BUILDING a road under the sea is a Herculean effort by any measure.
But when said road is near a dam that discharges 2,000 cubic metres of water per second - or about 50 swimming pools' worth per minute - the task becomes almost indomitable.
This is because the sheer volume of water being discharged causes the seabed to be churned and washed away.
Yet, that was exactly what Land Transport Authority director Chuah Han Leong and his team had to deal with when constructing the $4.3 billion Marina Coastal Expressway (MCE) which opens on Dec 29.
The 5km 10-lane highway joins the Kallang-Paya Lebar Expressway (KPE) and the Ayer Rajah Expressway. A mid-section of close to half a kilometre goes under the seabed of Marina Bay, just 130m south of Marina Barrage.
"We went about 25m below the waves, and just 130m from the barrage," the 58-year-old geotechnical engineer said.
Dealing with the water being discharged from the barrage was "a major challenge", he added.
"It's a hell of a lot of water coming through," the veteran said. "So we had to put in what's called gabions - rocks in cages - next to the temporary works, so that the seabed would not get eroded around the works."
The section had to be done in stages so that water flow was not impeded. "We had to leave at least 150m open to let the water flow out," Mr Chuah noted.
Another challenge was dealing with the seabed which consists of a thick layer of "peanut butter- like" marine clay. Piles had to go 85m down to anchor on hard rock.
But Mr Chuah is no stranger to such obstacles in his work. He has been involved in tunnelled road projects going below water bodies since the 1980s, starting with the Central Expressway (CTE) "when I was a young resident engineer in my early 30s".
"For the CTE, we went under the Singapore River - it was a 70m stretch. For the KPE, we went under the Geylang River, for 120m. And now for the MCE, we went under the sea, over a 420m stretch," he said.
All three underground roads went under or above MRT lines too. The CTE went above the East-West Line and the KPE went over the Circle Line, while the MCE is built above the North-South Line extension.
This would not have been possible if engineering provisions had not been made when the rail lines were laid. Mr Chuah said the MCE construction also includes provisions for future extensions.
It will have a westward branch heading towards the future Southern Waterfront (which will be built after the port moves to Tuas in 2027), and an eastward branch heading for Marina East.
Mr Chuah said all three projects were "memorable".
He said: "The CTE was my first, and it was Singapore's first tunnelled road. I was involved in the stretch near the Istana. We excavated under Stamford Canal.
"The KPE was interesting because I was involved from day one... The MCE was the same. I led a team of 120 people across many disciplines - that was satisfying." He added that it is also "satisfying to improve mobility".
"When people tell me that the KPE gives them a smooth journey and it helps them shorten their journeys, it's satisfying," he said.
At the same time, the engineer who started his career with the Public Works Department more than 30 years ago realises that infrastructure has its limitations.
While it relieves congestion, "you can never build enough roads". "The more roads you have, the more demand there will be," he said.
Hence, Singapore still needs to manage demand via measures such as the Electronic Road Pricing and the vehicle quota system.
"If you don't do these other things... eventually, they will all be choked up."