The engineering behaviour of the tropical clay soils of Dhaka, Bangladesh

Abstract

This research has evaluated the engineering behaviour of the tropical clay soils of Dhaka, Bangladesh. Attempts have been made to show the relationship of mineralogy and fabric with engineering properties. The engineering behaviour has been investigated by comparing the soil properties in a natural and destructured state at the same void ratio. Consolidation behaviour of the soils are discussed, based on oedometer and triaxial tests. Undrained and drained mechanical behaviour have been evaluated from triaxial tests in terms of stress-strain curves, stress paths, bonding effects, critical state conditions, stiffness and yielding behaviour. A framework for the tropical clay soils of Dhaka is presented. The tropical clay soils of Dhaka are intermediate to high plasticity inorganic clay. These soils are mainly composed of illite,. kaolinite, chlorite and some non clay minerals mainly quartz and feldspar. It was observed that these sods showed a random open microfabric of silt and clay. There was also some evidence that aluminosilicates, iron compounds and silica formed bonds between and within the grains. An apparent preconsolidation pressure of 170 kPa to 250 kPa was estimated for the natural soils, which is likely to be due to the bonded structure of the soils. The compressibility of the soil is very low to medium. The consolidation results are consistent with the mineralogy of the soils. It is established that the tropical clay soils of Dhaka are bonded. Bonding has an influence on the development of stress-strain and stiffness of these soils. Under undrained shearing, samples initially showed peak positive values of excess pore water pressure followed by negative values at higher strains due to the tendency of the samples to dilate. No negative pore water pressures were observed at high confining pressures. Only a few samples at low confining pressures reach the critical state at very large strains approximately in excess of 20%. High confining pressure samples may not have reached the critical state due to the formation of distinct shear surfaces. A significant difference between the natural and destructured failure surfaces was observed due to the presence of bonds in the natural soils. Differences in failure type were observed between the natural and destructured soils of three boreholes. It was observed that stiffness values gradually decreased with increasing strain. For the natural soils, two yield points could be identified at low confining pressures below the final yield. It was also observed that bond breakdown would occur in isotropic compression for tests at high confining pressures. At the final yield, the soil looses almost all of its stiffness due to bonding. After final yield, a soil’s behaviour is controlled only by friction. It was observed that three zones of behaviour could be identified for these soils in the stress space.