BY Jayashree kini mendes
Piercing through the foot of the Himalayan mountains at an altitude of 1,200 metres (3,937 feet), the Chenani-Nashri tunnel, also known as Patnitop tunnel, is an engineering marvel that connects Jammu to Srinagar. This vital artery is touted as Asia’s longest and has significantly cut the distance between Jammu and Srinagar by 31kms, while reducing the elevation and hairpin bends associated with the earlier road. The distance from Chenani to Nashri has now diminished to 10.9km (between two ends of the tunnel), instead of the existing 41km. Asia’s longest bi-directional tunnel has a 9.35m carriageway and a vertical clearance of 5m. Besides being an engineering marvel, it’s also an economic boon.
Over the last few years, India has taken upon herself to construct structures that are meant to shock and awe. The Rs 3,700 crore Chenani-Nashri tunnel is one such project, in that, it’s India’s first tunnel that is deemed ‘safest’. This is because the main tunnel is connected to an escape tunnel that will allow for assured evacuation in case of any untoward incidents, hitherto rife in the area. The uniqueness of this project is the total of 29 cross passages at 300m intervals in the tunnel, which includes seven vehicular cross passage and remaining 22 pedestrian cross passages. Length of each cross passage is 34m. That is just one part of this ambitious project.
Before the tunnel, commuters found it hard to travel seamlessly as the highway remained closed for 40 days a year due to bad weather conditions. That problem has now been solved. This initiative will not only reduce travel time, but also boost economic activities and tourism in the Indian state of Jammu & Kashmir.
It was way back in 2004 that the National Highways Authority of India (NHAI) decided to four-lane the Udhampur to Banihal section of National Highway 1A, now part of National Highway 44. Louis Berger in joint venture with LRP Consultants was chosen to prepare the feasibility study, detailed project report (DPR) and engineering design. The idea was to widen the existing two-lane road to a four-lane highway, including identifying suitable options for the four-laning of the 41km (25.5 mile) mountain section, with the possibility of a tunnel.
Kshitish Nadgauda, senior VP and MD for Asia, Louis Berger, “At that time, the main task was to consider the feasibility of four-laning so as to increase the capacity of the road, and the second was to ensure that it would be safer and an all-weather road, enabling the safe and smooth flow of traffic throughout the year, including during adverse weather conditions especially in the monsoon season and during winter. Even more challenging was to preserve the environmental and social characteristics of the area where the road passed through.” In particular, the chinar trees in the mountainous section of the road.
Any highway project requires a thorough traffic study with projections for the next couple of decades. Louis Berger undertook the study to gauge future traffic volumes of the four-lane road and for the design of the roadway pavement. Nadgauda says, “We used the best surveying technology at the time to accurately map the roadway corridor along its alignment so as to capture existing features such as the extent and ground levels of the existing road, utilities, culverts and other features. Several alternate alignments and configurations were studied, including the tunnel option that was finally selected for implementation. For the tunnel, we worked with a specialist using numerical modelling so as to come up with a safe, structurally sound and optimised tunnel design. We used the finite element method to ensure that the tunnel lining support we designed would be optimal. A thorough understanding of the surrounding geological conditions was essential. In addition, the tunnel incorporates a state-of-the-art control center for operations and safety. The final configuration also included sections in deep cut with steep slopes, necessitating adequate slope stabilisation measures to prevent landslides.”
Although NHAI had decided that a tunnel would be built, there was little work done on its configuration. Technical design criteria were established governing the design of the project, including the design speed (a norm for every highway facility), which then dictates the horizontal and vertical geometry, including curve radii and gradients.
It is important to keep a sharp eye for details as every small change can affect the cost of the project.
PANACEA FOR MODERN ILLS
As the fastest project to be ever completed in the Himalayas, stakeholders involved in the project had to ensure that 3D monitoring gave them enough insight and accurate understanding into the rock mass behaviour, potential risks involved, and check the effectiveness of the countermeasures and cost-effective solutions to speed up the process. For instance, the contractors realised that the tunnel had an overburden of 1,100m, which in layman terms implies a mountain of 1,100m over the tunnel, and could create pressure from all sides when digging through them.
The unpredictable Himalayan geology with its wide variation in rock classes caused frequent over-breaks, besides settlement, tilt, etc, in the surrounding rocks to overshoot estimates. The rock mass took longer than expected to minimise its settlement and further works had to be put off. As in a typical mountainous area, the team also encountered aquifers (water bearing rocks) during linear excavation, which caused seepage during the excavation thus leading to further delays. To make it dry, the contractors had to put a water proofing membrane behind the concrete walls. Behind the membrane there is a system which collects the water coming out from the mountains and takes it to a central drain. Once collected the water is saved for captive use such as fire fighting and construction activities. The rock water is safe for drinking as well.
Much of these were attended to by the consultants and contractors involved in the project. IL&FS Transportation Networks (ITNL) was awarded on a Design Build Finance Operate & Transfer (Annuity) basis for a concession period of 20 years (including a 5 year construction period).
Speaking of ITNL’s role, Mukund Sapre, executive director, IL&FS Transportation Networks, says, “We incorporated a dedicated 100% subsidiary SPV through which we have been involved in the project since inception and are responsible for project financing, design and execution. In addition to this, ITNL is also the supervision consultant, construction contractor and O&M contractor throughout the concession period.”
Explaining the geological uncertainty in the Himalayan terrain, Sapre says, “We overcame some large problems because we went with NATM rather than TBM. Moreover, the behaviour of flysch (mixed rock faces) formations is not easily determined and needed careful and experienced blasting. The workers often had to deal with inclement weather (cloud burst, floods, excess snowfall), law and order situation, among others, that caused a force majeure situation.”
THE GREAT BORE
Incorporating the latest technology, such as emergency evacuation and accident detection system, the tunnel has set a new benchmark for construction firms to provide all-weather roads in tough terrain. Considering that it’s part of the Jammu-Srinagar highway’s four-laning project and among 12 other similar tunnel projects being constructed there, it was necessary that the authorities get it right.
The tunnel is India’s first and the world’s sixth road tunnel with a transverse ventilation system enabled by ABB drives and controlled by a software from ABB. Such ventilation systems for such long tunnels are crucial to maintain clean air, permissible carbon dioxide levels and expel harmful vehicle emissions. Sapre says, “The tunnel has been designed and built as per the international standards and codes and is equipped with safety features used worldwide. The tunnelling methodology to construct the tunnel is NATM – New Austrian tunnelling method (i.e. drill and controlled blast). Safety features such as installation of SOS box, PA system and FM systems, lighting and ventilation system, monitoring through CCTV cameras, fire extinguishers and firefighting systems, cross passages for safe evacuation in case of fire, speed limit and overtaking restrictions are all installed. The Integrated Tunnel Control System (ITCS) are automatically monitored and activated. The ITCS allows for takeover in case of manual failure. It has two backup power systems, one at each end of the tunnel. It is designed for a fire equivalent to the fire in a fully loaded truck i.e. 30MW fire.”
A further safer bet was to design, engineer and supply low harmonics variable speed drive (VSD) system for ventilation. The VSDs and motors are installed at the north (Nashri) and south (Chenani) portals for air supply and exhaust. The VSDs are also equipped with inbuilt redundancy to ensure minimum downtime.
Authorities are further planning 12 such more tunnels to reduce the travel time between Jammu and Srinagar. The Chenani-Nashri took five years to complete. India’s penchant for triggering off a spate of infrastructure projects might soon culminate in the country seeing a sea change in the operations of highways in India.