Light at the end of the tunnel? Bengaluru Metro's Pink Line likely to be ready by next year

He explained that the presence of boulders in the soil is a frequent obstacle during tunnelling.

“Granite boulders are particularly hard, with a Uniaxial Compressive Strength (UCS) of 150 megapascals (MPa), while dolerite boulders are even harder, with a UCS of 300 MPa. When boulders are smaller than the cutter head of the tunnel boring machine (TBM), they rotate with the machine, damaging the cutting tools. This not only damages the tools but also causes ground settlement and leads to the accumulation of boulders in the cutting chamber” he said.

Gudge added that tunnelling through the Central Business District, specifically between Shivaji Nagar to Vellara (2.2 km), has been the most challenging part of Namma Metro’s Phase 2 underground section.

“The challenging geological conditions forced us to replace 799 cutter discs on this section. This is needed when discs wear out or are required to cut through rocky soil. The route's mix of rock and soil causes frequent disc replacements, as the discs mounted on the Tunnel Boring Machines are damaged by hard rock or boulders. Each cutter disc costs between Rs 1.5 lakh and Rs 3 lakh, and most are beyond repair” said Gudge.

In October 2024, Bengaluru Metro completed the tunnelling work for the city’s longest 14 km underground section of the Pink Line, which is part of Phase 2 of the project.

Officials said that more than 90 percent of the civil and track laying work has been completed. However, there is still pending work, including station construction, signalling, telecommunications, traction power supply, air-conditioning systems, lifts, and escalators.

Although the length of the underground section is 14 km, the total tunnel length involved is 21 km, as there are twin tunnels.

This underground section is part of Bengaluru Metro's 21.3 km Pink Line, which is a combination of 13.8 km of underground and 7.5 km of elevated sections, connecting Nagawara on the Outer Ring Road in the north to Kalena Agrahara on Bannerghatta Road in the south.

BMRCL officials plan to open the Pink Line in two phases. The first phase, which is a 7.5 km elevated section from Kalena Agrahara (Gottigere) to Tavarekere (Swagath Cross), is expected to open by December 2025. The second phase, which is a 13.8 km underground section connecting Dairy Circle to Nagawara, is expected to open by December 2026.

Also, read: Bengaluru Metro's Pink Line to be operational in two phases

In August 2023, Bharat Earth Movers Limited (BEML) had received a rolling stock order worth Rs 3,177 crore from BMRCL, which includes 96 coaches for the Pink Line. BEML is expected to supply the prototype train for the Pink Line by October 2025.