The tunnelling work along the remaining stretch of East West Metro, from Bowbazar to Sealdah and back to Bowbazar, near the site of the tunnel boring machine (TBM), Chandi is expected to be completed by March 2021. As the TBM Chandibecame defunct after hitting an aquifer last year on 31 August, its twin, Urvi, will take up the task of excavating the remaining portion, covering a length of around 1.8 km.
The tunnelling work had to be stopped after Chandi, while burrowing through the earth below Durga Pithuri Lane hit an aquifer on 31 August, last year. More than two hundred residents of the area near Nirmal Chandra Street had to be evacuated and shifted to different city hotels following the development of numerous cracks and crevices in the old and dilapidated buildings of Bowbazar.
After getting a nod from Calcutta High Court on February 11, the Kolkata Metro Rail Corporation Limited (KMRCL) is now gearing up to resume the tunnelling work in the next few days. To be able to start full-fledged work, the engineers are initially excavating the earth little by little while monitoring the slightest changes due to the work. According to an international expert the TBM Urvi restarted the tunnelling task with an initial excavation of 1.2 metres and so far has burrowed roughly 20 metres in the last four days.
Considering the cave-ins that turned more than two hundred residents of Bowbazar homeless overnight, the KMRCL and its team of engineering are taking every step very cautiously, to ensure that the incident is not repeated again. To assure smooth functioning of the TBM along its remaining path, the German burrower had been upgraded and equipped with features to take immediate action in case of any untoward incident. In case of a leakage while burrowing the TBM has been upgraded with features to tackle the problem within 60 minutes.
“What was seen in the Bowbazar incident, after 60 minutes there was nothing untoward on the surface side and it was not until several hours afterwards that we could see that the site may become dangerous and at that point the decision was taken to evacuate it. So this time we are making provisions inside the TBM so that the crew have got more kits and more equipment that they can use according to the type of inflow.”
“The TBM Urvi has been upgraded with 50 per cent more capability to ensure that the ground holds during tunnelling work,” said Mr Leonard John Endicott, a geotechnical expert appointed by KMRCL. Apart from upgrading the TBM, monitoring equipment have been enhanced to measure not only minute cracks but also, tilting of the structures along the route that are a major challenge before the international experts. In addition 20 extra boreholes have been made to study the nature of soil and ground water conditions in Bowbazar.
The boreholes have been made at the site of the cave-in and on the remaining stretch to compare and study the condition of soil and water at both places. According to the experts of the committee, the stretch from the present location of the second TBM near Nirmal Chandra Street to Sealdah, covering around one kilometre, is expected to be completed in about five months. The TBM Urvi will then take a u-turn to reach the first TBM Chandiin about three months. It is expected that Urvi would thereafter to take around four months’ time to complete the remaining stretch before reaching the first TBM. The damaged TBM, Chandi, will be dismantled and removed from the underground shaft.