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Title: Mechanical behaviour and modelling of unsaturated soils
Author: Zeljko, Cabarkapa
ISNI:       0000 0004 2696 0405
Awarding Body: London South Bank University
Current Institution: London South Bank University
Date of Award: 2001
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Interest in unsaturated soil has dramatically increased in recent years and considerable efforts have been made to develop a better understanding of the mechanical behaviour of unsaturated soils in terms of elastoplastic critical state constitutive models. Experimental work has also increasingly focused on the mechanical behaviour of this class of geomaterials but in this case it is the response at intermediate and large level of strains that has been the focus of interest. This is partly due to the need to combine several complex techniques simultaneously, with compatible levels of accuracy, in which established unsaturated soil testing techniques have to be combined with the most advanced techniques recently developed for saturated soils. The objective of the research is to conduct an experimental programme that would fill a gap in the existing knowledge of small strain behaviour of unsaturated soils. The research program consists of testing saturated and unsaturated silt that can be prepared either by compaction or by desaturation of initially saturated samples. From the experimental point of view, the mechanical behaviour of unsaturated soils is studied in this research using controlled stress path triaxial cells. The design of a new triaxial apparatus requires a reliable method for controlling or measuring matric suction, development of an accurate procedure to monitor the volume change of the specimen and an adequate system for stiffness measurement. Three new triaxial apparatus are designed and developed for unsaturated soil testing with simultaneous control of the stress and suction, as well as modification carried out to perform dynamic stiffness measurements - bender element tests. In all laboratory soil testing a major concern is the internal state of the soil sample. It is commonly assumed that the soil sample is uniform for the purposes of interpreting the experimental results and deriving soil properties. However, any tests involving the flow of water into or out of the soil sample must inevitably lead to an internally non-uniform state and this is particularly pronounced in unsaturated soil because of the presence of air in the soil sample. The STOMP simulator (Subsurface Transport Over Multiple Phases) has been used in order to simulate isothermal and nonisothermal two-phase flow drying and wetting processes. The STOMP simulator solves coupled conservation equations for water mass, air mass and energy that describe subsurface flow over multiple phases. Computer models are now routinely used in research and practice to predict the movement of water and air into and through the unsaturated zone of soils. Such models can be used successfully only if reliable estimates of the flow and transport properties of the soil are available such as the soil water characteristic curve and the hydraulic conductivity. Traditionally, the soil water characteristic curve is determined in the laboratory using a pressure plate apparatus in which vertical or confining pressure cannot be applied. A new triaxial apparatus is developed to investigate the influence on the stress state on the soil water characteristic curve. Although soils encountered in engineering practice are often unsaturated, most of the geotechnical analyses are usually based on the saturated shear strength parameters. This kind of approach is conservative to varying degrees in that the influence of soil suction is often neglected. However, there are many examples in practice where suction is responsible for maintaining the stability of slopes. The Author made an attempt to develop a model for the shear strength of an unsaturated soil. The proposed description includes the non-linear shear strength of saturated soils and takes into account influence of mean net stress on soil water characteristic curve as well as the fact that angle of shearing resistance in respect to matric suction is not constant as is commonly assumed in the literature. The numerical model is validated against experimental results found in the literature and excellent agreement for the whole range of unsaturated soils including: clays, clayey sand, silt, till and sand, are achieved. The proposed model can be used in the limit equilibrium slope stability analyses as well as in the development of a constitutive model for unsaturateds oils.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available