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Highways: Cost efficient

Innovations on Singapore’s Marina Coastal Expressway have enabled faster, safer, more cost-effective construction, while complying with famously tough safety requirements.

With civils work nearing completion, Singapore’s 5.1km Marina Coastal Expressway (MCE) is a game changing project linking existing expressways in east and west Singapore with the New Downtown area in Marina Bay.

The MCE traverses an area of man made land consisting of 30m to 40m of under-consolidated marine clay with the consistency of toothpaste, underlain by firm Old Alluvium and capped with 15m of reclamation fill. Some sections are in cut and cover tunnel.

Marine clay is highly plastic and flows when subject to force. As excavation advances, external earth pressure becomes progressively greater. Retaining walls want to cave in and the base of the excavation wants to heave up

Sophisticated, robust engineerig is called for to prevent undesirable ground movement.

“The structural strength of the pipe piles would in theory have allowed the contractor to eliminate all but one layer of struts”

Nick Mace, Mott MacDonald

After the sudden and fatal collapse of a cut and cover tunnel in similar ground conditions at Nicoll Highway in 2004, tighter safety requirements were imposed on cut and cover construction (NCE 29 April 2004).

As a result, for the MCE the indicative design limited lateral deflection of the retaining walls to 0.5% of the excavation depth - a maximum 75mm.

To achieve the 0.5% target, the Land Transport Authority required the installation of two layers of ground improvement underlying the formation level. These were to be anchored by bored reinforced concrete piles. Sheet pile retaining walls were to be supported by I-section soldier piles toed 2.5m into the Old Alluvium. In addition to the deep level restraint provided by the strata of improved ground, the retaining walls were to be propped as excavation advanced with five layers of struts at depth intervals of 3m.

Mott MacDonald teamed with contractors Samsung and Ssangyong to bid for four of the six MCE packages.

The teams came up with a combination of innovations that pared more than 10% off the client’s cost estimates for contract 482 won by Ssangyong and contracts 483 and 486 won by Samsung.

Valued at SG$930M (£470M), C482 includes 500m of depressed road, 500m of road tunnel, a stub for a future new tunnel alignment and, underneath the MCE, a short section of light rail tunnel for the planned Mass Transit Railway North-South Line. C483, valued at £362M, includes 950m of tunnel and a ventilation building. And C486, valued at £321M, includes 800m of tunnel, a ventilation building and an area of additional land reclamation.

Cost saving opportunity

To strengthen the surrounding ground Ssangyong and Samsung saw an opportunity to save cost and add value by using deep cement mixing (DCM) below the position of the base slab, instead of jet grout ground improvement.

Mott MacDonald examined the technical performance of both options and found that DCM worked better. On MCE the slender drill strings used for jet grouting would be up to 25m long, making them liable to deviation from their designed path. This presented a risk that grout would not penetrate evenly, resulting in localised weaknesses.
DCM uses augers to churn cement slurry into the ground. The larger diameter and resulting stiffness of the auger guaranteed better accuracy and therefore superior quality ground improvement.

Underlying the stiff layer of improved ground are bored reinforced concrete piles at 6m centres longitudinally. On most previous cut and cover projects in Singapore, engineering solutions have considered the performance of piles in compression only.

“But the piles work in tension too,” says Mott MacDonald geotechnical manager Nick Mace.

“Analysis showed that uplift exerted by earth pressure on the DCM layer would cause it to heave. But we found the amount it would heave was reduced by the action of the piles in tension. This is something that’s usually ignored - but by making the piles stiffer, we were able to anchor down the DCM layer.”

The performance of the DCM layer in combination with the piles meant that its thickness could have been reduced to 8m without compromising the required stiffness.

However, for comfort 10m of treatment was carried out. This was still significantly less expensive than jet grouting and benefited pile design.

“Increasing the thickness from 8m to 10m enabled a 20% reduction in pile diameter, delivering a 36% saving on concrete,” says Mace.

Pipe pile innovation

In another departure from convention, the contractors proposed constructing retaining walls using 1.2m to 1.5m diameter pipe piles instead of the common sheet and soldier pile combination.

“Pipe piles are far stiffer, making it easier to comply with the LTA’s very tight wall deflection criteria,” Mace explains.

The MCE retaining walls use soldier piles alternating with short and long pipe piles. The long pipes are embedded 2.5m into the Old Alluvium.

“The structural strength of the pipe piles would in theory have allowed the contractors to eliminate all but one layer of struts from the temporary works,” Mace elaborates. “We strongly argued against using the four to five strut levels specified in the indicative design. Two levels were more than adequate.”

With the first strut level just below ground level, the second was installed at mid-height, 7m down, with deep level restraint provided by the massively strong DCM ground improvement layer. On each contract, the elimination of every strut layer has saved £5M, yielding a £45.5M combined benefit.

The reinforced base slab effectively acts as an additional strut and Mott MacDonald successfully argued for removal of the second level of steel struts once the slab had been created. This allowed the tunnel’s permanent reinforced concrete side and central walls to be created in single full-height pours.

“Normally you’d be creating the walls in 3m lifts, each advance being restricted by the next layer of struts,” Mace explains. “This has enabled us to advance faster and reduce the number of cold joints, which benefits the strength and durability of the finished tunnel.”

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