Few visitors flying into Singapore are aware of the major construction work going on at the city republic's airport.
The Land Transport Authority (LTA) and Singapore's civil aviation authority, conscious of the airport's worldwide award winning status, have been at pains to be discreet.
Behind high painted screens and rows of potted plants, a dramatic piece of engineering and architecture is now taking shape.Huge excavations and delicate underpinning, when finished, will create a building that is anything but low profile: a highly visible metro station forming the entry point for a much needed link to the city's extensive mass transit system.
The station, designed by New York architect Skidmore Owings & Merrill, will dominate the space between two existing terminals and a planned third.Two 36m high, glass walled, column-free atria will rise from each end of the underground station. Between them, below ground, a dramatic 200m long curving glass and steel bridge, engineered by Ove Arup in New York, will link the second and third terminals over the platforms.
Much complex engineering is required before the fabricated sections for the 16m by 60m atria structures can be assembled. Two giant excavations are under way, and a twin box tunnel is being driven between the foundations supporting Terminal Two and its car park, removing ground less than 2m from major pile supports.
The two 25m deep main excavations are the most advanced part of the works. The first, which will house a rail tunnel crossover and barrette foundations for the future Terminal Three, is being built using conventional soldier piles and walings.
The main station box, which includes both atria, is more complex, explains LTA project manager for the station, C Murugamoorthy. It comprises three rectangular interlinked spaces excavated inside 35m deep diaphragm walls.
The excavation cuts across the main entry road to the airport, so had to be divided into two sections to maintain road and landside passenger links.
Main contractor, a joint venture of Japanese firm Kumagai Gumi and Singapore company Semcorp, has finished most of the eastern half of the box and is now concreting among the steel struts supporting temporary decking above.
Construction of the second atrium space is continuing beneath Terminal Two and its three-storey car park, which have required underpinning. Beneath the car park, the LTA designed a raft structure to hold up the lift and service core of the building when the excavation cuts away piles. Temporary barrette piles on one side of the excavation and a diaphragm wall on the other support 14m long steel span needle beams and the slab.
Beyond this box, more underpinning is needed to allow construction of two overrun tunnels for the trains.These extend beneath the car park, a Skytrain link to Terminal One, a bus ramp and an airside finger pier. They are being built as boxes within diaphragm walls just 1.5m away from the piles of the airport structures above.
The original plan was to use grouting and compensation grouting to deal with any ground movements caused by wall construction.However, the contractor's geotechnical engineer Richard Davies Associates (RDA) proposed an underpinning alternative using an observational method of construction.
A major uncertainty was the behaviour of the 'Old Alluvium' beneath the reclaimed ground at a depth of 7m. Although often described as a very weak rock with a good standup time, it was not known how this cemented sand would behave.
RDA's Alison Norrish explains that the two main concerns were stability and maintaining friction.Would the ground move into the excavated hole made for the wall panel, and would the de-stressing of the ground cause a loss of confining stress on the pile, reducing its friction-sustained load bearing capacity?
'We took advantage of some juggling in the programme which meant diaphragm walls for the main excavation went in earlier, ' Norrish says.
A pile the same size as the car park piles was sunk in identical ground alongside the main excavation, with two others acting as tension piles to allow load to be imposed. Extensive measurement was used to assess the pile's behaviour as the wall panels were excavated, and, later, as it was overloaded by 50%.
The results were better than expected. RDA thinks this may be due to an arching effect in the ground around the pile.
'We set some parameters for the construction as a result, ' says Norrish.'Panels had to be as small as possible - just over 2m long - and done one at a time. 'Ideally they should be installed no closer than 2m to a pile.' In the event two were just 1.75m away.
'We also discovered that time was important, ' she says. 'The low permeability in the material means that pore water pressure goes negative for a while, but it does begin to drain, so panels had to be excavated and concreted in 24 hours.'
If panel construction went wrong or if the building moved excessively, a special steel 'Zimmer frame' could be assembled under the car park floor to provide temporary support. 'As a first line of defence the panel hole would be backfilled with weak concrete, ' says Norrish.Nearly £1M of instruments was installed to monitor movements.
The diaphragm walls were formed by Korean subcontractor Sambo using two low headroom hydrofraise machines, and went in without incident. Excavation is now under way within the parallel walls, and slabs have been formed 4m down around piles to provide support for the surface building. The piles will be cut off beneath the slabs once the loads have been transferred to the walls and excavation continued to form a 6m deep box for the tunnel itself. In all, 47 piles will be cut away.
To complicate matters, a central 20m long section of each of the three tunnels has to be formed by hand mining within a sheet piled cofferdam area. A slab will go in, below which workers will construct the tunnel box in 2m top-down increments. This method is also being used for the section of tunnel passing through the foundations of the Skytrain viaduct and a bus ramp, where there is no space for rigs or piling machines.