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.