Atal Rohtang Tunnel: Construction of the World’s Longest Highway Tunnel at 10,000 feet

🕑 Reading time: 1 minute

Atal tunnel (Rohtang) is a roadway tunnel constructed at an altitude of 3060 m above mean sea level, making it the highest highway tunnel in the world. At 9.02 km length, it is also the longest bi-directional single-tube roadway tunnel of India. 

The tunnel was formerly known as the Rohtang tunnel and was renamed after India's former prime minister Atal Bihari Vajpayee. It is located under the Pir Panjal mountain range of the Himalayas, near Rohtang pass in Himachal Pradesh, India.

The project is considered a key development for the Indian defense forces as it connects the Manali and Lahaul valley. The tunnel will also reduce the distance between Manali and Leh by 46 km. Earlier, the road connectivity was limited to only one season of the year as the Rohtang pass remains closed in the winters due to heavy snowfall.

It is a horse-shoe shaped tunnel with raised footpaths on both sides of the lanes. It is equipped with a semi-transverse ventilation system and a service tunnel for maintenance and emergency exit.

The tunnel was constructed using the New Austrian Tunneling Method (NATM) and is designed for a maximum speed of 80 km/h. Excavation activities were carried out every year from June to December as construction was not possible during the rest of the year due to the accumulation of heavy snow at the tunnel portal.

1. Geology of Rohtang Tunnel

The Himalayas are full of geological uncertainties because of faults, folds, and shear zones due to ongoing tectonic activities. Therefore, it is essential to study the geology of the Himalayas before constructing any major structure in the region.

1. The site has a complicated geology marked with intermixed rocks along the alignment of the tunnel. The types of rocks consist of quartzite schist, phyllite, and magmatic gneiss.
2. The average dip direction and plunge of these rocks were 220° and 250°, respectively. It shows that the rocks are uniformly dipping throughout the alignment of the tunnel.
3. Two main shear zones were encountered during the construction of the tunnel. Tunneling in the shear zones is difficult due to the probability of rockburst. 

Properties of Rohtang Tunnel Rocks

The presence of good quality rocks is desirable for tunneling process, and the index properties of rocks can help assess the quality of rock. The index properties of rocks present along the alignment of the Rohtang tunnel are tabled below:

Rock Type/Properties	Phyllite	Quartzite schist	Magmatic gneiss
Density (gm/cc)	2.69	2.73	2.68
Porosity	0.48	0.49	0.61
Specific gravity	2.77	2.73	2.73
Sonic wave velocity (km/sec)	3.66	4.0	2.53
Slake durability index (%)	98.94	98.78	98.70
Water content	0.19	0.19	0.22

Density and sonic wave velocity are highest in quartzite schist; thus, it is denser than other rocks. However, the slake durability index for all the rocks is almost the same, which indicates that the rocks are highly durable.

For half of the year, the periphery of the Rohtang tunnel is covered in snow, and the porosity of rocks is in the moderate to high range. Therefore, the possibility of water ingress during the time of construction was very high.

The compressive and tensile strengths of rock mass are very important for designing tunnels. Mostly, the wear rate of the cutter head in tunnel boring machine (TBM) and the strength of rock mass decide the amount of explosives needed for NATM. The strength properties of rock mass available along the tunnel alignment are given below:

Rock Type/Properties	Phyllite	Quartzite schist	Magmatic gneiss
Uniaxial compressive strength (MPa)	106	112	47
Brazilian tensile strength (MPa)	13	11	6
Cohesion (MPa)	4	3.5	3
Friction angle	340	370	390
Modulus of elasticity(GPa)	42	43	15

2. Problems Encountered during Tunneling and their Remedial Measures

Tunneling in the young Himalayan mountains is challenging due to the existing geological structures and tectonic activities. During the construction stage, many unforeseen events had occurred, which forced the team to change the excavation methods and the support system. These problems have been described below: 

1. The tunnel was subjected to a maximum overburden of 2000 m. A huge overburden pressure created deformation of the crown area, bending of lattice girders, crack in shotcrete, and rockfalls. 
2. Adhesion between concrete and schist rock was very poor. Therefore, the shotcrete layer was losing its adhesive properties causing it to disintegrate at the locations where the schist rock was present.
3. Mainly in the crown region, two joint sets were aligned in the perpendicular direction. Thus, the wedge failure at the crown region was common. 
4. During the construction of the tunnel, there was a sudden collapse of a 50 m stretch of the roof, but fortunately, it didn't cause any loss of life. The heading portion of the tunnel was 100 m ahead of the collapsed area. It took almost two months to repair the collapsed section. Repair works were carried out by reinstallation of lattice girders, reinstallation and extension of rock bolts, and shotcreting with wire-mesh.
5. During construction, the primary challenge was the availability of time as the tunneling advancement was possible only from June to December. This is the main reason that the tunnel construction took more than ten years.

2.1 Major Problems Encountered during Tunneling

Other than the issues mentioned above, some major complications were observed, which have been discussed below:

2.1.1 Rock Squeezing

The rock mass tries to get mobilized for stability after excavating an opening in it. However, during tunneling in poor rocks at higher overburden pressure, fracture of rock mass occurs before mobilization. This process is called as squeezing of rocks.

At the Rohtang tunnel site, mica-schist rock was the poorest, causing squeezing of rock mass at those locations.

2.1.2 Rock Bursting 

Rock bursting is generally observed in hard rock due to an increase in overburden pressure. In the Rohtang tunnel, mild to moderate rock bursting conditions were observed.

2.1.3 Delay in Support Installation 

After clearing the muck from the heading area, lattice girder, wire mesh, and shotcrete were applied. The rock bolts were later installed after six or seven rounds of shotcrete. This led to deformation around the tunnel periphery.

2.1.4 Stress Concentration around the Tunnel Opening

If the tunnel alignment is along the orientation of the foliation plane, then the stress concentration is observed more around the tunnel periphery. This condition was encountered at multiple locations during the Rohtang tunnel drive. An increased stress concentration created the bending of lattice girders.

2.1.5 Leaving Shotcrete Slot during Primary Support Installation

Generally, the slot in shotcrete acts as an apparent lining stress controller. Therefore, it was created during the initial years of tunnel advancement. However, this process proved to be detrimental because, in the long term, it allowed the propagation of cracks in the surrounding rock mass. 

2.2 Remedial Measures

To avoid the collapse of the tunnel, the following remedial measures were taken:

1. Slots in shotcrete were no longer provided to act as apparent lining stress controller.
2. Rock bolts perpendicular to the foliation plane were provided, especially in the location where stress concentration was high. 
3. Continuous three-dimensional monitoring of the supports and tunnel deformation was carried out to determine the excessive deformation.
4. Rock bolts and wire mesh were properly tied, and shotcrete was applied in a monolithic manner. This ensured a uniform distribution of stress concentration around the periphery of the tunnel opening.
5. Ground response curves were developed to determine the allowable deformation before the installation of the main support system.
6. To prevent the roof collapse, single or double pipe roofing was installed in advance. 
7. Rock bolts were installed just after clearing the muck. It reduced the deformation around the tunnel periphery.

3. Construction Process of Rohtang Tunnel 

NATM was preferred over TBM to construct the Rohtang tunnel. The main reasons for not choosing TBM are: 

1. High deformations and pressures were expected on the tunnel. Thus, the TBM may have got jammed in such situations. 
2. The transportation of heavy machine parts over Himalaya's bridges was risky because the Himalayan roads are not designed to bear heavy loads. Also, poor road conditions and tight hairpin bends make the transportation dangerous.  

The construction process of the Rohtang tunnel is discussed in two segments. In the first segment, the general tunnel construction sequence using NATM is discussed, and the second segment describes the construction timeline. 

3.1 Tunnel Construction Using NATM 

NATM is not a construction method for tunneling, but a strategy that streamlines all the construction activities, with safety and economy being the main principles. Flexibility in the operation and construction works is the main advantage of the NATM technique. It can be used for hard rock, soft rock, and mixed ground conditions. The general tunnel construction sequence using NATM is described below:

1. Firstly, the drill holes are marked on the face of the tunnel based on the blasting pattern. Thereafter, the holes are drilled using manual excavators if the face is soft soil/rock, or with boomer machines if the face is hard rock. 
2. Further, the explosives are inserted into the drilled holes and connected to detonators which are connected to one prime connection. 
3. The prime connection where all the detonators are connected is detonated, which creates a chain reaction of explosions.
4. After the complete removal of the dust and harmful gases suffused due to the blast, the loose materials are removed from the face of the tunnel. 
5. After the removal of loose material, primary supports are installed on the tunnel periphery. Shotcrete or rock bolts are used as primary supports. 
6. Water ingress inside the tunnel is a major concern as it may lead to the failure of the structure. To avoid this situation, waterproofing membranes are provided after the installation of the primary support system. 
7. If the strata demand more support and structural stability, supports in the form of lattice girder, steel ribs, and a second layer of shotcrete can be provided. 
8. Finally, the precast concrete linings are installed throughout the periphery of the tunnel section. 

3.2 Construction Timeline of Rohtang Tunnel 

The idea for the construction of the Rohtang tunnel was first conceived over 15 years ago. A joint venture of Afcons and Strabag won the Engineering Procurement & Construction (EPC) tender for Rs 1,458 crore from Border Road Organisation (BRO) in 2009.

The portals of the tunnel were constructed from the north and south directions as an intermediate starting point was impossible due to the topography. While it was possible to work with limitations from the south portal throughout the year, the north portal could only be reached via the pass. Therefore, only six to seven months of each year were effectively available for the work. The construction timeline of the Rohtang tunnel is discussed below:

1. In 2010, the tunnel excavation work began from both sides of the portal. Two arm boomers and excavators were used for tunnel excavation. After six months of work, around 1 meter of tunnel was completed. 
2. In November 2012, the construction work came to a halt for more than a year due to water ingress into the tunnel, and continuous dewatering led to the delay. 
3. On 16 October 2013, the roof area of the tunnel collapsed. Before this incident, 1.95 km of the tunnel had already been excavated. It took more than two months to repair the collapsed area.
4. In 2014, half of the tunnel construction work was completed. The total tunnel lengths of 2.25 km and 2.7 km were excavated from the north and south portal respectively.
5. Finally, the excavation work was completed on 13th October 2017. Thereafter, the laying of concrete load, installation of electromagnetic fitting, lighting, intelligent traffic control system, ventilation system, and other miscellaneous works were executed.  

FAQs

Read More

Statue of Unity: Structural and Construction Features of the World’s Tallest Statue

Tunneling Failures – Causes and Remedies [PDF]

Structural Details of Burj Khalifa – Concrete Grade and Foundations