The tunnelling of the first tunnel of Mumbai’s Metro III project is completed. The tunnel is the 3.82 km long south bound tunnel running between CST and Mumbai Central stations via the station boxes at Kalbadevi, Girgaon and Grant Road. Hindustan Construction Company Ltd. (HCC) was the contractor in charge and according to a statement by the company, completed the drilling with an average drilling rate of around 8.2 meters per day.
The completion of the drilling activities marks a significant milestone for Mumbai’s metro network as this tunnel passes through some of the most thickly populated areas of the city. The HCC team faced several other challenges like geological and urban condition challenges, including the project’s proximity to the sea, tunnelling through heterogeneous reclaimed land and tunnelling below some of the oldest residential buildings that are over 100 years old without disturbing them. MMRCL had awarded the contract to HCC in July 2016, for the Design and Construction of Twin Tunnels by Tunnel Boring Machine (TBM) method and construction of 4 Under Ground Metro Stations at C.S.T, Kalbadevi, Girgaon and Grant Road.
HCC procured two TBMs from China with 117 m long single shield dual-mode hard rock with a 6.68 m diameter and weighing around 848 MT. It is equipped to operate in both, open or closed mode, in the predominantly fresh and slightly to moderately weathered Basalt and Breccia that was anticipated in the Mumbai Metro project. After months of geotechnical investigations, planning and a lengthy procurement phase, HCC team successfully launched Vaitarna I (the first TBM) from the project site at Chhatrapati Shivaji Terminus on December 04, 2017. The initial drive for Vaitarna I was completed on January 11, 2018 and the Main Drive started in the 2nd week of February 2018. This TBM requires around 32 people to operate, which includes TBM operators, engineers and labourers.
As the alignment of the tunnel passes through one of Mumbai’s most thickly populated residential areas, a thorough geological survey was conducted along the entire stretch of the project. Prior to the tunnelling work, a structural survey was carried out in a 30m influence zone on either side of the alignment to assess the impact of tunnelling on the buildings. Various monitoring instruments such as Inclinometers, Magnetic Extensometers, Piezometers, Rod Extensometers, Precise level markers, Bi-Reflex targets and Crack meters, were installed at regular intervals to check and regulate ground movements to the designated limits during and after the tunnelling work.
Currently, the construction of station buildings is underway. While all the station buildings are unique in design, the CST station is completely a cut and cover station, the remaining three stations would be made by NATM method.