SINGAPORE: Sections of Circle Line tunnels serving Mountbatten, Dakota and Paya Lebar stations will be partially closed for nearly three months in early 2026 for strengthening works, following a phenomenon known as “tunnel squatting”.
In a nutshell, this means longer waits of up to 30 minutes for commuters during peak periods.
The trains will arrive at 10-minute intervals – up from the usual three – at the affected stations which will use a single open tunnel. A new shuttle bus service will operate during weekday peak periods to take commuters to unaffected parts of the train network.
This will mark one of the longest scheduled MRT service disruptions to date.
But why does “tunnel squatting” happen in the first place, and what happens if it is left unchecked? To find out, CNA spoke to experts to understand all there is to know about this phenomenon, which is not unique to Singapore’s underground MRT network.
What is “tunnel squatting” and why does it need to be fixed?
Some stretches of the Circle Line tunnels were built in soft soil, such as marine clay, which gradually compresses over time, making them more prone to deformation. This causes the tunnels to become more elliptical or oval-shaped – what is known as tunnel squatting.
Mr David Ng, chairman of the Civil & Structural Engineering Technical Committee at the Institution of Engineers, Singapore, likened soft clay to a sponge filled with water to maintain its shape.
“(When) the water is squeezed out, the sponge will be compressed,” he told CNA on Monday (Dec 1).
He added that about 25 per cent of Singapore’s land area is built on marine clay, suggesting that a similar proportion of MRT tunnels is constructed in the same soil.
Singapore’s MRT tunnel design already factors in tolerances for marine clay, he said, but the stretch of the Circle Line near the coastline is built in “very soft” and “very sensitive” marine clay.
“(It) has actually exceeded the tolerance that we have actually designed for,” said Mr Ng.
Tunnel squatting arises from geological processes, including changes in the surrounding environment and groundwater surrounding the marine clay.
If severe, it can cause concrete to crack or spall, with debris potentially falling onto passing trains and posing safety risks. The Land Transport Authority (LTA) said that if left unaddressed, the issue may lead to disruptions in train operations and long-term structural defects.
Minor defects, such as signs of water leakage, have already been observed in the more affected tunnel sections.