Taichi KUMAGAI

Study on the Long-distance Ground-flow Mechanism in Palu City during the 2018 Sulawesi Earthquake, Indonesia

Takaaki IKEDA

An earthquake of Mw 7.5 occurred in central Sulawesi, Indonesia, on 28 September 2018. In the Palu, occurred strong seismic and suffered damage due to liquefaction, tsunamis up to 3 m high, and long-distance ground-flow. This long-distance ground-flow occurred in several areas and caused extensive damage in the Palu. The damage is rarely damage that the maximum slope was gentle, at about 3.9%, and that the maximum flow area was about1.5 km, which is unprecedented in the world.
The mechanism of the ground-flow is not simple and has been considered by various researchers. One consideration is that the ground may have been covered by groundwater due to the results of post-disaster borehole tests, damage and topographical conditions, and that an impermeable layer may have trapped the covered groundwater. It is thought that during the earthquake the impermeable layer may have ruptured, causing the pressurised groundwater continue to supply the groundwater. In this study, the ground in which the impermeable layer is destroyed were assessed using numerical analysis. Specifically, using the results of borehole tests in the flow area, considering four cases: total de-liquefaction, liquefaction above the impermeable layer, liquefaction below the layer and liquefaction of both the above and below layers.
The results of the analysis showed that the shear wave velocity in the layer above the impermeable layer was small, which caused large strains; liquefaction in the layer above the impermeable layer caused the largest strains; and liquefaction in the layer below the impermeable layer did not cause large strains in the whole ground. Therefore, it is considered that the liquefaction of the upper layers above the impermeable layer caused large shear strain, which may have led to the inflow of groundwater that had been under pressure due to damage to the impermeable layer, resulting in the damage described above. In addition, a simplified liquefaction assessment was carried out in non-flow areas, which showed that liquefaction was less likely to occur in non-flow areas, suggesting that the occurrence of liquefaction may be related to the damage caused by the earthquake.
In the future, effective stress analysis considering the other case. And, it is also considered necessary to conduct experiments using field samples and the analytical parameters into the field.