Unsaturated Embankment Research Articles

A three-phase two-point MPM for large deformation analysis of unsaturated soils

This paper presents a three-phase two-point formulation of the material point method (MPM) for modeling unsaturated soils involving large deformation. In the formulation, solid phase material is represented by one layer of material points while liquid and gas phases are modeled by another layer of material points. The formulation is a u-U formulation. It eliminates advection term between solid and liquid phases and is capable to address large relative deformations between the two phases. The advection term between liquid and gas phases are assumed to be small and not considered. The proposed formulation is validated with numerical models for small and finite deformation problems. The capacity of the method for study of geomechanics problems is demonstrated with simulation of seismic-induced ground liquefaction with an unsaturated embankment. The method is able to simulate large ground deformation due to soil liquefaction and generation of excess pore pressure, and captures pore pressure dissipation through the rapid water drainage through high-permeable soils.

Numerical simulation of rainfall-induced deformations of embankments considering the coupled hydro-mechanical behavior of unsaturated soils

This paper presents numerical simulations of the deformation response of unsaturated embankments subjected to rainfall infiltration using a coupled hydro-mechanical constitutive model for unsaturated soils. The constitutive model accounts for the influence of degree of saturation on the stress–strain behavior and the influence of void ratio on the water retention behavior. The constitutive model was implemented in the finite difference program FLAC, calibrated using triaxial test data, and validated using measurements of wetting-induced deformations of embankment models from centrifuge tests. The validation demonstrates that the constitutive model can capture the coupled hydro-mechanical behavior of unsaturated soils under wetting conditions. Simulations of the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration indicate that the differential settlement across the top surface of the embankment between the centerline and shoulder increases significantly during rainfall infiltration, which could result in severe damage to overlying transportation infrastructure. As the rainfall infiltration increases, shear strains accumulate and form a potential failure surface at a shallow depth of approximately 2 m from the slope surface. Insights into the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration gained will be useful for considering climate change effects in design and construction of compacted embankments.

Numerical Analysis of Soil Deformation in Earth Dams Using a Stiffness Variation Technique

Small earth dams have been used since ancient times and are often the main source of water in semi-arid regions. In this particular type of engineering work, soils saturated by water present changes in their mechanical properties (especially in the first filling), which usually cause a simultaneous loss of stiffness and shear strength. In this context, the software UNSTRUCT presents itself as a very useful tool, as it allows calculating the final state of stress x strain for a plane strain state, using the Finite Element Method, both for saturated and unsaturated embankments, considering an elastic model and the effect of suction (and its variation), through a stiffness variation technique. The present research used UNSTRUCT to assess the behavior of the embankment of a hypothetical earth dam, varying the features of its typical cross-section: homogeneity; internal drainage structures; geometry along a longitudinal profile; and the influence of the compaction level. The results showed that the soil responded better to strains for the scenario of better compaction along the dry branch of the compaction curve, as well as with the use of a vertical drainage element. It was also observed that the strains were higher when greater sections considered, which, in a real situation, would cause distortions along the longitudinal profile of the dam. The software UNSTRUCT proved to be efficient in obtaining and visualizing the modeling results, also making it possible to analyze the development of failure over time after the dam was filled. With UNSTRUCT, a more realistic analysis of the behavior of the hypothetical dam – considering factors that are specific to unsaturated soils – was possible. Worth mentioning also that the software allowed the appraisal of multiple changes in the dam’s geometry and parameters.

Water retention characterization of non-woven geotextiles: An application for wicking materials

Non-woven geotextiles are commonly used in soil embankments for separation, filtration, and drainage purposes. While these conventional geotextiles can effectively drain gravitational water from saturated embankments, they often struggle to drain capillary water when the embankments are unsaturated—a typical state during their service life. This inefficiency can lead to water accumulation, potentially resulting in moisture-induced damage. To overcome this, a new geotextile, termed "wicking non-woven geotextile," has been introduced. This material is engineered to drain capillary water from unsaturated embankments more effectively. This paper aims to offer a thorough review of the unsaturated behavior of geotextiles, discussing characterization methods and experimental techniques. Furthermore, we will present the results of a capillary rise test, which will help determine the geotextile-water retention curves (GWRC) for a wicking geogrid composite

Analytical method for evaluating the tensioned membrane effect in the geosynthetic-reinforced unsaturated embankment soil subjected to localized sinkholes

In karst regions, the occurrence of sinkholes is a common cause of roadway failures, leading to significant embankment deformation and instability. The advantages of geosynthetic reinforcement have been proven in practice in numerous countermeasures to lessen the impact of localized sinkholes, and associated design methodologies have been developed. However, some existing design methods assume there is no deformation in geosynthetics embedded in soils, and the effect of matric suction in soils is not considered either. This study suggested an analytical approach that took into account the strain in the geosynthetic reinforcements as well as the impact of matric suction. The developed design method was used to conduct parameter studies under various matric suction conditions. The method was validated against the experimental results, and the feasibility of the developed method was also compared with the existing design methods. The findings demonstrate that, in comparison to the method created here, existing design methodologies produce more conservative results. The strain and the deflection of the geosynthetic calculated using the suggested method are lower than those provided by the current design methodologies.

Numerical investigation on the effect of initial water content on dynamic responses of unsaturated sandy and clayey embankments

High water contents and poor compaction during construction have been reported to cause severe embankment damage during earthquakes. In this paper, a fully coupled soil-water-air finite element method is adopted to reveal the failure mechanism of unsaturated embankments subjected to earthquakes. And the dynamic responses of sandy and clayey unsaturated embankments with different water contents and dry densities are carefully investigated. It is found that the sandy embankments compacted above the optimum water content (OWC) during construction have a sliding potential triggered by liquefaction during earthquakes, while the sandy embankments compacted at or below the OWC only show slight subsidence due to the contribution of suction to soil strength. The stability of the clayey embankments worsens with increasing water content. The clayey embankments with a high water content exhibit substantial subsidence at the crest and bulging at the laterals, which are different from the failure mode of sandy embankments during earthquakes.

Reliability analysis of compacted embankment with geocomposite under infiltration

Embankment failures can be prevented by introducing geocomposites to act as drains. The effect of the geocomposite layer on the pore pressure distribution and surface displacements of an unsaturated embankment upon infiltration has been studied numerically using deterministic and probabilistic approaches. The inclusion of the geocomposite layer leads to an increase of suction below the interface and a decrease in suction above it by functioning both as a capillary barrier and a drainage layer, thereby reducing the surface displacements upon infiltration. The load in the form of rainfall and the resistance such as suction of the embankment material being variable leads to a variability in the displacements; therefore, reliability analysis has been carried out using hydraulic permeability and soil water characteristic curve (SWCC) parameters as random variables. To assess the probability of failure (Pf), a surrogate model based on augmented radial basis function has been used. Probabilistic analysis revealed that the embankment with geocomposite has less Pf compared to the embankment without geocomposite considering the rainfall infiltration. Moreover, sensitivity analysis predicted that SWCC parameters influence the Pf of embankment containing geosynthetics under infiltration to a larger extent.

Evaluation of climate change on the collapse potential of unsaturated cement-treated laterite soil for disaster risk reduction

Extreme weather events and intense rainfall may alter the climate, which would probably affect the geotechnical constructions such as unsaturated embankments. Basically, soil moisture content determines the strength of the unsaturated soil, with wetter soils often being weaker. Although it has been proved that unsaturated condition substantially impacts the shear strength and volumetric behaviour of soil, its implications are rarely investigated or taken into account in the design. As a result, changes in temperature and rainfall loads will have an influence on geotechnical constructions and develop long-term seasonal deformations that might severely jeopardize safety and maintenance. Therefore, it is crucial to assess the effects of the climate on soil behaviour for each location through adequate geotechnical laboratory tests. Johor, Malaysia has a large area and abundant tropical soils. Hence, this study aimed to elucidate the influence of climate change on soil behaviour in the tropical regions of Johor. To impose Malaysia's climate, a series of modified suction-controlled oedometer tests are conducted under different matric suctions. The outcomes revealed that the low and high matric suction has significantly impacted the untreated and cementtreated soil. However, the great reduction of soil settlement is mostly from the coupling effect of saturation and stabiliser.

Bearing capacity of shallow footing on an unsaturated embankment upon infiltration

Due to rapid urbanization and population growth, construction activities have been increased on slopes of unsaturated hilly regions. Thus, constructing shallow footings on these slopes is a common method for supporting infrastructure construction. The main factor causing instability of these unsaturated slopes is the loss of suction upon infiltration resulting in footing failures. Thus, in this study, a shallow footing restingon an unsaturated embankment modelled using Barcelona basic model (BBM) has been numerically analysed to investigate the influence of various factors affecting the bearing capacity upon infiltration. The influence of various critical design parameters, like the distance from the crest of the slope and water table positions, slope angle, and infiltration rate, has been studied comprehensively. As the footing distance increases from the crest (setback distance), soil provides higher bearing capacity upon infiltration due to the confinement. Moreover, it is noticed that the bearing capacity reduces monotonically as the water table rises above the toe, thus depicting the Prandtl-type of failure. Further, as the slope angle increases, bearing capacity decreases at various footing distances upon infiltration. The effect of infiltration rate on bearing capacity of footings depends on the air entry value of the soil. As the air-entry value increases, bearing capacity reduces drastically upon infiltration. This approach helps the design engineers consider these factors while constructing footings on unsaturated slopes.

Non-intrusive reliability analysis of unsaturated embankment slopes accounting for spatial variabilities of soil hydraulic and shear strength parameters

Existing reliability analyses of embankment slopes usually considered the spatial variabilities of shear strength parameters and saturated hydraulic conductivity separately. Additionally, the variability in the soil–water characteristic curve (SWCC) was ignored. A non-intrusive approach is proposed for reliability analyses of unsaturated embankment slopes under four different cases on a steady-state seepage analysis. The spatial variabilities of hydraulic parameters (including fitting parameters, a and n, of SWCC and saturated hydraulic conductivity) and shear strength parameters are accounted for simultaneously. To illustrate the proposed approach, a hypothetical embankment under unsaturated seepage is investigated. The soil parameters of the embankment and foundation are modeled as lognormal random fields and lognormal random variables, respectively, to take into account the inherent uncertainties. Parametric sensitivity studies are then performed to account explicitly for the influence of the spatial variation of hydraulic parameters on the reliability of embankment slope stability. The proposed approach can act as a practical and rigorous tool for estimating the reliability of embankment slopes with small levels of probability of failure (i.e., 10−4–10−3). An interesting finding that the probability of failure of the unsaturated embankment slope decreases with the variability of the fitting parameter n of SWCC is observed and explained.

An Approach to Assess the Stability of Unsaturated Multilayered Coastal-Embankment Slope during Rainfall Infiltration

This study aims to develop a simple but effective approach to investigate the stability of an unsaturated and multilayered coastal-embankment slope during the rainfall, in which a Random Search Algorithm (RSA) based on the random sampling idea of the Monte Carlo method was employed to obtain the most dangerous circular sliding surface, whereas the safety factor of the unsaturated slope was calculated by the modified Morgenstern–Price method. Firstly, two typical distributions of matric suction were illustrated and the associated methods for determining the strength parameters of unsaturated soil were developed. Based on this, the Morgenstern–Price method was further modified to calculate the safety factor, and RSA was adopted to locate the most dangerous sliding surface of the unsaturated multilayered coastal-embankment slope. Finally, the slope breaking process under rainfall infiltration was simulated through continuously searching the critical slip surfaces under different groundwater levels by RSA. The results indicated that the stability of the unsaturated embankment slope was gradually deteriorated with the increase of rainfall infiltration. It was also found that both of the distributions of the matrix suction (ua-uw) and the suction angle (φb) had significant effects on the safety factor of the embankment slope. Basically, linear distribution of (ua-uw) along the depth and linear relationship between φb and (ua-uw) should be adopted in assessing the stability of the unsaturated multilayered coastal-embankment slope.

Influence of Compaction Moisture Content on Wetting-Induced Settlement of Embankments

This paper presents a numerical investigation on the wetting-induced settlement of embankments using a finite difference program. The unsaturated compacted fill was simulated using the Barcelona Basic Model. A field unsaturated embankment subjected to full saturation was simulated as the worst case scenario for embankment design. Simulation results indicate that the embankment fill undergoes settlement as a result of wetting, which could lead to differential settlement between the embankment approach and bridge abutment, and cause damage to the approach slab. The influence of compaction moisture content on the wetting-induced settlement of embankments was also investigated. Results indicate that even though embankments with higher compaction moisture content could result in initial larger settlement under self-weight compression, the potential for subsequent wetting-induced settlement could be significantly reduced.

Effect of Drain for Dynamic-stability Enhancement of Unsaturated Road Embankment

Effect of Drain for Dynamic-stability Enhancement of Unsaturated Road Embankment

Dynamic Deformation Behavior of Unsaturated Road Embankments Subjected to Seepage Flow

Dynamic Deformation Behavior of Unsaturated Road Embankments Subjected to Seepage Flow

Analysis of the effect of groundwater level on the seismic behavior of an unsaturated embankment on clayey ground

Numerous river levees on clayey soil grounds were damaged by the 2011 off the Pacific coast of Tohoku Earthquake. In order to investigate such damage, the behavior of an unsaturated embankment on clayey ground was simulated during its construction, during an earthquake and after the earthquake. The simulation was carried out using a soil–water–air coupled finite deformation analysis code, with attention being focused on the effect of groundwater level. The results indicated that if the groundwater level is high, a saturated area (settlement-induced saturation area) is formed at the base of the embankment due to penetrative settlement during/after construction. In addition, the mean skeleton stress is low compared with the low groundwater level. As a consequence, in the embankment on ground with the high groundwater level, the co-seismic deformation is greater and the mean skeleton stress decreases sharply, particularly in the settlement-induced saturation area during the earthquake. It was also shown that after the earthquake, the groundwater level rises because water flows toward the unsaturated embankment from the settlement-induced saturation area and/or the saturated clayey ground. If the groundwater level is high, in particular, a phreatic line is formed temporarily within the embankment.

Preliminary research on the theory and application of unsaturated Red-layers embankment settlement based on rheology and consolidation theory

The settlement of an embankment body is always a significant technological issue in highway and railway construction. The embankment body is usually constructed using unsaturated soil, whose deformation consists of three parts, instantaneous deformation, primary-consolidation deformation and secondary-consolidation deformation. The primary consolidation and the secondary compression deformations are not two separate processes, but they influence each other, that are the deformation of the embankment body is the coupling-effect result of unsaturated-rheology and consolidation. Based on the results of uniaxial compression creep test of crushed Red-layers mudstone soils and the principle of the single stress state variable in unsaturated soils, the theory of unsaturated embankment settlement of Red-layers is put forward. Physical centrifugal tests and numerical simulation of centrifugal loading process are adapted not only to study the relationships of the settlement of the embankment body filled with different compaction coefficients versus embankment filling height, as well as the settlement after construction versus time, but also to predict the settlement after construction. Based on these studies, a project case, the settlement characteristics of a section of the railway line for passenger traffic from Chongqing to Suining have been studied by numerical simulation considering actual filling process. These show that the physical centrifugal tests and site monitoring data largely coincide with the corresponding numerical simulation results, showing the high validation of the proposed coupling-theory.

Soil-water-air coupled seismic behavior accompanying internal water level variation of an unsaturated embankment with an enclosed saturation area on cohesive soil ground

Soil-water-air coupled seismic behavior accompanying internal water level variation of an unsaturated embankment with an enclosed saturation area on cohesive soil ground

Study of dynamic stability of unsaturated embankments with different water contents by centrifugal model tests

It has been pointed out that the damage to unsaturated embankments caused by earthquakes is attributed to the high water content brought about by the seepage of underground water and/or rainfall infiltration. It is important to study the effects of the water content on the dynamic stability and deformation mode of unsaturated embankments in order to develop a proper design scheme, including effective reinforcements, for preventing severe damage. This paper presents a series of dynamic centrifugal model tests with different water contents to investigate the effect of the water content on the deformation and failure behaviors of unsaturated embankments. By measuring the displacement, the pore water pressure and the acceleration during dynamic loading, as well as the initial suction level, the dynamic behavior of unsaturated embankments with an approximately optimum water content, a higher than optimum water content, and a lower than optimum water content, are discussed. In addition, an image analysis reveals the displacement field and the distribution of strain in the embankment, by which the deformation mode of the embankment with the higher water content is clarified. It is found that in the case of the higher water content, the settlement of the crown is large mainly due to the volume compression underneath the crown, while the small confining pressure at the toe and near the slope surface induces large shear deformation with volume expansion.

함수비에 따른 불포화 도로성토의 동적 안정성 평가

함수비에 따른 불포화 도로성토의 동적 안정성 평가

Water characteristic curve of soil with bimodal grain-size distribution

Soil–water characteristic curve (SWCC) is the most fundamental and important soil property in unsaturated soil mechanics. It has been used for analyzing slope stability due to the infiltration of rainfall into slopes and water flow in unsaturated embankments. Generally, SWCC is obtained by laboratory tests. However high cost, long duration and difficulty of the tests impede the application of unsaturated soil mechanics to practical design or analysis. Therefore, several equations have been developed to predict the SWCC using grain-size distribution (GSD) curve. However, most of the equations were limited to soils with unimodal characteristics and the parameters of the equations are not related to the physical properties of the soil. In this paper, an equation to predict SWCC for soils with bimodal characteristics is proposed. The parameters of the proposed equation are related to the physical properties of soil and the variables of SWCC closely. The proposed equation is evaluated with data from the literature and laboratory tests carried out in this study. In addition, the computer codes for the computation of the predicted bimodal SWCC are presented.