Abstract:
As a result of the rapid increase in population and the development taken place in the city of Colombo and suburbs during the last few decades, lands with good
subsoil condition have been already utilized. The lands required for development
of industrial, commercial and residential facilities and development of
infrastructure facilities such as highways in Colombo and suburbs are presently
found mainly in low-lying areas where the sub soil conditions are not very
favorable for civil engineering structures. The geotechnical engineer is faced with the problem of finding an economical solution to this problem. This may involve improving the ground condition or designing suitable types of foundations, or both. A prominent feature of these low laying areas which are in the flood pains of main rivers and lakes is the presence of sub soils that are predominantly organic and possess unique physical and engineering properties quite different from those of inorganic soils. It has been revealed by the geotechnical investigations carried out by various institutions in the past, that these low laying areas have highly compressible peat and soft inorganic clays of thickness varying from lm to 15m. Peat encountered in these sites possesses organic contents in the range of 20% to 30%. As such, they should be referred to as peaty clays and not as peat. The most significant features of peaty clays are very high water contents and void ratios and low specific gravity. As a result they are of very low shear strength and high compressibility. They are characterised by the very high secondary consolidation settlements. The ground water table is almost at the ground level in most of the situations. These sub-soils clearly are susceptible to undergo very high settlements even under a small imposed load and to experience catastrophic shear failure.
