Bullet train tunnel work reaches major milestone in Mumbai

The National High Speed Rail Corporation Limited (NHSRCL) on Sunday said the first Tunnel Boring Machine (TBM) cutterhead has been lowered at Vikhroli in Mumbai’s eastern suburbs for the construction of a tunnel as part of the Mumbai-Ahmedabad bullet train project.

The 13.6-metre diameter cutterhead, weighing 350 tonnes, marks the final stage in the primary assembly of the TBM’s main shield for the high-speed rail corridor project.

According to NHSRCL, two TBMs weighing more than 3,000 tonnes each are being assembled for the construction of a 16-km section of the 21-km-long Mumbai tunnel. The stretch includes a 7-km undersea tunnel beneath Thane Creek, which will become India’s first undersea rail tunnel.

“The TBMs are the largest machines deployed so far for rail tunnel construction in the country. The cutterhead has been designed to excavate a single tunnel large enough to accommodate both the up and down lines of the bullet train corridor. The 350-tonne component arrived at the site in five separate shipments and was assembled using around 1,600 kg of high-precision welding,” NHSRCL said.

The corporation said the cutterhead has been fitted with 84 cutter discs that will cut through the rock face, 124 scrapers to clear muck at the tunnel face, and 16 bucket lips to collect and direct excavated material into the pipeline system for removal from the tunnel.

NHSRCL said the TBM will excavate an approximately 6-km stretch from Vikhroli to Bandra Kurla Complex (BKC), passing beneath densely populated urban areas and the Mithi river before reaching the under-construction Mumbai bullet train station at BKC.

The agency added that several monitoring systems are being deployed during excavation to ensure safe tunnelling and protect nearby structures.

“Various monitoring instruments are being deployed to ensure safe tunnelling and protect nearby structures during excavation. These include Surface Settlement Points (SSP), Optical Displacement Sensors (ODS), strain gauges and seismographs for vibration and seismic wave monitoring,” it said.

NHSRCL also described the cutterhead as one of the largest components being used in the project. With a diameter of 13.6 metres and a weight of 350 tonnes, the unit is designed to excavate a single tunnel carrying both the up and down lines of the high-speed rail corridor.

The corporation said the component was transported to the site in five separate shipments before being assembled using 1,600 kg of high-precision welding.

The unit is equipped with 84 cutter discs, 124 scrapers and 16 bucket lips for excavation and muck removal operations. NHSRCL said the cutting discs are the primary tools used to cut through the rock face, while the scrapers help clear muck from the excavation face.

“The bucket lip is the opening in the cutterhead through which muck enters to the muck chamber of the machine. It helps in collection, directing flow of the muck onto the pipeline system for removal from the tunnel,” the release said.

The TBM will travel nearly 6 km from Vikhroli towards Bandra Kurla Complex beneath the city landscape and the Mithi river before being retrieved at the under-construction Mumbai bullet train station.

NHSRCL said instruments including Surface Settlement Points (SSP), Optical Displacement Sensors (ODS), BRT (By reflect target/3D targets), strain gauges and seismographs are being used to monitor excavation, vibrations and nearby structures during tunnelling work.