Contractors and engineers on the new Kallang-Paya Lebar Expressway (KPE) in Singapore had an enforced break last year following the collapse of a cut and cover section of Mass Rapid Transit Circle Line next to Nicoll Highway (see news this month). 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 it one of the biggest cut and cover projects seen in Singapore, and possibly all of Southeast Asia. The highway will be an important link in the island state's network, giving much needed relief to the central expressway.
Deep excavation in Singapore's soft marine clays is never straightforward, and the KPE has some fairly complex sections. It runs within metres of crowded apartment blocks, crosses a major river and passes underneath Mass Rapid Transit system viaducts. At one point 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, client for the Circle Line project and the S$1,741M (£580M) KPE, called an immediate halt to all other cut and cover tunnelling jobs.
Project director Marcus Karakashian says: 'Work was stopped everywhere while we looked at quality and compliance of works and temporary works with designs.' The highway is a design and build contract. '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 project.
Karakashian says: 'There are six contracts in total, 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 required few changes for additional safety and work resumed 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 wide channel clear.
'It is important not to impede the fl ow, ' Karakashian says. The river is important for drainage during Singapore's tropical rains which can cause flooding.
Once the first section of the box is complete, a diversion channel was created over the top of it. Two 8m wide, sand fi lled cellular cofferdams 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 and 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 cofferdam we have formed an excavation cofferdam of soldier piles and sheet piles; this is supported with four levels of struts as excavation proceeds, ' Karakashian says. Big compound beam struts replace the big 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 to 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 being installed to a maximum 60m below ground. It widens out in plan at the far end to 70m to allow for slip roads.
Once all that is done a bridge will be created on top of the box to link 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 line.
Singapore's long term planning came to the rescue here; the highway route was already pencilled in the 1980s. The MRT viaduct columns had been extended downwards, with a pile cap formed much lower than usual to allow for construction of an underpass.
This allowed 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, ' Karakashian says, 'and there were quite severe headroom restrictions for piling, and limits to crane turning circles.' The next contract, by Samsung Corporation, is even more complex.
Here the tunnel winds through a major housing area with apartment blocks, several major roads to pass under and a section of canal along the same alignment. The 'canal' is a 27m wide drainage channel to cope with tropical rain runoff and must remain in use throughout work.
'So we are moving it from side to side, allowing a 'half-box' construction to take place, ' Karakashian says. 'Once one side is done the canal is moved over and the other side is done. At the end [the canal] will be restored to a centre line.' Each half is 20m wide.
Nearby buildings add further difficulty. Damage limitation must only be 'slight'. But in one area there are signifi cant worries. The eightstorey Block 122 sits only 7m from the work over a 45m deep buried valley in the old alluvium, which is fi lled 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, ' Karakashian says.
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 remaining jobs are straightforward, despite a total of seven interchanges. The first sections should open next year and the whole road in 2008.