New Delhi: A major milestone in Delhi Metro's Phase-4 construction work was achieved on Friday as the first-ever tunnelling stretch of Phase-4 was completed at Krishna Park Extension on the Janakpuri West – RK Ashram Marg corridor with a Tunnel Boring Machine (TBM) breaking through after boring a 1.4-kilometre long tunnel. "The main tunnelling work on this stretch was commenced in April earlier this year after completing the initial drive. Despite the many constraints posed by the COVID-19 pandemic, work was continued and this major milestone could be achieved, Delhi Metro said in an official statement.
This tunnel breakthrough using a mammoth 73 metre long TBM was achieved on Friday in the presence of Delhi Metro's Managing Director Dr Mangu Singh and other senior officials.
Two parallel circular tunnels for up and down movement are being constructed on this stretch which is a part of the 2.2-kilometre-long underground section from Janakpuri West to Keshopur. Tunnelling work on the other parallel tunnel shall commence soon.
This new tunnel is a continuation of the earlier Magenta line tunnel that was already constructed for the presently operational Botanical Garden – Janakpuri West corridor. The tunnel has been constructed approximately at a depth of 14 to 16 metres. Over a thousand rings have been installed in the tunnel. It has an inner diameter of 5.8 metres. The alignment of the tunnel runs along the Outer ring road and below the multi-storied built-up structures.
The tunnel has been built with the proven technology of EPBM (Earth pressure balancing method) with concrete lining made up of precast tunnel rings. The tunnel rings have been cast at the fully mechanized casting yard setup at Mundka. These concrete segments were cured with a steam curing system to achieve early strength.
All necessary safety precautions were taken while constructing the tunnel below the built-up structures by monitoring the ground movements with highly sensitive instruments fixed on the nearby structures. As part of the phase-4 work approved so far, about 27 kilometres of underground lines will be constructed. The Janakpuri West – RK Ashram Marg corridor will have underground sections of 7.74 kilometres in total.
A TBM is a machine used to excavate tunnels with a circular cross-section through a variety of soil and rock strata. They can be designed to bore through anything from hard rock to sand. TBMs have revolutionised the way tunnelling work is done all over the world as now tunnels can be bored without disturbing the buildings and other structures on the surface.
TBMs are especially useful for underground tunnelling work in congested urban areas. Delhi Metro has been using TBMs for its tunnelling work since Phase 1. In Phase 3, when about 50 kilometres of underground sections were built, about 30 TBMs were pressed into service in the national capital.
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