At US$4.4bn, the Singapore Circle Line extension is one of the most ambitious tunnelling projects currently being undertaken in South East Asia. It involves extending the line 33km, over a five-year period, at a rate of up to 24m/day. When complete, the line will cut travel times and allow commuters to bypass busy interchanges like City Hall and Raffles Place and will link with the North-South, East-West and North-East Lines.
The last stages of the work, Four (CCL4) and Five (CCL5), are currently well underway and on schedule to open from 2010.
Discover B2B Marketing That Performs
Combine business intelligence and editorial excellence to reach engaged professionals across 36 leading media platforms.
CCL5 Contract
The CCL5 Contract has proved to be one of the most testing parts of the whole Circle Line construction and main contractor Sembawang Engineers & Constructors Pte Ltd (SEC) have had to contend with a range of engineering challenges. Nearly all of these have been down to the complex geological profile of the areas tunnelled. This phase of the line has encountered highly variable ground conditions and has involved excavating highly alternating sedimentary and alluvial deposits.
Simon Hoblyn, who has been with the Singapore Land Transit Authority (LTA) for three years and a total of 20 years in the tunnelling industry, is the project manager working alongside LTA director Rama Venkta on CCL5. Rama Venkta says that the work has been challenging: “The ground conditions are predominately sedimentary rock, but have been extremely variable in weathering. A significant number of interfaces and valleys of alluvial soft clays and fluvial sands were also encountered during the tunnelling works. One section may involve boring through hard rock material while the next soft clay. This has meant we have had a lot of factors to consider while tunnelling in such mixed face conditions along this part of the Circle Line.”
The much publicised collapse of Singapore’s Nicoll Highway in 2004 as a result of earlier Circle line extension work (for which prosecutions followed, see box on p24), was found to be the result of misinterpretation of local geology, making the current correct reading of the rock formations that much more crucial.
US Tariffs are shifting - will you react or anticipate?
Don’t let policy changes catch you off guard. Stay proactive with real-time data and expert analysis.
By GlobalDataAnother key issue has been groundwater levels, since tunnelling activity has the potential to partially drain water-laden clay, causing unwanted ground movement and settlement. To minimise disruption, SEC has employed three 6.6m diameter Herrenknecht EPBMs equipped with rock cutterheads dressed with 41 x 17” cutterdisks.
“The clay acts a bit like a sponge – the moment you drain away the water it has the capacity to shrink. You therefore need to use the right equipment or make adequate allowances. We used pressure-balanced TBMs and intensive monitoring to ensure that the watertable was kept within allowable limits,” says Venkta.
Delay during design, as a result of the Nicoll Highway collapse, and challenging ground conditions has caused station construction along the course of the Circle Line to be either re-scheduled or re-sequenced. As a consequence, some 7.5km of critical path tunnelling, which was originally slated to take 16 months with three TBMs, will take 22 months instead.
Hoblyn says CCL5 has benefited from Singaporean buildings’ generally high quality foundations but has encountered difficult ground formations. This has meant careful equipment planning and execution at each stage of the work.
“You can have very hard rock in one section of the alignment and soft clay in the next, so you need to know exactly what the geology is so that you can plan ahead the sort of equipment that you are going to need and when to change it,” said Venkta.
Urban considerations
Aside from the geology, CCL5’s contractors have had to allow for the fact that Singapore is a crowded island and they consequently have had to navigate their way underneath a range of structures, from public roads to residential housing and commercial blocks.
CCL5’s four underground stations, two cut and cover crossover boxes and an interchange station, linked by 3.5km of twin bore tunnels with 500m of single bore and overrun tunnel, are substantially located underneath public roads and within a built-up corridor. Along one section, the Circle Line follows an elevated 3-lane dual carriage highway, passing close to and between its piled foundations with at times less than 2m clearance from the TBM’s extrados to the bored piles of the viaduct pier.
Advanced planning allowed for the selection of a suitable tunnelling corridor and mitigation against the effects of tunnelling, such as negative skin friction on the piles due to tunnel-induced settlement. State-of-the-art monitoring is supplemented with traditional instrumentation to keep track of progress and ensure the safety of overlying structures and the public in general. Without such work, settlement of the soft clays could have compromised the piles. “We would only be talking minor movement of perhaps 50mm surface settlement, but even this small amount of movement could generate additional loading on the piles resulting in settlement to the pier.”
To date, strict procedures and vigilance have led to the successful excavation of 80% of the total alignment, with minimal surface disruption.
One of the CCL5 Contract Herrenknecht EPBMs Map showing the various phases of Singapore’s Circle Line PROJECT FACTFILE
Cost
Total cost of Circle Line Extn – S$6.7bn (US$4.4bn)
Value of CCL5 Contract – S$335M (US$222M)
Timescale of CCL5
Originally scheduled to take 16 months. Due to re-sequencing of works, following the Nicoll collapse, tunnelling will now last 22 months
Workforce
The project team for CCL5 consists of 20 LTA management, engineering and inspection staff; 25 contractor’s management, engineering and supervisory staff; and 230 tunnel workers
Contractors
Sembawang Engineers & Constructors is main contractor and Tri Tech Engineering for instrumentation
Tunnelling techniques
EPBM; Rock TBM; Soft ground SCL; Rock SCL; Cut & Cover
Machinery used
3 x 6.6m Herrenknecht EPBMs, TBM supply via diesel locos and rolling stock, plus gantry crane
NICOLL COLLAPSE
On 20 April 2004 a tunnel support structure within one of the open cut stations on the Circle Line Extension gave way. The resulting 30m (100ft) collapse spread across six lanes of Singapore’s Nicoll Highway, killing four people and injuring three (T&TI, February 2007, p31).
An investigation showed that a key reason was the misinterpretation of local geology and overestimated soil shear strengths. The structure was consequently under designed to resist lateral earth pressure. Errors in detailing the structural connections for the bracing system were made and the collapse occurred when the lower level of bracing became overloaded and there was inadequate capacity to redistribute the loads among the remaining supports.
Although large wall deflections occurred during the excavation, the measured strut loads were smaller than expected. As a result, project engineers were apparently unaware of the potential for a catastrophic failure.
A committee of inquiry found main contractor Nishimatsu Construction Company as well as Singapore’s Land Transport Authority responsible for the collapse. Subcontractors were reprimanded and issued warnings in connection with the accident. The incident was a major contributory factor to delays in the line extension and the affected station has been shifted about 100m (330ft) away from the accident site.
Four men faced charges in the wake of
the Nicoll Highway Collapse. The Nicoll Highway incident came as a result of cut- and cover work, so the implications for TBM tunnelling work concerned the evaluation of risk assessment.
