North-East Line
Construction of the MRT North East Line comprises the main tunnelling work in Singapore at present. It involves some 20km of tunnel and 16 stations, all underground (Fig 1). A longitudinal section showing the geological features of the route is given in Fig 2.
The civil works have been split into 12 separate design+construct contracts. One is for the depot; six include one or two stations with associated bored tunnels; three include one or two stations with cut+cover tunnels; and two are for a single station each. These latter two station contracts have been singled out because of their complexity and are for the Peoples Park Station in a congested part of Singapore and the deep interchange station at Dhoby Ghaut.
Cut+cover tunnel construction is mostly at the northern end of the route in reasonably firm Old Alluvium.
The six contracts to use bored tunnel methods have a total of 11 twin bored drives with EPBMs and one twin bored tunnel drive from River Valley to Dhoby Ghaut using semi-mechanical open face machines. The first drive started in September 1998.
In total, 16 TBMs will be employed, all provided with air locks for face entry and continuous grouting systems to minimise settlements. Precast concrete tunnel linings of 5.8m i.d. are to be used throughout, generally having five segments plus key to form a ring 1.2m wide or, in the case of two of the contracts, 1.5m.
Double gaskets of EPDM and hydrophilic material will be used to seal the segment joints. The only exceptions to the universal use of precast concrete linings are for a 100m length of tunnel which overlies the future underground road scheme bored tunnels, where SGI linings will be used and a short section of NATM with in-situ concrete lining to complete a crossover where a tunnel width greater than 5.8m is required.
The TBMs will be launched from purpose built start shafts or from the station ends and will encounter the same wide range of ground conditions as found on the first phase MRT tunnels. Tunnelling will also be carried out for the first time in the Old Alluvium, which has a high quartz content and is potentially abrasive. About half of the TBMs will be used on one drive only and the other half on two drives.
On completion of their final drive, the machine skins will be left in the ground, tight up to the outside of the station end wall. The TBM skeleton and equipment will then be moved through the completed tunnel. This may seem wasteful, but the station design places most of the key plant rooms at the station ends and, to meet the overall programme, these plant rooms must be completed early and accessed by the M&E contractors, precluding recovery shafts inside the station to withdraw the TBMs.
All tunnel works are scheduled for completion in mid-2000.
MRT Extension to Changi Airport
A 6.5km extension from the existing MRT line west of Tanah Merah to Changi Airport is now under construction (Fig 1). The tunnel section begins after the intermediate station at the Exhibition Centre (also under construction) and comprises a 500m cut+cover section and 3.5km of twin bored tunnels, 5.4m i.d.
The line was designed by the LTA. Programme considerations resulted in the LTA purchasing by competitive tender two TBMs ahead of main tunnel contract award. This TBM contract was novated to the main contractor and the machines delivered in early 1999. The first drive started in July and the second was due to begin in October. Anticipated ground conditions are mainly Old Alluvium. comprising weakly cemented sandstones and mudstones with considerable weathering. Sections of the drive are likely to encounter soft marine clays and clayey sand/sandy clay in buried valleys.
The TBMs are 6.2m o.d. and care has been taken to ensure that they will be able to withstand the highly abrasive conditions.
The extension will be opened to traffic in 2001.
