Bengaluru: In a setback for Namma Metro's Pink Line, Tunnel Boring Machine (TBM) Tunga had to be stopped on Thursday after a sinkhole emerged on the road surface in northeastern Bengaluru's KG Halli.
The German-made Herrenknecht EPB machine (S839B) is currently in its final drive, tunneling a 938-metre stretch from KG Halli to Nagavara.
Bhadra, its sister TBM, is also on its last assignment, boring the parallel tunnel.
This is the final section to be bored on the 21.26-km Pink Line, which will connect Kalena Agrahara on Bannerghatta Road to Nagavara via Dairy Circle, MG Road and Shivajinagar.
Bangalore Metro Rail Corporation Limited (BMRCL) aims to open the line by March 2025.
Around 8.30 am on Thursday, metro authorities noticed a sinkhole in the area above the tunnelling section.
The sinkhole — 2.5-3 metres wide and 3 metres deep — likely occurred due to an air pocket or loose soil, a senior BMRCL official said. An air pocket is a cavity containing air.
The air pocket likely resulted from the pressure or vibrations caused by the TBM as it moved forward, the official added, noting that sinkholes are a common occurrence during metro tunnelling.
This is the third sinkhole on the Pink Line since 2023. The previous sinkholes appeared on Brigade Road and in Pottery Town.
As a result of the sinkhole, authorities stopped TBM Tunga, another official privy to the workings of the machine said.
While BMRCL authorities backfilled the sinkhole with concrete, they had to reopen it to repair old water/sewerage pipelines that suffered damage in the process.
"The repair is underway and will be followed by grouting. Backfilling the sinkhole again will need four more days. The TBM will not operate during this time. We will then proceed slowly," the official told DH.
The road will remain partially closed to traffic, the official added.
The terrain between KG Halli and Nagavara is proving challenging for both machines. It consists of soil, rock and sand. "The progress is slow because of the geological conditions," the official said.
Tunga is expected to emerge at Nagavara, the terminal station of the Pink Line, on October 30 and Bhadra on November 30.
Tunnelling progress
TBM Tunga, which began its final drive on February 1, 2024, has bored 469 metres (335 rings) out of 938 metres, averaging 3.19 metres per day.
TBM Bhadra, which began its final drive on April 2, 2024, has bored 310.8 metres (222 rings) out of 939 metres, averaging 3.61 metres per day.