Contractors and engineers on the new Kallang-Paya Lebar expressway (KPE) took a short break last year following the catastrophic collapse of the Nicoll Highway cut and cover rail tunnel (News last week). The expressway involves a substantial amount of deep excavation, not far from Nicoll Highway.
Three quarters of the 12km long, dual three-lane motorway is being put underground, making the new road one of the biggest cut and cover projects seen in the city, and possibly the whole of south-east Asia. The highway will be an important link in the island city state's overall highway network, giving much needed traffic relief to the busy central expressway.
Deep excavation in Singapore's soft marine clays is never entirely straightforward, and the KPE has some fairly complex sections. It runs within metres of crowded apartment blocks, crossing a major river and passing underneath Mass Rapid Transit system viaducts. At one stage the cut and cover box structure has to carry a bridge, and at another a 2km long canal.
When the Nicoll Highway cave-in happened, Singapore's Land Transport Authority, which is client for that project as well as the S$1,741M (£580M) KPE, called an immediate halt to all other cut and cover tunnelling jobs, says project director Marcus Karakashian.
'Work was stopped everywhere for a while, during which we looked at quality and compliance of works and temporary works with designs.' Like many projects in Singapore the highway is being built on a design and build basis.
'And although there was a requirement for an independent checking engineer on permanent works that had not applied to temporary works, ' Karakashian says.
German consultant Meinhardt was rapidly appointed as checking engineer for the temporary works on the river crossing, which is one of the most difficult sections.
Contractor Sembcorp Engineers & Constructors has a £78M contract for the crossing, which is part of a package for the first 2km section of the work.
'There are six contracts in total, ' says Karakashian, 'from the south, where the road has a junction with the East Coast Parkway, to the north, where it joins the Tampines Expressway.' The river works actually required few changes for additional safety - 'we put in some extra evacuation stairways', says Karakashian - and the contractor was able resume work quite quickly.
At that point, in spring last year, the crossing was in its first phase with a tunnel box being built in a deep excavation on the north bank. Work proceeded behind a cofferdam which left most of the river's curving 70m channel clear.
Later in the year, with the first section of the box complete, a diversion channel could be created over the top of it. Two 8m wide, sand-filled, cellular coffer dams have now been formed in the river, one upstream and one down. Both used fairly deep sheet piling because, like many areas in Singapore, the crossing has a deep layer of soft marine clay sitting over a thinner sand layer with denser clay beneath. To key into the structurally competent old alluvium below that meant going down 27m on the upstream side of the river and 42m on the downstream side.
'Inside the main cofferdam we have formed an excavation cofferdam of soldier piles and sheet piles; this is supported with four levels of struts as excavation proceeds, ' says Karakashian. Big compound beam struts replace the large tubular struts which used to be seen on Singapore sites.
Much of the second stage excavation is still to be done, with the tunnel box roof to be formed once a second strut layer is in.
Top-down construction will then continue to base slab level. After that is formed, the bottom struts will be taken out allowing the wall be cast in two stages, removing another strut layer on the way.
The box sits 19m to 22m below ground and will be supported on bored piles, currently being formed down to a maximum 60m below ground. It widens out in plan at the far end to 70m to allow for slip roads.
Once this is complete, a bridge will be created on top of the box to link to a local road across the river. Further up the route the road passes underneath the viaducts of an elevated section of the mass rapid transit system and the tunnel had to be formed around the supporting columns of the busy operational line.
Fortunately Singapore's long term planning came to the rescue here, with the highway route already pencilled in during the 1980s. The MRT viaduct columns had been extended downwards, with a pilecap formed much lower than usual to allow for construction of an underpass. This allowed the joint venture contractor on this section, Sembcorp/Daewoo Engineering & Construction, to get on with a major excavation around the columns. 'We had to use a lot of instrumentation, ' says Karakashian, 'and there were quite severe headroom restrictions for piling, and limits to crane turning circles.' The next contract up, by Samsung Corporation, is even more complex, however. Here the tunnel winds through a major housing area with multiple apartment blocks alongside, and has to pass under several major roads and a section of canal along the same alignment.
The 'canal' is a very large, 27m wide, drainage channel to cope with tropical rain runoff from a big catchment and must remain usable throughout the work.
'So we are moving it from side to side, ' says Karakashian, 'allowing a 'half-box' construction to take place. Once one side is completed the canal is moved over and work starts on the other side. At the end it will be restored to a centre line.' Each half is 20m wide.
Nearby buildings add further difficulty. The contract states that damage limitation must only be 'slight'. But in one area there are significant concerns. The eight story Block 122 sits just 7m from the work, above a 45m deep buried valley in the old alluvium, which is filled with very soft marine clay. It is highly sensitive to disturbance.
Over a length of 300m, as it passes Block 122, the tunnel box is being founded on transverse diaphragm walls at 7m intervals.
These 1m thick walls tie into longitudinal side walls and a central wall.
'An important precaution is to try and maintain the ground in its normal condition, ' says Karakashian. Part of that is to prevent drainage of the sand layer beneath the impermeable clay, which could occur when it is penetrated.
'We have insisted the contractor set up recharge wells along this section to prevent drainage, ' he says.
By comparison the remaining jobs are straight forward, despite a total of seven interchanges to be built. First sections of the route should open next year and the whole is due on stream in 2008.