Effect of herbaceous plants on the response of loose silty sand slope under rainfall

There is a large number of loose soil slopes backfilled with silty sand in southern China that are prone to landslide hazard under rainfall. Recently, herbaceous plants have been adopted as an environment-friendly slope amendment; for instance, grass vegetation can act as a reinforcement in a short time because of its rapid growing ability. This study presents a series of model tests, aiming to investigate the influence of ryegrass plantation on the response of loose silty sand slopes subjected to various rainfall conditions. During continuous rainfall, matric suction and pore water pressure were monitored by the sensors inside, while displacement field of the whole soil slope was obtained through PIV technique. Test results suggest that the deformation of silty sand slope is closely related to the loss of matric suction, and there is a sudden rise of pore water pressure at the time of failure. It is also observed that a higher ryegrass plant density increases the critical rainfall return period that a model slope could withstand, while the deep global failure may also alter to a shallow sliding due to the increasing soil-root composite integrity. Finally, although the increase of rainfall intensity amplifies the magnitude and rate of deformation for a vegetated slope, silty sand slopes are more prone to failure under a lighter but longer rain than a heavier but short-term rainfall if both events share the same return period.