Lateritic soils often contain high content of iron and aluminium oxides (i.e., sesquioxides) which enhance the aggregation of the fine particles. Moreover, different mineralogy emerges depending on the environmental conditions under which these soils are formed. The formation of aggregates and the variations in mineralogy can induce significant changes in the pore size distribution (PSD) and consequently affect the hydro-mechanical behaviour of lateritic soils. Presently, the influence of sesquioxide content and mineral types on the aggregates of the lateritic soil and its hydromechanical behaviour is not well understood.

This study investigates the influence of sesquioxide contents and mineral types on the microstructure, stress-dependent water retention curve (SDSWRC) of two la...[ Read more ]

Lateritic soils often contain high content of iron and aluminium oxides (i.e., sesquioxides) which enhance the aggregation of the fine particles. Moreover, different mineralogy emerges depending on the environmental conditions under which these soils are formed. The formation of aggregates and the variations in mineralogy can induce significant changes in the pore size distribution (PSD) and consequently affect the hydro-mechanical behaviour of lateritic soils. Presently, the influence of sesquioxide content and mineral types on the aggregates of the lateritic soil and its hydromechanical behaviour is not well understood.

This study investigates the influence of sesquioxide contents and mineral types on the microstructure, stress-dependent water retention curve (SDSWRC) of two lateritic clays from Nigeria (NLG) and Ghana (GLH), and the compressibility and shear strength of the NLG. Mineralogical and chemical characterisation were carried out using x-ray diffraction (XRD) and x-ray fluorescence (XRF) respectively to distinguish both lateritic soils. Thereafter, the stress-controlled pressure plate apparatus was re-designed to measure their SDSWRCs. In addition, compression tests and constant suction shear tests were out on saturated and unsaturated samples prepared from the NLG. Results are interpreted within the context of elastoplastic theory and Extended Mohr Coulomb theory. The complementary microstructural studies included mercury intrusion porosimetry (MIP), Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) tests.

It was found that the water retention ability of the studied lateritic clays is lower at higher stress than at zero stress. This finding is not consistent with the commonly reported behaviour for other soils, where the retention ability increases with stress levels. The behaviour of the lateritic clays is mainly due to the higher content and state of the sesquioxides found in the soil in comparison with other soils. Furthermore, the GLH showed a consistent decrease in AEV with stress levels while the NLG showed a decrease in the AEV only at 40 kPa net stress, and the followed by an increase up to the highest stress considered. A direct relation was observed between content of sesquioxides, D_p and D_h for the weathered soils, which implies that hysteresis is strongly related to state of sesquioxides, hence, can be used to predict aggregation potential. The size of the hysteresis loop for GLH increased by 47% as stress increased from 0 kPa to 80 kPa and then decreased by 18% as stress reached 120 kPa. On the other hand, the NLG shows an increase of 200% as stress increased from 0 kPa to 200 kPa. This implies that variations in suction has a profound impact on the volume change on the studied clays, with the size of the loop indicating the reduction in shear strength contribution due to increase in suction. The different responses in AEV and the size of hysteresis were found to be as a result of the different responses of the aggregates in the soil. This is as a result of the differences in the hardness of the mineral types providing binding strength to the aggregates owing to the different parent rocks from which the soils were formed.

The shear test results showed that no unique relationship exists between cohesion (c’), Φ’ and Φ^b and suction levels. It was also observed that an optimum suction corresponding to an optimum S_r exist and need to be considered in engineering designs. Similarly, current elastoplastic models fail to explain the shear behaviour observed, mainly because the sesquioxides in the lateritic clay studied has caused the fine content to form aggregation into large-size particles as shown by the SEM images. Hence, pore size distributions are altered at both micro and nanoscale as revealed MIP and BET results. Thus, soil tends to show a granular behaviour at lower S_r, nevertheless, the aggregates tend to deform under compression and shear loading. Hence, distinct phase transformation was observed in the shear behaviour. Furthermore, the shape of the loading–collapse (LC) yield locus is shown to be different from the generally accepted form. Because, when a saturated and an unsaturated sample are compressed and shared, the aggregates were relatively destroyed in the former while they were preserved in the latter as shown by the SEM images. The preserved aggregates, however, appears not to contribute to shear resistance (Φ’). Hence, the Φ’ was observed to decrease with suction level, while c’ increases. This crucial finding of sesquioxides and mineralogy on behaviour of lateritic soils have not been considered in engineering designs so far.