#TITLE_ALTERNATIVE#

• Main Author: (NIM 32294021), MARSUDI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/8542
• Institution: Institut Teknologi Bandung

Negative impact may occur if the utilization of natural resources is not properly managed. One of the negative impacts is land subsidence that is caused by excessive pumping of groundwater and by filling of embankment over alluvium plain such as occurring in Semarang. <br /> <br /> <br /> Geologically, Semarang plain consists of alluvial deposit, delta plain sediments as well as tidal flat. These deposits comprise alternating sand, silty sand, and soft clay with lenses of gravel and volcanic sand. Such geological condition enables Semarang plain to have significant potential of groundwater availability. The presence of soft clay that is thick and located in the upper part at 2 m - 30 m bellow ground surface tend to accelerate the process of consolidation. <br /> <br /> <br /> Drinking water required for the local inhabitants of Semarang City (1.974.392 peoples), as well as for industries, and other purposes in 1996 is approximately 88.705.000 m3/ year. Most of the supplied water is originated from groundwater. Significant amount of groundwater pumping has resulted in a lowering of groundwater level that reached up to 1.5 m to 22 m ugds in 1996. <br /> <br /> <br /> The lowering of ground water level results in increase of effective stress of the ground. When the effective stress exceeds effective stress previously received by the ground, soil will consolidate and compact that leads to land subsidence in normally consolidated zones. <br /> <br /> <br /> It is predicted that the land subsidence that has been occurring in the Semarang plain is caused by two factors; i.e. lowering of ground water level due to pumping, and increase of overburden pressure due to earth filling for embankment. <br /> <br /> <br /> Tectonic condition of Java Island that was active in the Late Pliocene to Middle Pleistocene period has produced complex geological structures in the south of the <br /> <br /> study area. It has not been clearly understood whether these active fault structures influence the process of land subsidence in Semarang alluvial plain. Micro seismicity map of Central Java indicates that seismicity of the study area ranges from 3.0 to 5.0 of the Richter's scale. <br /> <br /> <br /> The piling of earth material for coastal reclamation in the study area was started in 1980, and it covered the area of PRPP, Tanah Mas, Bandarharjo, Tanjung Mas Harbor, Tambaklorog. The reclamation was then followed for other location in 1996. The reclamation was conducted by filling of earth material from I m to 5 m and followed by development of buildings for housing and offices. <br /> <br /> <br /> In order to calculate and to predict the rate of land subsidence due to lowering of groundwater level and due to earth filling, calculation of consolidation settlement (MI) for clay layers is conducted. It is conducted based on the transfer of stresses from excess pore water pressure to effective stress of soil. The process of one dimensional consolidation (1-D) from Terzaghi is numerically performed using the finite different method proposed by Kai Sin Wong (1988). This numerical simulation enables the calculation of land subsidence for ground condition having multi layers of clay soil. <br /> <br /> <br /> The calculation results of consolidation settlement due to lowering groundwater level and based on the 1-D model is verified by actual field observation and compared by using the visco-elastic consolidation model from Taylor-Merchant (1940) that was subsequently developed by Maathuis H. etal, (1996). Calibration for the models are performed by adjusting input parameters. The input parameters for soils are adjusted until the values of consolidation settlement are equal or closed to the actual field observation. Using the adjusted parameters, prediction of the rate of land subsidence is then conducted. <br /> <br /> <br /> Prediction of land subsidence based on the (1-D) consolidation model for the year of 2013 ranges from 87 to 190 cm. It is obtained that the amounts of land subsidence become larger toward north - north east direction. It is also found that the amount of <br /> <br /> land subsidence for the Semarang plain increases toward coastal area. This finding is appropriate with the pattern of groundwater lowering, distribution of the thickness of soft clay layers, thickness of earth filling, and the presence of sand layers in the delta plain sediment. <br /> <br /> <br /> Calculation results of land subsidence based on the 1-D model from Terzaghi are closed to those of the visco-elastic model from Taylor-Merchant up to 20 years. Beyond 20 years, the visco-elastic model yields lower values of subsidence. <br /> <br /> <br /> Based upon the results of numerical simulation using 1-D model, it can be concluded that the land subsidence around the coastal area is mainly caused by increase of load from earth filling for reclamation and from buildings. It's contribution ranges from 52 % to 59 % of the total subsidence. Whereas at areas located far away from the coastal area, the subsidence is largely affected by the lowering ground water level. It's contribution varies from 52 % to 64 % of the total subsidence. <br /> <br /> <br /> The results of numerical simulation indicate that the analytical method for consolidation processes and the numerical simulation based on 1-D model that considers the change of effective stress due to the lowering of groundwater surface and due to the presence of earth filling can be applied to accurately predict the ground settlement. <br /> <br /> <br /> It can also be expected that the simulation of land subsidence model provides valuable contribution to the knowledge concerning land subsidence. The simulation may be applied for other areas particularly for Resent alluvial plains or areas consisting of unconsolidated / normally consolidated sediments. <br /> <br /> <br /> Furthermore, results of this study can be used as an important consideration in the development of policy and planning related to the development of infra structures and land reclamation and related to water resource management and conservation in coastal areas, not only for Semarang but also for other urban coastal areas in Indonesia. <br />