Stress-Strain Behavior and Residual Strength of Petobo Sand with Variable Fines Content after the 2018 Palu Liquefaction

Desiana Vidayanti, Paulus Pramono Rahardjo, Ramli Nazir

Abstract


Liquefaction was one of the main causes of severe ground deformation during the 2018 Palu earthquake, particularly in the Petobo area where large-scale flow liquefaction occurred. Despite extensive field investigations, no laboratory-based residual shear strength data for Petobo soil have been available to explain the exceptional mobility of the flowslide. This study investigates the stress–strain response and residual strength of Petobo silty sand containing different proportions of fines through monotonic Consolidated Undrained (CU) triaxial tests. Reconstituted specimens were prepared using the moist tamping method with fines contents of approximately 9% and 26.4%, and tested under three levels of initial mean effective stress.The higher-fines specimen generated excess pore water pressure more rapidly and showed stronger contractive behavior at small strains. However, at larger strains, the 26.4% fines specimen mobilized greater residual strength, expressed as the ratio of residual deviator stress to initial mean effective stress (0.79-0.94), compared with the 9% fines specimen (0.85-0.89). These results indicate that fines increase contractive tendency during initiation but enhance resistance against large post-liquefaction deformation. The influence of fines on post-liquefaction behavior is therefore nonlinear and dependent on deformation stage and initial density. This study provides the first laboratory-based residual strength data of Petobo silty sand within the Critical State Soil Mechanics framework, clarifying its post-liquefaction resistance characteristics. The findings improve understanding of flow behavior in silty sands, offer mechanistic insight into the 2018 Petobo failure, and supply essential parameters for calibrating constitutive models and supporting liquefaction hazard mitigation in similar alluvial deposits in Central Sulawesi

Keywords


Contractive behavior; Critical state; Flow liquefaction; Silty sand; Undrained triaxial test

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