ABSTRACT
With the development of underground tunnel infrastructure in urban cities congestion is inevitable. New tunnel systems are being constructed in close proximity to existing systems providing challenges of minimising movements to the existing systems. The North East Line Project (NELP) presented Singapore with its first such challenge, at Chinatown and Dhoby Ghaut. At Chinatown, the NELP twin bored tunnels passed less than 5 metres beneath the existing East-West Mass Rapid Transit (MRT) tunnels. Safe operation of the existing line had to be ensured throughout construction. This paper focuses on the measures adopted to maintain minimal movements, a study of the movements generated and their impact on the existing underground line. These measures centered on a protective pipe roof between the new and existing tunnels and ground control through the use of an Earth Pressure Balance Machine (EPBM). Extensive monitoring of the ground and existing tunnels was undertaken at “real time” frequencies. Induced movements were compared to ten predetermined limit states to ensure the safe operation of the line. The recorded movements and their influence on the existing MRT are presented and assessed. The magnitude of the movements illustrates how, with good construction control and appropriate techniques, such tunnelling can be achieved with minimal impact.
