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Downtown looks up in Singapore

Construction of Singapore's new Downtown Line is drawing on lessons from the 2004 Nicoll Highway collapse.

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 at a rate that make rhetoric redundant. Who needs to be convinced to leave the car at home when kilometres of state-of-the-art underground railway are being added to the network each year?

But construction of Singapore's 33.3km Circle Line made international news in 2004 when a section of cut-and-cover tunnel adjacent to the Nicoll Highway station collapsed during excavation, killing four workers. The disaster halted construction while safety systems and construction methodologies were reviewed.

Four years on, however, and the pace of construction is blistering, with Circle Line works ongoing and construction of the city's new Downtown Line just starting.

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 congested central business district, connecting the North-East Line with the Circle and East-West lines.

The route passes close to Nicoll Highway, and lessons learned from the 2004 incident 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 contractor joint venture Samsung-Soletanche Bachy, which is building Cross Street Station under a design and build contract.

Mott MacDonald is also independent checking engineer and construction supervisor for Bayfront Station, being built beneath Singapore's vast Integrated Resort development. Contractor for both the resort and 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 grout piles 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 grout pile 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 by conventional "bottom up" construction, with propping provided by steel struts supported at mid-span by king posts and bearing on to waling beams installed across the face of the diaphragm walls. Struts are installed every 3m depth.

Downtown Line stations are now being constructed "top down", with the permanent reinforced-concrete roof slab performing as a giant strut. This enables the elimination of struts, the installation of which is time-consuming and poses safety risks.

As well as improving safety in Singapore's humming commercial district, top down construction helps minimise one of the major challenges of open cut excavation – disruption to traffic. With bottom up construction the hole in the ground is excavated leading to the carriageway being closed. With top down construction, 50% of the roof slab can be cast before the hole is excavated and half the carriage can be kept open under a contra-flow system. The carriageway is then reopened under the newly cast roof slab while the other half is cast.

Cross Street Station
Previous tunnelling challenges in Singapore have been presented by the depth of excavation. The challenge at Cross Street, by contrast, is the lack of depth. "What is different about this line is that there are previous structures in the ground that the tunnels have to negotiate or interface with," says Whiting. "That makes things interesting."

Downtown Line excavation depths are typically 24m to 25m, "but within our section the Downtown Line crosses over the existing East-West MRT line", Whiting explains. "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.

"The East-West Line has somewhere between 10m-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, meaning that the tunnels are dependent on uniform ground pressure for their structural integrity. Excavating so close to an existing tunnel can lead to a loss of earth pressure, which can cause deformation and, potentially, collapse. In soft marine clay loss of earth pressure is a significant concern.

"You can't just dig a big hole, as the clay will move too much and you'll get excessive deformation in the existing tunnels. 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.

Top down construction provides a means of preventing movement of the surrounding ground, so keeping changes in equilibrium extremely local, with thick diaphragm walls braced by the jet grout pile slab and monolithic roof slab. Adjoining sections of running tunnel are also being constructed top down.

The depth of the line is having wider impacts. Maximum rail gradients mean it is impossible for the Cross Street section of the Downtown Line to swoop to a deeper level before it reaches the location of Cross Street Station. As a result there is insufficient headroom for a conventional station design, in which the ticket hall, concourse and plant rooms sit above the platforms. Instead a stepped layout is being adopted, with ticket halls, concourses and plant rooms ranged either side of the running tunnels.

click here for diagram

Structurally the layout is more complex than a conventional double-deck station box structure, but it has the advantage of reduced lift, stair and escalator heights.

And even though building above existing tunnels is technically difficult, the fact that excavation is so much shallower than normal means that temporary works required will be much more economical.

Any savings on tunnel and station excavation will help offset the cost of a major temporary structure – a 407m long viaduct along Cross Street that will carry two lanes of traffic and free-up space underneath for the construction of the station.

Cross Street is one of Singapore's major traffic arteries, carrying three lanes of traffic in each direction. Traffic studies commissioned by the LTA indicates that loss of road capacity to construction work would cause gridlock. Samsung-Soletanche Bachy will therefore precede tunnelling works with bridge construction works.

The viaduct will be founded on columns within the station box and station design has been accelerated to finalise column positions. This means that Samsung-Soletanche Bachy can get on with superstructure erection.

Bayfront Station
The Downtown Line is key to development of the Marina Bay area, which will include a new financial centre and a colossal leisure and entertainment complex known as the Integrated Resort, housed in twin towers connected at roof level by a botanical garden.

Mott MacDonald is providing the LTA with supervising engineers' services for design and construction of Bayfront Station, which will sit under the Integrated Resort, and will carry out independent checks on earth-retaining structures.

Marina Bay was reclaimed during the early 1990s and ground across the site consists of 10m to 17m of fill overlying the soft Kallang Formation marine clays, flowable sands and silts.

The Kallang Formation has a typical standard penetration test N-value of 0, notes Mott MacDonald team leader Vince Goh. It is underlain by dense alluvium. While excavation has taken place in such soft ground before, it is at the more challenging end of the geotechnical spectrum, he says.

Excavation will be to a depth of 24m and to counter the effects of the weak soil, thick diaphragm walls ranging from 1m to 1.2m are being deployed. They are being stiffened and braced locally by cross walls and a jet grout slab.
Cut and cover tunnels connecting to Bayfront Station range from a modest 8m to 24m deep and will require up to seven levels of strutting to the diaphragm wall.

Construction of Bayfront Station is taking place on a fast-track 24-month programme, with the basic structure due to be finished by the end of next year. Completion of the station works is being driven by the schedule for construction of the Integrated Resort. There are also interfaces with Marina Bay's combined services tunnels.

Basement works for the Integrated Resort are already taking place around Bayfront Station, which started on site in January. Excavation of the combined services tunnel is currently under way beneath the new Downtown Line Bayfront Station and tunnels. The services tunnel needs to be completed before contractor Sembawang Engineers & Constructors can really put its foot on the accelerator.

WHO IS DOING WHAT

Contract 905.
Tunnels between Promenade and Marina Bay: Shimizu Corporation, £38.3M

Contract 906.
Bayfront Station and tunnels: Sembawang Engineers & Constructors, £172.7M

Contract 907.
Landmark Station and tunnels: Taisei Corporation, £85.8M

Contract 908.
Cross Street Station and tunnels: Samsung-Soletanche Bachy, £83.9M

Contract 909.
Chinatown Station and tunnels: Gammon Construction, £59.8M

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