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South East Asia | Singapore's rectangular TBM

Rectangular Box Jack TBM

Singapore has embarked on a huge programme of public transport expansion aimed at encouraging people out of their cars and onto buses and trains. The country has an area of just 71,800ha and a growing population, which is currently around 6M.

“It is a very small country, but there is a lot of private transport use,” explains Henry Foo, who is a project director on one of Singapore Land Transport Authority’s most ambitious new public transport schemes, the Thomson East-Coast Line. “We would like to do much more to encourage people to take public transport.”

The Land Transport Authority plans to double the length of the island’s rail network from 178km to 360km by 2030, as well as introducing 80 new bus routes, 24 of which will run parallel to existing mass rapid transit lines. The aim, says Foo, is to have 75% of all journeys in peak hours to be made using on public transport, compared with 62% now; and for 80% of households to be within 10 minutes walk of a train station. 

Rectangular Box Jack TBM

Rectangular Box Jack TBM

The Chinese TBM will drive underpasses beneath a busy five lane road

The existing light rail and MRT network consists mainly of radial routes converging in the central business district in the south. Many of these are being extended under the Land Authority’s masterplan, but there will also be an entirely new north/south line, a regional line and a cross-island line running from the north east to the west.

One of these new routes is the 43km Thomson-East Coast line, which will run north/south, and then along the south east coast, taking in 31 stations and seven interchanges with other lines. It will be underground for its entire length.

All of the stations will be built in cut and cover, with tunnel boring machines (TBMs) being used to excavate the running tunnels between stations. The pedestrian adits within the stations are mainly being excavated using cut and cover or pipe roofing by micro TBM.

But at two stations – Havelock and Stevens, in the congested central area – the Land Transport Authority and its contractors will be using a rectangular cross section TBM for passenger underpasses, to improve construction productivity. The underpasses were due to be built in cut and cover, but this would have involved ground treatment to control settlement, as well as protecting and diverting utilities and installing piles and beams to support the excavation. 

TBM launch shaft

TBM launch shaft

The machine will push under the road from an adjacent launch shaft

Instead, the Authority looked for trenchless technologies that could successfully excavate lengths of up to 150m in soft soils at shallow depth, and found two possible rectangular TBM options in Japan and China. The two machines operate quite differently, with the Japanese machine including segmental lining erection within the shield, while the Chinese machine uses a technology that is similar to pipe jacking, with lining segments being pushed into the tunnel.

Both machines are suitable for soft and homogeneous ground, but the Japanese machine requires a minimum overburden of at least 1.5 times the depth of the excavation. As this is not the case at the Singapore sites, the Authority commissioned the rectangular box jack TBM from Chinese contractor China Railway Equipment Engineering Group, with China Railway Tunnelling Group the sub-contractor doing the tunnelling works.

Thomson east coast line

Thomson east coast line

The contract was awarded in December 2014, and the TBM was due to arrive at Havelock station last month [February], as part of contractor Gammon’s station construction contract.

“Havelock station is a busy interchange,” explains Foo. “The underpass has to go under a busy five lane road, between houses and offices and a hotel. “We had originally expected it to be done using cut and cover, but cut and cover is a lot of work. It would have meant lots of traffic diversions, utility diversions, a heavy support system, and a lot of noise and dust.”

These problems are all overcome by using the rectangular box jack machine, which has the added advantage of a 30% saving in manpower compared with cut and cover. 

The earth pressure balance TBM incorporates a system to pump bentonite into the excavation in front of the cutter head. The machine is propelled by jacks pushing against a wall inside a jacking pit, and the 47t rectangular precast concrete lining sections are added as it moves forward to create a monolithic concrete box.

“It is much simpler and safer, and there are huge productivity benefits, as well as high quality workmanship,” says Foo.

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