Numerical Analysis of Land Subsidence in Semarang Low Area Due to Groundwater Extraction

Semarang City, located in the lowlands area of Central Java Province, is densely populated. This will certainly have an impact on the carrying capacity of the land as a place to live, including the need for water consumption in the form of groundwater extraction for household or industrial needs. Continuous groundwater withdrawal will result in continuous land subsidence. This study is intent to searching how much settlement were currently occurred because of soil consolidation, and how much will occur because of soil consolidation and ground water extraction. The method used in this study is to perform deep well modeling using numerical software to determine the prediction of land subsidence due to soil consolidation and groundwater extraction. The lower Semarang area in this study is divided into two zones, first zone namely the area of Semarang North West at Tugu Subdistrict, and the second zone is at East Semarang Subdistrict. The results of consolidation analysis in Tugu Subdistrict and East Semarang Subdistricts showed that without pumping, the subsidence reached 37.4 cm and 37.3 cm in 3 years or 12.5 cm/year and 12.4 cm/year, respectively. The rate of settlement without pumping is in the range of soil rate settlement measured by Abidin, et all [8]. With pumping, the settlement increased to 33.8 cm/year at Tugu Subdistrict and 38.8 cm/year at East Semarang Subdistrict. The percentage of subsidence relative to the compressible soil thickness ranged from 0.32% to 0.51% without pumping and 0.87% to 1.58% with pumping. Based on the results obtained, it appears that at the study site, the use of pumps can accelerate the consolidation process and increase the rate of land subsidence each year. Along with the development of the area in Semarang City, the use of pumps in the area can increase the rate of soil settlement. To maintain soil settlement in the long term, it is recommended that a better fresh water supply system has to be implemented, so that the use of pumps can be minimized or stopped.