Fast Determination of Soil Behavior in the Capillary Zone Using Simple Laboratory Tests

INE/AUTC 13.1

Development of Landslide Warning System

Landslides cause approximately 25 to 50 deaths and US$1 - 2 billion worth of damage in the United States annually. They can be triggered by humans or by nature. It has been widely recognized that rainfall is one of the major causes of slope instability and failure. Slope remediation and stabilization efforts can be costly. An early warning system is a suitable alternative and can save human lives. In this project, an early warning system was developed for a 40-foot-high cut slope on the island of Hawaii. To achieve the objective, subsurface investigations were performed and undisturbed samples were collected. For the purpose of unsaturated soil testing, new testing apparatuses were developed by modifying the conventional oedometer and direct shear cells. The unsaturated soil was characterized using two separate approaches and, later, the results were discussed and compared. The slope site was instrumented for the measurement of suction, water content, displacement, and precipitation. The collected climatic data along with the calibrated hydraulic parameters were used to build an infiltration-evapotranspiration numerical model. The model estimations were compared with the field measurements and showed good agreement. The verified model was used to determine the pore-water pressure distribution during and after a 500-years return storm. Later, the pore-water pressure distribution was transferred to a slope stability software and used to study the slope stability during and after the storm. Based on a 2D slope stability analysis, the slope can survive the 500-year storm with a factor of safety of 1.20. Instrument threshold values were established for water content sensors and tensiometers using a traffic-light-based trigger criterion

Revisiting the thermodynamics of hardening plasticity for unsaturated soils

A thermodynamically consistent extension of the constitutive equations of saturated soils to unsaturated conditions is often worked out through the use a unique 'effective' interstitial pressure, accounting equivalently for the pressures of the saturating fluids acting separately on the internal solid walls of the pore network. The natural candidate for this effective interstitial pressure is the space averaged interstitial pressure. In contrast experimental observations have revealed that, at least, a pair of stress state variables was needed for a suitable framework to describe stress-strain-strength behaviour of unsaturated soils. The thermodynamics analysis presented here shows that the most general approach to the behaviour of unsaturated soils actually requires three stress state variables: the suction, which is required to describe the invasion of the soil by the liquid water phase through the retention curve; two effective stresses, which are required to describe the soil deformation at water saturation held constant. However a simple assumption related to the plastic flow rule leads to the final need of only a Bishop-like effective stress to formulate the stress-strain constitutive equation describing the soil deformation, while the retention properties still involve the suction and possibly the deformation. Commonly accepted models for unsaturated soils, that is the Barcelona Basic Model and any approach based on the use of an effective averaged interstitial pressure, appear as special extreme cases of the thermodynamic formulation proposed here

A FINITE ELEMENT MODEL FOR COUPLED DEFORMATION-FLOW ANALYSIS OF UNSATURATED SOIL-STRUCTURE SYSTEMS AND ITS VALIDATION

The unsaturated soil mechanics is one of the emerging fields that require extensive studies to understand its behavior under various loading and environmental conditions. Unsaturated soil consists of three bulk phases: solid, liquid and gas and three interfaces: solid-liquid, liquid-gas and gas-solid. It is generally accepted that the interaction among various bulk phases and interfaces has to be taken into account in the characterization of unsaturated soils. The behavior of soil-structure systems is complex and the complexity further increases when the structure is located in unsaturated soil. Numerical methods such as the finite element method are ideally suited for elucidating such complex behavior of unsaturated soil-structure systems. In recent years, various forms of finite element formulations and numerical tools have been developed for studying the behavior of unsaturated soils. Among these, TeraDysac, a framework based finite element software developed by Ravichandran and Muraleetharan is found to be an effective tool for analyzing soil-structure interaction in a fully coupled manner. This software consists of two decoupled codes: dysac and udysac. dysac is for the analysis of saturated soil-pile system and udysac is for the analysis of unsaturated soil-pile system. The original udysac code has simplified (reduced formulation) and complete finite element formulations. Although the complete formulation represents the real condition more closely, it is highly nonlinear and cannot be used for solving practical problems within a reasonable amount of computational time. On the other hand, the simplified formulation is computationally efficient and numerically stable. However, because the relative accelerations and relative velocities of both water and air phases are neglected at the governing equation level, its applicability to solve coupled mechanical-flow problems, is limited. Also the damping matrix does not naturally appear at the governing equation level, resulting in predicting unreasonably high accelerations. In this research, the simplified formulation is improved by incorporating a viscous damping model. The improved simplified formulation seems to predict the unsaturated soil-pile interaction response reasonably well, compared to the simplified formulation. As a major development, a partially reduced finite element formulation for coupled deformation-flow analysis of unsaturated soil-structure systems is developed and implemented in TeraDysac. Soil-Water Characteristic Curve (SWCC), which represents the moisture-suction variation of unsaturated soils, is one of the constitutive models necessary for numerical modeling of unsaturated soil systems. In this research, limitations of commonly used SWCC models such as the Brooks and Corey, van Genuchten, and Fredlund and Xing models are extensively analyzed and limitations/disadvantages are identified. Based on this and also to avoid the identified limitations, two new SWCC models are developed and presented in this dissertation. The capability of the new SWCC models in fitting the measured data of different types of soil is investigated. The comparisons show that the new models are effective and can be used to fit the experimental data well over the entire range of degree of saturation. The numerical stability and the performance of the new models in finite element simulations are investigated by implementing these models within TeraDysac and simulating both static and dynamic problems. These studies showed that the new models are numerically stable and effective in calculating the moisture-suction variation in finite element simulations. Permeability coefficients of fluids occupying the pore space of unsaturated soils greatly influence the deformation and flow behaviors of unsaturated soils. The permeability coefficient varies with degree of saturation or volumetric water content of the unsaturated soils. The other properties that affect the permeability coefficient are void ratio and particle/pore size distribution. Accurate evaluation of the permeability-degree of saturation or permeability-suction relationship is very important to study the coupled deformation-flow behaviors of unsaturated soils using numerical tools. However, experimental studies of coupled deformation-flow problems such as slope failure after rainfall, and contaminant transport will be time consuming and may require advanced equipments. As a result, experimental studies will not be an effective choice. The properties which affect the permeability coefficients also affect the soil water characteristic of unsaturated soils. Therefore, soil water characteristic curve models can be effectively used to calculate permeability-degree of saturation or permeability-suction variation. In this research, a simple mathematical equation is developed using the model parameters of S-R SWCC models for determining the permeability-suction variation. The predictive capability of the permeability model is verified by comparing with experimental data of eight different soils found in the literature. This proposed model is capable of predicting the relative permeability of water in unsaturated soil over a wide range of degrees of saturation. An effective coupled deformation-flow analysis finite element model for unsaturated soils, should consist of the following elements: (1) governing equations and corresponding finite element formulation that represent the physical phenomena of unsaturated soils more closely and capable of calculating deformation-flow characteristics in a fully coupled manner, (2) realistic and accurate constitutive model that represents the stress-strain behavior of unsaturated soil skeleton, (3) soil-water characteristic curve (SWCC) model that represents the moisture-suction relationship in unsaturated soils, and (4) permeability model that represents the flow of fluids in unsaturated soils. Upon successful completion of a finite element model development, the model must be validated against experimental measurements before using it as a viable tool. In this research, the finite element model is validated against experimental data obtained from a series of centrifuge tests; conducted at the University of Boulder, Colorado. The comparison of the numerical simulation results and the centrifuge measurements shows that the accuracy of the coupled deformation-flow analysis finite element model can be considered to be adequate for both elastoplastic and elastic simulations. Based on this research study, it can be concluded that the coupled deformation-flow analysis finite element model, which is implemented in TeraDysac, can be effectively used to analyze the elastic and elastoplastic behavior of unsaturated soils and soil-structure systems

Laboratory and numerical investigations of soil retention curves

Imperial Users onl

Détermination expérimentale de la contrainte effective pour les sols granulaires non saturés

Abstract: Understanding the stress distribution within a soil under different loading and deformation scenarios is necessary for any resistance-serviceability analysis in geotechnical engineering. In saturated soil, Terzaghi’s effective stress theory forms the basis for studying the stress distribution within the soil from the theoretical and practical viewpoints. For unsaturated soils, however, the presence of suction and its corresponding effect on hydro-mechanical behaviour of the soil complicates the efforts to choose a proper stress-state framework that is theoretically sound and experimentally applicable. Although the validity of available stress-state frameworks was the subject of many studies for unsaturated cohesive and fine materials, the unsaturated cohesionless and granular soils are less studied in the literature despite their extensive use in practice particularly in medium to high range of deformation. Much research is still required to understand how suction contributes to the behaviour of unsaturated soils with regards to the design and construction. The objective of this study is to perform a series of laboratory experiments including direct shear, triaxial and one-dimensional deformations tests on selected granular soils in unsaturated conditions. By performing an extensive literature review and using the obtained experimental results, the most appropriate stress-state framework for describing the unsaturated behaviour of granular soils will be found and evaluated. The modifications necessary for improving this stress-state framework are studied in this research. The major conclusions obtained from the current study include: (1) the hydro-mechanical behaviour of granular soils under the influence of suction is similar to fine materials. (2) the modified effective stress-state framework provides better interpretation of the hydro-mechanical behaviour of unsaturated granular soil compared to experimental stress-state frameworks (3) among modified effective stress approaches, a modified suction stress characteristics method can present more meaningful results given its simplified thermodynamic assumptions while having a more realistic microscopic definitionLa connaissance du régime de contrainte dans les milieux de sol sous différents scénarios de charge et de déformation est nécessaire pour toute analyse de résistance-entretien en géotechnique. En mécanique des sols saturés, la théorie de contrainte effective de Terzaghi constitue la base de l'étude du régime de contrainte dans le sol d'un point de vue théorique et pratique. Pourtant pour les sols non saturés, étant donné la structure naturelle du sol, la présence de la succion du sol et son effet correspondant sur le comportement hydromécanique du sol complique les efforts pour choisir un cadre d'état de contrainte approprié qui est théoriquement solide et expérimentalement applicable. Bien que la validité des cadres d'état de contrainte disponibles ait fait l'objet de nombreuses études pour les matériaux fins et cohérents non saturés, les sols non saturés sans cohésion et granulaires sont moins étudiés dans la littérature malgré leur utilisation extensive dans la pratique. De nombreuses recherches sont encore nécessaires pour comprendre comment la succion du sol contribue à améliorer la fiabilité des sols non saturés dans la conception et la construction. L'objectif principal de cette étude de recherche est d'évaluer l'application des cadres d'état de contrainte les plus couramment utilisés pour traiter le comportement hydromécanique des sols granulaires non saturés à l'aide de résultats expérimentaux. Les principales conclusions tirées de cette recherche sont les suivantes: (1) la nature du comportement du sol granulaire sous l'influence de la succion du sol est semblable à celle des matériaux fins, en particulier les sols limoneux, mais avec moins d'intensité et de complication (2) le cadre de contrainte effective modifié est un outil supérieur dans l'interprétation du comportement hydromécanique des sols granulaires non saturés par rapport au cadre des deux contraintes d'états indépendants (3) parmi les approches de contraintes effectives modifiées, un cadre de caractéristiques de contraintes de succion modifiées peut présenter des résultats plus pertinents étant donné ses hypothèses thermodynamiques simplifiées tout en ayant des définitions microscopiques plus réalistes

Conceptual development and numerical modelling of vegetation induced suction and implications on rail track stabilisation

The effects of tree roots on soil suction and ground settlement are investigated. This paper highlights the inter-related parameters contributing to the development of a conceptual evapo-transpiration and root water uptake equilibrium model that is then incorporated in a comprehensive numerical model. The developed numerical model based on the finite element analysis (ABAQUS) considers fully coupled flow-deformation behaviour of soil. Field measurements obtained by the authors from a field site in western Victoria and from past literature are used to validate the model. The predicted results show acceptable agreement with the field data in spite of the assumptions made for simplifying the effects of soil heterogeneity and anisotropy. The numerical analysis proves that the proposed root water uptake model can reliably predict the region of maximum matric suction away from the tree axis. The paper also compares the natural favourable effect of tree roots with the stabilising mechanisms of geosynthetic vertical drains subjected to vacuum pressure. Although this analogy is only justified for shallow vertical drains, the comparison still emphasises the obvious economical advantages of native vegetation

Behaviour of stiff clayey soils using fracture mechanics approach

Most of the conventional elastic plastic models of soils are based on continuum mechanics, however, for stiff, hard soils and soft rocks discontinuities develop under load, and since the models assume continuity, they would cease to apply. These discontinuities had not been accounted for in the continuum-based elastic plastic models. On the other hand, fracture mechanical theory may be used to advantage to replicate their behaviour. The behaviour of soil commonly is interpreted from conventional triaxial apparatus, whereas, testing of soil using the plane strain device would be more useful information, as more geotechnical field problems are basically occur in these situations.The present study has dealt with the investigation on the behaviour of saturated over consolidated clay as well as partially saturated clay, which represent the stiff and hard brittle clay by the use of a new biaxial device modified from conventional triaxial apparatus. In general, the apparatus was able to produce data which are in a good accordance with known soil behaviour of stiff clay. Shear band localization occurred in all test specimens of over consolidated clay. Specimen initiated to be discontinuous upon reaching the peak stresses. It is evident that specimen of partially saturated containing fissures had weaker shear strength as well as compressive strength.From point of view of the discontinuities that take places in the stiff clay, a model based on the unified model (Lo et al, 1996) and the elasto-plastic shear fracture model (Lo, et al, 2005) was used in this study. The problem may be dealth with one of brittle fracture of a three-phase specimen, where the matric suction is disrupted by tensile or shear loading. As a result the fracture toughness of the specimen would vary according to matric suction changes. A problem of plane strain compression testing was carried out to implement the model. The crack propagation simulation was resulted the same pattern with the experimental results on partially saturated kaolin clay

Finite element analysis of non-isothermal multiphase porous media in dynamics

This work presents a mathematical and a numerical model for the analysis of the thermo-hydro-mechanical (THM) behavior of multiphase deformable porous materials in dynamics. The fully coupled governing equations are developed within the Hybrid Mixture Theory. To analyze the THM behavior of soil structures in the low frequency domain, e.g. under earthquake excitation, the u-p-T formulation is advocated by neglecting the relative acceleration of the fluids and their convective terms. The standard Bubnov-Galerkin method is applied to the governing equations for the spatial discretization, whereas the generalized Newmark scheme is used for the time discretization. The final non-linear and coupled system of algebraic equations is solved by the Newton method within the monolithic approach. The formulation and the implemented solution procedure are validated through the comparison with other finite element solutions or analytical solutions

Soil-environment interactions in geotechnical engineering

The range of problems that geotechnical engineers must face is increasing in complexity and scope. Often, complexity arises from the interaction between the soil and the environment – the topic of this lecture. To deal with this type of problem, the classical soil mechanics formulation is progressively generalised in order to incorporate the effects of new phenomena and new variables on soil behaviour. Recent advances in unsaturated soil mechanics are presented first: it is shown that they provide a consistent framework for understanding the engineering behaviour of unsaturated soils, and the effects of suction and moisture changes. Building on those developments, soil behaviour is further explored by considering thermal effects for two opposite cases: high temperatures, associated with the problem of storage and disposal of high-level radioactive waste; and low temperatures in problems of freezing ground. Finally, the lecture examines some issues related to chemical effects on soils and rocks, focusing in part on the subject of tunnelling in sulphate-bearing rocks. In each case new environmental variables are identified, enhanced theoretical formulations are established, and new or extended constitutive laws are presented. Particular emphasis is placed on mechanical constitutive equations, as they are especially important in geotechnical engineering. The lecture includes summary accounts of a number of case histories that illustrate the relevance and implications of the developments described for geotechnical engineering practice