performed with dry and/or saturated assumptions under two-dimensional (2D) conditions. However, in practice, soils are often unsaturated and slopes usually fail in 3D fashion. Three-dimensional (3D) effect and suction-induced effect are essential to be considered to produce more realistic solutions in the slope stability analyses. In this regard, an analytical framework is developed in this paper for assessing the 3D seismic stability of unsaturated soil slopes. A semi-analytical method including the simplified method (SM) and the layer-wise summation method (LSM) is developed to deal with the highly variable nature of suction. The validity of the analytical framework in assessing the soil slope stability is checked with published solutions. Examples are given to illustrate the 3D effect and the effect of suction on the slope seismic stability. The described framework allows the user to quantify the effect of changes in moisture content on effective stress distribution and effective unit weight distribution in the slope and thus, changes in the slope stability.

Effects of Moisture Content on the Dynamic Response and Failure Mode of Unsaturated Soil Slope Subjected to Seismic Load

Understanding the relationship between the moisture content of unsaturated soil and the instability and dynamic response of slope under the action of seismic load can bring convenience to the field of slope safety assessment. The effect of moisture content of the unsaturated soil on the failure mode and dynamic response of slope under seismic loading was investigated using shaking table tests. In this regard, three different moisture contents of 5%, 8.1%, and 12.5% were considered. The experimental results show that the amplification factor of acceleration (AFA) on the slope surface increases with the increase in both relative elevation and excitation amplitude of acceleration (EAA). As the moisture content of unsaturated soil increases, AFA tends to decrease. An empirical correlation describing the relationship among AFA, relative elevation, dimensionless EAA, and moisture content of unsaturated soil was proposed. The excitation frequency of acceleration (EFA) has a significant impact on the AFA at the middle and upper parts of the slope. The deformation of slope surface is able to reflect the failure mode of slope, which can be used in slope monitoring work. The slope with different moisture contents of unsaturated soil has different instability modes. With the increase in soil moisture content, the slope needs more energy to reach the unstable state under seismic loading. There are three typical regions of principal stress field in the slope exposed to seismic loading, which control the evolution of initial slip plane inside the slope.

3D stability of unsaturated soil slopes with tension cracks under steady infiltrations

SummaryMost classical approaches for evaluating the stability of soil slopes with cracks are performed under two‐dimensional (2D) condition. Three‐dimensional (3D) effect and suction‐induced effect of unsaturated soils are generally neglected in assessing the slope stability. This paper develops a 3D limit analysis method to evaluate the stability of an unsaturated soil slope with tension cracks under steady infiltrations. The boundary‐value problem is formulated based on the 3D rotational failure mechanism by taking the effects of suction, effective unit weight, and tension crack into account. A simplified method is proposed to calculate the work rate of unsaturated soil weight. A layer‐wise summation method is developed based on the divergence theorem to calculate the internal energy dissipation rate. Detailed discussions are conducted to investigate the effects of suction and Poisson's ratio on the stability of unsaturated soil slopes. Examples are given to illustrate the 3D effect, the suction‐induced effect as well as the effects of infiltration and water in cracks on the slope stability.

Stability analysis of an unsaturated expansive soil slope subjected to rainfall infiltration

Shallow failures occur frequently in both engineered and natural slopes in expansive soils. Rainfall infiltration is the most predominant triggering factor that contributes to slope failures in both expansive soils and clayey soils. However, slope failures in expansive soils have some distinct characteristics in comparison to slopes in conventional clayey soils. They typically undergo shallow failures with gentle sliding retrogression characteristics. The shallow sliding mass near the slope surface is typically in a state of unsaturated condition and will exhibit significant volume changes with increasing water content during rainfall periods. Many other properties or characteristics change such as the shear strength, matric suction including stress distribution change with respect to depth and time. All these parameters have a significant contribution to the expansive soil slopes instability and are difficult to take into consideration in slope stability analysis using traditional slope stability analysis methods based on principles of saturated soil mechanics. In this paper, commercial software VADOSE/W that can account for climatic factors is used to predict variation of matric suction with respect to time for an expansive soil cut slope in China, which is reported in the literature. The variation of factor of safety with respect to time for this slope is computed using SLOPE/W by taking account of shear strength reduction associated with loss of matric suction extending state-of-the art understanding of the mechanics of unsaturated soils.

Experimental study on soil water characteristic curve of unsaturated loess in Xining area

The soil water characteristic curve is an important hydraulic characteristic parameter of unsaturated soil, and plays an important role in the analysis of unsaturated soil strength and slope stability. At present, there are few studies on soil water characteristics in Xining area. In this paper, the matrix suction value of undisturbed loess in Xining area is tested by filter paper method under different water content, then the soil water characteristic curve is obtained. The specific analysis of the test results shows that the soil sample at the same depth increases the matrix suction during the dehydration process, and the volumetric water content of the soil sample decreases gradually. This paper lays a foundation for the study of the impact of rainfall infiltration on the shear strength of slope soil and the stability impact.

Landslide analysis of unsaturated soil slopes based on rainfall and matric suction data

Shallow slope failures that occur as a result of a decrease in matric suction after water infiltration from intense rains (unsaturated soil conditions) are the main causes of slope instability in tropical and subtropical regions. This study analyzes the occurrence of shallow landslides in a highway cutting of residual soil originating from aeolian sandstone. Characterization of failure mechanisms and stability analyses have been carried out in different scenarios and supported by surface and subsurface investigations, instrumentation and monitoring of rainfall, matric suction, and water level, field tests, and laboratory tests. The results indicate that the reduction in matric suction induced by rainwater infiltration is the triggering mechanism of slope failure. Conventional slope stability analyses and analyses incorporating unsaturated seepage models present results compatible with the hydrological and geotechnical data collected in the study. On the basis of these analyses, and considering the frequent rainfall events in the study area, a critical geometric configuration is proposed for the triggering of landslides in highway cutting slopes of residual soils of aeolian sandstone.

Influence of Raised Groundwater Level on the Stability of Unsaturated Soil Slopes

Influence of Raised Groundwater Level on the Stability of Unsaturated Soil Slopes

Stability analysis of unsaturated soil slopes under random rainfall patterns

The significance of rainfall pattern in the assessment of rainfall-induced landslide is widely recognized. However, much work so far is limited to several simplified typical rainfall patterns. In this study, the random rainfall pattern (RRP) is introduced and generated using random cascade model based on the rainfall event characterized by average rainfall intensity and duration. The stability of unsaturated slope considering RRPs is studied from three perspectives: deterministic analysis by means of safety factor under different generated RRPs, probabilistic analysis through conditional failure probability considering the diversity of generated RRPs based on Monte Carlo method and risk assessment analysis by introducing annual failure probability (AFP) considering also the occurrence frequencies of rainfall events. Three typical rainfall patterns are introduced for comparison analysis. The results show that slope stability is sensitive to the RRP and is strongly depend on the temporal distribution of rainfall intensity in RRP. High likelihood of slope failure may occur considering the variety of RRPs even though the slope is in a stable state in terms of deterministic analysis. The AFP considering RRPs increases rapidly with increasing rainfall duration and is significantly different from those under typical rainfall patterns. The findings lead to the conclusion that RRPs should be considered in the estimation of unsaturated slope stability.

Probabilistic Study of Rainfall-Triggered Instabilities in Randomly Heterogeneous Unsaturated Finite Slopes

Water infiltration destabilises unsaturated soil slopes by reducing matric suction, which produces a decrease of material cohesion. If the porosity of the soil is spatially heterogeneous, a degree of uncertainty is added to the problem as water tends to follow preferential paths and produces an irregular spatial distribution of suction. This study employs the finite element method together with Monte Carlo simulations to quantify the effect of random porosity on the uncertainty of both the factor of safety and failure size of an unsaturated finite slope during and after a rainfall event. The random porosity is modelled using a univariate random field. Results show that, under partially saturated conditions, the random heterogeneity leads to a complex statistical variation of both factor of safety and failure size during the rainfall event. At any given time, the uncertainty about failure size is directly linked to the uncertainty about the position of the wetting front generated by infiltration. Interestingly, the statistical mean of the failed area is smallest when the mean of the factor of safety is lowest. In other words, the slope becomes more likely to fail, but the size of the failure mass tends to be limited. The study also investigates the sensitivity of failure uncertainty to external hydraulic parameters (i.e. initial water table depth, rainfall intensity) and internal soil parameters (i.e. permeability and water retention characteristics). In general, the sensitivity increases when the effect of these parameters on the spatial variation of suction is stronger.

Influence of Rainfall Intensity and Its Pattern on the Stability of Unsaturated Soil Slope

Rainfall infiltration is an important factor that leads to slope instability. In this study, a numerical model for slope is built to investigate the effects of rainfall intensity and its pattern on slope stability. Results show that rainfall intensity and its pattern significantly influence slope stability due the following instances: (1) the safety factor of slope initially decreases and then stabilizes during rainfall, which is defined as the effective rainfall days; moreover, when the rainfall intensity is less than the saturated permeability coefficient of soil, the effective rainfall days decreases with the increase of rainfall intensity. (2) Under the same rainfall intensity, the ultimate safety factor of sand slope when the effective rainfall days is reached is greater than that of the clay slope. (3) When the rainfall intensity is less than the saturated permeability coefficient of soil with the same total precipitation, a longer rainfall duration indicates a larger slope safety factor; conversely, when the rainfall intensity is greater than the saturated permeability coefficient of soil, a longer rainfall duration indicates a smaller slope safety factor.

Upper-Bound Limit Analysis of Unsaturated Soil Slopes under Rainfall

AbstractThis paper presents an analytical framework for the stability analysis of unsaturated soil slopes under rainfall. The conventional log-spiral upper bound limit analysis for dry and saturate...

A practical approach for reliability analysis of unsaturated slope by conditional random finite element method

The paper presents a method for reliability analysis of slopes in unsaturated soils. Conditional random fields are simulated by consideration possible fluctuations of the measured soil properties. To predict the unsaturated soil behaviour, suctions are estimated and implemented in a finite element analysis. Numerical results of a case study demonstrate that the use of conditional random fields, affects not only the standard deviation of the safety factor distribution, but can also cause considerable variation in the mean value. It is concluded that unsaturated conditions can result in higher slope reliability, but with potentially more extensive failure zones.

Unsaturated soil slope characterization with Karhunen–Loève and polynomial chaos via Bayesian approach

Field measured data reflect real response of soil slopes under rainfall infiltration and can provide representative estimates of in situ soil properties. In this study, an efficient probabilistic back analysis method for characterization of spatial variability of soil properties is used to investigate the effects of field responses with various monitoring schemes on characterization of spatial variability in unsaturated soil slope. A hypothetical heterogeneous slope of spatially varied saturated hydraulic conductivity subjecting to steady-state rainfall infiltration is analyzed as a numerical example. The spatially varied soil saturated hydraulic conductivity is parameterized by the Karhunen–Loeve expansion (KLE) with a given covariance. The random variables corresponding to the truncated KLE terms are considered as variables to be estimated with Bayesian inverse method. Synthetic pore water pressure data corrupted with artificial noise are utilized as measurement data. Nine schemes with various locations, spacings and depths of monitoring sections are discussed. The results show that the local variability can be reduced substantially around the monitoring points of pore pressure. The spatial variability can be estimated more accurately with a smaller spacing of measurement points. When measurement points are installed with a spacing of 16.5 m, the posterior average COV of ks field is around 2% and the RMSE of the MAP field is only 5.90 × 10− 7 m/s. For schemes with different depths, the RMSEs of the MAP field does not change much but the posterior uncertainty of the estimated field is reduced with the increase of borehole depth.

Analysis of Infiltration-Suction Response in Unsaturated Residual Soil Slope in Gelugor, Penang

Rainfall infiltration on residual soil slope may impair slope stability by altering the pore-water pressure in the soil. A study has been carried out on unsaturated residual soil slope in Gelugor, Penang to determine the changes in matric suction of residual soils at different depth due to rainwater infiltration. The sequence of this study includes the site investigation, field instrumentation, laboratory experiment and numerical modeling. Void ratio and porosity of soil were found to be decreasing with depth while the bulk density and dry density of soil increased due to lower porosity of soil at greater depth. Soil infiltration rate and matric suction of all depths decrease with the increase of volumetric water content as well as the degree of saturation. Numerical modeling was used to verify and predict the relationship between infiltration-suction response and degree of saturation. Numerical models can be used to integrate the rainfall scenarios into quantitative landslide hazard assessments. Thus, development plans and mitigation measures can be designed for estimated impacts from hazard assessments based on collected data.

Three-dimensional stability analysis of slope in unsaturated soils considering strength nonlinearity under water drawdown

In slope stability analyses, the soil has been frequently considered dry or saturated, whereas the soil is in fact unsaturated in many cases. Owing to the existence of matric suction, the strength properties of unsaturated soils are greatly different from dry or saturated soil, and thereby leading to diverse stability conditions of the slope. Based on the kinematical approach of limit analysis, in this study, a three-dimensional (3D) stability analysis was conducted for slopes in unsaturated soils. The effects of matric suction distribution patterns, the nonlinearity of the strength under soil-water characteristic curves and the water drawdown were investigated to explore the shear strength and stability of a slope in unsaturated soil. It was found that, when the matric suction is uniformly distributed, the stability factors of the slope increase linearly along with the matric suction. In addition, when the matric suction increases linearly with the depth, the enhancing effect of the matric suction on the slope stability intensifies as the variation magnitude of the matric suction increases, especially for a gentle slope with a narrower width. For nonlinear patterns, different soil-water characteristic curve (SWCC) models will lead to diverse shear strengths and stability conditions of the slope. The water drawdown has significant effects on the functions of the matric suction as well as slope stability.

Stability of unsaturated soil slopes subjected to external load and rainfall

The aim of this study is to investigate the effect of the amount of the external load and failure form for the slopes of unsaturated clayey and alluvial soils. Also, the impact of the rainfall on the stability of these slopes was also studied. Laboratory physical model was designed as a box with dimensions of 1580 × 1400 × 2000 mm. Heavy rainfall with the intensity of approximately 10 and 5 mm/hr with duration 16 and 8 h for soil A and B, respectively, was imposed. Soil B was very sensitive to the changes in the total suction comparing with soil A. The effect of rainfall reduced the load capacity from 510 and 359.8 to 183 and 11 kPa of soil slopes A and B. Therefore, a reduction in load capacity of 64 and 97% was obtained.