Construction materials used in tunnels vary as per the design and construction methods selected for each project. In addition, the construction material is determined by factors such as geology, cost and potential disruption of other activities. The different types of materials used in tunnel construction are explosives, concrete, steel, shotcrete material, lattice girders, geomembranes, rock bolts/anchors, admixtures, fibres and rock reinforcement.
The demand for a particular type of material depends on its availability, site conditions, transportation costs, project location, manufacturing capabilities and local taxes. Other key factors in the selection of construction materials are tunnel life expectancy and capital costs.
Sewerage tunnels carry waste and hydrogen and sulphur gases that can lead to a quick deterioration of tunnels. Further, chemicals such as accelerators and grouting agents used during construction contaminate groundwater, and cause noise and vibration in transportation tunnels.
Recent trends in construction material
The quality of construction material used in tunnelling has improved over the years. The growing complexity of tunnel construction in the hilly regions and urban cities has necessitated the use of new and advanced materials.
Delhi Metro Rail Corporation Limited has used Tech Geo PR 30 non-woven geotextile in the tunnels of Delhi metro. The material provides protection, separation and drainage between the top shotcrete lining and the yellow geomembrane sheet. The material was manufactured by TechFab India Industries Limited.
In the Pir Panjal tunnel located in the Pir Panjal range in Jammu & Kashmir, geotextiles and waterproofing membranes have been used. Specialised work was carried out simultaneously across the block for the installation of the waterproofing membrane and geotextiles. They were installed in the walls of the tunnel to enable the groundwater to get channelled through the waterproofing membrane to the perforated PVC pipes. These pipes are placed in the corner of the kicker and help in draining out the water collected through the diversion of the waterproofing membrane to the main drainage gutters. The drainage pipes (perforated PVC pipes wrapped in non-woven geotextiles) enable controlled drainage in the area. Geotextiles have also been used to mitigate the problem of slope failures and cracks on the portals of the Chenani Nashri tunnel that occurred because of water seepage. The geotextile membrane waterproofing was installed to prevent water percolation and erosion through seepage from the tunnel crown.
For the construction of the Rohtang tunnel, a unique self-adhesive waterproofing PVC membrane, LOGICROOF V-GR FB SA, was selected. It is a multilayer glass fibre reinforced PVC membrane with a laminated geotextile fleece on the bottom surface, which is pre-impregnated at the plant with a special adhesive and protected by a release film. Its upper layer provides high resistance to precipitation and ultraviolet rays. Further, LOGICROOF V-GR FB SA is a self-adhesive PVC membrane that has no analogues in India and is simple and easy to install.
The application of steel fibre to reinforce concrete is also gaining traction due to its advantages such as durability and sustainability. As compared to conventional steel reinforcement, the production of steel fibre reinforced concrete for tunnel linings is simpler and the formation of cracks during handling and installation is considerably reduced. Further, owing to higher resistance against corrosion and cracking stresses as well as greater impact toughness, steel fibre reinforced concrete is more durable. The Maharashtra State Road Development Corporation used Bajaj Reinforcements’ fibre reinforced shotcrete in the Samruddhi tunnel.
The way forward
Going forward, there will be increased tunnelling activity in difficult geological zones as well as congested urban areas, for which new and advanced construction materials are expected to be developed. With respect to the method of tunnel construction, drill-and-blast will continue to remain the preferred method of tunnelling in India, at least in the short to medium term. Thus, there will be a higher demand for construction materials such as explosives, steel, cement, rock bolts, fibre and aggregates.
There will be some key challenges as well that will require the urgent attention of tunnel contractors. The challenges include selecting the most appropriate construction material; finding construction material manufacturing units near project locations, particularly in the Himalayan region and the Western Ghats; dealing with difficult site conditions such as weak zones, silty or clayey strata; and disposing of waste/backfilling construction material.