The effects of Vetiver grass (Chrysopogon zizanioides) roots on the soil-water retention curves (SWRCs), permeability (k) function, and saturated permeability, k_sat, 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.

This paper presents a methodology to investigate the collapse behavior of unsaturated soils using suction-monitored oedometer tests. By incorporating independent suction measurement, the oedometer apparatus is capable of following the same stress paths as in double oedometer tests, while continuously monitoring the suction. The proposed method has been used to investigate the collapse behavior of a compacted silty clay and to confirm the uniqueness of the loading-collapse surface as identified from loading and wetting paths. A new mathematical form of the yield surface within an elastoplastic framework is proposed on the basis of test results over a wide range of suctions (0 to 30,000 kPa ) and net stresses (up to 7,000 kPa ). The fundamental assumptions of the newer type of elastoplastic framework, which incorporate the degree of saturation within their stress variables, are evaluated, and the limitations of such models are identified. The collapse behavior of samples with different fabrics induced by di...

The effects of climate change including intense rainfalls have resulted in many slope failures. The bioengineering method using vegetation is an environmentally friendly alternative for slope stabilisation. This paper presents the effect of root reinforcement on the stability of vegetated slope under rainfall condition. A transient seepage and slope stability analysis was conducted based on the finite-element and limit equilibrium methods. Field monitoring on a residual soil slope in Thailand was numerically modelled. In transient seepage analysis, the soil-water characteristic curve and the saturated permeability measured from laboratory and in situ tests were used, and the results were comparable with the monitored values from the site. For slope stability analysis, the effective shear strength parameters from direct shear tests were used to model the well-compacted and non-compacted soil slopes. The additional shear strength of soil from the root cohesion was modelled for vegetated soil slope. The results show that the contribution of root cohesion to slope stability was more significant for the non-compacted soil condition than for the well-compacted one. Both modelled and monitored pore-water pressure reached the highest values about 1 d after the daily rainfall had reached its peak. This finding has a significant implication for landslide warning based on rainfall.

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).