Conventional tunneling (NATM) in urban environment is always a challenge where surrounding buildings or structures are sensitive to settlement even though ground improvement usually serve as solution. In this case study, ground improvement from ground surface was not able to be carried out due to existence of multiple underground utilities. With mentioned restriction, chemical grouting was applied inside the tunnel during excavation. However, huge water inflow was encountered, and water table drawdown caused severe settlement within days. Additional chemical grouting was applied to refrain further settlement with continuous excavation. After the completion of excavation with water plugged off, water table recovered and 1/3 of settlement recovered. As a result, the effective conventional tunneling in urban environment is to limit water inflow and shorten the exposure period of the ground.

1. Introduction

When tunneling in urban environment, minimizing settlement and prevention of damage on surrounding buildings, structures, roads and underground utilities is always the top priority. Ground improvement is commonly carried out beforehand and usually from the surface. In this paper, cross passages between main line were required for emergency purpose for Thomson-East Coast Line as shown in Figure 1. The cross passages will connect to main, Adit tunnel and finally to escape shaft, while both cross sections and Adit were excavated by New Austrian Tunneling Method (NATM). The effectiveness of various measures for conventional tunneling in urban environment shall be discussed.

2. Project overview &Challenges

In Contract T207, 48m Adit tunnel which connect the main tunnel and escape shaft 1 (ES-1) is an important exit passage for emergency purpose. The geology consists of sandy SILT of GV-GVI Bukit Timah GRANITE (SPT:20-30) with shallow overburden (approximate 14m) and proximity with water reservoir. Adit tunnel could be excavated by NATM within settlement limit with the existing properties. However, as precaution measure, ground improvement by Jet Grouting Piling (JGP) from surface to eliminate the possibility of unexpected water ingress.

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