Contractor Ed. Züblin has achieved breakthrough ahead of schedule on the second and final of two drives on Contract 706 of the North East MRT Line in Singapore.
Contract 706 includes the construction of four single-rail tunnels with a total length of 3.3km in two separate sections of twin tunnels. For each section, an EPBM was employed.
Tunnelling started on the Farrer Park to Boon Keng (FRP) drive on October 23 1998 and was completed on November 3 1999. Excavation of the second section, the Little India to Dhoby Ghaut (LTI) section, started on February 22 1999. Breakthrough occurred at the turn of the millennium after the installation of 985 rings.
The FRP was constructed through two marine clay valleys that interrupted the predominant old alluvium. The LTI tunnelling team encountered mainly mud and sandstone.
The tunnel lining, with an i.d. of 5.5m and a thickness of 0.25m, consists of 1.5m wide precast concrete segments with one ring composed of 5 segments plus a key.
Mucking out and segment transport in the tunnels was carried out with tunnel trucks because of space restrictions in the starter shafts. Although this system caused interruptions to TBM advance when trucks/muck skips needed to be changed, the best advance rates achieved by Ed. Züblin were 28.5m in a day, 141m in a week and 406.5m in a month.
The breakthrough into an adjacent station was the first on the North East MRT Line and was a welcome change from the previous three times when the TBMs ended up in front of diaphragm or soldier pile walls, where the last 1.5m had to be excavated manually and lined with shotcrete in order for the TBM to be dismantled and moved back through the tunnels.
