CCL Stage 6: Building an MRT tunnel directly under Keppel Viaduct with live traffic overhead

SINGAPORE – As vehicles rumbled across the Keppel Viaduct, their occupants oblivious to the engineering work taking place beneath them, part of the bridge’s foundation was being cut to make way for tunnels serving the new Circle Line (CCL) Stage 6 stations.

This was because two of the viaduct’s existing support piers were in the path of the tunnels, so the Land Transport Authority (LTA) had to construct micro-piles – each with a diameter of 400mm – and a supporting beam to bear the bridge’s load.

Following that, a total of seven piles belonging to the two piers were cut and drilled through to enable the building of tunnels between the new Keppel and Cantonment stations.

Speaking to reporters at a briefing on June 17, LTA said the preparatory works for the load transfer lasted eight months to a year – although the cutting itself was completed in a few days.

Part of the sixth and final stage of the CCL, Keppel, Cantonment and Prince Edward Road stations will close the loop between HarbourFront and Marina Bay stations. They will start operations on July 12.

Across these stations, a special trenchless pipe roofing method was used to construct six underpass linkways that run beneath busy roads such as the nine-lane Keppel Road.

One of these linkways is almost 100m long and is located at Cantonment station.

This underground mining method involves installing a temporary roof comprising steel pipes to build an underpass beneath it, removing the need to dig a hole at ground level.

It is more challenging to execute compared with traditional open excavations, but was done to minimise disruptions to traffic and utilities, as well as noise and sound pollution.

Over at Prince Edward Road station, part of its tunnels was built just 3.9m away from the foundation of a nearby commercial building.

Due to the tight underground space and proximity to nearby high-rise buildings, the tunnels and platforms of this station had to be built in a stacked manner.

At Cantonment station, measures were put in place to safeguard the former Tanjong Pagar Railway Station building, before tunnelling works were carried out underneath.

The new MRT station is under the former railway station’s platform.

Integrating systems throughout the entire CCL

From September to December 2025 and from April to May 2026, testing to integrate the systems across the existing CCL network, the three upcoming stations and the Kim Chuan Depot expansion was carried out.

Each round of tests required 10 hours of line closure, including two hours to switch from live to test systems and another two hours to switch back.

This was why train services on the CCL ended earlier at 11pm on some days and started later at 9am on others, for a combined duration of 5½ months.

More than 100 staff were involved each night, coordinating the tests from the stations, the operations control centre and the depot.

Integration is needed to allow the existing CCL network’s systems – which have been operating since 2009 – to work seamlessly with the newer equipment and software on CCL Stage 6 and the depot expansion as one connected system.

It includes upgrades to software and changes to physical equipment.

After the final systems integration took place on April 12, operator SMRT has been carrying out trial runs to ensure that the line can operate safely and reliably under actual conditions.

These include running trains across the entire line, testing different operating scenarios, staff familiarisation and emergency preparedness exercises before the new stations open.

Integrating CCL Stage 6 was more complex than other rail extensions, said LTA, as it completes the full loop on the CCL.

This entailed testing for continuous operations in both clockwise and anticlockwise directions across a range of service patterns and operating scenarios.

Careful coordination across multiple systems was needed, and more scenarios had to be tested, including how the passenger information display presents service information. This is because the full line will operate as one integrated network, said LTA.

As a result, systems integration tests on the CCL took longer than other rail projects.

In the event of a breakdown at one CCL station, LTA said partial services in both directions will still be in operation as long as the trains can turn around.

For the depot extension to be ready in December 2025, sections of the wall at the existing depot had to be torn down to link the existing depot with its expanded section.

Additionally, some tracks had to be realigned while the depot’s systems and services had to undergo integration. This is to allow trains to move across the entire depot for greater operational efficiency.

One challenge LTA faced – unique to the CCL – was ensuring that train operations at the existing depot and on the CCL proceeded as usual throughout this refurbishment process.

The Covid-19 pandemic affected the construction of CCL Stage 6.

Originally slated for completion by 2025, the opening was pushed back by 1½ years to mid-2026.