5 years on, E-W finishes shaft work that led to collapse in Bowbazar
Times of India | 3 September 2024
Kolkata: The East-West Metro on Sunday successfully completed the most crucial construction challenge in the problematic Sealdah-Esplanade section: joining of the twin tunnels and the evacuation shaft coming up at Durga Pituri Lane through micro-tunnelling method. Five years ago, it was at this very spot in Bowbazar, where macro-tunnelling (done with the help of giant tunnel borers) had caused large-scale subsidence and houses had fallen like a pack of cards.
Engineers heaved a sigh of relief when the passenger evacuation shaft or egress shaft’s most challenging aspect was completed on Sunday despite multiple geological threats.Much as implementing agency Kolkata Metro Rail Corporation (KMRC) is eyeing an early-2025 completion of this most-troubled section and is in the last leg of the East-West Metro project, everyone was bracing for the job of linking the east-bound and west-bound tunnels with a cross-passage and the evacuation shaft.
Cross-passages are smaller tunnels, through which passengers walk over from one underground tunnel to another. In this case, the smaller tunnel leads to the evacuation shaft through which passengers will climb up during emergencies. The technique adopted was the new Austrian tunnelling method (NATM) or hi-tech micro-tunnelling mechanism. It is deployed when the surrounding soil is porous and supporting tools are needed to maintain the stability of the soil while the tunnels are built.
On Aug 31, 2019, a giant tunnel boring machine (TBM), burrowing through the same Bowbazar site to build the west-bound tunnel, punctured a sand aquifer causing large-scale subsidence. As water gushed out, at least 25 houses collapsed and several others were damaged. More than 700 residents were evacuated. Around 400 are still homeless, because two more incidents of subsidence followed, one of them at the same spot on Durga Pituri Lane in May 2022.
“Naturally, we were extremely cautious while embarking on the micro-tunnelling job below Durga Pituri Lane, considered to be the Ground Zero of the Aug 2019 subsidence. We spent sleepless nights when we started the NATM work to link the twin tunnels and the egress shaft last Wednesday (Aug 28). Finally, there is light at the end of the tunnel. It is safe to say that there isn’t any underground geological threat in the Sealdah-Esplanade section any more,” said Pinaki Mukherjee, project manager, ITD ITD Cementation, which is contracted to build the 2.5km Esplanade-Sealdah stretch.