Assessment of Petobo Flowslide Induced by Soil Liquefaction during 2018 Palu–Donggala Indonesian Earthquake

This paper presents the results of the subsurface investigation and liquefaction assessment of the Petobo flowslide, induced by soil liquefaction during the Mw 7.5 Palu–Donggala earthquake of Indonesia on 28 September 2018. The investigations, including drilling, standard penetration tests, electrical resistivity imaging survey, dynamic probing, groundwater table monitoring, etc., were conducted along the main road that passes through the middle of the flowslide area. Liquefaction assessments and flowslide simulations were carried out with three assumed scenarios. Scenario 1 describes the condition if the flowslide were to be retriggered at the cease of sliding due to the same earthquake striking the site. Scenarios 2 and 3 attempt to examine the influence of locally raised groundwater levels due to the infiltration of the Gumbasa irrigation system and widespread paddy fields of the site as a result of soil liquefaction and the flowslide. Subsurface investigations revealed that, within a 30-m depth of the ground, the sliding area generally consists of sandy deposits of SM/SP/SW, except for the toe portion where thick layers of silt (ML) and clay (CL) were found. The results of field testing and liquefaction assessment identify potentially weak and liquefiable zones which enable the construction of a speculated slip surface for the flowslide. Both liquefaction assessment and flowslide simulations verify that soil liquefaction would be triggered during the earthquake as a result of locally raised groundwater tables, and that a gentle slope (≈3°) that provides static shears would lead to long-distance sliding after shaking. Should the locally raised groundwater tables not exist, the results indicate no soil liquefaction and no flowslide.