Land Subsidence in Central Calcutta

In this paper, two approaches have been mentioned to calculate subsidence. One is based on the linear theory (considering elastic property of the material) and the other on the logarithmic theory. In the linear theory, coefficient of volume compressibility (denoted as m_v) indicates the nature of soil towards compression, and in the logarithmic theory, compression index (denoted as C_c) indicates the same. The top 30 m of the subsurface soil stratification in Calcutta generally indicates softer clayey soil in the first 15 m having higher m_v values and relatively stiffer clayey soil between 15 m and 30 m having lower mv values. Further below, the compressibility of the layers diminishes due to increasing overburden pressure also. In the analysis section for subsidence in two localities in Central Calcutta, firstly, data of soil characteristics and properties in different layers and change in piezometric level or depth of water table from 1956 to 2000 are represented. Based on these data, intergranular pressures for different layers are calculated, and finally total subsidence in the said time span is estimated using the linear theory. This comes out to be 0.4932 m (average) for Central Calcutta region from 1956 to 2000, and hence, estimated average subsidence rate is 11.208 mm/year.