An experimental study of stress-dependent water retention behaviour of two lateritic clays with different minerals

Abstract

Lateritic soils are associated with a high content of sesquioxides, which enhance the aggregation of fine particles. The formation of aggregates can induce significant changes in the pore size distribution (PSD) and consequently affect the hydro-mechanical behaviour of lateritic soils. The influence of mineral types on the extent of aggregation and the stress-dependent water retention behaviour of lateritic clays is not well understood. In this study, the stress-dependent water retention curves (SDSWRCs) of a compacted lateritic sandy fat clay with halloysite (i.e., GLH) obtained from Ghana were measured at different net vertical stresses using an improved stress-controlled pressure plate apparatus. The results were then compared with those of another lateritic sandy lean clay with goethite (NLG) obtained from Nigeria. Additionally, scanning electron microscopy (SEM) was employed to examine the microstructure associated with these soils. The experimental results show that GLH has a higher ability to retain water than NLG at any given suction, even though the former has a higher void ratio than the latter prior to the drying-wetting cycle. This is because of the presence of relatively small aggregates and small-size macropores in GLH, as revealed by SEM images. On the other hand, the water retention capacity of both soils reduces with elevated stress levels, except at net vertical stress of 120 kPa for GLH, with a corresponding decrease in the rate of desorption. Furthermore, the overall increase in the total degree of hysteresis measured is almost 2 times higher for NLG (i.e., 116%) than GLH (i.e., 72%) in the stress and suction range considered, contrary to the decreasing trends commonly reported in the literature. It was found that the sizes of aggregates in GLH reduced (‘finer soil’) as stress level increased and caused more water to be retained. Conversely to GLH, the aggregate size remained unchanged for NLG, as revealed by aggregate size analysis. This is mainly because of the differences in the strengths provided by the unique minerals in the two soils (i.e., halloysite for GLH and goethite for NLG) to the aggregates, due to the different parent rocks from which the soils were formed.