Microstructure Study on Ageing Effect of Expansive Soil

Abstract

The safe disposal of the nuclear waste is one of the important criteria for the nuclear power stations. Every station has its own disposal site. Bhabha Atomic Research Centre (BARC) operates this waste disposal system in India. The latest technology uses canisters and engineered barriers for the safe disposal of the High Level radioactive Waste (HLW). After many experimentations, most of the researches preferred that expansive clay is best suitable for engineered barrier, due to its special characteristics such as, swelling behaviour, low hydraulic conductivity, water retention properties etc. The present study is the investigation of ageing effect on the bentonite of different compacted dry densities under different suction pressures. Water equilibrium technique is adopted for equilibration of samples using salt solutions. The water retention properties of compacted dry densities 1.3, 1.5, and 1.7 gm/cc of bentonite under the suction pressures in the range of 4 - 82 MPa were compared. Water holding capacities of these densities are almost identical. Suction developed within compacted specimen is independent of dry density. Microstructural observations of these samples were made using Scanning Electron Microscope (SEM) for 1, 60, and 90 days. By maintaining constant moisture content using desiccators for 90 days, the particles within the sample are become homogeneous with time. Gap between the clay plates is increased due to gradual introduction of water molecules into it. This results in the increase of intra aggregate porosity within the particle and decrease in inter aggregate porosity leading to decrease in permeability. The conclusion obtained from SEM results is only approximation, the accurate one can be obtained from other advanced techniques like Mercury Intrusion Porosimetry (MIP) test.