Subsurface Study of Flowslide Liquefaction in Petobo, Palu, Indonesia

Abstract

A huge flowslide due to liquefaction occurred at Petobo on September 28, 2018. Many building structures were collapsed, tilted, buried, or moved away up to a distance of 800 m or more. Flowslide occurred at slopes of around 3° and the affected area was approximately 1.64 km2. Magnitude and intensity of earthquake shaking, soil and groundwater conditions etc., would have contributed to the phenomena of the flowslide. Drilling, SPT with hammer energy measurements, laboratory testing on grain-size distributions and groundwater monitoring were performed after the incident. This paper discusses subsurface conditions and the assessment of liquefaction susceptibility. The geometry of ground surface was developed based on topographic survey and DTM data. Results show the materials at Petobo site consist primarily of loose silty sands and sandy silts in the middle and the debris flood areas. At the crown, the soils are mostly gravelly sands or sandy gravels. The groundwater is generally very close to the surface in the middle and the toe areas. The liquefaction susceptibility was assessed by Seed/NCEER method. At BH-1, located near the crown, liquefaction would be more susceptible in layers with depth generally more than 10 m. In the middle areas, BH-2 would likely be liquefied due to the earthquake at depth of less than 10 m. At BH-3, situated near the toe, the liquefaction susceptibility appears low, where only few separated depths are computed with low factors of safety.