ABSTRACT

It is difficult to locate a new infrastructure on a good ground due to the over-population in urban areas throughout the world. Renovation or retrofit of old infrastructures should often be carried out in close proximity of existing structures. Good quality material for constructions is becoming a precious resource to be left for the next generation. Due to these reasons and environmental restrictions on public works, ground improvement is becoming a necessary part of infrastructure development projects both in the developed and developing countries. This situation is especially pronounced in Japan, where many construction projects must locate on soft alluvial clay grounds, artificial lands reclaimed with soft dredged clays, highly organic soils and so on. These ground conditions would pose serious problems of large ground settlement and/or instability of structures. Apart from clayey or highly organic soils, loose sand deposits under the water table would cause a serious problem of liquefaction under seismic condition. When these problems are anticipated to violate the performance and function of the structure, the foundation ground is called a ‘soft ground’ and needs to be improved. The required performance and function of the ground are, however, different for different structures. It is not appropriate to define a ‘soft ground’ by its geotechnical characteristics alone, but by incorporating the size, type, function and importance of structure, and construction period. Only if the type of structure is specified it is possible to define ‘soft ground’. Table 1.1 provides a rough idea of ’soft ground’ for several types of structures in terms of water content, unconfined compressive strength, SPT N-value, ground thickness and bearing capacity (Japanese Society of Soil Mechanics and Foundation Engineering, 1988).