STUDY OF FLOW SLIDE MECHANISM DUE TO LIQUEFACTION CASE STUDY OF PALU CITY'S PETOBO AREA

• Main Author: Ibrahim Tanjung, Mahdi
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/87094
• Institution: Institut Teknologi Bandung

Massive flow slide with deformations of approximately 1 km occurred on gentle slopes in Petobo during the liquefaction of the 2018 Palu Earthquake. The enormous lateral deformations due to liquefaction are not fully understood. A comprehensive understanding of the mechanisms of this phenomenon is essential for mitigating similar disasters in the future. This study describes the mechanics of flow slide in Petobo and how liquefaction can cause extremely lengthy flow slide there. Normally, liquefaction typically causes lateral displacement of a few to tens of meters, but in Petobo, the lateral deformation was greater than anticipated in comparison to other documented occurrences. Data from geotechnical investigations, including as trench tests, MASW, CPT, drilling, and laboratory testing, served as the basis for this study. Liquefaction tube tests were also conducted to investigate the void redistribution mechanism due to liquefaction. In addition, the influence of thickness, relative density of the liquefaction layer, kcap/ksand permeability ratio, and non-uniformity of permeability in the liquefaction layer were obtained through numerical analysis utilizing FLAC software with the PM4SAND material model. Petobo flow slide can be separated into two blocks according to patterns of deformation. The First Block is a highly liquefied block, while the Second Block is a block that has experienced overturning and lateral extension. The groundwater level in the First Block is shallow, while in the Second Block it is deeper. The results of field and laboratory investigations in the flow slide area confirmed the presence of a cap layer above the potentially liquefied layer. In the flow slide area, the layer that has the potential for liquefaction tends to be thicker than the area that does not experience flow slide. A potentially liquefied layer may consist of several liquefied layers with different permeabilities due to differences in fine grain content. Some conditions found at non-flow slide locations include thin layers of sand alternating with clay, silt, or gravel; beneath the cap layer there is a coarser layer such as gravel, or a very dense layer; and the slope is relatively flat. In general, the results of numerical simulations correlate with findings in the field or laboratory tests. The results of numerical simulations show that the liquefied layer experiences a reduction in the void ratio, but the upper part of the liquefaction experiences an increase in the void ratio along with the dissipation process. The void ratio (volumetric strain increase) of the layer directly beneath the cap layer increases with the thickness of the liquefied layer. The lower the relative density, the greater the potential for increasing the void ratio of the layer below the cap layer. The larger the permeability ratio kcap/ksand, the smaller the maximum volumetric strain that can occur, conversely, the smaller the kcap/ksand, the larger the maximum volumetric strain that can occur. When kcap/ksand is less than 1x10-2, the permeability ratio has a negligible impact on the volumetric strain that is generated. Therefore, a variation in fine grain content of greater than 10% can be considered as a cap layer based on the correlation between permeability and fine grain content. Due to the non-uniformity of permeability in the liquefaction layer, an increase in the void ratio first occurs between the liquefaction layers which then propagates up below the cap layer. In addition, the effective stress of almost 0 can be distributed between the liquefied layers which have lower permeability during the dissipation process. Layers with reduced shear strength may thicken as a result of this process, increasing the likelihood of deformation. It is anticipated that the findings of this study will advance our understanding of liquefaction-induced flow slide and serve as a foundation for assessing the potential of flow slide in other areas.