Rainfall has been considered the cause of the majority of slope failures and landslides that happened in regions experiencing high seasonal rainfalls. The mechanism of the failures was mainly due to the lost of matric suction of soils by rainwater. This paper presents the results of a laboratory model study on the effect of slope angle and surface cover on water infiltration into soil and soil matric suction. A field infiltration test is carried out for comparison. A parametric study is also done to examine the effect of permeability ratio, development of perched water table and rainfall intensity on the factor of safety against instability of a soil slope. Results of the model study show that different surface covers on slopes have an effect on the water infiltration. Generally the covered surface (grass or geosynthetic net) has a lower infiltration rate compared with the bare (no cover) surface. On the effect of slope angle, it was observed that water infiltration decrease with increase in the slope steepness. With regards to the movement of the wetting front, it appears that water infiltration is more at the toe compared with the top of the model slope. Based on the parametric study, it is found factor of safety of the slope against instability drops for slope with higher ratio of permeability for the permeable and impermeable stratum. As the perched water table is formed, the factor of safety decreased. The rainfall intensity also has a marked effect on the slope factor of safety. The higher the intensity of the rainfall, the higher is the infiltration rate into the soil, hence the lower is the factor of safety against slope instability.

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Vegetation can signiˆcantly contribute to stabilise sloping terrain by reinforcing the soil: this reinforcement depends on the morphological characteristics of the root systems and the tensile strength of single roots. This paper describes an investigation on the reinforcing eŠect of soil-root matrix in the laboratory using a modiˆed large shear box apparatus (300 mm×300 mm). Four diŠerent species of plant namely Vertiveria zizanoides, Leucaena leucocephala, Bixa orellana and Bauhinia purpurea were planted in special boxes containing residual soil compacted to a known density. The results show that roots signiˆcantly contribute to the increase in soil shear strength. The presence of the roots only aŠects the apparent cohesion of the soil and no signiˆcant change in angle of friction is observed. L. leucocephala shows the outstanding increase in its root strength in which the strength varies with depth and time e.g., under soil suction-free condition (matric suction＝0), the roots have increased the cohesion by 116.6z (0.1 m), 225.0z (0.3 m) and 413.4z (0.5 m) after six months of growth. In twelve months, it is observed that the increase in cohesion is more than three-fold of the six months growth period at 0.1 m depth. The results also indicate that shear strength is in‰uenced by root proˆle and to some extent, the physiological parameters of the plants.

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