The “Unanimous opinion” of a four-member expert committee was placed before a Division Bench of Chief Justice, Calcutta High Court, today recommending shifting of a Tunnel Boring Machine (TBM) beneath Nirmal Chunder Street by five meters for maintenance from a work-inprogress tunnel of East West Metro.

The manoeuvre will not result in soil subsidence and will as such not affect life or property above the ground over the distance over which the machine is proposed to be shifted or otherwise, according to the report placed before the Division Bench of Chief Justice T B N Radhakrishnan and Justice Arijit Banerjee of the court.

The committee has suggested several additional safeguards which include sealing of the ground around the TBM with polyurethanae grout and carrying out the work under supervision of the committee. The committee will be represented on site by the chairman throughout the operation, it was suggested. The machine will be operated by staff from Herrenkneckt, the manufacturer.

The overall maintenance work will be supervised by general consultant’s tunnel manager Mr Andrew Allen of Australia who was appointed on 11 November, it was stated. Additional monitoring points will be installed on the surface at the location and multiple position extensometer settlement monitoring points will be installed at depth, it was stated. Monitoring will be conducted continuously during and after the maintenance operation, it was further stated.

The TBM in question beneath Nirmal Chunder Street was excavating the eastbound tunnel since 1 September, but its maintenance is needed after standing idle for 10 weeks, the report stated. To facilitate and carry out maintenance, it is necessary to move the machine forward by five meters, while remaining beneath Nirmal Chunder Street. To maintain operating components, the cutter head of the TBM needs to be rotated.

This will involve some excavation and the machine will be moved forward to keep the cutter head pressed against the soil face to support the soil ahead of the machine for normal operations, the report stated. The TBM is presently located in a horizontal curved alignment. To carry out maintenance, it is required to straighten the machine by moving it for five meters so that the seals at the rear of the machine are aligned in the middle of the range of the operation, the report stated. The nature of the maintenance includes checking the main bearing by rotating the cutter head, checking the hydraulics system, the articulation joint, the tail seal systems.

It is also necessary to rotate the cutter head again and check whether it fits snugly against the vertical soil face, inspect the tail seals and carry out maintenance work if necessary and to consider further add-on safety devices, it was stated. Tunnelling for Kolkata Metro East-West Line between Esplanade and Sealdah stopped on 1 September when an inflow of water and soil was encountered by the TBM while excavating the west-bound tunnel.

A crisis management team on behalf of KMRCL was formed led by Mr Leornard John Endicott (Geotechnical expert from Hong Kong), Mr Neelakantan Kumar Pitchumani (Geotechnical expert from Chennai) Mr Guy David Christopher Bridges (Tunnel boring expert from Hong Kong) and Mr Suman Dutta (Structural expert from Kolkata) as members to review the situation, mobilisation and for execution of restoration measures. The counsel of the petitioners said that its expert seeks to go through the report. The matter will come up for further hearing on Monday.