A comprehensive experimental study has been carried out to investigate the volumetric and shear strength behaviour of a compacted silty clay, which exhibited collapse-on-wetting behaviour. The experiments consisted mainly of suction-monitored triaxial tests, carried out in a new apparatus. The test results are interpreted on the basis of an elasto-plastic framework using the conventional stress variable approach (net stress and suction) as well as an approach that takes into account the degree of saturation within two modified stress variables. The evidence for critical states was examined, based on the stress–dilatancy relationships. It was found that a critical state was observed experimentally in cases where the samples contracted over much of their stress paths, but was not truly established in other cases. Relationships between the shear strength and specific volume at the ultimate state were established for samples over a wide range of degrees of saturation (from fully saturated to the air-dried state).

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The effects of Vetiver grass (Chrysopogon zizanioides) roots on the soil-water retention curves (SWRCs), permeability (k) function, and saturated permeability, ksat, have been investigated on clayey sand (SC) and low-plasticity silt (ML). For ML soil, when the root biomass per soil volume was lower than 6.5 kg/m3, the saturated permeability increased, the air-entry suction decreased slightly, and the SWRC became steeper with increasing root contents, probably due to the formation of cracks caused by wetting and drying cycles during the plant growing period. Nevertheless, roots appeared to decrease the saturated permeability and increase the air-entry suction of ML soil, after reaching this threshold with a root content of about 6.5 kg/m3 as roots occupied the macropores and tended to suppress cracks and swelling. For SC soil for all root contents, only a slight variation of the saturated permeability with root content could be observed for the upper bounds on saturated permeability. However, the lower bounds on saturated permeability appeared to decrease as root content increased. It has also been shown that for a suction range beyond 30 kPa, the influence of roots on permeability appeared to be less significant.