Strong earthquakes occurred in Central Sulawesi, Indonesia, in late 2018, causing an inheritance disaster, soil liquefaction on Gumbasa Irrigation Canal at Petobo, Sulawesi. Soil liquefaction is a phenomenon of a decreasing soil bearing capacity triggered by strong vibrations in certain soil conditions. It immediately changes the soil characteristic from solid to liquid. Liquefaction vulnerability analysis was done using Idriss-Boulanger’s simplified procedure based on SPT value in several spots. The Petobo liquefaction zone has seven boreholes, five of which are located near the Gumbasa Irrigation Canal. The soil sample at those boreholes was taken to the laboratory for further soil testing using grain size analysis. The simplified procedure is intended to calculate the safety factor using Cyclic Resistance Ratio, Cyclic Stress Ratio, and Magnitude Scaling Factor. The liquefaction vulnerability analysis resulted in the AB 1 – AB 3 area near Gumbasa Irrigation Canal, which liquefied. Meanwhile, LP 1 and LP 4 are contrary. LP 1 is located upstream of the canal, whereas LP 4 the downstream. Grain size analysis yields a consistent result that AB 1 – AB 3 soil is quite scattered inside the liquefiable constraint.
The soil liquefaction disaster in Petobo, Central Sulawesi, caused massive terrain damage. With a total displacement of up to 800 meters and a total sliding area of 1.43 km2, the soil structure was deformed. The 7.5 Mw earthquake with the maximum PGA of 0.45 was causing flow-induced liquefaction with a distance from the crown to the foot of the landslide as far as 2 km. The research aims to find out how well the post-earthquake soil can withstand the potential to liquefaction if it is hit by various earthquake levels. This research will conduct several simulations to determine the soil performance against earthquakes. The empirical calculation method is carried out by referring to Idris-Boulanger to determine the liquefaction safety factor. Based on several scenarios that have been carried out, post-earthquake land can be re-liquefy when shaken by an earthquake with a certain level below the magnitude of the 7.5 Mw 2018’s Palu earthquake. There are a total of six different scenarios of earthquakes with various level of PGA. The soils still have the possibility to liquefy with the PGA of 0.20 and can be considered safe as the PGA value of 0.13. Mitigation efforts, including soil remediation, must be carried out before reconstructing phase of the Petobo irrigation canal starts.
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