Kolkata: Metro completes last East-West cross-passage in subsidence zone

KOLKATA: The last cross-passage in East-West Metro’s Esplanade-to-Sealdah subsidence zone has been completed, paving the way for linking the two disjointed sections of the Green Line. Now that the cross-passages on the section are built, work on the Esplanade-Sealdah stretch is expected to be over soon, following which the two currently operational sections—one between Sector V and Sealdah and the other between Howrah Maidan and Esplanade—will be connected and services on the entire 16.6km corridor will be flagged off.
“We made sure there was no ground settlement and damage to the buildings in the vicinity,” an engineer said.Contractors ITD-ITD Cementation and Kolkata Metro Rail Corporation (KMRC) had to go slow and take a month to build the last of the cross-passages—5B—in the cave-in zone as they had to be alert and watchful. On April 18, water seepage was detected at two spots during micro-tunnelling to build a cross-passage below Hind on Ganesh Chandra Avenue. Construction was stopped immediately. Expensive chemical and cement grouting were done for three days and micro-tunnelling resumed on April 22. A high-rise at 1A, Subodh Mullick Square was evacuated for a week from April 22 as a precaution.

Cross-passages are small tunnels built at an interval of 250m on underground metro lines for passengers to cross over from one of the main tunnels to the other during emergencies, like fire. Though these smaller tunnels are an important safety precondition for an underground metro corridor, KMRC had to scrap three of the eight cross-passages originally planned in the Esplanade-Sealdah section after the subsidence on Oct 14, 2022. East-West Metro work between Esplanade and Sealdah went through three such incidents from 2019 to 2022.
Over the past one months, apart from micro-tunnelling, concretizing of small tunnel rings were carried out to create these smaller tunnels connecting the bigger twin tunnels.
The original site for cross-passage 5B under Nirmal Chunder Street turned out to be as porous as three proposed beyond this area. When several months of grouting did nothing to firm up the soil consistency, the location was shifted to a more stable ground condition, below the cinema. But this place, too, needed extensive chemical and concrete grouting. On April 16, the hi-tech micro-tunnelling, with the help of new Austrian tunnel method or NATM, could start. “In less than four days, water seepage was detected at two spots and the work was stopped. A high-rise with weak foundation was evacuated. From April 22, we worked slowly, lest there was subsidence. Thankfully, there was none,” the engineer said.