Evolving Landscape: Improving tunnelling practices in the road segment

Improving tunnelling practices in the road segment

The roads and highways sector has witnessed limited tunnel construction activity so far. Most of the tunnel construction works in the road sector have been taken up in the Himalayan region. As a result, these projects have faced frequent delays, difficulty in mobilisation of machinery and equipment, geological complexities and cost overruns. In the past few years, there has been increased used of advanced tunnelling technologies such as tunnel boring machines and the New Austrian Tunnelling Method in the road sector. This has resulted in timely completion of road tunnel projects.

Some of the key projects completed in the sector are the Chenani-Nashri tunnel in Jammu & Kashmir, the Aut-Kullu tunnel in Himachal Pradesh and the Mumbai-Pune Expressway in Maharashtra. The 9 km Chenani-Nashri tunnel constructed between Udhampur and Ramban is the longest tunnel in the country. It is a single-tube bidirectional tunnel, comprising a 9.35 metre carriageway with a vertical clearance of 5 metres and a parallel escape tunnel.

At a recent conference organised by India Infrastructure, Colonel Manish Kapil, joint director, tunnels and bridges, Border Roads Organisation, and PurnachandaBhave, general manager, Afcons Infrastructure Limited, discussed the progress of the Rohtang and Nagpur-Mumbai Samruddhi Expressway tunnels, the challenges faced and the lessons learnt from these projects…

Rohtang tunnel

At present, one of the most important tunnels under execution in the road sector is the 8.8 km Rohtang tunnel. It is being built under the Rohtang Pass in the eastern PirPanjal range on the Leh-Manali highway. Once completed, it will be the longest traffic tunnel at an altitude of over 10,000 feet in the world. It will reduce the distance between Manali and Keylong by about 46 km.

The Rohtang tunnel is a single-tube, double-lane, horseshoe-shaped tunnel that will ensure all-weather connectivity to Lahul Valley from Manali. Funded by the Ministry of Defence, construction work on the tunnel commenced in September 2019 and is being executed by a joint venture of Afcons Infrastructure Limited and Austrian construction company Strabag.

The project has faced several challenges such as the tunnel alignment crossing below glacial lakes, 46 avalanche locations near the tunnel alignment, construction through crystalline thrust, construction being possible only through two portals (and not during the winter season), ventilation issues, breakdown of loose material, collapse of tunnel face, and geological surprises. Of these, a key issue was the high ingress of water from the Seri Nullah leading to a delay of about five years in project execution. A number of steps were taken to deal with the water ingress. A waterproofing system made of geotextiles was installed to protect the tunnel membrane. Some of the other steps taken were the construction of a deep insert, pre-grouting and chemical grouting, plugging the water gush and installation of large pipes for roof protection, besides the adoption of techniques including the central pilot drift method, the side drift excavation method, the double-layer skin to skin pipe roof method and the excavation sequence for heading. While initial progress of the project was quite satisfactory, a plethora of issues faced in project execution delayed it by many years. It is now expected to be completed by September 2020.

Tunnel under Nagpur-Mumbai Samruddhi Expressway

The Nagpur-Mumbai Samruddhi Expressway project involves the development of a 700 km six-lane expressway with paved shoulders. The project also includes the construction of five tunnels measuring 10.48 km in length. Of this, an 8.12 km tunnel is being constructed by Afcons Infrastructure. The scope of work for Afcons also involves the design and construction of a road viaduct, two interchanges, eight box culverts, and two ventilation shafts, besides mechanical and engineering works inside the tunnel.

Construction works on the tunnel commenced in 2019. So far, excavation works for each tube from the north portal and 0.6 km in each tube from the south portal stand completed. Though project progress has been impacted due to a number of factors, it is expected to be completed on time.

Lessons learnt and the road ahead

Typically, not enough stress is laid on geological studies and investigations for the construction of tunnels in the country. As a result, project implementation goes haywire, causing long delays. In order to bring about an improvement in the construction of road tunnels, especially in difficult terrains, emphasis needs to be laid on understanding the basics of rock mechanics. Detailed investigations and exploration are of paramount importance. Adopting new methods of investigation on a case-to-case basis is important too.

There is also a need to have some flexibility in tunnel contracts to make room for the disruptions caused by unanticipated geological issues. Covering all the aspects of tunnel construction in the contracts is equally important. In this regard, a dispute redressal mechanism is required to avoid inordinate delays in project implementation. Further, it is imperative to recognise the limitations of empirical methods in understanding rock mass behaviour during tunnel widening. Technical expertise in tunnel construction is equally important. Also, data sharing to facilitate knowledge dissemination across all levels should be encouraged.