Civils works are wrapping up Singapore’s Marina Coastal Expressway (MCE). Primarily in tunnel, the project has had to innovate to handle with exacting project specifications. The 5.1km long, dual five lane highway will link existing expressways in east and west Singapore with the New Downtown area in Marina Bay. The city-state is awash with 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," says consultant Mott MacDonald’s geotechnical manager Nick Mace.
The tunnel is situated in an area of manmade land consisting of 30-40m of under-consolidated marine clay with the consistency of toothpaste. This is underlain by firm Old Alluvium and topped with 15m of reclamation fill. The project is founded on a reinforced concrete base slab, supported with permanent side walls and finished with a cast roof before ground is reinstated over the top.
The method can become complicated in soft ground. 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 wants to heave up. Sophisticated and robust engineering methods are needed to maintain control and stability.
Cost savings within tight parameters
Singapore’s construction safety culture has been shaped by the sudden and fatal collapse of a cut and cover tunnel in similar ground conditions at Nicoll Highway in 2004 (Tunnels, February 2007, pp31-34) . The event graphically demonstrated the huge power of the forces at work and the potential danger of design or construction errors. In the aftermath, government client body the Land Transport Authority (LTA) acted to improve safety by setting requirements for temporary works, including geotechnical parameters, retaining wall sizes and ground improvement.
When design and build contracts for construction of MCE were put out to tender in 2008 the indicative design required that lateral deflection of the retaining walls was limited to 0.5 per cent of the excavation depth – a maximum 75mm. "Historically, two to three times that amount of movement would have been acceptable, providing you could demonstrate that the deflection wouldn’t be detrimental to surrounding structures," Mace says. To achieve the 0.5 per cent target, the LTA 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 from in front 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 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 client’s criteria made it quite challenging to find areas where we could add value for a winning tender," Mace admits. However, a combination of innovations pared more than 10 per cent off the client’s cost estimates for contract 482 won by Ssangyong and contracts 483 and 486 won by Samsung. Valued at SGD 930M (USD 761.2M), 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 SGD 716M (USD 586M), includes 950m of tunnel and a ventilation building. And C486, valued at SGD 635M (USD 519.7M), includes 800m of tunnel, a ventilation building and an area of additional land reclamation.
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. Jet grouting involves drilling into the ground and then injecting cement grout at high pressure so that it penetrates and mixes with the surrounding ground. Following a carefully designed pattern, jet grout ‘columns’ are joined up to form a continuous layer of improved ground. 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. However, the technique did not allow the contractors to easily create the two distinct strata of stronger ground specified by the LTA. Mott MacDonald carried out extensive modelling and analysis to assess the performance of a single DCM layer.
Savings on piles
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," Mace notes. "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."
Stiffening the piles required additional reinforcement. To avoid unnecessary use of steel and the resulting cost, the team analysed the forces acting on every one of the 2,500 piles across all three contracts. Steel quantities are worked out as a proportion of the pile area. "We assumed a minimum of 0.5 per cent reinforcement and worked up from there," Mace states. "Locally there are some very high uplift forces and as much as 4 per cent steel relative to the pile area is used. On average, the ratio is 2.5 per cent."
The performance of the DCM layer 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 treatment was carried out. This was still significantly less expensive than jet grouting and benefited pile design. "The bond between each pile and the slab relies on the contact area between them," Mace explains. "Increasing the slab thickness from 8m to 10m enabled a 20 per cent reduction in pile diameter, delivering a 36 per cent saving on concrete."
Retaining wall innovation
In another departure from convention, the contractors proposed constructing retaining walls using 1.2 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 alternating short and long pipe piles, with the long pipes embedded 2.5m into the Old Alluvium.
The solution offered other advantages. "It’s too hard to drive the piles into the Old Alluvium so we usually have to prebore," says Samsung’s design manager for C486, Seok San Lim. "Preboring for a soldier pile is a fiddly operation – you have to bring in an auger first, then come back with your hammer rig and get the alignments spot-on. With a pipe pile, you drive down to the Old Alluvium, then prebore into the alluvium down the pile itself, using it as a casing. It’s a very neat, fast operation."
Looking to the end of the construction programme, the piled retaining wall will be extracted for reuse. DCM could not be carried out right up to the retaining wall, so jet grouting was carried out to seal the gap between the wall and the ground improvement slab. The contractors knew from experience that H-section soldier piles are often locked solid by jet grout, which ‘grips’ five of each pile’s six faces, and cannot be pulled out. Circular pipe piles by contrast are only 50 per cent exposed.
Saving on struts – SGD 90M (USD 73.7M)
"The pipe piles were a similar price to a sheet and soldier pile combination but much more efficient," says Lim. "Their structural strength would in theory have allowed the contractors to eliminate all but one layer of struts from the temporary works," Mace says. "In the context of the prevailing industry culture, we elected not to push for this option, but strongly argued against using the four to five strut levels 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 DCM ground layer. On each contract, the elimination of every strut layer has saved SGD 10M (USD 8.2M), yielding a SGD 90M (USD 73.7M) combined benefit.
Singapore’s tunnelling industry uses a modular I-beam strutting system, with bolted connections. 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 per cent.
Productivity has been improved by creating more working room. Construction equipment can move freely within the excavation. And concreting operations have been simplified: "Normally the lowest layer of struts would be just above the final formation level so you’d be laying reinforcement and pouring concrete for the base slab around a grid of temporary steelwork," explains Mace. "On MCE, there’s 7m clearance between the base slab and the nearest struts."
The reinforced base slab of the highway tunnel effectively acts as an additional strut and Mott MacDonald successfully argued for removal of the second level of steel struts once it 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."
Junction challenge
C482, built on a curving alignment, is complicated by the inclusion of a stub-tunnel branching off from the MCE itself at 45°, plus a second tunnel which crosses underneath MCE at 45° in the other direction.
Throughout the rest of the MCE, retaining walls are parallel, standing a consistent 60m apart. However, the stub tunnel junction introduces angles and, as the retaining walls diverge, increases the distance from one side of the tunnel to the other to a maximum 135m. Over this distance it would be difficult, if not impossible, to prevent dangerous deflection along the length of struts. There was the added complexity that loading from the diverging walls would produce longitudinal forces in the retaining system.
The solution developed with subconsultant LSW was a reinforced concrete ‘super beam’ continuing the line of the main MCE retaining wall. This beam divided the excavation into manageable spans and collected longitudinal load from the struts, transferring the thrust into the wall.
Directly beneath the junction, the second tunnel, for Singapore’s Mass Transit Railway, is a modest 15m wide compared to the MCE’s 60m. It has reinforced concrete diaphragm walls toed into the Old Alluvium. The tops of the diaphragm walls are propped by the overlying DCM ground improvement layer installed for the MCE. This has created a very strong box. Work compounds either side of the MCE have enabled Ssangyong to excavate and line the MTR tunnel concurrently with the road tunnel.