Long-distance flow mechanism of gentle slopes under seepage due to liquefaction-induced water film during 2018 Sulawesi earthquake, Indonesia

Unprecedented liquefaction-induced slope failure occurred during the 2018 Indonesian Sulawesi earthquake in which more than 2000 people were killed by sliding debris flows in very gentle slopes with a 2 % gradient. In order to clarify the mysterious mechanism of how long-distance debris flows could occur on such gentle slopes, transient seepage analyses were conducted focusing on the impact of a thin water film of a limited horizontal length which was supposed to emerge during liquefaction in a layered soil profile beneath a low-permeability cap layer of the slope influenced by stationary seepage. Consequently, the water film was found to play a key role in transmitting higher pressure head to the tip at a lower elevation with a marginal head loss, leading to the downslope extension of the water film and associated boiling failure in the cap layer. The water film that formed during liquefaction was significant in realizing the long-distance flows down the gentle slopes during and even after liquefaction as long as the water film was sustained. This mechanism should be newly recognized as a serious threat to a society due to liquefaction in gentle slopes during strong earthquakes.