Mumbai Metro machine arrives on site

The twin 1.mile (2.8km)-long tunnels will pass through mixed ground and possible water pressures of up to 2 bar. The geology includes basalt rock and transition zones consisting of black carbonaceous shale, tuff, and breccia. Rock strengths are anticipated to range between 15 MPa and 125 MPa (2,200 and 18,100 psi) UCS. The machines will bore with only 50 to 70ft 15 to 20m of cover above the tunnel and the structure will be lined with reinforced concrete segments in a 5+1 arrangement.

The TBMs will begin their excavation in hard rock mode. “Each cutterhead is optimized for operation in rock, as this is what is expected. The machines can also operate in soft ground thanks to the screw conveyor with bulkhead gate and discharge gate,” said JP Bayart, a Robbins project engineer. “The face of each machine is equipped with six muck buckets and six large internal muck loading plates. This design, in combination with the screw conveyor located at the centerline of each machine, will allow for the option of fully emptying the cutterhead chamber, resulting in minimal wear when EPB mode is not required. Muck will be removed from the tunnels via muck cars.”

Assembly and launch preparations for the TBM began on 20 June and are estimated to take about six weeks. The second TBM underwent factory acceptance testing at the end of May and will arrive at the jobsite at the end of July for its assembly. Jim Clark, Robbins Projects Manager, India said, “The target to complete the boring operations is 20 months, which includes the additional time required for the short start-up using umbilicals on the initial drives, dragging the machines and re-launching through three stations.”

L&T plans to work crews on double shifts to cover a full day of operations in order to keep to this timeline. The Metro Line 3 project as a whole is expected to be completed by 2021.