Mumbai Metro 3: Tunnelling below Mithi river to begin in February

Staff ReporterUpdated: Wednesday, May 29, 2019, 03:12 AM IST
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Mumbai: The Mumbai Metro Rail Corporation (MMRCL), which has undertaken the Colaba-Bandra-SEEPZ Metro line (Metro 3), is all set to start crucial tunnelling work below the Mithi riverbed at the Bandra Kurla Complex (BKC) from February 5. This was announced by S K Gupta, MMRCL director (projects), on Monday.

The work will be completed in eight months, by October 12, 2019, he said. The area will have three tunnels; two tunnels will be dug using tunnel boring machines (TBMs) while the 150-metre long tunnel below the riverbed, will be dug using the New Austrian Technology method (NATM).

“The NATM tunnelling will be carried out in three parts — top heading, benching and invert. The first step includes creating a support system by using various ways, to stabilise the rocks. The situation in this area is different from other areas due to the water above,” said Gupta.

Every monsoon, the Mithi river overflows, causing a flood-like situation in the Kurla-BKC area. To avoid waterlogging in the metro station premises, which could disrupt services, preventive measures like the installation of de-watering pumps, channels to drain water are being adopted.

“When the water level is high, hydrostatic forces are high and the tunnel design accounts for these factors. The tunnelling will be watertight, as will the station. Entry and exit points will be above the highest flood level margin in this area. Therefore, water cannot come in from anywhere. However, even after this, if some water does enter the station, a small network of the drains will carry it to the pump, which will throw out the water.”

Explaining the difference between TBM and NATM technology, Gupta said, unlike the tunnels builty by the TBM, those built by NATMs are arch-shaped. By using small excavators, instead of the regular ones and with controlled blasting, a tunnel will be created below the Mithi river.

The two other tunnels, each of which will be 1.2-kilometres long, will be created with TBMs. Gupta said, “Here, the TBM will be deployed with the earth pressure boring machine, with foam that will form a paste. The paste will create positive pressure on the face of the rock and as a water barrier and maintain the pressure.”

The MMRCL also proposes reserving a spot called the ‘knockout section’, which will connect the bullet train terminus of the BKC to this metro station. The deadline for the completion of Phase 1 of Metro 3 is December 2022. “Since this station will be the terminus, it will have four stations, so that trains can reverse from here,” said Gupta.

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