RISET Geologi dan Pertambangan
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Mineralogy of granites from Hukurila area, Ambon Island, Indonesia: An insight into petrogenesis
Full text linkThe Hukurila area in the Leitimor region of Ambon Island hosts one of the granite bodies. The granite is surrounded by Jurassic-Cretaceous peridotites. Although granites in Ambon Island have been intensively investigated, their origin remains interesting to study. This work offers petrography and X-ray diffraction data of granite from Hukurila area and contributes to understanding petrogenesis in Ambon Island. Granites from Hukurila area are white to light brown and have a medium- to coarse-grained holocrystalline texture, with quartz, potassium feldspars, plagioclase, biotite, and muscovite being the most common minerals. Potassium feldspars are sometimes found in aggregated larger crystals in the outcrops. While cordierite, zircon, apatite, and mullite were also observed as accessory minerals under the microscope. Mullite in granites from Hukurila area indicates that the rocks were subjected to high temperatures. Aluminium-rich minerals (i.e., cordierite and mullite) in granites from Hukurila area suggest S-type granite with significant crustal contamination during their formation
Volcanostratigraphy and Geological Hazard Potential of the Karaha-Sadakeling Volcano Complex, West Java, Indonesia
Full text linkThe purpose of this study is to determine the type and distribution of volcanic rocks both laterally and vertically and to analyze the potential hazards of the Karaha-Sadakeling Volcanic Complex. Volcanostratigraphically, the volcanic evolution from old to young is Cakrabuana Crown, Sadakeling Crown, Ewaranda Crown, and Talagabodas Crown. The recognizable landforms are crater traces, volcanic cones, lava flow ridges, and alluvial plains. The north-south oriented volcanic cone morphology of the study area parallels an approximate major fault that passes between the Cakrabuana Complex to the northeast, and the Karaha-Sadakeling Complex. The northern part is dominated by Sadakeling Crown volcanic deposits, the central part is composed of Ewaranda Crown deposits, and the southern part is dominated by TelagaBodas deposits. The magmatic evolution shows the process of magma assimilation and mixing with primitive magma. Potential volcanic disasters can include pyroclastic fall, pyroclastic flows, and lava flows. Primary eruptions are inferred to originate from Karaha Crater, which exhibits ongoing volcanic activity
Rock Mass Quality Analysis of Soko Cave, Temayang District, Bojonegoro Regency, East Java based on Q-System, Rock Mass Rating, and Geological Strength Index Methods
Full text linkThis study aims to analyze the rock mass quality of Soko Cave, located in Temayang District, Bojonegoro Regency, East Java, as a basis for evaluating geotechnical stability for tourism purposes. Three rock mass classification methods were used: Q-System, Rock Mass Rating (RMR), and Geological Strength Index (GSI). Data were obtained through field surveys, geological mapping, cave geometry measurements, and observation of discontinuities using the scanline method at 11 observation stations. The rock mass quality was generally classified as good to very good, with the Q-System method producing the highest score, followed by GSI and RMR. The differences in results were due to the different parameters used by each method. Basedon these findings, the rock mass in Soko Cave was considered naturally stable and did not require additional support structures, making it safe for geological tourism development. This study not only compares methods but also emphasizes the importance of using the three systems complementarily to provide a realistic and applicable picture of the stability of rock masses in carbonate caves. The GSI method shows potential as a reliable approach for this environment, although further validation with a broader data coverage is needed
Evaluation of andesite slopes stability using pseudostatic limit equilibrium method in Lembang active fault zone, West Java, Indonesia
Full text linkEarthquakes and rainfall can trigger landslides in many regions of Indonesia. Rock slopes of andesite outcrops in Gunung Batu and Graha Puspa areas coincide with the Lembang active fault zone in West Java. The region is also subjected to high-intensity rainfall. Thus, the rock slopes are prone to failure during earthquake shaking and heavy rainfall. To mitigate the hazards associated with slope failure in residential areas close to the rock slopes, it is necessary to assess the slope failure hazard at the andesite hill slopes. The study presented in this paper aims to analyse the stability of the andesite slopes using the pseudostatic limit equilibrium method and evaluate the effect of variations of regional seismicity and water content on the stability of the slopes. Pseudostatic analysis considered the peak ground acceleration (PGA) and the calculated horizontal seismic coefficient (kh). The limit equilibrium method was focused on toppling and wedge failure cases. Based on the analysis, the andesite slopes in Gunung Batu and Graha Puspa are stable (FoS ≥ 1.1) in factual conditions (dry-static). In contrast, all slopes have the lowest FoS values (less stable-unstable) under the saturated-pseudostatic conditions. The threshold values of kh and %w (percent water fill) for the slopes’ instability were obtained by varying the regional seismicity and water content conditions. It is recommended that numerical slope stability modeling (i.e., finite element method) be conducted to improve the accuracy of the models
Deterministic Modeling of Pasir Panjang Debris Flow Using Smoothed Particle Hydrodynamics (SPH)
Full text linkThe debris flow can be modeled as non-Newtonian flow using physical and mathematical modeling methods based on rheological property measurements and movement parameters. The numerical simulation in this research used smoothed particle hydrodynamics (SPH) to solve the momentum and energy equations. The debris flow modeled in this research is located in the Bentarsari Basin, which is surrounded by hills composed of volcanic breccia from the Kumbang Formation (Tpk) and tends to be more susceptible to landslides and may become debris flows. The results of the numerical simulation begin with the collapse of the natural dam and become debris flow with 8.8 m maximum thickness. The debris flow destroyed the bridge in the 40s with 100 kPa pressure. A speed of 6 m/s was reached at 120 s. A small hill in the deposition area was hit by debris flow at 130 s causing 18 casualties. The debris flow enters the deposition area at the 150 s. The debris flow average velocity shows relatively transverse pattern (1-126 s), constant pattern (126-289 s), and random pattern (after 289 s). The debris flow average pressure shows steep negative gradient pattern (1-47 s) and relatively flat pattern (after 47 s). The average debris flow thickness shows a steep negative gradient pattern (1-13 s) and a gentle negative gradient pattern (after 13 s). The results of debris flow modeling using SPH can simulate the debris flow segregation process, which usually cannot be captured by conventional debris flow modeling software
Evaluation of Liquefaction Potential According to Resistivity and CPT Data
Full text linkThis study investigates the liquefaction potential in Tambak Wedi Subdistrict, Surabaya, employing the electrical resistivity geophysical method with a dipole-dipole configuration and the Cone Penetration Test (CPT). The electrical resistivity method is utilized to acceess the resistivity values of soil layers and to generate a 2D subsurface profile. The CPT method is applied to confirm the soil layer types and to compute the safety factor (SF). The soil data from both methods are subsequently analyzed to evaluate the liquefaction potential based on the soil resistivity and SF values. The analysis incorporates a Peak Ground Acceleration (PGA) of 0.3g and considers an earthquake magnitude of 7.5 Mw. The findings from this study reveal that the soil layers ranging from sandy to organic soil, with dominant silt-sandy and clay-silt layers present up to a depth of 10 meters, and clay-silt and clay layers from 11 to 20 meters. Except in the first 2 meters depth, the calculated SF is less than 0.6, indicating a high liquefaction potential in the region. The assessment of liquefaction potential in this study involved the calculation of N-SPT, Liquefaction Potential Index (LPI), and Liquefaction Severity Index (LSI). These findings underscore the importance of incorporating sitespecific geotechnical evaluations into disaster risk reduction strategies, as they provide critical input for the development of effective mitigation plans aimed at minimizing potential loss of life and economic impact
Reconstruction of Shear Surface Based on Joint Roughness Coefficient and Its Relation to Rock Shear Strength
Full text linkRock strength is a fundamental parameter in rock mechanics, serving as the basis for predicting rock behavior under different loading conditions. Among the various approaches to characterizing rock strength, shear testing of discontinuities plays a crucial role. The Joint Roughness Coefficient (JRC) provides an empirical link between joint surface roughness and shear strength parameters. In this study, joint surface profiles were reconstructed statistically based on JRC parameters and subsequently reproduced using three-dimensional (3D) printing technology. The printed molds were employed to cast laboratory specimens with pre-formed shear surfaces, which were then tested under direct shear conditions. The shear strength parameters obtained from the tests were analyzed in relation to their corresponding JRC values. The results demonstrate that the reconstructed and 3D-printed surfaces were successfully fabricated and accurately replicated joint roughness geometries. Direct shear tests confirmed the expected trend, with shear strength increasing alongside JRC. These findings indicate that shear surfaces can be prefabricated and manipulated with controlled roughness, providing a reliable and reproducible platform for investigating the mechanical behavior of rock joints
Assessment of Bedrock PGA Values based on Probabilistic Seismic Hazard Analysis in Purworejo, Indonesia
Full text linkPurworejo Regency, Central Java, is directly adjacent to the Indian Ocean in the South, where there is a subduction zone between the Indo-Australian Plate and the Eurasian Plate. The movement of these plates has the potential to cause earthquakes, so for mitigation purposes with the design of earthquake-resistant buildings, PGA analysis and Earthquake Hazard Curve are needed. Data is sourced from the 2017 Indonesian Earthquake Hazard Source Map Book using Hazard Analysis software (USGS PSHA) GMPE modified. The attenuation function or GMPE (Ground Motion Prediction Equation) used is also the same as that used to create the Indonesian Earthquake Hazard Map of earthquake source mechanisms consisting of subduction, fault, and background earthquake with a return period of five hundred, one thousand, two thousand and five hundred, five thousand, ten thousand years. The results show that for a 2500-year return period, the most contributing earthquake source in the Purworejo Regency is a subduction earthquake source, with Kaligesing Subdistrict having the highest PGA value of 0.209 g and Bruno Subdistrict owning the lowest PGA at 0.186 g. The distribution of PGA values at this return period shows that the PGA values increase as the location approaches the subduction earthquake source from North (0.15 – 0.20 g) to South (0.20 – 0.25 g). Based on the PGA value, the Purworejo Regency area could feel the occurrence of earthquake vibrations. However, it is still necessary to mitigate the occurrence of earthquakes by building earthquake-resistant buildings to minimize losses when a major earthquake occurs in the futur
Land Subsidence due to Groundwater Extraction and Natural Consolidation in the Bandung Basin, West Java, Indonesia
Full text linkBased on GPS and InSAR data, it is known that there is land subsidence in the Bandung Basin of 1.1 - 16.9 cm/year (GPS) and 0.9 - 1.7 cm/year (InSAR). Several types of land subsidence can be expected to occur in the Bandung Basin. However, the detailed characteristics and exact mechanism of land subsidence from a geological perspective in the Bandung Basin are still unknown. Two factors that can cause land subsidence are groundwater extraction and natural consolidation. It is known that there has been a significant decrease in groundwater levels in Bandung since the 1970s due to the very high intensity of groundwater extraction. It is also known that the Bandung Basin comprises one of them, the Kosambi Formation, which consists of organic clays with very high compressibility and undergoes a natural consolidation. Based on these facts, we determine the distribution of land subsidence rates due to groundwater extraction and natural consolidation, along with the percentage of their respective contributions in the Bandung Basin. Research materials include groundwater level depth data and organic clay consolidation data from previous studies. The results showed that the average land subsidence rate due to groundwater extraction was 1.85 cm/year, with a contribution percentage of 44.30%. In comparison, the average land subsidence rate due to natural consolidation was 0.92 cm/year, with a contribution percentage of 15.76%