The scorching equatorial sun beats down on the quiet streets of Singapore. City dwellers hurry from one air-conditioned safe haven to another, avoiding the sweltering heat wherever is possible.
The rich developments on the Marina Bay Sands sit side by side with the colonial classics such as the infamous Raffles Hotel. Brilliant glass buildings with crazy architecture define Singapore as a modern city on the forefront of technology and thinking.
In 1983, when engineers first set to work on the city-state’s mass rapid transit (MRT) metro, Singapore had a population of approximately 2.5M. The government instructed engineers to develop a network fit for a little over 3M. The city’s population last year broke 5M. This underestimate is the reason why the MRT operates six carriage trains when eight would be more appropriate.
Downtown Line stage two director Ng Kee Nam, who works for client the Land Transport Authority, says, “When LTA launched its Land Transport Master Plan it decided to make rail the backbone of the public transportation system in Singapore. The plan [launched in March 2008] is to double the network in the next 10 years from 195km to more than 300km.”
The LTA is responsible for all transport in Singapore. The roads in Singapore are not congested, traffic flows with relative ease compared with major metropolises elsewhere in the world. This is because of the very powerful traffic control methods the government has at it disposal. There are three methods employed: a more than 100 per cent tax on car purchases making it very expensive to own a car in Singapore; a permit for car ownership called a Certificate of Entitlement, which can cost thousands of dollars (Singaporean and US) and a congestion charging zone that discourages drivers from entering the city.
To continue with such a tough anti-car stance it is vital the city can offer a reliable and comprehensive public transport option and this is the main driver behind the LTMP and the extension to the twin bore Downtown Line (DTL).
The extension
Stage one of the DTL had only just been announced when the LTA began working on plans for its extension. DTL will be the fifth MRT line in Singapore. When fully completed, the line will be about 42km long with 34 stations. It will be a fully-automatic and driverless system similar to the North East Line and the Circle Line, and will be fully underground.
Somewhat similar to the East West Line, the line will serve the Bukit Timah area and eastern Singapore and bring commuters to the developments in the downtown area. The line is coloured brown on the rail map.
Stage one of the DTL is targeted for completion in 2013, while stage two is targeted for 2015 and stage three is targeted for 2017. When completed, the line is expected to serve some 500,000 commuters daily and reduce travelling time from Bukit Panjang to Expo to about 65 minutes.
As construction on stage one nears an end and stage three is still in planning, stage two is preparing for its peak next year. Construction started on stage two with a groundbreaking ceremony at Beauty World station in July 2009. Nam says, “Downtown Line two is 16.6km with 12 stations and one depot. The project is split into 10 main civil contracts and is fully underground.”
At the northern end of the line the tunnels run under less populated areas of the city. There is ample space to run the tracks above ground. Perhaps the government has learnt a lesson from its earlier mistake of opting for six-carriage trains and is now keen to future proof the project. LTA project manager for civil construction team one, Simon Hoblyn, says, “The instructions came for the whole lot to go underground so that’s what we’re doing.”
The Singapore government’s decision not to hold public consultation on the alignment of the tunnels or the location of the stations was crucial to meeting the political promises in the LTMP. The 2015 delivery target for stage two is the single greatest driver for the project.
Hoblyn says, “The philosophy has been to put program first so that we can deliver the tunnels on time. So we have not tried to build particularly long stretches or drive through stations or have complicated interfaces. We have actually come up with more TBMs with a typical drive distance of 1km or less to try and ensure we deliver the job on time. Occasionally we do reuse the TBMs but it is normal for each TBM not to drive more than 1km in total. At the end of the drive the machines are dismantled underground and taken away.”
The TBMs will not be removed through the stations, as one LTA requirement is to separate the station and tunnel works. Holbyn explains, “Station and tunnelprograms often don’t coincide. So wherever possible we have given the TBMs their own launch shafts and at the interfaces we do not guarantee the contractor access through the stations and so they will need to take the machine back through the tunnel.”
The stage two alignment, which runs as deep as 35m below the city, has been broken into two sections, civils team one and civils team two. It runs relatively flat with a maximum gradient of 3 per cent. Team one cover the northern end furthest from the city and team two cover the southern end, nearest the city.
The two teams are currently enabling the tunnelling works with shafts going down. Some TBMs are under manufacture while others have been delivered and waiting to go underground. Tunnelling is set to get underway in April.
Fit for purpose
Much of the program for this project is based on lessons learnt and one of the greatest lessons learnt on earlier lines, especially the Circle line, is just how harsh Singapore’s tunnelling environment is. The Bukit Timah granite is a fiercely hard rock and in many stretches along the stage two alignment the face dips in and out of soft ground and very hard rock.
The toughest conditions are in the mixed faces – hard rock in the invert and weathered rock in the face. The rock itself can range from typically 100-250mpa, very fresh and very abrasive. Along the entire alignment the rock is dipping in and out of the face.
Team one uses nine TBMs, seven of which are slurry machines and two of which are EPBMs. Team two uses 10 TBMs, two slurries and eight EPBMs, explains Hoblyn.
Hoblyn says, “We were careful where we saw a high percentage of Bukit Timah granite. We’ve had experience from Circle Line and other projects here and it’s strength is very hard and where it is mixed face, where the rock has become weathered beyond rock grade three and four the EPBs do not operate efficiently, they struggle to maintain face pressure and they struggle to maintain excavation volume control. So we as LTA took the decision when we put the contracts out to tender to specify slurry machines where we saw Bukit Timah granite.
“The majority of Bukit Timah granite is coming down from the northwest that dips towards the interface [between civils team one and civils team two] so they have a few slurry machines there. Where we are tunnelling more in the soil of granite we use the EPBs.”
On DTL stage two the client has left it to the contractor to purchase the TBMs but did specify some of the details.
The first stretch to get underway will be McConnel Dowell’s two 1km drives in the central section of the project in April. The TBMs are 6.6m external diameter Herrenknecht slurry machines.
“They are really quite powerful at 1750kw drive. The power was specified by the contractor as there is a lot of rock expected and a lot of mixed ground expected, they want the assurance that the TBM is able to perform no matter what. It is part of our philosophy of wanting the job to finish on time – it is better to be looking at it than for it,” says Hoblyn.
Herrenknecht country manager Dirk Schrader adds, “It is about 30 per cent full face rock, 40 per cent mixed face and the rest will be soil…Our approach was to learn from experiences in dealing with the Bukit Timah granite and increase the power, increase the torque. We changed the slurry circuits, improved the cutterhead design and cutter tools spacing. So a lot of improvements were made to avoid the experiences of the past.”
Once the machine is in the ground it cannot be changed but the geology will and in Singapore it is notorious for changing drastically within a metre. “I haven’t come across any other geology that is as complex and difficult as Singapore, to design a machine that can overcome all the unknowns was a challenge,” says Schrader.
To cope with the unknowns he says, “We increased the power and the torque. We changed the cutterhead design. We changed the opening ratio of the cutter face. We also narrowed the spacing between the cutters in order to treat the rock as a rock. We increased the number of cutters in the face and centre and we increased the number of cutters in the gauge area, we increased the gauge cutters on the last track and the second to last track to ensure we always have a certain over cut.
“Previously we only had one cutter on the final track, just one cutter to cut this over cut. If you don’t change this cutter properly the machine will get stuck and that’s what happened on earlier projects. Increasing the number of gauge cutters increases the amount wear the machine can withstand without reducing the operation skills. We had to design the machine as a hard rock machine that could cope with softer sections, which means there is a huge compromise.”
The machine is fitted with 17-inch wedge lock type disc cutters with an 18-inch tip. The cutters are pressure compensating to help cope with the mixed conditions. The face is fitted with wear detection on the cutting wheels and cutter tools and a double chamber man-lock, which is in compliance to EU regulations, to allow access to the face.
To finish the tunnel the LTA has specified it’s standard tunnel segments in a 5:1 ratio. The segments are 6350mm long and some 1380mm wide. The tunnel will be made water tight with gaskets.
Map showing the geology of Singapore and the route of the underground rail system First TBM launch chamber for Downtown Line stage two
