Material Selection: Improved inputs for tunnel construction

Improved inputs for tunnel construction

Materials used in tunnel construction vary significantly with the type of tunnelling method or technique deployed. Generally, non-mechanised techniques such as drill-and-blast entail a higher material cost as compared to mechanised techniques. The different types of materials used in tunnel construction include 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 that play a vital role in the selection of construction materials are tunnel life expectancy and capital costs. Besides, internal factors also influence material selection for tunnel construction. For instance, sewerage tunnels carry waste and hydrogen and sulphur gases that can cause very quick deterioration of tunnels. Besides, there are also cases of leakage of groundwater into tunnels which reduces their life. External factors too play an important role in the selection of construction materials. These include chemicals such as accelerators and grouting agents used during construction contaminate groundwater and cause noise and vibration in transportation tunnels.

At present, explosives are the most important raw material used in tunnel construction. Broadly, the four variants of explosives used are low explosives, high explosives, packaged explosives and bulk explosives. The second most important raw material used in tunnel construction is steel. This is primarily used for steel rib supports, lattice girders, mesh reinforcement, rock bolting and anchoring, and self-drilling anchors.

Another key raw material used in tunnel construction is concrete. At present, various additives are used to improve the durability and strength of concrete. The different types of cement available in the country are ordinary Portland cement, fly ash-based Portland pozzolana cement, calcined-based Portland pozzolana cement, sulphur-resistant cement, etc.

The quality of construction material used in tunnelling has evolved over the years. The growing complexity of tunnel construction in the Himalayan and peninsular regions has necessitated the use of new and advanced materials.

New types of materials are also being used to improve the durability and strength of tunnels. Steel fibre-reinforced concrete is being used in tunnel construction. It provides ductility in tension and compression as well as high resistance against the spalling of concrete. Micro silica or micro fine cement is being used in pre-grouting to effectively control water seepage. Further, the use of synthetic fibres with micro silica in shotcreting improves strength and reduces rebound. A case in point is the Allain Duhangan hydropower project where the material was used for grouting. Artificial ground freezing is being used for soft ground tunnelling to create a watertight barrier as a support system. Earlier, rib and lagging were used as the principal tunnel liners to strengthen the material on the external part of the tunnels. Today, primary or safety shotcreting is done, which requires no lagging. This is followed by rock bolting.

Recently, tunnel contractors have started using a metakolin mineral admixture for high strength concrete and grouting. Self-compacting concrete has also emerged as a cost-effective option for casting heavily reinforced elements and complex geometrical shapes. Besides, retarder chemical is being used to increase the setting time of cement slurries. Geotextile membranes are also being used to make tunnels waterproof.

Conclusion

With increased tunnelling activity in difficult geological zones as well as congested urban areas, 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 higher demand for construction materials like explosives, steel, cement, rock bolts, fibre, aggregates, etc.

That said, some of the key challenges that require the urgent attention of tunnel contractors 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 challenging site conditions such as weak zones, silty or clayey strata; and disposing of waste/backfilling construction material.