Date of Award

5-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Civil Engineering

Advisor

Ravichandran, Nadarajah

Committee Member

Andrus , Ronald D.

Committee Member

Juang , Hsein C.

Committee Member

Khan , Abdul A.

Abstract

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.

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