In dead of night, tunnel boring machines on the move in Mumbai

Led by a 144-tyre trailer, pulled by a 500-horsepower Volvo truck, the convoy restricts its movement to between 11 pm and 5 am. The biggest trailer is loaded with a 300-tonne main drive, the critical and heaviest part of a TBM that powers the cutter head used to excavate tunnels. At least three more multi-axle trailers carry the other heavy components including the main shield.

In fact, a total of 219 trailers have been deployed to move the TBMs from JNPT to the two sites where they will be deployed — Vikhroli, a suburb of Mumbai, and Savali in Ghansoli area of Navi Mumbai.

The multi-axle trailers manage to cover a maximum of 10 km each night. The smaller, flatbed trailers, carrying containers of smaller machine parts, make the journey to Vikhroli/ Savali in one night, in about four hours.

All the shipments of the TBMs are expected to leave the port by April 12. While a total of 154 trailers have arrived at the two sites so far, 65 are yet to reach.

Once fully assembled, a TBM will measure 96 metres long, with a diameter of 13.6 metres, which is almost equal to a four-floor building. For comparison, the TBM used in the Delhi Metro project typically has a diameter of 6.6 metres.

Earlier, the National High Speed Rail Corporation Limited (NHSRCL), the implementing agency of the high-speed rail project, had commissioned a detailed survey of the stretch of road from the port to the two sites. The survey mapped all the bridges, underpasses, creeks, toll plazas, barricades, traffic diversions, height of street lights, and the width and weight-carrying capacity of roads along the routes. It was then decided to take a longer route, without disturbing daytime traffic movement, avoiding major roads like the Mumbai Trans Harbour Link and the Eastern Express Highway.

Explaining the challenges faced during the route survey, Pranet Kumar, an employee of the logistics company involved in the task, said: “The first survey was conducted almost a year ago, and we carried out another one recently. On the JNPT road, stone barricades were blocking the route and had to be removed. At the 28.3-km mark, there is the Kalundri River Bridge, but since the weight of our consignment is too high, we had to divert the route. At the Khandeshwar railway overbridge, as the structural adequacy was uncertain, a detailed analysis was conducted before allowing heavy-axle movement. Similarly, on Kalamboli Link Road, a side pole had narrowed the road to 7.2 metres. At Kalamboli Circle, there was a low-hanging overhead cable, which was lifted using a T-crane during the movement of the multi-axle trailer,” he said.

At the wheel of the biggest trailer is Mohan Singh, a resident of Ballia. “The last time I carried such heavy machinery was in 2017 — a 466-tonne turbine, from a BHEL plant in Haridwar to Krishnapatnam in Andhra Pradesh, which took 10 months. The entire transportation process is very complex. It takes at least 10 people to keep the vehicle moving. A minor error can get you stuck for many days,” he said.

“Once all the parts arrive, it will take 97 days to assemble a TBM and start excavation work. The tunnel will cross under many residential areas, there are around 600 residential buildings which fall along the stretch. The project involves a 7-km undersea tunnel as well. Under such circumstances, tunnelling through blasting, which is called the New Austrian Tunneling Method (NATM), is risky. This is why TBMs have been brought, we need a high level of precision,” said a senior official of NHSRCL.

“You can imagine the sensitivity of this project from the fact that there is an 18-storey building along the tunnel alignment, where the gap between its foundation and the upper surface of the tunnel — what we technically call the overburden — is just 5 metres. In such a situation, there is a risk of the tunnel collapsing. However, this work can be carried out using a TBM. It will be like Mumbai city will have a normal life on the surface, while a tunnel will be excavated beneath it,” said the official.

A senior official at NHSRCL said this is the first time that a semi-automatic TBM will be used in the country. It will carry out drilling and segment deployment at the same time. Currently, in projects where TBMs are used, like the Rishikesh-Karnaprayag rail line, only one task can be performed at a time.

The 508-km bullet train project includes a total tunnel area of 27.4 km (21 km underground, and 6.4 km of surface/ mountain tunnels). Almost 5 km of the 21-km underground tunnel has been excavated through the NATM blasting method, while the remaining excavation will be done by TBMs.

“With NATM, we were able to excavate almost 2.5 metres per day. With TBM, we will excavate 16-20 metres per day,” said the NHSRCL official.

A TBM is expected to consume about 10 MW of electricity daily, for which new electricity transmission lines have been laid at both the sites. It is also expected to require 23 lakh litres of water daily. A water treatment plant is coming up at Vikhroli, which will use the city’s rainwater to run the TBM.

At the Vikhroli site, a 1,700-tonne crane towers in the sky, ready to lift the TBM’s heavy components. An excavation shaft has been prepared, measuring 56 metres in length, 56 metres in depth, and 40 metres in width. This is where the TBM will be assembled.

Inside the pit, rails are being laid, gantries are being installed, and blaring sirens signal the completion of each stage of work. Vipul Ghosh, a worker at the site, uses his total station machine to check the coordinates of each task. “In this space, even a millimetre cannot go wrong. If a bolt is being tightened, we have to ensure its coordinates are precise. There is no scope for error,” he said.