Circle Line Tunnels to Close for Months Amid ‘Tunnel Squatting’ Phenomenon
A significant disruption is looming for commuters on Singapore’s Circle Line as sections of tunnels serving Mountbatten, Dakota, and Paya Lebar stations will undergo extensive strengthening work beginning in early 2026. The closures, expected to last nearly three months, are a response to a geological issue known as “tunnel squatting,” which threatens the long-term structural integrity of the underground network.
The Land Transport Authority (LTA) anticipates that commuters will face extended wait times of up to 30 minutes during peak hours, with train intervals increasing from the usual three minutes to 10 minutes at the affected stations. To mitigate the impact, a new shuttle bus service will operate during weekday peak periods, connecting passengers to unaffected portions of the rail network. This planned disruption marks one of the longest scheduled service interruptions in the history of Singapore’s Mass Rapid Transit (MRT) system.
Understanding ‘Tunnel Squatting’ and the Risks
The issue of tunnel squatting isn’t unique to Singapore, but it’s a critical concern for the city-state’s underground infrastructure. Certain stretches of the Circle Line were constructed through marine clay, a type of soft soil prone to compression over time. This compression causes the tunnels to gradually deform, taking on a more elliptical or oval shape – the defining characteristic of tunnel squatting.
“Soft clay is like a sponge filled with water to maintain its shape,” explained Mr. David Ng, chairman of the Civil & Structural Engineering Technical Committee at the Institution of Engineers, Singapore, on December 1. “When the water is squeezed out, the sponge will be compressed.” He further noted that approximately 25 percent of Singapore’s landmass is built on marine clay, suggesting a comparable proportion of MRT tunnels are similarly constructed.
While Singapore’s tunnel designs already incorporate allowances for the behavior of marine clay, the section of the Circle Line near the coastline is built on particularly “very soft” and “very sensitive” marine clay. According to Mr. Ng, “It has actually exceeded the tolerance that we have actually designed for.”
The underlying cause of tunnel squatting is a complex interplay of geological processes, including shifts in the surrounding environment and fluctuations in groundwater levels around the marine clay. If left unchecked, the consequences could be severe.
“If severe, it can cause concrete to crack or spall, with debris potentially falling onto passing trains and posing safety risks,” the LTA stated. The authority emphasized that failing to address the issue could lead to ongoing disruptions to train operations and the development of long-term structural defects. Preliminary observations have already revealed minor defects, including signs of water leakage, in the most affected tunnel sections.
The upcoming strengthening works are a proactive measure to ensure the continued safety and reliability of the Circle Line, safeguarding Singapore’s vital public transportation network.
