MODELLING OF LAND SUBSIDENCE ALONG TANAH MAS -PELABUHAN SECTION SEMARANG CITY USING FINITE ELEMENT METHOD

ABSTRACT Land subsidence has been an apparent problem in the northern part of Semarang city, Central Java, which has caused enormous physical and economical impacts. This area is underlain by alluvium sediments of clay, silt, sand and gravel. The nature of the alluvium soil is soft and highly compressible which induces natural consolidation to occur. The lowering of groundwater table due to exploitation and loads of building and earth fill accelerate the rate of consolidation settlement. A study of land subsidence modeling was carried out in the northern part of Semarang city. Collection and analysis of data on the geology, hydrology, soil properties and monitored settlements were conducted. Modeling of land subsidence involved stress deformation analysis using finite element method. The settlements computed for selected section of the city of Semarang were compared with measurements of settlement in the city area. The simulation results appear to be in reasonably good agreement with the measurement results. Simulation results at selected section indicated that the contribution of lowering groundwater table factor accounts for less than 50% of total monitored subsidence. Meanwhile simulation results using application of external loading and lowering of groundwater table showed better agreement with the monitored subsidence. Information of magnitude and factors causing land subsidence is important particularly for city planning purposes

JEDNODIMENZIONALNA ANALIZA SEIZMIČKIH VRIJEDNOSTI NA SMJESTIŠTU NEKOMERCIJALNOGA NUKLEARNOG REAKTORA, SERPONG, INDONEZIJA

One dimensional seismic response analysis on the ground surface of the Non-Commercial Power Reactor (RDNK) site based on the mean uniform hazard spectrum (UHS) and disaggregation analysis has been conducted. The study’s objective was to perform an analysis on site-specific response spectra on the ground surface based on existing mean UHS and disaggregation data of the site that correspond to a 1,000 and 10,000 year return period of earthquakes in compliance with the national nuclear regulatory body requirements of Indonesia. Detailed site characterization was defined based on secondary data of a geotechnical drill-hole, seismic cross-hole, downhole data, and microtremor array data. The dynamic site characteristic analysis was presented along with strong motion selection and processing using two types of strong motion datasets. An investigation of strong motion selection, spectral matching, and scaling has been presented as an essential step in ground motion processing. One-dimensional equivalent linear analysis simulation was performed by computing the processed ground motions. A seismic design spectrum and ground surface response spectra from the two datasets of strong motion, both corresponding to a 10,000 and 1,000 year return period, are presented at the end of this study. This study has shown that in order to establish the appropriate seismic response design spectrum, site-specific data and seismic hazard analysis must be immensely considered.Jednodimenzionalna seizmička analiza na tlu u okolici nekomercijalnoga nuklearnog reaktora temeljena je na izučavanju srednjih, uniformnih spektara rizika te analizi dezagregacije tla. Analiza je načinjena na smjestištu koje je određeno svojim posebnim seizmičkim spektrom. Spomenute varijable izučavane su s obzirom na povratno razdoblje potresa od 1000 i 10 000 godina te su uspoređene s nacionalnim zakonodavstvom o sigurnosti nuklearnih postrojenja u Indoneziji. Analizirano područje detaljno je opisano iz sekundarnih podataka koji su obuhvatili geotehničke i sezimičke bušotinske podatke te mjerenja mikropotresa. Dinamička svojstva smjestišta predstavljena su usporedno s podatcima iz dvaju skupova mjerenja snažnih gibanja. Njihovo izučavanje spektralnom analizom te odabirom mjerenja opisano je kao temeljni korak u izučavanju gibanja tla. Jednodimenzionalna analiza načinjena je računalnom obradbom. Prikazani su seizmički i površinski spektri, za oba skupa podataka i oba povratna razdoblja. Pokazano je kako ove metode trebaju nužno biti korištene u analizi seizmičkoga rizika na smjestištima ove vrste

Swelling potential of volcanic residual soils in Sumatra (Indonesia) in relation to environmental issues

Swelling potential characterization of clay-silt soil is an essential issue in stabilization, settlement, consolidation, and land suitability studies. This article attempts to explain the swelling characteristics of soils around the area of West Lampung, Lampung Province, Sumatra, Indonesia, in relation to environmental issues. An investigation in relation to the soil swelling potential was carried out using 15 disturbed soil samples collectd in the study area. The methods used were analyses of clay mineral geochemistry, physical characteristics, and the free swell ratio. These results showed that the soil in the study area was Quaternary tropical volcanic residual soil. These soils were formed in a proximal volcanic hydrothermal alteration environment. The soils of the study area have characteristics of high plasticity, a reddish-brown colour, and are clayey silt grained (MH) (USCS). The soils had loose physical characteristics in dry conditions; however, these soils tends to be plastic and sticky in wet conditions. Evidence of groove erosion was found at the soil surface. Based on XRD analysis, kaolinite, halloysite, and montmorillonite were types of clay minerals found in the soil. The soil had a clay content of 11.05–78.9%, a liquid limit value > 50%, a plasticity index value of 16.7–36.9%, a shrinkage value of 14.24–36.89%, a soil activity of 0.38–2.47; and an FSR value of 0.69–0.95. These characteristics have implications for swelling soil potential. The results showed that the soils in the study area had medium to very high swelling potential. These results suggest a risk of erosion in the area, which could cause soil degradation and a change in water quality. These soils are likely to affect land productivity and aquifer replenishment and will cause negative environmental and economic impacts. Thus, soil improvement techniques are needed. It is important to maintaining vegetative cover these soils and revegetation may be required