India’s ambitious first bullet train project, connecting Mumbai and Ahmedabad, has taken a significant leap forward. The bullet train begins its journey at Mumbai’s Bandra-Kurla Complex (BKC), immediately diving underground and traveling through a 21-kilometer stretch that runs beneath the sea. While this project has garnered much attention for its high-speed travel and modern infrastructure, a critical, lesser-known component is the construction of a secretive tunnel network that is vital for the success of this engineering marvel.
The 21-Kilometer Mega Tunnel
The bullet train tunnel between BKC and Shilphata, measuring 21 kilometers in total, is a key part of the project. This route, featuring a 7-kilometer undersea section through Thane Creek, is the first of its kind in India. The tunnel is designed as a single-tube structure, accommodating two tracks for both up and down train movements. Construction is progressing rapidly, with the tunnel expected to revolutionize transportation between Mumbai and Ahmedabad.
The Role of the ADIT Tunnel
One crucial part of this mega tunnel is the Additionally Driven Intermediate Tunnel (ADIT), a 394-meter inclined tunnel located at Ghansoli. Excavation for the ADIT began on December 6, 2023, and was completed in just six months, using 214 controlled blasts and over 27,500 kg of explosives under expert supervision. The ADIT, which extends 26 meters deep into the ground, will expedite the construction of a 3.3-kilometer tunnel using the New Austrian Tunneling Method (NATM), allowing for simultaneous excavation on both sides.
The ADIT’s 11-meter by 6.4-meter internal dimensions are wide enough to allow vehicular access to the main tunnel during both construction and operations. Additionally, the ADIT will serve as an emergency evacuation route and provide maintenance access after the tunnel becomes operational. A number of monitoring instruments, such as Surface Settlement Points (SSP), Optical Displacement Sensors (ODS), strain gauges, and seismographs, were used to ensure the safe excavation of the tunnel and to protect nearby structures from any impact during the construction process.
The Tunneling Techniques: TBM and NATM
Out of the total 21-kilometer tunnel, 16 kilometers will be constructed using Tunnel Boring Machines (TBM), while the remaining 5 kilometers will rely on the more traditional NATM. The use of TBMs, with massive cutter heads measuring 13.6 meters in diameter (in contrast to the usual 6-8 meter heads used in metro tunnels), is another groundbreaking element of this project. These machines will bore through the tough geological layers, creating the tunnel with precision. The NATM method, on the other hand, involves a series of controlled blasts to clear rock, making it suitable for shorter sections of tunnel construction where heavy machinery cannot be deployed.
The construction of the tunnel is not only an engineering feat but also a logistical challenge, especially in a densely populated and geologically complex region like Mumbai. TBMs will operate from multiple entry points, including BKC, Vikroli, and Ghansoli, working simultaneously to complete different sections of the tunnel. The undersea section is particularly challenging, and the project team has deployed state-of-the-art technology to monitor the impact of tunneling on the surrounding environment, ensuring safety and structural integrity.
A Rapidly Progressing Mega Project
The bullet train tunnel is an essential part of the broader Mumbai-Ahmedabad High-Speed Rail Project, one of India’s most expensive and technologically advanced infrastructure undertakings. The ADIT tunnel, though relatively small in size, plays a critical role in facilitating the main tunnel’s construction, providing key access points for machinery and personnel. As the project moves ahead, more breakthroughs are expected, including the completion of the undersea tunnel section beneath Thane creek, which will further cement this project as a landmark in India’s transportation history.
Once completed, the bullet train is expected to drastically reduce travel time between Mumbai and Ahmedabad, offering passengers a world-class, high-speed rail experience. With an extensive tunnel network, including the ADIT and the main 21-kilometer tunnel, this project stands as a testament to India’s growing infrastructure capabilities and its vision for the future of transport.
This bullet train, with its groundbreaking undersea tunnel and cutting-edge technology, will set a new benchmark for large-scale transportation projects in the country.
Mumbai–Ahmedabad High Speed Rail Corridor (MAHSR) or Mumbai–Ahmedabad HSR is an under construction high speed railway line operated by the National High Speed Rail Corporation Limited.
