The fabric evaluation of two Indian bentonites subjected to different suctions and consolidation pressures

Abstract

The study highlighted the fabric evaluation (i.e., pore size distribution (PSD) and morphology) of monovalent and divalent Indian bentonites, subjected to different suctions and consolidation pressures. Oedometer tests were conducted on bentonites specimens in two different initial conditions, i.e., slurry and compacted conditions. The slurry and compacted specimens were prepared at water contents equal to liquid limit (LL) and optimum moisture content (OMC), respectively. The water retention properties of the bentonites were determined in both unconfined (slurry) and confined (compacted) conditions using pressure plate apparatus and WP4C dew-point potentiameter. The influence of different consolidation pressures and suctions on pore structure and morphology of the bentonites were investigated by mercury intrusion porosimetry (MIP) and Field Emission Scanning Electron Microscope (FESEM) observations. For both the bentonites, the bimodal pore size distribution was observed over the applied consolidation pressure between 7.2 kPa and 2289.6 kPa in both slurry and compacted specimen conditions. However, an increase in consolidation pressure lead to a significant decrease in macropore volume. The compression index of the bentonites was found to be strongly correlated with a percentage decrease in macropore intruded volume (R2 = 0.91) and total intruded pore volume (R2 = 0.89), however, independent of micropores volume. The FESEM images and MIP results evidenced that for both the bentonites more homogeneous morphology and effective filling of the pores took place in slurry conditions as compared to initially compacted specimens. For the same void ratio, the hydraulic conductivity of slurry specimens was noted to be slightly lower than the compacted specimens. In slurry conditions at suctions below 3 MPa, the water retention capacity of monovalent bentonite was noted to be higher than that of divalent bentonite, whereas the opposite trend was found in compacted conditions. Irrespective of slurry or compacted conditions, the increase in suction lead to the transformation of bimodal PSD into a unimodal pore family for divalent bentonite, whereas trimodal to unimodal PSD was noted for monovalent bentonite in compacted specimens. The initial state (slurry or compacted) and the boundary condition of the specimen, as well as fabric (pore size distribution and morphology) of the bentonites, were found to have a notable influence on the compressibility, hydraulic conductivity and water retention properties.