As tunnels are complex structures, their construction requires different types of materials. The materials used vary significantly depending upon the design of the tunnel, construction method or technique deployed and the prevalent geological conditions. Some of the most commonly used materials in tunnel construction are explosives, concrete, steel, shotcrete material, lattice girders, geomembranes, rock bolts/anchors, admixtures, fibres and rock reinforcement. Further, in recent years, with several tunnelling projects being undertaken in areas with complex geological conditions and advancements in tunnel design, the use of new and innovative materials has gained traction.
Material advancements
A large number of tunnelling projects in the country are being executed in challenging terrains with difficult geological conditions. Addressing these challenges not only requires the adoption of innovative tunnelling methods but also the use of new-age construction materials that promise high pressure absorption and fire and crack resistance. Explosives, which are widely used for rock blasting purposes during tunnel construction, have undergone significant advancements. While dynamite has traditionally been used for blasting, safer and sustainable alternatives such as emulsion-based and water gel-based explosives have emerged in the past few years. Further, the use of cement has also witnessed rapid changes. Reinforced concrete mixed with glued steel fibres is being used as shotcrete for permanent/temporary lining, cast-in-situ lining and precast segmental lining works. The homogeneous solution provides uniform thickness and helps in stabilising deformations by improving ductility. Besides, it also offers higher energy absorption capacity and makes the tunnel crack resistant. In the past couple of years, the use of synthetic fibre reinforced concrete has also gained traction. The choice of structure (steel fibre, mesh or synthetic fibre), however, depends upon the performance criterion.
Tunnel contractors have also started using a high-strength mineral admixture, metakaolin, for grouting purposes. It is a low-cost solution as compared to other cement mixtures such as ordinary Portland cement and fly-ash-based Portland pozzolana cement. Besides, the use of microfine cement for grouting applications is also being tested. With a maximum particle size of 10 microns, microfine cement is able to penetrate very small openings such as soil pores to improve strength and reduce permeability. The grout material is injected using either the pre-injection or post-injection method. While microfine cement is environment-friendly, economical, and compatible with all rock and soil conditions, its adoption is still at a nascent stage.
In recent years, there has also been an increase in the adoption of geotextile membranes for waterproofing of tunnels. In addition, artificial ground freezing coolants such as glycols, calcium chloride brine and liquid nitrogen are being used for taking up tunnelling works in soft soils. Steel and fibre reinforced polymer active anchors are also being used to increase tunnel strength. Besides, mechanical single bolts, self-drilling rock bolts, etc. are being utilised to reduce project lead time and installation difficulty. The use of ceramic tunnel coatings in road tunnels too offers many advantages including increased light reflectance, ease of cleaning and protection of concrete from exhaust deposits, de-icing salts, motor exhaust fumes, rubber tyre dust, etc.
Noteworthy initiatives
Over the years, various tunnel projects in the country have made use of advanced construction materials. For instance, steel reinforced fibreshotcrete has been used in tunnels developed as part of the Udhampur-Srinagar-Baramulla rail link project. Besides, self-drilling anchors were also utilised in the project to stabilise loose rock mass after excavation.
Further, a waterproofing system made of geotextiles has been installed in the Rohtang tunnel in Himachal Pradesh. The project faced several challenges due to high water ingress in the Seri Nullah fault zone. The proofing technique has helped in minimising water seepage in the tunnel. Geotextiles have also been used to mitigate the problem of slope failures and cracks on the portals of the ChenaniNashri tunnel that occurred because of water seepage.
Conclusion
In the past few years, there has been a substantial increase in tunnelling activity in the country. Besides, several tunnel projects are currently being implemented or have been planned in the hydropower, railway, roads and highways, urban mass transit, and water supply and sewerage sectors. Also, with an increase in the complexities in tunnel construction, new materials have emerged. However, their adoption is still at a nascent stage with tunnel contractors relying mostly on conventional materials. Further, testing material suitability before adoption for actual construction is essential to avoid accidents. Therefore, pre-qualification tests, energy adsorption tests, material availability and site conditions should be taken into account before starting construction with a particular material.