Microstructure evolution of a compacted lateritic silt upon drying–wetting and loading

Abstract

Lateritic soil usually contains a large amount of iron and aluminium, which promotes the formation of large-size aggregations and further affects soil fabric. It has been demonstrated that the microstructure of soil influences the hydraulic and mechanical behaviour significantly. As a commonly used filling material with special mineral composition, the microstructure change of lateritic soil under common environment actions, such as wetting and loading, is thus of great importance. In this study, the microstructure properties of a lateritic silt under drying–wetting and one-dimensional compression were investigated using mercury intrusion porosimetry and scanning electron microscopy. Test results show that large-sized aggregates are formed in the compacted lateritic silt with 10% of hematite. These aggregates induce a bi-modal pore size distribution (PSD) which includes both micropore and macropore peaks. The different macropore distributions under various dry densities lead to scattered soil–water characteristic curves in a suction range less than 20 MPa. Changes in volume and pore structure mainly occur in the suction path within the “transition zone” where the dominant capillarity affects only the macropore. Drying causes an increase in macropores due to the decreased volume of aggregates, while has limited effect on micropores, whereas wetting weakens the bonding between aggregates, and induces a reduction in macropores. The compacted lateritic silt shows obvious collapse upon soaking, in which the collapse grows up to a peak and then turns to decrease with increasing stress level. Both loading and soaking lead to the collapse of macropores, whereas soaking tends to affect a broader range of pore sizes. It is inferred that, the strong particle cementation produced by hematite leads to a strong micropore structure, which is almost unaffected by the suction change under both confined and unconfined conditions. The PSD change due to the variation in suction is different from that observed on other compacted fine-grained soils, which verifies the effects of special mineral composition (iron and aluminium) on microstructure and further the hydro-mechanical behaviour of lateritic soil.