Atal Tunnel: An engineering wonder in the Himalayas

The Atal Tunnel in India was finally completed in 2020, after 14 years of work and with enormous challenges, including heavy snowfall, avalanches and severe geographic conditions. Located in the Pir Panjal Range of the Himalayas, it is now the world’s longest highway tunnel above an altitude of 10,000 ft. The engineering team on the project, SMEC, shares the story behind this once-in-a-lifetime milestone.

‘A very special project’

The need for a road through the Rohtang Pass, which is located at an altitude of over 13,000 ft in the Pir Panjal Range of the Himalayas, was discussed as early as 1860 by the Moravian Mission, revealed SMEC. Nearly 160 years later, the Atal Tunnel (formerly called the Rohtang Tunnel) has become a reality.

In 2006, SMEC was engaged by India’s Border Roads Organisation and the Ministry of Defence to provide design, engineering and advisory services on the project. The company also took on a subsequent role as the Independent Engineer (IE), carrying out supervision, project management and contract management for all electrical and mechanical installations.

“This was a very special project for SMEC because it has a significant impact on the people living here and what a contribution it is to India,” said Prashant Agrawal, senior general manager (hydropower & dams) at SMEC. “Our team of more than 25 experts from different countries has come together to provide our global expertise to our client, so that this huge undertaking can become a reality.”

The opening of the 9-km tunnel is a relief for almost 20,000 people in the Lahaul and Spiti Valleys, as they are no longer isolated from the rest of the country due to snowfall, avalanches and landslides. The travel distance between the two valleys and Manali has also been reduced by 46 km, with the travel time cut by up to four hours.

SMEC pointed out that despite its successful effort, the remoteness, accessibility and extreme altitude of the project presented immense challenges to geological works, tunnelling and mechanical and electrical installations.

Difficult tunnelling

The topographical layout of the mountain range through which the tunnel was planned, including some 5,200 m high peaks, precluded almost any practical possibility of a shaft or adit to investigate the ground conditions along the alignment, explained SMEC. Thus the entire investigation was undertaken by desktop studies and supplemented by aerial (helicopter) reconnaissance.

“The geological investigations were quite difficult because the mountains have peaks up to 5,200 m covered in snow. They are the High Himalayas,” said Robert Goldsmith, chief technical principal engineering geology at SMEC. “So our geological investigations were limited initially to extensive desk studies, satellite imagery and so on. And a few geological traverses up into the rugged mountain valleys.”

The fire and life safety requirements were equally challenging, given that the two tunnel portals were located in different climatic conditions, added SMEC. The southern portal was in a lush, forested location and the northern portal in an arid climate. This resulted in extreme atmospheric pressure differentials between the two portals and thus high wind velocities in the tunnel.

“The resulting extreme atmospheric pressure differential between the two portals could result in up to 30 km/hr high velocities,” said Hans Bleuler, tunnel consultant at SMEC. To allow effective fire suppression, the north portal was fitted with a hydraulically operated door, which allowed for control of air velocity within the tunnel during a fire event.

“With the tunnel being driven under 5,200-m-high peaks and the anticipated variability of the rock, we had to include provision for flexible support system to allow for up to 600 mm convergence of the tunnel during excavation,” added Mr Bleuler.

The location of the proposed 9-km tunnel meant that the excavation could be attempted only from the two portals, with the north portal not accessible for more than half the year due to snow, avalanches and blizzards, said SMEC. Access over the 4,000 m high Rohtang Pass was limited to the brief summer period.

SMEC investigated two viable construction methods, including by tunnel boring machine (TBM) and by drill & blast. In the end, the drill & blast method was chosen, which would offer the flexibility to adopt different kinds of support systems as excavation progressed.

‘Unique design’

SMEC also highlighted the design of the tunnel. It featured an independent escape passage with a separate ventilation system, aligned to current international best practice. This escape passage was located below the tunnel pavement and included airlocks at each access point.

“This meant the design of the tunnel cross section was quite unique as we had to effectively separate the tunnel into three compartments, using concrete roof slabs and in-situ pavement slabs,” said SMEC.

“The crown of the tunnel formed a large tunnel ventilation duct used for providing adequate air supply during operations and smoke extraction capability during a fire event. The road section included regular access points to the emergency egress, the space below the pavement slab was used for the emergency evacuation of road users during a fire event as well as trafficable access for firefighting service personnel and equipment.”

Finally, the south portal needed to be secured by an avalanche protection gallery since it is located in an active avalanche catchment area, continued SMEC. “This allowed us to develop the entire southern ventilation, tunnel control and maintenance facility within this avalanche gallery, designed using international (Swiss Avalanche Design Institute) guidelines.”

With its multi-disciplinary team of specialists, SMEC completed the tender design and construction began around late 2009 to 2010.

Complex geological conditions

The Himalayas are a thrilling but daunting environment when it comes to geological setup and challenges to large civil works, explained SMEC. On this project, the team was dealing with complex rock types of schist, gneiss, phyllite and quartzite as major rock types with interbedded layers and faults.

“Because of the remote nature of the project, it was very challenging to get significant geotechnical investigations undertaken for the full length of tunnel,” said Luke Drowley, manager, tunnels – NSW at SMEC.

“There were large lengths of tunnel where no borehole information was available, and so we relied on interpretation of surface geology. To account for this, we developed a suit of rock support classes to cover the full range of expected ground conditions that we would encounter. These support classes ranged from light rock bolts and shotcrete through to heavy lattice girders and thick shotcrete linings.”

According to SMEC, one of the most challenging sections was the Seri Nallah fault zone, located about 1.5 km into the mountain from the south tunnel portal. Here, the soft material and conditions were more adverse than the team predicted prior to construction. Tunnelling through this section was so difficult that it took almost five years to complete.

The primary rock support was designed under seven different support classes, including a combination of rock bolts, lattice girders and shotcrete, with the provision of forepoles and piperoof for more challenging ground. “For the fault zone, however, we had to introduce additional support measures and change the construction sequence,” said SMEC.

“This section was excavated with a circular or oval shape rather than the standard horseshoe shape. There was heavy water ingress of more than 100 l/s owing to the glacial lake. The tunnel in this reach was excavated after installing a piperoof and excavating the face in small intervals while supporting it with shotcrete, face bolts and a central core.

“With high deformations, it was also necessary to close the shotcrete ring as soon as possible. So a temporary invert was installed after the excavation of the heading, which was followed by bench excavation and finally excavation of the deep invert.”

Life-changing project

“After 14 years of our persistence and collaborative effort with our client and contractors, we’re really pleased to see this tunnel getting completed,” said Mr Agrawal. It now facilitates two-way traffic and can accommodate up to 3,000 vehicles per day in any weather conditions, at a maximum vehicular speed of 80 km/hr.

The Atal Tunnel is truly a life-changing achievement, helping to improve the quality of life for the people living in the area, and ultimately benefitting the economy of the country in general.

“Previously, access over the Rohtang Pass was limited to only six to eight months out of each year, due to heavy snowfall, avalanches and landslides. What this tunnel offers – safe, year round road travel through the Rohtang Pass – is a first for India,” said SMEC. “It enables the previously isolated communities of the Lahaul and Spiti Valleys to travel and sell agricultural produce in towns, as well as boost tourism by providing all-year access to Leh and Lahaul-Spiti Valley.”

“We are really proud to have helped make an impact on so many lives by delivering this Atal Tunnel,” concluded Mr Agrawal.

Photos © SMEC

Note: The longer version of this article has been published in the Mar/Apr 2021 issue of Southeast Asia Construction. Click here to read the article.