Mumbai subway goes beneath Mithi River

The construction of a 153-meter long metro tunnel below the Mithi River in Mumbai has begun.

The project is carried out by Mumbai Metro Rail Corporation (MMRC), the contractor of Colaba-Bandra-Seepz Metro 3 corridor. This metro line, stretching for 33.5 km., will be India's 2^nd to have a tunnel under a river after Kolkata's subway that passed beneath the Hooghly river.

The New Austrian Technology Method (NATM) will be utilized and the excavation will be conducted in three phases (top heading, benching and invert). The project will have to tackle severe issues including poor soil conditions and water table. "The NATM tunneling 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 stabilize the rocks. The situation in this area is different from other areas due to the water above," said S.K. Gupta, MMRC director. According to officials, the construction will be completed in 8 months. 

During monsoon period, the river overflows and causes floods. To ensure the operations will be conducted normally and the water pressure will not dramatically increase, MMRC will make the facility waterproof and will install pumps to drain water. "When the water level is high, hydrostatic forces are high and the tunnel design accounts for these factors. The tunneling 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."

The ground under the river consists of soil and weak rock which could result in instabilities and water leaks due to high permeability. MMRC will install an "umbrella" of steel bars above the excavation to support the ground that will also be strengthened by grouting to reduce permeability.

Two more tunnels are going to be constructed in the area but Tunnel Boring Machines (TBMs) will be used instead of NATM. In this case, "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," Mr. Gupta stated.