Circle Line commuters can expect delays of up to 30 minutes from January to April

Key Concepts

• Tunnel Squatting: A phenomenon where soft soil surrounding tunnels settles and exerts pressure on tunnel walls, causing deformation.
• Soft Marine Clay: A type of soil found in coastal areas, characterized by its low strength and susceptibility to settlement, posing engineering challenges for tunnel construction.
• Strengthening Works: Proactive engineering measures undertaken to reinforce tunnel structures and prevent further deformation.
• Steel Plates: A material used in the strengthening works to reinforce the affected tunnel sections.
• Scheduled Service Disruptions: Planned periods of reduced train service to allow for essential maintenance and construction work.
• Groundwater Changes: Fluctuations in the water table surrounding marine clay, which can contribute to tunnel squatting.

Tunnel Strengthening Works on the Circle Line

1. Overview of Disruption

• Timeline: From January to April of the upcoming year.
• Impact: Commuters on the Circle Line can expect delays of up to 30 minutes.
• Reason: Strengthening works in the tunnels between Py Labour and Mount Batton stations.
• Significance: This is described as one of the longest scheduled service disruptions to date.
• Daily Ridership: The Circle Line serves nearly half a million people daily.

2. The Engineering Challenge: Tunnel Squatting

• Root Cause: Certain stretches of the Circle Line tunnels are situated in coastal grounds composed of soft soil.
• Mechanism: Over time, this soft soil settles, leading to pressure against the tunnel walls and subsequent deformation. This phenomenon is termed "tunnel squatting."
• Global Context: This is identified as an engineering challenge for SEC (Singapore's Land Transport Authority) tunnels in soft marine clay, a condition observed in other cities globally.
• Monitoring and Maintenance: LTA has been monitoring and implementing maintenance works for over a decade.
• Proactive Measures: The current works are described as proactive steps to strengthen the tunnel structure.

3. Specific Affected Sections and Remediation

• Most Affected Tunnels: Two tunnels between Mountbatton and P Labba stations have been identified as more affected.
• Observed Defects: Minor defects, such as water leaks, have been observed in these tunnels.
• Repair Status: While these minor defects have been fixed, the tunnels will undergo further strengthening.
• Strengthening Method: The tunnels will be strengthened with steel plates as a preventive measure.

4. Methodology for Strengthening Works

• Tunnel Closure: To provide engineers with sufficient time for work, the tunnels will be shut one at a time for approximately three months.
• Safety Assurance: LTA states that the tunnels remain safe for rail operations during this period.
• Train Service: Trains will continue to run every 10 minutes in the open tunnel.
• Consequence: This will result in longer waiting times across the entire Circle Line.
• Underground vs. Aboveground Works:
  • Authorities can strengthen tunnels from above or underground.
  • Reason for Underground Works: The presence of numerous residential areas near stations like Dakota Station makes underground execution necessary.
  • Other Considerations: Soil conditions and tunnel size also influence the choice of methodology.

5. Contributing Factors and Design Tolerances

• Percentage of Tunnels in Soft Soil: Approximately 25% of MRT tunnels are built in soft soil.
• Design Intent: "In our tunnel design, we already allow for some tolerances in in in such soft marine cate."
• Exceeding Tolerances: In the specific stretch of the Circle Line near the coastline, situated in very soft and sensitive marine clay, the settlement has "actually exceeded the tolerance that we have actually designed for."
• Environmental Influence: Changes in the environment and groundwater surrounding the marine clay can contribute to tunnel squatting.

6. Future Outlook and Completion

• Importance of Works: The strengthening works are deemed crucial to ensure the safety of commuters moving forward.
• Completion Target: LTA aims to complete these works before the remaining Circle Line stations open in the first half of 2026.

7. Mitigation of Disruption

• Shuttle Buses: Shuttle buses will operate during peak periods on weekdays to ease disruption.
• Commuter Assistance: Additional staff will be deployed to guide commuters.

Synthesis/Conclusion

The Circle Line is set to experience significant service disruptions from January to April due to essential strengthening works on tunnels between Py Labour and Mount Batton stations. These works are necessitated by "tunnel squatting," a phenomenon where soft marine clay surrounding tunnels causes deformation. While the affected tunnels have been monitored and maintained for over a decade, proactive measures involving steel plates are now required to reinforce two specific tunnels that have exceeded design tolerances due to settlement. The strengthening will be conducted underground to minimize impact on residential areas, with one tunnel closed at a time for approximately three months. Despite train services continuing every 10 minutes in the open tunnel, commuters should anticipate delays of up to 30 minutes. Shuttle buses and increased staff assistance will be provided to mitigate the inconvenience. These crucial works are targeted for completion before the full opening of the Circle Line in the first half of 2026, ensuring long-term commuter safety.