NATURAL COMPACTION OF SEMARANG DEMAK ALLUVIAL DEPOSIT

• Main Author: Sarah, Dwi
• Format: Dissertations
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/38685
• Institution: Institut Teknologi Bandung

Land subsidence has become a global problem threatening the development of large cities across the world, including cities in Indonesia. Physical damages due to land subsidence have caused enormous economic costs and hindering the sustainable development of the area. Semarang Demak plain has been reported to experience land subsidence since the 1980s until now. The land subsidence is caused by the combination of natural compaction and anthropogenic factor (groundwater extraction and building loads). The Recent sedimentation of the alluvial deposit forming the Semarang Demak plain resulted in some near surface sediment that has not been fully compacted yet. Understanding of the natural compaction factor is important to comprehend the land subsidence mechanism in Recent sediment. This study aims to prove the occurrence of natural compaction in the Semarang Demak alluvial plain, to explain the factors influencing the natural compaction, and to obtain the rate of natural compaction. Natural compaction takes place in clays with pore pressure in excess of hydrostatic (overpressure). Overpressure can be developed in clays which are deposited very rapidly, where the pore pressure has not yet attained equilibrium state while the overlying overburden keeps increasing. Overpressure condition has transient nature and will dissipate towards lower pressure, causing the sediment to compact naturally. The dissipation of overpressure would still occur even after the sedimentation has ceased. Combination of Cone Penetration Test with pore water measurement (CPTu) and laboratory consolidation test has proved the occurrence of overpressure at shallow deposit (maximum of 35 m depth) in the study area. Methods employed in this research included the integration of subsurface data (borehole and CPTu logs), physical properties and consolidation tests of clay, clay mineralogy, groundwater quality testing, and modeling of natural compaction rate. Clay as a very compressible stratum determines the mechanism of natural compaction. The compressibility of clay is influenced by the clay mineralogy, cation type and exchange capacity, fabric, and salinity of the porewater. Result shows that the Semarang Demak clay is classifed into high activity clay, dominated by Na-montmorillonite that has high cation exchange capacity. The transgression and regression of the sea due to the global sea level fluctuation during the sedimentation of Semarang Demak plain causes the occurrence of fresh and brackish groundwater as the result of deposition. Study result shows that the variety of groundwater condition affects the compressive behavior of clay. Clay with brackish porewater tends to compress faster and easier due to its parallel fabric. The parallel fabric facilitates the faster dissipation of porewater and is mechanically more compressible. Subsurface profile of Semarang Demak plain reveals the interfingering between deltaic and tidal deposit on the upper part that lies unconformably on top of the Damar Formation. The deposit consists of sand, silt and clay layers intercalated with lenses of sand and gravels. The clay layer occupies the upper part with thickness varying from 5 to 60 meter that is getting thicker towards the north and east. Modeling results shows that the rate of natural compaction varies spatially, ranging from 0.0 - 2.2 cm/year. The natural compaction in Semarang city is relatively lower (<1.1 cm/year), probably due to depletion of the pore pressure by the rapidly declining piezometric pressure of the heavily exploited Damar Formation aquifer. The natural compaction rate is higher in the east (0.9 - 2.2 cm/year) due to the less anthropogenic force in this area. The novelties of this research include the identification of overpressure in shallow deposit, quantification of natural compaction rate, and the interaction of chemo-hydro-mechanics of Semarang Demak clay. The scientific contribution is related to the knowledge of the chemo-hydro-mechanical properties of the Semarang Demak clay and its susceptibility to natural compaction. The output of this research is advantageous for the Semarang Demak stakeholders as the basis of regional management plan in the subsidence prone area, and could be used as a reference for other coastal areas experiencing similar problem.