Ground level view of cut and cover tunnel construction

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Marina Coastal Expressway, Singapore

Singapore’s MCE is a game-changing project. Built mainly in tunnel, the 5.1km long, dual five lane highway links existing expressways in east and west Singapore with the New Downtown area in Marina Bay. The city state has become famous for mammoth cut and cover tunnels in the soft ground close to its reclaimed shoreline. But with a width of 60m, this project is considerably bigger and more challenging than anything that’s been done here before.

We worked closely with two contractors to review conventional construction methods and develop more efficient alternatives that have enabled nearly 3km of tunnel to be delivered safely, with cost and time savings.

Soft ground tunnelling challenge

Cut-and-cover excavation involves installing temporary longitudinal retaining walls and then excavating the ground between them. A reinforced concrete base slab, permanent side walls and a roof are then cast in the excavation before ground is reinstated over the top. The method can become complicated in soft ground.

MCE traverses an area of man-made land consisting of 30-40m of marine clay, underlain by firm Old Alluvium and capped with 15m of fill. Marine clay has the consistency of toothpaste 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. Robust engineering methods are called for to prevent undesirable ground movement.

Better value for the public purse

Government client body the Land Transport Authority (LTA) stipulates requirements for temporary works, including geotechnical parameters, retaining wall sizes and ground improvement. When design and build contracts for MCE were put out to tender in 2008 the indicative design required that lateral deflection of the retaining walls would be no greater than 75mm. To achieve the target, the LTA required two layers of ground improvement underlying the formation level, founded on bored reinforced concrete piles. Sheet pile retaining walls were to be supported from in front by I-section soldier piles toed into the Old Alluvium. In addition, the retaining walls were to be propped as excavation advanced with five layers of struts at depth intervals of 3m.

We teamed with contractors Samsung and Ssangyong to bid for four of the six MCE packages. A combination of innovations pared more than 10% off the client’s cost estimates for contract 482 won by Ssangyong and contracts 483 and 486 won by Samsung.

Value engineered ground improvement

Ssangyong and Samsung saw an opportunity to save cost and add value for LTA by using deep cement mixing (DCM) instead of jet grout ground improvement. We examined the technical performance of both options and found that DCM worked better. Jet grouting involves drilling into the ground and then injecting cement grout at high pressure so that it mixes with the surrounding ground. Following a carefully designed pattern, jet grout ‘columns’ are joined up to form a continuous layer of improved ground. However, 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.

40% saving on piles

Underlying the stiff layer of ground improvement are bored reinforced concrete piles. 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. Analysis showed that uplift exerted by earth pressure on the DCM layer would cause it to heave and bend. But we found that this 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.

Stiffening the piles required additional reinforcement. To avoid unnecessary use of steel and resulting cost, we analysed the forces acting on every one of the 2500 piles across all three contracts. Normally reinforcement would be designed to cope with the maximum load and applied to each and every pile. Instead we assumed a minimum and worked up from there. This delivered a 40% saving on steel reinforcement across the project.

The DCM layer restrains movement of the retaining walls. Its performance in combination with the piles meant that its thickness could have been reduced to 8m while providing the stiffness required. However, for comfort 10m of DCM treatment was carried out. This was still significantly less expensive than jet grouting and benefited pile design. The bond between each pile and the DCM layer relies on the contact area between them. Increasing the slab thickness from 8m to 10m enabled a 20% reduction in pile diameter, delivering a 36% saving on concrete.

US$70M saving on struts

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. This meant that two levels of struts were more than adequate, rather than the four to five strut levels specified in the indicative design.

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 almost US$24M, yielding a US$70M combined benefit. Reducing the number of struts from five to two layers has offered a huge time saving and dramatically improved worker safety, with lifting and manual handling operations cut by 60%.