Tunnel construction has witnessed significant developments in the past few years, globally and in India, in terms of both new tunnels being constructed and new tunnelling techniques. The methods/techniques employed in tunnelling vary according to geographical conditions, cost, etc. These new technologies are being used to enhance the pace of construction.
Keeping in mind the long-term benefits to society as a whole, tunnels are considered to be among the best long-term solutions for various problems, especially in mountainous terrain. This is especially applicable for road tunnels, where the tunnel cost is estimated to be recovered in five to six years’ time. Austria and Norway are two countries that are known to have vast experience in tunnelling. Both nations have made use of shotcrete and rock bolts as support, but the methods developed by them differ in philosophy. These methods are the New Austrian tunnelling method (NATM) and the Norwegian method of tunnelling (NMT) respectively.
New Austrian tunnelling method
In India, NATM is more popular than NMT. This is predominantly due to the successful completion of the approximately 10 km long Pir Panjal railway tunnel (T80) using NATM, as part of the Udhampur-Srinagar-Baramulla Rail Link project. NATM is considered to be suitable for soft ground that is not dominated by jointing and overbreaks. Monitoring is also a key factor in NATM. As in other methods, it uses the surrounding rocks and ground as the main load- bearing components.
Norwegian method of tunnelling
NMT is used where hard rocks and overbreaks are present. Excavation is carried out using the drill-and-blast method. The temporary support forms a part of the permanent support, and in weak zones, reinforced ribs of shotcrete (RRS) are used. The Q-system is used for predicting rock mass quality and support needs, which are updated during excavation. Monitoring is, however, done only in the critical zones. The application of rock mass classification is considered to be a forward predictive method, significantly differing from NATM. NMT effectively minimises the use of concrete, reducing the production of carbon dioxide. One of the main distinguishing features of NMT is its single shell characteristic, compared to the classical double shell in NATM. Further, NMT is much more cost effective than NATM.
In NMT, great emphasis is placed on the geological and geotechnical aspects of the project. Different numerical methods, such as the Distinct Element Method, are applied to optimise the support in a tunnel.
NMT is very appropriate for joint hard rock-weak rock scenarios as well. Temporary support is provided based on the rock mask classification or the Q-system, and includes fibre-reinforced sprayed concrete, rock bolts and RRS. Monitoring is carried out when deformation is expected. The permanent support consists of the temporary support and water shielding (single shell). The cost is around $10,000-$15,000 per metre in Norway. Considering that in Norway labour costs are much higher, NMT is likely to be more cost-effective than other methods of construction in India.
Global practices
In Norway, the landscape consists of high mountains, and long and deep valleys. Rock tunnels have been found to be an efficient solution to infrastructure challenges. Annually, approximately 3 million cubic metres of rock is excavated there. Norway has experienced the construction of 7,000 km of tunnels via NMT. This includes about 3,000 km of tunnel length comprising around 750 railway tunnels, 1,000 road tunnels and 36 subsea tunnels, plus 4,000 km of hydropower tunnels. The world’s largest man-made cavern, the Gjovik Olympic Cavern Hall in Norway, was constructed using NMT. It spans 61 metres, with a height of 24 metres. The 24.5 km long Laerdal tunnel, the world’s longest road tunnel, also made use of NMT. The tunnel passes through Precambrian gneisses, with a rock cover of up to 1.5 km. There were severe rock stress problems and the total cost of the tunnel amounted to $120 million. The conditions were similar to the Rohtang tunnel in India. India has also seen the use of half tunnels. These are seen on routes on the way to Manali. Half tunnels are excavated as overhangs within the slopes of hard rocks.
Q-system and RRS
NMT uses the Q-system. RRS is used depending on the Q-system. RRS makes use of rock bolts and crossbars instead of lattice girders. RRS can match an uneven profile better than lattice girders or steel ribs. One of its benefits is the ability to put a band aid on any overbreak. It is also easier to transport, as the bars can be made at the site, inside the tunnel. NMT can also be used in weak zones. Instruments such as load cells and strain gauges can be used to monitor the load on the RRS, but it differs from NATM in that there is no continuous monitoring system. RRS has different applications depending on the range of the Q value. These ranges differ between hydro tunnels, railway tunnels and two-lane highway tunnels. RRS has been installed at the Adit-3 tunnel of the Rishikesh-Karnaprayag project.
The way forward
Tunnels play an important role in enhancing connectivity between regions. They are particularly advantageous in mountainous regions. There are two main tunnelling methods, NATM and NMT. A combination of the two tunnelling techniques can also be used. This would be helpful in geographical areas with rocks in both good and bad condition. Tunnelling methods have evolved over time due to increasing construction, varying geographical conditions and other factors. As seen from Norway’s experience, NMT is an advanced method of tunnelling with numerous notable benefits. While NATM is more popular in India, NMT is witnessing increasing uptake. Further, taking the cost aspect into consideration, NMT is likely to be less expensive than NATM and other methods. The scope of its application in India is likely to expand, going by current trends.
Based on remarks by C.S. Khokhar, Executive Director, Geotechnical, AECOM, at a recent India Infrastructure conference
