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The thesis includes details of research carried out on various aspects, of the behavior of soils: ranging from its characterization to various analytical methods (including constitutive relationships), together with enhancement of engineering properties using ground improvement. The analysis of geotechnical properties is a crucial part of any geotechnical design and engineers should consider a careful approach, particularly when classifying the soil in term of its physical constituent such as index properties and particle size distributions. Chapter 2 of this thesis is dedicated to the author's contributions in relation to improved methods of measuring the plasticrty index of fine soils. It also highlights the effects of non-standard procedures adopted in the laboratory with respect to index properties and particle size distribution of fine grained soils. The chapter also includes a discussion of other recommended laboratory procedures with respect to suction measurements, acceleration of consolidation time, assessment of consolidation parameters and data collection using wireless technology. The construction industry is facing considerable problems in securing land for development for two different reasons: (a) soaring land prices in key areas, and (b) restrictions on greenfield sites for construction. The above difficulties have forced the construction industry to consider derelict and marginal land for developments and these sites are often treated with ground improvement methods prior to construction. Commonly used techniques include deep dynamic compaction, grouting, lime stabilization, granular columns and soil mixing. Chapter 3 is dedicated to some interesting experimental observations in relation to the employability of ground improvement techniques using granular columns for various ground conditions. The structure (caused by previous stress history, depositional environment, mineralogy, etc) is an important aspect of the soil that has significant influences on its stress-strain properties, permeability, strength and indeed the in situ stresses. Chapter 4 is dedicated to the assessment of anisotropy caused by various aspects, and its implications on the stress-strain characteristics. Chapter 4 also highlights some constitutive frameworks with respect to pre-yield pressure-volume characteristics and proposes a logical and rational relationship for the earth pressure coefficient at rest, Ko. This chapter also reports some interesting observations in relation to long term strength of overconsolidated clays. In many geotechnical designs, the soil is assumed to be saturated. However the soils in arid and semi-arid regions are often unsaturated, and indeed the same applies to deposits in wet regions, largely due to rapid changes in the climatic conditions associated with prolonged dry summers and wet winters. In addition, most civil engineering projects involve the use of compacted fills in various forms. These fills are placed in unsaturated conditions. The behavior of unsaturated soils is complex, and it cannot be evaluated using the classical theories which are developed for saturated soils. Chapter 5 of this thesis is dedicated to an in-depth approach to modeling the behavior of unsaturated soils which includes: constitutive framework, laboratory techniques and practical implications of using compacted fills for construction purposes.

An innovative technique of electro osmosis coupled with vertical surcharge loading to accelerate the consolidation and stiffen Kaolin (China Clay Grade E) was investigated in this study. The geotechnical properties of this China Clay Kaolin Grade E and the design of electro osmotic consolidation chamber are discussed together with an explanation of the procedural concept of the electro osmotic consolidation chamber (i.e., the preparation of the apparatus and the clay sample, assembling of the electro osmotic consolidation chamber; and the experimental work). The plastic limit, liquid limit and plasticity index were 35%, 53% and 18% respectively. Therefore, China Clay Kaolin Grade E is classified as MH soil, and it is predominantly a silt with high plasticity. The specific gravity of the soil is 2.65. To ensure the kaolin is saturated, all samples were prepared in a similar manner with deaired water to produce a slurry at 150% of the liquid limit (initial moisture content of 79.5%). The electro osmotic consolidation chamber was cylindrical and consisted of the body, the base and the top cap. The body and the base of the chamber were constructed of polyvinyl chloride (PVC) tube with a wall thickness of 10.9 mm, 345 mm high and 251 mm inner diameter. The electro osmotic consolidation chamber was assembled together with a 45 mm thick flange and collar. The top cap of this chamber was based on that of a Rowe cell of similar diameter. Twenty one tests were performed in this study with an applied voltage and one test was a control test. The test samples in the twenty one tests were all consolidated to three different phases. In Phases 1 and 2, the samples were consolidated at 15 kPa while in the Phase 3, 50 kPa was used. The electro osmotic process was only performed during Phase 2. The time of treatment, numbers of electrodes, the arrangement of electrodes, and the applied voltages were investigated in these tests. Results from these tests indicated that the China Clay Kaolin Grade E in a 79.5% slurry form responded well to electro osmotic treatment and that electro osmotic process increased the overall stiffness of the soil as indicated by the reduced relative settlement in Phase 3 with a pressure of 50kPa. The water content around the anodes was less than that at the cathode creating zones of higher average constrained stiffness. The tests demonstrated that the longer the time of treatment, the greater the numbers of anodes, the shorter distance between the electrodes and the higher the applied voltages associated with electro osmosis increased the average stiffness of the soil mass confirming the concept of an electro osmotic pile.

Theoretical and experimental investigations have been carried out to study the influence of stress path on the deformation and consolidation of over-consolidated clays - with particular emphasis on London clay - in relation to settlement of structures. It is shown that for a proper understanding of the deformation of a soil beneath a foundation the soil should be tested in the laboratory under the same set of effective stresses that it will undergo in the field and that the influence of lateral stresses cannot be ignored. The stresses and displacements in non-homogeneous soil media, beneath circular and strip footings, have been calculated from Gibson's analytical solutions. A numerical method is suggested for determining the "immediate" (elastic) settlement of structures founded on a medium whose modulus of elasticity varies with depth. The influence of lateral stresses on the deformation characteristics of undisturbed London clay has been studied from both oedometer and triaxial tests while the stress path for one-dimensional compression is determined from specially designed oedometers. The effect of small pressure increments on the compressibility of London clay is also studied in the oedometer. The experimental results are examined in the light of their influence on the settlement of structures and a method of settlement analysis is proposed that takes into account the stress path of the elements of soil beneath a foundation: comparisons are made with the existing methods of analysis. The pre-consolidation pressures of London clay are determined from the stress deformation characteristics of samples loaded to high effective stresses.

La présence de carrières souterraines abandonnées dans le nord de la France, à proximité de zones urbanisées, est à l’origine de problèmes importants touchant à la sécurité des personnes et des constructions, d’une part, et à l’élaboration de plans d’urbanisme d’autre part. Dans ce cadre, cette thèse présente une étude numérique de comportement mécanique et stabilité de carrières souterraines creusées il y a longtemps dans la craie du nord de la France. Elle a pour objectif de proposer une approche complémentaire aux observations et investigations naturalistes. Ces dernières confirment la présence de familles des joints stratifiées dans la craie du nord de la France et d’autres fractures qui se développent due à la dégradation. Ainsi, le premier objectif est de rendre compte du comportement anisotrope induit par la présence de ces joints. Un critère de rupture orienté est développé et différents mécanismes de plasticité sont considérés pour générer cet effet. Une modélisation numérique tridimensionnelle d’une excavation est mise en œuvre. Une procédure de réduction des paramètres de résistance au cisaillement est proposée pour les joints afin d’étudier leur influence sur les mécanismes de déformation et de rupture et sur la stabilité. Les effets du temps et les mécanismes de dégradation induits par différents facteurs aggravants ont aussi un rôle important. Une approche de dégradation dépendante du temps a été développée pour l’analyse des mécanismes de déformation et de rupture en fonction du temps et pour l’estimation de la stabilité à long terme. Enfin, pour l’estimation préliminaire de la stabilité des carrières, des formules empiriques et des abaques sont proposés. / The presence of unexploited underground cavities in the region of North France regarding their close proximity to urbanized areas, poses serious problems in terms of security of both people and constructions on one hand, and induces a strong constraint against the planning and urban development on the other hand. In this framework, this thesis presents a numerical study of the mechanical behaviour and the long term stability of underground cavities excavated in the chalk substratum of North France. It aims to provide a complementary understanding to geological investigations and observations carried out. This latter shows that stratified joints are present in the chalk layers as well as fractures developing due to the degradation of chalk with time. Hence, the first target is to account for the anisotropic behaviour induced due to the presence of these joints. An oriented yield criterion is developed where several mechanisms of plasticity are considered to generate this effect. A three dimensional numerical model of the excavation is executed. An approach for reducing shear strength properties of the joints is proposed to study the joints influence on the strain and failure mechanisms and on the stability state of the cavity. Degradation mechanisms induced upon time in the presence of different aggravating factors create also a critical effect. A non-homogeneous time-dependent-degradation model is developed to analyse the time-dependent deformation and failure mechanisms and to assess the long term stability of these cavities. At the end, empirical formulae and abacuses are developed for a preliminary estimation of the stability of underground cavities of North France.