Landslides are one of the most disastrous natural hazards that frequently occur in Indonesia. In 2017, Balai Sabo developed an Indonesia Landslide Early Warning System (ILEWS) by utilizing a single rainfall threshold for an entire nation, leading to inaccuracy in landslide predictions. The study aimed to improve the accuracy of the system by updating the rainfall threshold. We analyzed 420 landslide events in Java with the 1-day and 3-day effective antecedent rainfall for each landslide event. Rainfall data were obtained from the Global Precipitation Measurement (GPM), which is also used in the ILEWS. We propose four methods to derive the thresholds: the first is the existing threshold applied in the Balai Sabo ILEWS, the second and third use the average and minimum values of rainfall that trigger landslides, respectively, and the fourth uses the minimum value of rainfall that induces major landslides. We used receiver operating characteristic (ROC) analysis to evaluate the predictability of the rainfall thresholds. The fourth method showed the best results compared with the others, and this method provided a good prediction of landslide events with a low error value. The chosen threshold was then applied in the Balai Sabo-ILEWS.
The Sidoarjo mud volcano is a geological disaster which still erupting after 16 years located in a densely populated. The eruption of Sidoarjo mud volcano is the longest continous disaster that Indonesia has ever experienced. It is known that there is overpressure in subsurface that propagated to the surface throught faults. However, the overpressure generation leads to the increase of pore water pressure, so the effective soil stress decreases. This study aims to estimate the change of pore water pressure and effective stress on the subgrade of Sidoarjo mud volcano due to the subsurface pressure. Furthermore, this study considers the existing embankment and excess pore water pressure due to the consolidation process using Finite Element Method. The results show high active pore water pressure in these area is around -580 kPa, due to the consolidation process is -372 kPa and the contribution of subsurface pressure is -208 kPa. The anomaly of effective stress occur from a depth of -13 m to -30 m. Thus, the reduction of effective stress is around 6%-56% from the ideal conditions with the largest reduction occurred at a depth of -30 m.
The mapping of soil movement was examined by comparing an extension of the deterministic Soil Stability Index Mapping (SINMAP) method, and an overlay method with trigger parameters of soil movement. The SINMAP model used soil parameters in the form of the cohesion value (c), internal friction angle (φ), and hydraulic conductivity (ks) for the prediction of soil movement based on the factor of safety (FS), while the indirect method used a literature review and field observations. The weightings of soil movement trigger parameters in assessments were based on natural physical aspects: (1) slope inclination = 30%; (2) rock weathering = 15%; (3) geological structure = 20%; (4) rainfall = 15%; (5) groundwater potential = 7%; (6) seismicity = 3%; and (7) vegetation = 10%. The research area was located in the Buleleng district, in particular in the ancient mountain area of Buyan-Tamblingan, in the Sukasada sub-district. The hazard mapping gave a high and very high hazard scale. The SINMAP model gave a validation accuracy of 14.29%, while the overlay method with seven trigger parameters produced an accuracy of 71.43%. Based on the analysis of the very high and high hazard class and the validation of the landslide occurrence points, the deterministic method using soil parameters and water absorption gave a much lower accuracy than the overlay method with a study of soil motion trigger parameters.
Poboya area has many geological structures that result in distribution of strength and stress of rocks not evenly distributed, as a result, the rock mass strength becomes disturbed and slopes become unstable. The objective of the study was to determine the geological structure effect on slope stability and tunnel conditions on metamorphic rocks at the Poboya gold mine. The study was conducted in the Mantikulore sub-district, Palu, Central Sulawesi. Lithology composing study area is metamorphic rocks consisting of gneiss and schist, therefore, the analysis was carried out on both locations which have many geological structures. Numerical analysis was performed applying the finite element method with the RS2 program assistance. The findings show that the safety factor value of the existing slope at gneiss and schist location under static loading is 4.6 and 2.72, if there is an earthquake it becomes 1.07 and 0.77. The safety factor value under static loading with the joint is 4.58 and 2.03, while under dynamic loading with joint, it becomes 0.94 and 0.64. The geological structure effect which represented by the joint gave a big impact with a decrease of safety factor at gneiss about 0.43% under static loading and 80% under dynamic loading. Meanwhile, at schist, safety factor decreased 25% under static loading and 76% under dynamic loading. For tunnel stability, the existence of joint will increase the displacement of 65% at gneiss under static loading and 84% under dynamic loading, while at schist, it increases 25% under static loading and 54% under dynamic loading. This illustrates that geological structures under dynamic loading affect significantly slope stability of Poboya gold mine.
Bunaken Island is a quaternary deposit of Holocene age. Dominating sand layers, shallow groundwater table, and near active fault locations make the area highly susceptible to liquefaction. This study aims to determine the minimum ground acceleration that can potentially trigger liquefaction in the area. In this study, earthquakes originating in the North Sulawesi Thrust were modelled with various magnitudes. PGA was calculated using the attenuation function from : Liu and Tsai (2005), Abrahamson et al. (2016), Atkinson and Boore (2003), and Zhao et al. (2006). Each earthquake parameter was analyzed for its liquefaction potential using the simplified procedure by Idriss and Boulanger (2008), and then the minimum earthquake parameter value that can cause liquefaction was determined. The analyses show that the study site has the potential for liquefaction if more than Mw 5.8 earthquake occurs with a PGA value of above 0.17g. The BKN-BH02 borehole is the most critical point of the four boreholes made at the study site.
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