Bengaluru Metro completes tunnelling work on city’s longest underground section

BMRCL officials said that the maximum tunneling achieved in a single day was 27 metres by TBM Urja on April 25, 2022, between Cantonment and Pottery Town stations. The maximum tunneling done in a month was 308 metres by TBM Tunga between KG Halli and Nagawara in July 2024.

A senior Bangalore Metro Rail Corporation Limited (BMRCL) official told Moneycontrol: “In Bengaluru, tunnelling is challenging due to a mix of soft soil, hard rock and boulders, especially beneath buildings, requiring careful planning.”

He added that TBMs were operating 40-60 feet below the surface, with most boring an average of 3-5 metres per day, though progress varies with ground conditions.

“We’re tunnelling below densely populated areas with unknown and improperly sealed wells. When encountering loose pockets, the slurry we use can surface, causing issues. Some wells are unidentified, as property changes or incomplete records mean owners aren’t always aware of them.”

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

The subterranean section is part of Bengaluru Metro's 21.3 km Pink Line (13.8 km underground, 7.5 km elevated), which connects Nagawara on the Outer Ring Road in the north to Kalena Agrahara on Bannerghatta Road in the south.

Bengaluru Metro's Pink Line

 The Phase 2 underground section spans 13.8 km from the South Ramp near Jayanagar Fire Station to the North Ramp near Nagawara Station. It comprises 12 underground stations and twin tunnels totalling 21.9 km.

BMRCL officials plan to open the Pink Line in two phases: a 13.8 km underground section connecting Dairy Circle to Nagawara by June 2026 and a 7.5 km elevated section from Kalena Agrahara (Gottigere) to Tavarekere (Swagath Cross) by September 2025.

BEML Limited will deliver 96 coaches for services on the Pink Line between June 2025 and December 2026. Officials said they need to complete pending work including station work, track laying, signalling, telecommunications, traction power supply,  air-conditioning systems, lifts, and escalators.

Also read: India's most congested city, Bengaluru, faces train set supply crisis

Challenges aplenty

While the central business district areas like MG Road and Shivajinagar were bustling with life above ground, TBMs were tirelessly and silently cutting through the rock beneath. A geological investigation by BMRCL revealed that 34 percent of the underground stretch consisted of hard and weathered rock, 46 percent of soft soil and 20 percent mixed geology. Phase 2 tunnelling work, which began in July 2020, was divided into four packages awarded to contractors Larsen & Toubro, Afcons and ITD Cementation India. A total of nine TBMs—Urja, Varada, Avni, Lavi, Vindhya, Vamika, Rudra, Tunga and Bhadra—were deployed for the work.

Bengaluru Metro officials faced multiple challenges in the process. Tunnelling through densely populated areas with old buildings, particularly between Shivajinagar and Nagawara, posed additional difficulties. “Tunnelling in Bengaluru's geology is complex,” said an official. "Sometimes we don’t know what lies ahead—soft soil, high groundwater levels, hard granite or large boulders, primarily granite and dolerite."

TBMs frequently required interventions, including changing cutter heads worn down by hard granite rock. Challenges in mixed-ground conditions led to over-excavation, excessive wear on cutter tools and regular cutter-head maintenance.

One TBM, Rudra, was delayed for six months while tunnelling from Dairy Circle to Lakkasandra, as it encountered a 33-foot pile of garbage left over from a former quarry site turned landfill.

Also read: BEML receives Rs 3,177 crore rolling stock order from Bengaluru Metro

Other issues included sinkholes, road cave-ins, slurry leaks into houses, and cracks in nearby buildings. Officials also had to exercise caution due to illegal borewells and old, structurally weak buildings along the route.

The Pink Line faced major obstacles, such as tender cancellations due to high bids, land acquisition challenges, labour and oxygen shortages during the pandemic, protests against land acquisition and alignment, etc. The line’s colour was changed from red to pink in 2019 to avoid associations with “stop” in the mobility sector.

“Despite these challenges, Phase 2 tunnelling has been smoother than Phase 1. For instance, Green Line's 4 km underground section between Sampige Road and National College took four years” said an official.

BMRCL learns from Phase 1 tunnelling

BMRCL appears to have learned from the challenges of Phase 1 tunnelling. After discovering reported cartel-like bidding practices among contractors, BMRCL cancelled the first tender where contractors quoted high bids, opted for experienced contractors, conducted a thorough geological investigation and  went in for more slurry-based TBMs over the earth pressure balance TBMs used in Phase 1.

"Of the nine TBMs, only three—Tunga, Bhadra and Varada—were earth pressure balance TBMs, while the others were slurry-based.  All contractors had extensive tunnelling experience, and slurry-based TBMs handled mixed-ground conditions better than earth pressure balance TBMs,” an official said.

Also read: Bengaluru traffic: IISc bats for Inner Ring underground Metro over DKS’ tunnel road plan