ABSTRACT
The extensive Indo-Gangetic alluvial plains of Bengal, Bihar, Uttar Pradesh and Punjab fall within the region bounded by the Peninsula and the Extra-Peninsula of the vast Indian subcontinent. In the hilly regions of the state of West Bengal, sandstone, minor siltstone and shale are found in the plains of Darjeeling–Jalpaiguri districts followed to the north by a wide zone of slates, phyllites, carbonaceous beds (coal and coaly slates) and quartzites. The Terai zone is made of alternate layers of clay, sand, gravel and pebbles. Old igneous rocks, mainly granite and gneiss of the Archaean era and coal-bearing mudstone and quartzite rocks of the Carboniferous period, are found in the western part of this state with laterite soil in the plateau region. The Indo-Gangetic alluvial plain comprising the Bengal basin appeared during the Quaternary era. They are basically made up of sand, clay and peat beds. The subsoil profiles obtained from various exploratory borings suggest in favour of two distinct categories of subsoil, viz., one which is known as the normal Calcutta deposit and the other the newer river channel deposit or alluvial deposit. In some places, superposition of these two deposits is also encountered. The places of alluvial deposits are susceptible to liquefaction. The Sundarbans delta has been created by deposition of silt by its numerous rivers and the coastal fringe is made up of sand and mud deposited by rivers and wind. Pile foundations are suitable for construction of high-rise buildings in these regions. For other parts of this state, shallow foundations may be provided.
