Effects of sesquioxide content on stress-dependent water retention behaviour of weathered soils

Lateritic soils generally have high content of iron and aluminium oxides (i.e., content of sesquioxides), which are found able to enhance soil aggregation and therefore to alter pore size distributions. So far, however, there is no study on the influence of sesquioxides on the stress-dependent soil water retention curves (SDSWRCs) of unsaturated soils. In this study, SDSWRCs of compacted lateritic clay (LAT) were measured at various stress levels using a modified stress-controllable pressure plate apparatus. The results were compared with the SDSWRCs of two other weathered soils, including sandy silt (i.e., CDV) and a gravely sand (i.e., CDG). These three soils have different sesquioxide contents (i.e., 38% for LAT, 27% for CDV and 19% for CDG). Moreover, scanning electron microscopy (SEM) tests were carried out to analyse the microstructures of these three soils. At a given stress, it was found that the LAT had the lowest water retention ability among the three soils, even though it had the most clay particles. This is mainly because most clay particles in the LAT formed aggregates due to its high sesquioxide content, altering the pore size distribution. Consequently, the LAT specimen had many large-size inter-aggregate pores, as revealed by the SEM images. Furthermore, the influence of stress on the water retention ability of the LAT seems negligible in the stress range considered (0-120 kPa). On the contrary, the water retention ability of CDV and CDG increases with an increase in net stress, likely attributed to a reduction in average pore size under compression.