Robbins TBMs in final Mumbai Line 3 breakthrough20 July 2021
Robbins has announced that its 6.65m-diameter Crossover XRE TBM (the second of two) recently made its third and final breakthrough on India’s Mumbai Metro Line 3, following its sister machine’s final breakthrough made in late April.
Contracting joint venture Larsen & Toubro and Shanghai Tunnel Engineering Co (L&T- STEC) overcame unpredictable terrain, high-pressure water ingress, and government-imposed lockdown orders during the Covid-19 pandemic.
The two custom-built machines bored parallel 2.9km tunnels between stations at Cuffe Parade and CST, breaking through into several station sites along the way. It is the first time in India that Dual Mode, Crossover-type TBMs equipped with a horizontal screw conveyor and high torque/high speed (two-speed) cutterhead drives have been used. Crews are now preparing the two TBMs to bore the L&T Chennai Metro project.
Citing another first for India, Robbins claimed that along a 554m-long section, the Crossover TBMs were used uniquely to bench the NATM platform tunnel through basalt rock. This entailed removing the bottom section of rock left in the station after conventional excavation of the top section and required fine control of the TBM as only 25% of the cutterhead was excavating the rock mass, while the remaining 75% had no contact with rock or soil. In another first for India, the TBM was relaunched without using a reaction frame, instead pushing off from half segments erected during the benching of the NATM platform tunnel.
Despite the difficult conditions, L&T-STEC made good progress throughout tunnelling. The JV tackled the restrictions of an urban environment with only 15-20m of cover beneath sensitive structures which were monitored for vibration, movement and potential settlement.
There was also the complex geology: a mix of fresh, greyish basalt, soft volcanic tuffs, shale, and breccias – consolidated rocks of angular fragments of disintegrated volcanic rock. Also of concern was the tunnels’ proximity to the coastline of the Arabian Sea: at one point, TBM 1 was only 25m from the coastline, with invert level at around 22m below mean sea level. As a result, the Crossovers faced a significant amount of groundwater with up to 300lit/min ingress during excavation, yet the machines still managed impressive rates. TBM 2 even completed one push in a swift 14 minutes. Excavation speed proved not to be an issue and was only limited by the rate of muck removal.
Mumbai Metro Line 3 is expected to be completed in 2025, improving transit journeys, decreasing road traffic by 35% and reducing daily fuel consumption by 460,000lit.