Study on mechanism of retrogressive slope failure using artificial rainfall

• Experiments consisted of detailed observations of slope failure process.
• Initial failure is not always necessary from the region near the toe of the slope.
• The relationship between rainfall intensity, sliding initiation time, and sliding head
• Failure initiating near the toe propagates an unstable zone toward up-slope.
• Failure initiating toward the head reach may propagate down-slope and then up-slope.

Many slope failures have been observed in the mountainous environments of the world due to rainfall events. However, studies related to the correlation of rainfall intensity, sliding initiation time and the position of slip surface head are still lacking. Studies on the mechanisms of retrogressive slope failure that may occur in a series and the down-slope displacement of failure mass are also rare. In this study, slope failure experiments were performed using medium-grained silica sand S6 to prepare the model slope in a 5 m long, 30 cm wide and 50 cm deep rectangular flume. The experiments consisted of detailed observations of failure process consisting series of failures successively, with particular emphasis on the time of failure; shape, size and position of the slip surface; and final shape of the model slope after down-slope displacement of the failure masses. Moisture profiles at different points within the model slope soil domain and the profile of surface-water forefront were also reported. The observations showed that the slope failure initiating once at the region near the toe results in successive sliding failures in a series, thereby propagating an unstable zone toward the up-slope. When the slope failure initiates at the region toward the head reach, the successive failures may propagate toward the down-slope first, and then again toward the up-slope. This study also noticed a strong correlation between rainfall intensity, sliding initiation time and the position of its slip surface head. The relationship might depend on the shape and size of the model slope and the type of material used in the formation of the model slope. However, the results indicate the possibility of finding a relationship among rainfall intensity, sliding initiation time, and the position of its slip surface head in a natural slope failure prone zone. A landslide model test and application in real field, based on the similarity theory, may reveal the relationship more realistic. Then it would have a significant impact on policy making to mitigate the probable catastrophe during extreme rainfall-induced landslide disaster.