Mumbai: The construction of tunnels for Mumbai’s first underground Metro will take at least three years, starting from early next year. The 33.5km route has been divided into seven 5km sections. The contractor will bore the tunnel and construct stations in a stretch, said Ashwini Bhide, managing director, Mumbai Metro Rail Corporation Ltd (MMRCL).
Officials said at a designated spot, a 20metre (75feet) deep hole, 25×30 metre hole will be dug into the ground. Two tunnel boring machines will then be lowered. The machine, which consists of a cutter head, a screw jack excavator machine and a conveyor system, will be assembled, and boring will be done.
“A computer system, which is part of the equipment, will monitor the ground water level and earth pressure, and maintain the air pressure level, thrust applied, type of soil and excavation progress. It warns of tilts and shifts and ensures boring is along the centre,” an MMRCL official said.
In each 5km section, two tunnels will be dug simultaneously for the Up and Down lines. The tunnels will be separated by a six-metre wide layer of earth. The 3.8km tunnels will open onto stations, officials said. The excavated soil will be removed from the trench by the conveyor. The machine will carry out digging of 25-30 metres daily.
People along the 33.5km route will not be greatly inconvenienced, officials said, as the digging will be underground. “The machine will be brought to the surface in the station area, where it will be serviced, and then lowered from there. No external space will be used for construction work. The station area will also be used for storing the excavated soil. Around 150 trucks from the seven stretches will take the material daily out to be dumped in quarries,” Bhide said on Monday.
In the case of eight stations, the new Austrian tunnel method will be used. Once the station area is dug, the surface will be covered and traffic restored. Construction work will then continue underground.