Breakthrough last month of the last EPBM drive marked completion, without incident, of all 5.7km of twin tube running tunnel for the middle, Stage 3, of the Circle Line (CCL) beneath Singapore.

The final breakthrough on 21 March occurred on Contract 852 when the second 6.63m diameter Herrenknecht EPBM working for the Woh Hup/Shanghai Tunnel Engineering/Alpine Mayreder Bau JV emerged at the west end of Bartley Station to complete the 1200m drive from Serangoon Station in 345 days.

About three months earlier its twin machine completed the parallel tunnel in 349 days working two 12h shifts/day and averaging about 50m/seven day week for both drives.

The Bartley tunnels run 20-25m below ground and pass through variable soil/rock interfaces and high groundwater content in the residual clay material of the Bukit Timah Formation and the estuarine and fluvial clay overlay of the Kallang Formation. For a length of about 245m, the tunnels had to be driven to within 2.5m apart. The lead tunnel was braced with some 180 temporary supports and more than 1700 GFRP ground-stabilising dowels before the second TBM arrived. Effective performance of double screw conveyors in the machines limited average volume loss to 0.5% for most of the drives.

According to a Land Transport Authority (LTA) supervising engineer: “Only two localised areas reached 2.5% and 3% volume loss”. A belt-weighing device monitored spoil removal in real time.

Prior to the Bartley drives, the TBMs completed the tunnels from Serangoon station westward to Lorong Chuan Station driving primarily through competent old alluvium Kallang Formation. Average volume losses for these drives was recorded at 1%-1.5% as compared to an estimated 3%.

The 33km long, entirely underground, Circle Line with 29 stations is being constructed in five stages (T&TI, April 2006, p21). The middle, almost US$1.4bn, CCL Stage 3 from Bartley Road to Marymount Road, includes five underground stations, two of which, at Serangoon and Bishan, are interchanges with the North East and the North South Lines, respectively.

Construction of the five underground stations on CCL3 is almost complete and major electrical and mechanical equipment for the interchange stations is in an advanced stage of installation.

The Circle Line is due to open in 2010 to offer inter-suburban linkages to bypass the city centre and reduce travel times.