A Study on Pore Size Distribution of Compacted Expansive Soils

Compacted expansive soils, characterized with very low hydraulic conductivity and good contaminant retention capacity, have been widely used as barriers in landfills. They exhibit a double porosity structure with discrete interaggregate pores (macropores) and intra-aggregate pores (micropores) when compacted at optimum and dry of optimum water contents. The distribution of these macropores and micropores varies for different expansive soils depending on their grain size distribution and compaction characteristics, and thus, an in-depth study is necessary. This paper focuses on pore size distribution analysis using X-ray computed tomography (X-ray CT) and mercury intrusion porosimetry (MIP) tests on four expansive soils collected from different parts of Tamil Nadu, India. X-ray CT test gave the 2D image slices from top to bottom for all the specimens, and the acquired CT images of each soil specimen were segmented to separate the pores from the soil solids. The most probable threshold numbers for image segmentation were obtained using a newly developed methodology. The threshold numbers obtained decreased with increase in coarser fractions present in the soils. The thresholded binary images illustrated the pattern of larger pores in different expansive soils considered for the study. The MIP results showed a lower volume of macropores and a higher volume of micropores for soils with more clay content and higher dry density. A general insight into the range of macropores and micropores size distribution of compacted expansive soils with different gradation and compaction characteristics was achieved.