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Subway stories

Construction of Singapore's new Downtown Line involves squeezing cut and cover tunnels between existing underground railway lines and the surface.

While politicians the world over talk about the benefits of public transport, Singapore acts, forging ahead with construction of new mass rapid transit (MRT) lines. One of the lines now under construction is a 4.3km section of the Downtown line.

Previous tunnelling challenges in Singapore have been presented by the depth of excavation. The challenge close to the new line's Cross Street station, by contrast, is the lack of depth.

Downtown Line excavation depths are typically 24m to 25m, "but within our section, the Downtown Line crosses over the existing East-West MRT line", says Mott MacDonald project director for Cross Street Station design Chris Whiting.

"At the crossing point the existing line runs in two tunnels that are stacked one above the other. That makes it impractical to go underneath it would be immensely deep.

Instead, the line must be squeezed between the East-West Line and the surface.

"The East-West Line has somewhere between 10m and 12m of cover, which provides just enough space to put the Downtown Line in over the top. But it's extremely tight. Clearance between the two sets of tunnels will be less than 2m. Depending on finalisation of the design it could be less than 1m."

The East-West Line tunnels are segmentally lined so they rely on uniform ground pressure for their structural integrity. Excavating above an existing tunnel can cause heave and even collapse and in Singapore's soft marine clay, loss of earth pressure is a significant concern.

"Work will have to be carried out using restricted amounts of excavation at a time, with earth pressures restored in one area before moving on to tackle another," says Whiting.

Where the Down Town Line crosses the East-West Line tunnels, the solution has been to use a top down construction method using a series of techniques to prevent ground heave.

Thick temporary concrete diaphragm walls 35m deep will be constructed either side of the East-West Line tunnels to protect them from any movement induced by excavation of Downtown Line tunnel sections on either side.

Where the lines actually cross, the Downtown Line's permanent diaphragm walls will be extremely shallow, terminating just above the crown of the upper East-West Line tunnel. This will require delicate excavation.

When sections of the Downtown Line either side of the crossing point have been excavated to formation level, areas adjacent to the temporary diaphragm walls will be deepened. This will provide working space for the installation of a mat, comprising 300mm diameter pipes reinforced with steel I-beams, immediately above the East-West Line tunnels.

A pair of reaction frames will then be placed in trenches aligned with the outer edges of the East-West Line tunnels. The frames will bear onto the pipe mat, and will be fitted with jacks.

With these in place, the heavily reinforced roof slab will be cast and excavation of the Downtown Line crossing can begin.

As spoil is removed through hatches in the roof slab, the jacks, reaction frames and pipe mat will be used to resist any heave detected by instrumentation in the East-West Line tunnels.

The pipe mat will form part of the new tunnel's permanent base slab. The lower part of the temporary diaphragm headwall will be left in-situ. But once the permanent structure is complete, the upper part of the temporary headwall will be broken out.

Project overview

The Downtown Line will eventually be 40km long with 33 stations connecting the north-west and east regions of Singapore. The city's Land Transport Authority (LTA) is starting with a 4.3km section snaking through the central business district.

The route passes close to Nicoll Highway, and lessons learned from the serious tunnel collapse there in 2004 are evident in the planned design and construction of the new line.

UK consultant Mott MacDonald is designing one of the line's six new stations, Cross Street, plus the adjoining cut-and-cover tunnel for design and build contractor joint venture Samsung-Soletanche Bachy.

The consultant is also independent checking engineer and construction supervisor for Bayfront Station, being built beneath Singapore's vast Integrated Resort development (see p20). Contractor for the resort and the station is Sembawang Engineers & Constructors.

Singapore has expanded across a vast tract of reclaimed land over the past four decades and the area served by the first section of the Downtown Line extends from the original shoreline to the present day one. Most tunnelling and station box excavation is taking place in soft marine clay - a material described as being similar to toothpaste.

Braced excavation, calling for heavy-duty temporary works, has been a standard feature of Singapore MRT construction projects. But since the Nicoll Highway collapse extra robustness has been added, says Mott MacDonald project director for Cross Street Station design Chris Whiting.

Panels making up cast-insitu concrete retaining diaphragm walls for tunnel and station excavations are up to 1.2m thick. This is to resist the walls bending under the large earth pressure forces imposed by the potentially flowable clay.

Slabs made up of jet grouted columns have been installed to strengthen and stiffen the clay at base-slab level before excavation commences. The slabs provide propping action at low level, resisting earth pressure as excavation takes place.

Not only are jet grouted mats being made deeper, but for stations, additional diaphragm wall panels are being installed at right angles, acting like buttresses to the retaining walls. "They provide considerable extra stiffness," Whiting says.

Historically MRT stations have been built using "bottom up" construction, with props every 3m supported at mid-span by king posts bearing on to the face of the diaphragm walls.

Downtown Line stations are now being constructed "top down", with the permanent reinforced-concrete roof slab performing as a giant prop.

Click here for diagram of Crossing the East-West Line: Construction Method

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