BENGALURU: Over 95 per cent of civil works for Namma Metro’s longest underground (UG) corridor in Phase 2 have been completed and the stretch is expected to be opened for the public by the end of 2026, said Bangalore Metro Rail Corporation Limited (BMRCL).
“The UG corridor with a length of 13.75 km connects Dairy Circle in the South with Nagawara in North Bengaluru. The total tunnel length is 21 km,” said Subrahmanya Gudge, Chief Engineer, BMRCL, who led media persons to the underground station at MG Road on Monday.
Remaining works include laying of tracks and setting up of tunnel ventilation system (TVS), while air-conditioning system, traction, electrical and mechanical works are pending, he said.
Bengaluru’s geology a challenge
The main challenge for tunnelling is Bengaluru’s geology. Unlike Mumbai, where the basalt rock is even, in Bengaluru half the portion is soil, while the rest is rock. While soil is easy to excavate, rocks are hard and take time to bore and may lead to over-excavation, leading to settlement of roads and cracks in nearby buildings, Gudge explained.
He said the tunnel-boring machine was stopped 193 times and brought up for repair just between Shivajinagar and MG Road stations and the cutter heads had to be replaced multiple times. He said in Delhi, for the same distance, the machine would have stopped for repair a maximum of 10-12 times.
Learnings from the past helped finish tunnelling early
Gudge said there were many learnings in the underground corridor in Phase 1 and those helped them finish this tunnelling work early, without causing much damage.
“If we have options for soil investigation every 20 metres of tunnel stretch, we could plan better. However, due to the presence of buildings, we did not have that option,” he explained.
“Geological patterns are not even across the city. With the help of ‘tunnel instrumentation’ and monitoring, which involve the use of various instruments to assess the behaviour of the tunnels (which are not visible to our eyes) during the work, we took necessary remedial measures to prevent damage,” he said.
They have designed and executed the work in such a manner that after it is completed, there is absolutely no problem, he added.
He said stations are constructed by a bottom-up method except for a portion of Langford Town station, where a top-down method has been used. For the tunnelling, nine tunnel boring machines were used along with six slurry TBMs and three earth pressure balancing TBMs. The tunnelling work began in August 2020 and ended in October 2024, he explained.