Significant development of Jakarta infrastructure is necessary to keep up with the economic growth. For the new Jakarta underground rail network, imminent extension of the North-South Line Phase 1 (NSL-P1) tunnel heads towards the northern part of city. One of the engineering challenges that exist in the area is the substantial amount of reported surface settlement over the years, which is indicated by the continuous sea wall improvement project and the increase of flooded area. The recent GPS survey reported an average subsidence rate of 5 centimeters per year and could reach a maximum of 15 centimeters per year on several hotspots in North Jakarta.
Being an underground structure, the rail tunnel would be affected by the settling environment and the reciprocity can be expected. Operational interruptions of the tunnel could occur in the future as a product of differential structural displacement. On the other hand, alterations in settlement rate or pattern must also be anticipated. The aforementioned reciprocity underlines the importance of structural and geotechnical assessments in the area. The assessment then can be used as a reference to determine and improve the safety level of the tunnel as well as the surrounding infrastructures.
The study was commenced with the investigation of prevalent driving factors of Jakarta land subsidence from the preceding researches. The initial stage of the study elaborated the concepts used in the analysis, including geotechnical data interpretation, soil consolidation and creep, numerical model formulation, and past studies regarding the loads on bored tunnels.
A segment of proposed North-South Line Phase 2 (NSL-P2) tunnel was selected for this study. The selection was motivated by the the amount of available geotechnical information and the severity of differential land subsidence in the respective area. Longitudinal and cross-sectional two-dimensional numerical model of the selected segment were developed in Plaxis 2D, based on the combination of in-situ soil tests and the outcome of earlier studies about Jakarta geotechnical characteristics. Into the model, four time-dependent groundwater level scenarios were assigned to simulate the surface settlement. As the research emphasizes on the long-term settlement, a 100-year study period was chosen and started in the year 2000.
Given that the NSL-P2 tunnel design has not been confirmed at the time of writing, the numerical study adopted an identical design to the NSL-P1 tunnel. A 6.65-m diameter concrete tunnel was added into the model at an average depth of 15-m. From the longitudinal numerical analysis, total structural displacement in time and additional longitudinal forces were obtained. Subsequently, further analysis was performed on the cross-section, in the transverse direction, which displayed most settlement at the end of the analysis period. At the cross-sectional perspective, the development of forces as well as soil stress around the tunnel ring due to settlement and structural deformation were acquired.
Finally, this study reached a general conclusion which explains that the land subsidence in Jakarta posed non-governing additional loads to the future NSL-P2 tunnel. A majority of the total surface settlement was caused by the consolidation and compression of the upper soil layers. However, special attention must be paid to the station-tunnel interface as substantial differential settlement could take place. To minimize further issues, several design recommendations are provided at the end of the research.