RISET Geologi dan Pertambangan
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Landslide Potential in Cihanjuang, Cimanggung, Sumedang, West Java Province
Full text linkLandslides occurred in the Cihanjuang area, Cimanggung, Sumedang Regency, West Java on January, 9th 2021 with 40 fatalities and several buildings severely damaged. This area is in the form of hills with a fairly steep slope and housing is built on it. Geologically, the research area is composed of Undecomposed Young Volcanoes (Qyu) in the form of tuffaceous sand, lapilli, breccias, lava, and agglomerates. The research aims to determine the potential and volume of landslides. The research method used is conducting geotechnical testing and carrying out topographical mapping. Geotechnical tests carried out were water content, specific gravity, sieve analysis, hydrometer, and direct shear strength. The geotechnical test results showed that water content: 16.37% – 31.26%, cohesion: 59.2 kPa – 112.7 kPa, and internal friction angle: 6.30° – 30.50°, percentage of gravel and sand 11.10 % – 34.90 %, percentage of silt and clay 65.10 % – 88.90 %. According to USCS, this soil sample includes the MH-CH-OH classification Topographical mapping was carried out covering an area of 7.006 hectares. The internal friction angle is relatively steep, and the percentage of gravel and sand is quite high, so water infiltration becomes easy, so the Cimanggung area has a high potential for landslides. From the topographic mapping, the volume of the landslide was around 161.981 m³. To maintain slope stability and avoid landslides, apart from planting strong-rooted plants in areas with high elevations, they also manage the drainage by making waterways on the northwest and southeast sides of the steep slopes to the main road
Hydrogeochemical characteristics of groundwater in Baleendah - Soreang, South Bandung, West Java Province
Full text linkThe Baleendah – Soreang area is part of the southern part of the Bandung Soreang Groundwater Basin, constructed by volcanic activity. Regional development in the Baleendah – Soreang area can decrease groundwater quality, influenced by natural and anthropogenic factors. Groundwater hydrogeochemical analysis is important in environmental management studies. This study aims to determine groundwater’s physical properties and hydrogeochemical facies in this area. The groundwater hydrochemistry analysis, determined by analysis of Stiff diagrams and Piper diagrams, shows that the groundwater in the area is classified as freshwater and has intermediate groundwater flow. In addition, the groundwater quality is affected by some anthropogenic activities
Petrology, Geochemistry and K-Ar Dating of Metamorphic Rock in Ciletuh Mélange Complex, West Java Indonesia
Full text linkThe Ciletuh Mélange Complex in West Java, Indonesia, provides evidence of early Cenozoic subduction. This study aims to investigate the stratigraphic position, geological structure, metamorphic facies, and protoliths of the metamorphic rock units. The research methods employed geological mapping, petrology, and geochemical analysis. The samples collected exhibit different facies, including zeolites, greenschist, and epidote amphibolite. Protoliths consist of metasedimentary rocks such as metapellites, metapsammitics, meta-calcilates, and ortho-metamorphs such as metagabbro and metabasalt. Retrograde metamorphism, indicated by epidote, chlorite, and calcite in amphibolite schists, suggests lower temperature stress conditions. Hydrothermal changes are evidenced by some samples’ occurrence of quartz and calcite veins. Geochemical analysis reveals that the provenance of metasedimentary rocks originated from a volcanic arc, while the metabasalt rock originated from an island arc tectonic environment. K-Ar dating indicates an age range of 55.2 - 37.8 million years ago, corresponding to the Early to Late Eocene. These metamorphic rocks are believed to have formed through regional metamorphism due to island arc subduction and the formation of accretionary prisms. Retrograde metamorphism signifies uplift or accretion processes after tectonic activit
Discerning geomorphological aspects of tsunami risk in Pangandaran, West Java, Indonesia
Full text linkThe geomorphological understanding of earth dynamics, including the relationship between landforms and their processes, was one of the earliest and most specific contributions to disaster prevention. Disaster geomorphology is one of the approaches in disaster studies, which includes aspects of landforms, processes, and results of physical processes that have the potential and can cause disasters. The landform is of risk factors that can turn natural hazards into natural disasters and determines the damage that disasters can cause to human activities. Following the 2006 South Java Tsunami, infrastructural development occurred massively in the 2006 South Java Tsunami inundation areas. Several tsunami risk mitigation efforts were conducted but solely based on the 2006 tsunami scenario and ignored the existence of more considerable tsunami hazards from the Sunda Megathrust. This consideration may lead to an increasing risk of future tsunamis. We evaluate and appraise favorable and unfavorable geomorphological features to reduce the risk of future tsunamis. Pangandaran has a unique landform compared to other areas on the south coast of Java, and this landform has the potential to reduce future tsunami risk. Typical landforms studied include coastal plains, alluvial plains, Tombolo, tied islands, and structural hills. The results show that the morphological features of the Tombolo and the coastal plain area are categorized as high risk when a tsunami occurs. The tied island is categorized as a favorable morphology where these morphological units have the advantage of elevation and efficient distance to the tsunami risk zone. Evacuation facilities are also needed, especially in the coastal plain and Tombolo areas (with a height of >20 meters), to reduce disaster risk, particularly mortality caused by tsunami events
Two-Dimensional Resistivity Modeling of Seawater Intrusion Along the West Flood Canal, Semarang
Full text linkThe northern coastal area of Semarang City has significant problems with a decrease in groundwater quality due to seawater intrusion. Increasing groundwater extraction and subsidence due to the city growth worsen the condition. It is necessary to collect information on the contaminated location to develop a strategy to decrease the spreading of seawater intrusion. This study aims to identify the presence of seawater intrusion zones in groundwater and estimate the spread distance of seawater intrusion to the land. We applied the electrical resistivity tomography method to obtain the image of the subsurface. A geo-electric survey with a dipole-dipole multi-electrode configuration was completed along the West Flood Canal. The subsurface model result indicated that the identified seawater intrusion zone is associated with a low resistivity value of less than 3 Ohm.m. The zone is about 0 - 70 meters deep near the coast and is thinning to about 2600 meters to the south. The result confirms that seawater has penetrated long distances to the land
Presupposition of Slip Plane Using the 2D Resistivity Method with Dipole-Dipole Array in Jahiang Village, Salawu District, Tasikmalaya Regency, West Java Province
Full text linkLandslide disasters frequently occur in Tasikmalaya Regency, with Salawu Village being at a medium to high ground movement vulnerability zone. Steep slopes and quite high rainfall dominate this area. This study aims to identify subsurface rock types and model the slip plane by applying the dipole-dipole configuration of the 2D geoelectrical method. The result can be used as a mitigation measure to minimize losses if a landslide disaster occurs. We measured the resistivity in five lines with spacing between electrodes of 5 m with a total length of 275 m. The data processing results indicate three classes of resistivity values: a resistivity value under 25 Ωm is interpreted as sandy clay, a value in the range of 20-105 Ωm as clayey/ tuffaceous sand, and a resistivity of more than 80 Ωm as volcanic breccia. The slip area is sandy clay with a slipping mass of clayey/ tuffaceous sand and volcanic breccia. Two of the surveyed lines have safety factor values less than 1.00 (the slope is approximately 40°, and the slip plane angle is between 33° and 35°), which means that the area has an unstable slope. The small safety factors should be a serious concern because landslides can occur at any time. Moreover, breccia rock is one of the landslide materials that would have a very destructive impact. To confirm the results of this study, in situ geomechanical tests are needed in this area
Assessment of Landslide Susceptibility in the Pagentan Area, Banjarnegara Regency: A Spatial Multi-Criteria Evaluation Approach
Full text linkLandslides are widespread natural disasters that occur across various areas in Indonesia. Among these areas, Pagentan and its surroundings in Banjarnegara Regency are identified as having significant potential for large-scale landslides. Therefore, this research was conducted to determine the susceptibility of ground movements. The method used to examine the surface geological mapping and analyze the soil movement susceptibility was the Spatial Multi-Criteria Evaluation (SMCE). This method is an applied science approach that employs spatial analysis and multi-criteria evaluation to support decision-making processes. Geological mapping was used to describe rocks, make geomorphological observations, measure geological structures, take stratigraphic sections, and collect rock samples. Multiple parameters were used to determine soil susceptibilities in the research area, such as slope, lithology, rock mass, elevation, land cover, road buffer, river buffer, and aspects, which were transformed into raster data for analysis. The susceptibility analysis classified the research area into four categories: low, medium, high, and very high. The low susceptibility zone includes Pandansari and Karangtengah. The moderate susceptibility zone includes Wonosroyo, Aribaya, Karangtengah, Pandansari, and Bantar. Most of the high susceptibility zones are in Bantar and Karangtengah. Meanwhile, the very high susceptibility zones include Gumingsir, Plumbungan, Kalitlaga, Kayuares, Nagasari, Karangnangka and Mentawan
Geological Condition and Landslide Vulnerability Zonation Using Frequency Ratio Method in Salaman District, Magelang Regency, Central Java Province
Full text linkSalaman District is an area with an area of 68.87 km2, which is included in the Magelang Regency area. The southwest part of this area features hilly topography, forming the slope of the Menoreh Hills. In contrast, the central and eastern areas have predominantly flat topography, with breccia rocks distributed widely. The district has a population of 74,429 people, and the majority earn their livelihoods as farmers. The primary commodities include cassava, as well as fruit crops like mango and durian. Other commodities originating from this area are teak wood, bamboo, mahogany, sengon and waru. From January 2020 to April 2021, rainfall in Magelang Regency ranged from 20 mm to 531 mm. Based on data obtained from the BPPD Magelang Regency website, Salaman District is one of the sub-districts that had the highest intensity of landslides in that period, with a total of 67 landslides. Given the significant number of landslide disasters, it is deemed crucial to conduct mapping and develop a landslide susceptibility map for the research area. The research utilizes direct data, including lithological information, geological structure, and geomorphology, as well as secondary data comprising landslide occurrence, topography, DEM, and land cover data in the research area. The method used in this research is the frequency ratio (FR) method. It is constructed based on the relationship between the location of the landslide incident and the factors that control the occurrence of landslides. The FR method is included in the bivariate statistical method, which is used to determine the relationship between two variables. Based on the geomorphological analysis, it was found that several landforms developed in the research area, namely River Body (F1), Alluvial Plain (F2), Structural Hills (S1), and Residual Hill (D1). The stratigraphy of the research area consists of six rock units, listed from oldest to youngest as follows: Kaligesing Pyroclastic Breccia unit, Kaligesing Andesite Lava unit, Kaligesing Epiclastic Breccia unit, Old Sumbing Laharic Breccia unit, Young Sumbing Tuf-lapilli unit, and Alluvial Deposits. The geological structures that develop in the research area are right strike-slip faults and left strike-slip faults, which have a west-east line. Based on the results of the analysis using the Frequency Ratio method, which uses parameters in the form of slope, lithology, structure distance, river distance, land use, and landslide occurrence points, the zoning in the research area is divided into 4, namely the Very Low Landslide Susceptibility Zone with a Landslide Susceptibility value. Index (LSI) 22.48 – 40.75; Low Landslide Susceptibility Zone with LSI value of 40.75 – 46.72; Medium Landslide Susceptibility Zone with LSI value 46.72 – 52.87; and High Landslide Susceptibility Zone with an LSI value of 52.87 – 67.27. After validating the landslide susceptibility map using the Area Under Curve (AUC) method, the results obtained were 70.95% and included in the good category, so the map was suitable for use