As a geologically complex activity, tunnel construction poses numerous challenges. Addressing these challenges requires the adoption of not just advanced tunnelling methods but also new-age materials. Diverse geological problems such as difficult terrain conditions, thrust zones, folded rock sequence, in-situ stresses, rock cover, ingress of water, and geothermal gradient need to be tackled using materials that promise high pressure absorption and fire and crack resistance.
Tunnel construction requires different types of materials. Steel fibre-reinforced shotcrete (SFRS), safer emulsion-based explosives, geosynthetics such as 5D steel fibres (for increased tensile strength), mineral admixtures, geotextile membranes, steel anchors and self-drilling rock bolts are some of the key advanced materials currently in use. Also, globally accepted microfine cement is being tested in the country to overcome issues of cracks and water seepage in tunnels. Still, the lack of awareness about suitability and the right application of these materials limit their use to a few select projects. This needs to be rectified to ensure increased application so as to prolong the life of structures.
The growing number of complex tunnels being constructed has replaced conventional raw materials with advanced materials. Explosives, one of the key materials used for blasting activities, have evolved from dynamite to emulsion-based and water gel-based explosives. These explosives are a safer and more sustainable alternative to destructive dynamite.
Cement, another key raw material used to provide strength to the tunnel structure, has undergone significant advancements. Reinforced concrete mixed with glued steel fibres is being used as shotcrete for permanent/temporary lining, cast-in-situ lining, pre-cast segmental lining and for other flooring works. The homogeneous solution gives a uniform thickness and improves ductility, offers a higher energy absorption capacity (500-1,000 joules), and makes the tunnel crack resistant. It creates a strong bond with the surface making the underground tunnel self-supporting. In addition, its application is easier and quick as it can be sprayed either on the mesh installation or on steel fibres. This eliminates the need for equipment such shutters, travellers and rebars.
The shotcrete mixture if sprayed on steel fibre has a thickness of about 10 cm and on mesh 13 cm, leading to more consumption and increased cost. Therefore, the choice of structure (steel fibre or mesh) can be made depending on the required performance criterion. For preparing reinforced cement, steel fibres such as 5D, 4D or the most commonly used 3D fibres are being utilised.
Other high strength mineral admixtures such as metakolin to be used in place of cement are also being used for grouting. Metakolin is not just high in strength but is also a low-cost solution as compared to other cement mixtures such as ordinary Portland cement (OPC), and fly ash-based Portland pozzolana cement, among others.
Further, compatible materials that can fill cracks/joints completely using the pressurised grouting technique are required. At present, OPC grout, which comprises larger particles, is primarily used to fill cracks, and thus gaps are left. To overcome this issue, grout with finer material referred to as microfine cement is being tested. The grout material is injected using either the pre-injection and post-injection method. During pre-injection of microfine cement, the stratum gets consolidated ahead of excavation thereby helping in intercepting water paths and avoiding a ground collapse. The post-injection method, on the other hand, consolidates the strata after excavation activity. This enables firming up of the strata and stopping of leakages. Irrespective of the type of method adopted, this innovative material is environment friendly, economical, easy to handle and compatible with all soil and rock conditions. However, its adoption is still at a nascent stage.
Waterproofing of tunnels is another key requirement for which geotextile membranes are being widely adopted. Besides, artificial ground freezing coolants such as glycols, calcium chloride brine or liquid nitrogen are being used for taking up tunnelling works in soft soils that lack compressive strength to withhold their own loads. These coolants create a watertight barrier as a support system for soil stabilisation.
To improve durability and add strength to tunnels, steel and fibre-reinforced polymer active anchors, and steel passive anchors are also used by tunnel contractors. Apart from this, expandable friction bolts, self-drilling rock bolts, and mechanical single bolts that allow faster installation are used to increased productivity.
Tunnelling activity in the country is increasing with the implementation of hydropower, irrigation, urban rail and railway projects. Though the materials used at present are still traditional, new materials are beginning to be used and are expected to evolve still further in the future. At the same time, awareness about the suitability and use of materials has increased among contractors. Testing material suitability before adoption for actual construction activity is essential to avoid accidents. For this, aspects related to pre-qualification tests, energy adsorption tests, material availability and site conditions need to be taken into account.