Iconic Structure: Asia’s longest bidirectional tunnel receives nod

Asia’s longest bidirectional tunnel receives nod

After the opening of the landmark Chenani-Nashri tunnel in Jammu & Kashmir, the state has yet another iconic structure in the making – the Zojila tunnel – which has been proposed to surpass the former’s length, and will become India’s longest tunnel and Asia’s longest bidirectional tunnel upon completion. The project aims to provide all-weather connectivity between Srinagar, Kargil and Leh. In January 2018, the Cabinet Committee on Economic Affairs approved the project and subsequently an MoU was signed between National Highways and Infrastructure Development Corporation Limited (NHIDCL) and IL&FS Transportation Networks Limited (ITNL). The engineering marvel will be the first of its kind, holding strategic importance for driving development and regional integration in the underdeveloped districts of the state.

What the project entails

The project will primarily entail the construction, operation and maintenance of a 14.08 km, two-lane, bidirectional tunnel, along with a parallel 14.2 km egress tunnel with approaches on the Srinagar-Leh section connecting National Highway-1 at km 95 and at km 118. It is being implemented on an engineering, procurement and construction basis by ITNL at a civil construction cost of Rs 48.99 billion. The tunnel will be aligned in the southwest-northeast direction along the Zojila Pass Valley, with the western portal of the tunnel having an elevation of 2,900 metres, a 37 metre covered tunnel and an approach road spanning 5.02 km. Further, a minor bridge of length 60 metres will also be constructed. The eastern portal, that is, the one towards the Leh region, will have an elevation of 3,310 metres, a 30 metre covered tunnel and an approach road spanning 0.8 km. The tunnel will also have a 150 metre snow gallery at km 2.85.

Overall, the gradient of the tunnel will be a continuous positive 2.89 per cent from the west to the east. There will be a separate egress tunnel, with adequate space for emergency evacuation, which will have a clearance of 3.5 metre width and 4 metre height, with provisions for bi-directional construction traffic. Besides, the project will entail the construction of three ventilation shafts, of which two will be 12 metres in diameter and the third 5.3 metres in diameter. There will also be cross passages at 250 metre intervals, lay-bys every 750 metres, and emergency niches every 125 metres. Further, jet fan cabinets spanning a length of 30 metres will be installed at an interval of 400-600 metres on both sides of the highway. Electrical supply cabinets will also be installed every 700 metres. Meanwhile, emergency telephone and communication systems will be set up at intervals of 125 metres on one side of the tunnel.

With regard to the construction site, “The project area exhibits an undulating and rugged topography with a highly mountainous and barren terrain,” says Ashutosh Chandwar, senior vice-president and regional head, north, ITNL. Further, the altitude varies between 2,800 metres and 4,500 metres above mean sea level, while the base of the hill faces is covered with slide debris and loose talus/scree deposits, produced mainly due to frost weathering. The region where the project is located has metamorphic rocks of sedimentary and magmatic origin, geologically related to the Tethyan facies.

Progress so far

The Zojila tunnel project was awarded to ITNL by NHIDCL on January 24, 2018. The appointed date, however, is still awaited from the latter.

Regarding project implementation, “All roads after Sonmarg have been closed due to heavy snowfall and are expected to be opened to traffic only after end-April 2018. ITNL will start mobilising its manpower, materials, and plants and machinery post the opening of the roads. Currently, the organisation is having technical discussions with various agencies and designers for finalising the implementation plan. Meanwhile, high quality pre-construction investigations (well adapted to the geological conditions and project characteristics) for evaluating the feasibility of the tunnel are under way,” says Chandwar.

Selecting the appropriate technology

Some of the major challenges expected under this project are those relating to the construction of the three vertical shafts and tunnel excavation work, for which deploying the appropriate technology will be key to effective implementation. In order to facilitate quick and reliable construction of the shafts, a vertical shaft sinking machine is likely to be used. The key strength of this unique mechanised equipment is that it has the flexibility to be used in both soft (unstable) and stable soils with compression strengths of up to 80 megapascals, and has therefore been considered the most suited for the task.

Regarding excavation works, the use of tunnel boring machines is being considered far too risky due to the presence of geological uncertainties, which could lead to delays and cost overruns. Further, the use of double-shield machines presents high risk of blockage in areas with squeezing. Therefore, given the poor rock quality and fault zones through which the main tunnel is to be excavated, the new Austrian tunnelling method (NATM) has been finalised as the most suitable technology for the main and egress tunnels. The egress tunnel will be used as the drive gallery to anticipate real geotechnical conditions. The NATM presents flexible support, which will allow easy adjustment to the different geotechnical conditions present along the tunnel.

The Zojila tunnel project is expected to be completed in a period of seven years. It will be implemented round the year in the eastern portal, while winter breaks (spanning a period of four months per year) will be required in the western portal and shaft sections of the project. Therefore, to extend the work period in the latter sections, snow galleries and isolate buildings will be needed. Initially, intermediate shafts will be mandatory for ventilation requirements, for which two have been proposed for construction.

Challenges on the anvil

Given the tremendous complexity of the project, challenges of a similar magnitude are likely to arise. ITNL is expecting the presence of five major fault zones and eight fracture zones along the tunnel alignment. Heavy inflow of water is also anticipated during the excavation of the tunnel, due to its alignment traversing beneath a river. The construction of two shafts of 14 metre diameter each will also be done over the riverbed and the area is likely to experience frequent avalanches and snow as high as 30 feet during the winter season. Further, the logistics and mobilisation of heavy machinery to the site and the supply of electricity and diesel during the winter season will be other challenges.

To add to the woes, winter breaks will be expected for all faces except the eastern portal. There also exists a lot of variation in the rock masses, for which any prior design represents only a prediction, so all the input data will have to be thoroughly verified on site as conclusions might change where the basis differs significantly. Further, due to a lack of geotechnical field and laboratory tests in part of the tunnel, engineering judgement in selecting input parameters for the evaluation of the geotechnical parameters will be key, based on the experiences with comparable ground conditions as registered in the Z-morh tunnel. Together, several geometrical tunnel sections, alignments and tunnel length make the adoption of the suitable tunnelling method critical for the construction of the project.

The road ahead

The tunnel is expected to be an engineering feat, with modern technical safety arrangements such as cut and cross ventilation systems, two axial fans, a fully transverse ventilation system, uninterrupted power supply, CCTV monitoring, variable messaging boards, traffic logging equipment, tunnel radio system, emergency telephone systems, etc. Further, the experience gained by the company in installing a fully integrated tunnel control system in the Chenani-Nashri tunnel will be used effectively in designing and constructing the Zojila tunnel.

Going forward, the experience and know-ledge of the Zojila area will be critical for the implementation of the project. Further, specific geotechnical analysis of the particular conditions will be essential for optimising the project from a technical, financial and environmental standpoint, while also reducing the associated risks in ensuring the required level of safety for the tunnel users. Although the experience gained from working in a geologically similar terrain during the construction of the Z-morh tunnel will give the company a certain advantage, accurate engineering judgement will play a key role in determining the project’s progress.