The Delhi Metro Rail Corporation (DMRC) on Saturday completed an underground tunnel between Chhatarpur and Chhatarpur Mandir stations on Phase-4 Golden Line.   A 97-metre-long tunnel boring machine (TBM) broke through on Saturday morning at Chhatarpur Mandir station after boring an 860-metre-long tunnel.

According to Delhi Metro,  two parallel circular tunnels for up and down movement have been constructed on this stretch as part of the Aerocity–Tughlakabad corridor. The breakthrough on the other parallel tunnel was achieved on August 21.

 This new tunnel has been constructed at an average depth of approximately 12 metres. About 613 rings have been installed in the tunnel, with an inner diameter of 5.8 meters, DMRC Principal Executive Director Anuj Dayal said.

 The tunnelling work involved numerous challenges such as shifting a 66 KV electrical high tension line. Additionally, the TBM had to cross below the existing Yellow Line viaduct without disrupting metro train operations on the line, he said.   The tunnel has been built using the technology of EPBM (earth pressure balancing method) with a concrete lining made of precast tunnel rings. These tunnel rings were cast at a fully mechanised casting yard set up at Mundka. The concrete segments were cured with a steam curing system to achieve early strength, Dayal said.

 All necessary safety precautions were taken during the construction of the tunnel below the existing viaduct and built-up structures. Ground movements were monitored with highly sensitive instruments fixed on nearby structures, ensuring that there was no settlement anywhere, he said.

 As part of Phase-4 work approved so far, 40.109 kilometres of underground lines are being constructed. The Aerocity-Tughlakabad corridor has underground sections totaling 19.343 kilometres, they said.

 A TBM is a machine used to excavate tunnels with a circular cross-section through various soil and rock strata. They can be designed to bore through anything from hard rock to sand. TBMs have revolutionised tunneling work worldwide, enabling tunnels to be bored without disturbing buildings and other surface structures, the officials said. TBMs are particularly useful for underground tunneling work in congested urban areas.   The DMRC has been using TBMs for its tunneling work since Phase-1. In Phase-3, when approximately 50 kilometres of underground sections were built, about 30 TBMs were deployed in the national capital, he added.