Stability Of Unsaturated Soil Slopes Research Articles

Effects of seepage on a three-layered slope and its stability analysis under rainfall conditions

The seepage and stability of unsaturated soil slopes under torrential rain conditions are key issues in geotechnical and hydraulic engineering. In this work, based on the generalized limit equilibrium method, the stability of nonhomogeneous slopes with three layers was investigated. The effects of rainfall on the volume water content, pore water pressure and the stability of the slope were evaluated, and rainfall cycles and rainfall intensities were considered. The results show that the stability of the slope decreased with increasing rainfall intensity until it was destroyed. The safety factor of the slope is lowest in a period of time after the rain stops. This condition means that the slope is more prone to damage. The seepage field and stress field of the slope were significantly changed with increasing rainfall, and finally, the slope became unstable.

Discussion on “Probabilistic characterization of the soil-water retention curve and hydraulic conductivity and its application to slope reliability analysis” by L. Wang, L. Tang, Z. Wang, H. Liu, and W. Zhang

The statistical characteristics of the soil-water retention curve parameters has an important role in unsaturated soil slope stability analysis under rainfall. However, such its application is questionable for reliability analysis of a hypothesis slope considering a steady state condition of seepage analysis.

Numerical Analysis of the Effects of Crack Characteristics on the Stress and Deformation of Unsaturated Soil Slopes

Cracks induced by evaporation or rainfall have a great influence on the stability of unsaturated soil slopes, which can lead to landslides during the rainfall process. In order to study the effect of crack characteristics on the evolution of stress and deformation of unsaturated soil slopes, a series of numerical analyses under different conditions were performed using a coupled elastoplastic finite element program that we developed for unsaturated soil. When carrying out the numerical analyses, the effective stress for unsaturated soil proposed by Bishop and an elastoplastic double-hardening constitutive model for the soil skeleton were employed. The varying parameters, including the crack location, the discharge speed, evaporation rate, infiltration rate, and tensile strength, were investigated to study the coupling process of pore water pressure and deformation in the process of evaporation and rainfall infiltration. The numerical results showed that the minimum pore water pressure of the soil slope at the end of evaporation/rainfall decreased gradually and the crack width increased gradually as the crack set closer to the slope; the larger the discharge speed of pore air, the greater the crack width. With the increase in the evaporation rate, the pore water pressure of the soil slope reduced and the crack initiated earlier and became wider. As the infiltration rate increased, the pore water pressure of the soil slope and the crack width increased, but the decreasing duration became shorter. The change of tensile strength had little effect on the pore water pressure, but the development of the crack width changed with evaporation and rainfall infiltration.

Stability analysis of unsaturated soil slope under rainfall and seepage conditions

Rainfall infiltration and canal seepage are the main causes of slope instability, the paper considered the factors such as soil type, soil-water characteristic curve and rainfall duration, and analysed the stability of unsaturated soil slope. The calculation results show that rainfall has a significant impact on the stability of the slope, as the matric suction in the unsaturated soil makes the slope safer. However, it reduced during rainfall, so the safety factor is greatly reduced when the continuous rainfall occurred. Moreover, when rainfall infiltration and canal seepage coupled, it is worse than just taking one factor into account, and the slope safety factor decreases from stable to unstable.

Stability of unsaturated soil slopes covered with Melastoma malabathricum in Singapore

Past studies have shown that slopes covered with vegetation are less susceptible to rainfall-induced slope failures. This research was focused on the unsaturated properties of soil consisting of mangrove roots and their effects on the stability of an old alluvium soil slope during rainfall. Melastoma malabathricum was selected in this study since this mangrove is best suited for tropical climatic conditions. The results of laboratory tests indicated that the air-entry value and shear strength of soil with roots of M. malabathricum were higher than those of the soil without roots. The saturated permeability of the soil with roots was lower than that of the soil without roots. Numerical analyses indicated that the slope without M. malabathricum experienced significant decreases in the factor of safety during rainfall than the slope covered with M. malabathricum. M. malabathricum can thus be used as a vegetative cover for minimising rainwater infiltration in slope layers with a slope angle up to 35°.

Investigation of the Parameters Affecting the Stability of Unsaturated Soil Slope Subjected to Rainfall

Çalışma kapsamında, yağışlı mevsimlerde şev duraysızlıklarının sıklıkla görüldüğü bir bölgede, doygun olmayan koşullardaki şevin duraylılığını etkileyen parametreler incelenmiştir. İncelenen parametreler; yağış yoğunluğu, yağış süresi, hidrolik iletkenlik ve şev eğimi olup; çalışmada bölgeye ait gerçek yağış verilerini de içeren bir veri aralığı kullanılarak, yarı-parametrik çalışma yapılmıştır. Bu şekilde şevin mevcut durumu ve parametre değişiminin etkisi aynı anda incelenebilmiştir. Yağışın etkisiyle oluşan emme basınçlarındaki değişim nümerik yöntemle elde edilmiştir. Şev analizlerinde negatif boşluk suyu basıncındaki değişim dikkate alınarak, Bishop limit denge yöntemi kullanılmıştır. Analizler sonucunda belirli bir yüksekliğin üzerindeki şevlerde yağışlı dönemde görülen göçmelerin nedeni ve incelenen parametrelerin duraylılığa etkisi gösterilmiştir.

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.

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.

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.

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.

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

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

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.

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.

The Probabilistic Stability Analysis of Saturated-Unsaturated MH and CL Soil Slope with Rainfall Infiltration

It is well known that the slope failure in unsaturated soil is the decrease in the matric suction, when the rain water infiltrates. However, the seepage mechanism and the stability of unsaturated soil slope have not been well established in geotechnical engineering. In this study, the suction profile and reliability index of saturated-unsaturated slope is studied at different rainfall condition with intensity in the range of saturated hydraulic conductivity of MH and CL soil. The finite element seepage model is uncoupled with slope stability model to calculate the reliability index at different time of long duration uniform rainfall. For CL soil slope, the reliability index is slightly changed during and after rain fall infiltration. For the MH soil slope the wetting front remains unsaturated up to a certain time of rainfall infiltration. After this time, the infiltrated rainfall reaches the ground water table and elevates it. The reliability index of the soil slope is thus reduced.

Incorporating hydro-mechanical coupling in an analysis of the effects of rainfall patterns on unsaturated soil slope stability

Rainfall-induced landslides can cause loss of life and damage to property, infrastructure, and the environment. Rainfall patterns affect the pore-water pressure of unsaturated soil slopes, and are related to the slopes’ stability. Four rainfall patterns were chosen to represent natural rainfall patterns for an examination of rainfall infiltration into soil slopes using numerical models incorporating coupled water infiltration and deformation in unsaturated soils. Our analysis showed that rainfall patterns play a significant role in the distribution of the pore-water pressure in soil slopes, and influence the slope stability. The pore-water pressure profile of soil slopes and the factor of safety are affected by the ratio of rainfall intensity and the coefficient of permeability. The depth and shape of the shallow sliding plane of the landslide is closely related to the rainfall pattern; moreover, the results showed a correlation between the factor of safety of the slope and the rainfall intensity. This relationship can be described by a dimensionless rainfall intensity. The nonlinear relationship can be used to estimate the slope stability resulting from rainfall infiltration when the hydro-mechanical coupling in unsaturated soil slopes is considered.

Influence of the spatial variability of shear strength parameters on rainfall induced landslides: a case study of sandstone slope in Japan

Rainfall-induced landslides frequently occur in humid temperate regions worldwide. Research activity in understanding the mechanism of rainfall-induced landslides has recently focused on the probability of slope failure involving non-homogeneous soil profiles. This paper presents probabilistic analyses to assess the stability of unsaturated soil slope under rainfall. The influence of the spatial variability of shear strength parameters on the probability of rainfall-induced slope failure is conducted by means of a series of seepage and stability analyses of an infinite slope based on random fields. A case study of shallow failure located on sandstone slopes in Japan is used to verify the analysis framework. The results confirm that a probabilistic analysis can be efficiently used to qualify various locations of failure surface caused by spatial variability of soil shear strength for a shallow infinite slope failure due to rainfall.

Limit Analysis of Unsaturated Soil Slope Stability Considering Intermediate Principal Stress and Strength Nonlinearity

Natural and man-made slopes are mostly in unsaturated soils, and the saturated or dry condition is just a particular case. This paper investigates the stability of unsaturated soil slopes. The influence of intermediate principal stress is considered by introducing the unified shear strength formulation, and the strength nonlinearity of unsaturated soils is addressed by means of two different methods. Based on the upper bound theorem of limit analysis, analytical expressions of the critical height of slope are deduced. Relevant numerical solutions are obtained through optimization and parametric analysis is then conducted. It is found that: the critical height of slope increases with the increase of UST parameter b or matric suction, while it decreases with the increase of slope angles. Besides, as regards the strength nonlinearity, it’s suggested to adopt Method II when parameter $$m$$ can be reasonably determined.

Reasonable selection of yield criteria for quantitative analysis of unsaturated soil slope stability

The yield criterion parameters of the soil material change with different values of the cohesion and the angle of friction because of sustained rainfall infiltration. Based on the Mohr-Coulomb (M-C) and Drucker-Prager (D-P) yield criteria, some reasonable yield criteria selections were discussed for quantitative analysis of unsaturated soil slope stability. Moreover, a critical point was found at the effective angle of friction equaling to 16.5° by transformation of parameters related to unsaturated soil under sustained rainfall. When the effective angle of friction more than 16.5° through parameter transformation of different yield criteria under natural condition, the calculation result of the safety factor was such that: f(DP1) > f(M-C) > f(equivalent M-C) > f(DP2) > f(DP3). While the effective angle of friction less than 16.5°, through parameter transformation, the safety factors were in the following order: f(DP1) > f(M-C) > f(DP2) > f(equivalent M-C) > f(DP3). The calculated results from a case study showed that the equivalent M-C yield criterion should be the best at evaluating soil slope stability before rainfall; the DP2 yield criterion should be selected to calculate the soil slope stability at the effective angle of friction less than 16.5° under sustained rainfall. The yield criterion should be selected or adjusted reasonably to calculate the safety factor of unsaturated soil slopes before and during sustained rainfall.

Effects of Airflow Induced by Rainfall Infiltration on Unsaturated Soil Slope Stability

The aim of this study was to investigate the characteristics of airflow induced by rainfall infiltration and their effects on unsaturated soil slope stability using numerical methods. The TOUGH2/EOS3 simulator was used to analyze the features of water–air two-phase flow in a soil slope under rain conditions. The results show that when the rain infiltrates the soil, the air is pushed into the soil by the advancing wetting front, the pore-air pressure in the unsaturated zone increases firstly, then the airflow is reversed, and the pore-air pressure decreases until it is close to atmospheric pressure. The features of water single-phase flow (i.e., the gas phase is neglected) were also simulated using the TOUGH2/EOS9 simulator. When the infiltration of rainwater into the soil slope in the water single-phase flow model is set as same as that in the water–air two-phase flow model, the water saturation and the capillary pressure distributions in the unsaturated zone are probably the same. Slope stability studies were then performed based on the simulated water–air two-phase and water single-phase seepage conditions, respectively. The slope stability analysis on a given slip surface based on the simulated water–air two-phase seepage condition shows that the safety factor decreases during rainfall, reaches the lowest value when rain stops, generates immediately a sudden rise, and then increases slowly to a steady value. The capillary pressure is beneficial to slope stability, while the pore-air pressure is unfavorable to slope stability. The variation tendency of the safety factor is similar to that of the negative total air pressure acting on the slip surface. The slope stability analysis on the given slip surface based on the water single-phase seepage condition shows that the safety factor is approximate to FS1 (is the safety factor for the case that does not take into account the air pressure based on the water–air two-phase seepage condition). Therefore, difference between the safety factor and FS1 based on the water–air two-phase seepage condition can approximately represent the effects of airflow induced by rainfall on unsaturated soil slope stability under the condition that the infiltration of rainwater into the soil slope remains unchanged.

Rainfall characteristics for anisotropic conductivity of unsaturated soil slopes

The purpose of this study was to investigate the effects of the anisotropic ratio on the stability of slopes using the reliability index approach. A numerical analysis of the relationship between the three rainfall patterns, advanced, normal and delayed, and the anisotropic ratios was designed. This study also considered three different soil properties (sand, silt, and clay) to simulate rain infiltration. In this study, probability analysis was used to evaluate the stability of unsaturated soil slopes. The finite element computer program Geo-Studio was used to simulate the process of rainwater infiltrating a slope. The pore-water pressure results evaluated from seepage analysis (SEEP/W) were imported into the slope stability program (SLOPE/W). Results for the anisotropic ratio of hydraulic conductivity indicate that when the anisotropic ratios become higher, the reduction in the reliability index is insignificant. In addition, the simulation results indicated that when saturated hydraulic conductivity (ks) was less than rainfall intensities (I), the percentage probability of the occurrence of a landslide was larger than when ks was greater than I. Finally, in the cases of anisotropic ks, stability of the high ratio soil slopes was not found to be sensitive to the reliability index variation during the simulation period. Moreover, when ks was greater than I, slope stability decreased earlier than was the case in the opposite situation.