Chennai’s metro rail expansion project is encountering significant challenges, as tunnel boring machines (TBMs) at several locations face the risk of being stranded underground. This issue arises amidst ongoing construction of the city’s extensive Phase-2 metro network, which spans 116.1 km, including a 42.6 km underground section with 45 stations.
According to a Chennai Metro Rail Limited (CMRL) official, TBMs currently lying idle in Mandaveli and Moolaikadai are at risk of remaining stuck underground due to delays in constructing the necessary retrieval shafts. Similar situations are anticipated in Perambur, Kilpauk, and Thirumayilai. The delay stems from late award of contracts for the construction of 28 stations, which has impacted the timely readiness of retrieval infrastructure. The issue poses a dual risk: the potential for cave-ins above ground and the slow progress of tunnelling operations. When a TBM remains inactive underground, the surrounding soil pressure diminishes, which could lead to surface subsidence. Additionally, if TBMs are not operational when needed for tunnelling, it may impede the overall progress of the metro project.
In Phase-2, CMRL is employing an ‘earth pressure balance’ method for tunnelling, where the TBM’s cutting head excavates soil while maintaining pressure to support the tunnel’s surface. As the TBM advances, precast segments are assembled to form the tunnel lining. Normally, a TBM is retrieved from a shaft at one end of a station and can be redeployed to another location, or refurbished for reuse. Former CMRL Director R Ramanathan highlighted that during Phase-1, when work was halted on Anna Salai, maintaining TBM operations was critical to avoid cave-ins. He emphasised the importance of synchronising the awarding of tunnelling and station construction contracts to prevent such complications.
The ongoing challenge underscores the need for strategic planning and timely execution of both construction and retrieval components to ensure the safety and efficiency of Chennai’s metro rail expansion. As the project advances, addressing these issues will be crucial in preventing operational delays and maintaining the integrity of the city’s transport infrastructure.
