Material Choice: Preferencse vary by tunnel type, design and construction method

Preferencse vary by tunnel type, design and construction method

Tunnel construction requires various types of materials including explosives, concrete, steel, shotcrete material, lattice girders and rock reinforcement supplies. Materials used in tunnelling vary with the design and construction methods chosen for each project. The demand for a particular type of material also depends on its availability, site conditions, transportation costs, project location, manufacturing capabilities, weather conditions and local taxes. Choosing the right construction materials is a critical process that must be undertaken early in the planning and design process.

Materials must meet the requirements of various tunnel types (railway, road, water, sewerage), location (young Himalayan, southern granite, coastal, undersea, rivers, etc.) and size (varying from 2-3 metres to 12-13 metres). The materials should be durable since it is not easy to replace tunnels once constructed. Typically, the target life for road and railway tunnels is 100-200 years, for hydraulic tunnels it is 40-50 years, and for sewerage tunnels it is 100-125 years. Materials should also enable speedy construction. Further, they must fulfill functional requirements – watertightness for road and railway tunnels, abrasion and cavitation for hydraulic tunnels and corrosion for sewerage tunnels.

Some of the factors that play a vital role in the selection of construction materials are tunnel life expectancy and capital cost versus maintenance cost. Internal factors, too, play a key role. For instance, sewerage tunnels carry waste, and the presence of hydrogen and sulphur generates gases and causes very quick deterioration of tunnels. There is also some leakage of groundwater into tunnels, which carries contaminated chemicals. Lack of oxygen can also impact tunnels. External factors also play a significant role in the selection of construction materials. Some of these are chemicals used during construction (accelerators and grouting agents) which can contaminate groundwater, noise and vibration in transportation tunnels, as well as toxic gases generated in these tunnels. Other important factors are availability of key materials (steel, cement, aggregates, etc.), site conditions (urban or rural), costs (including transportation), legislative issues, project location, availability of manufacturing capabilities nearby, road, rail and port facilities, weather and atmospheric conditions.

Types of raw materials

Explosives are the most important raw material used in tunnel construction. At present, India accounts for 5-6 per cent of the global demand for civil engineering explosives. Broadly, explosives are of four types – low explosives, high explosives, packaged explosives and bulk explosives.

Low explosives comprise sodium nitrate (72 per cent), sulphur (16 per cent) and coal (12 per cent), and burn at subsonic speeds, generate gases and do not generate shock waves. High explosives comprise oxidisers such as ammonium nitrate, fuel oil and sensitisers like methyl amino nitrate along with entrapped air bubbles. These generate high temperatures and high pressure gases, which travel at speeds faster than sound. Packaged explosives are in the form of cartridges, varying in diameter and size, depending on blasting requirements. These have become popular in India in the past two to three decades. More recently, a bulk explosive, ANFO (comprising ammonium nitrate and fuel oil), is becoming popular and replacing cartridges as it is easy to handle and charge. These are essentially non-explosives taken on a bulk delivery vehicle to the mine site and pumped down bore holes as per the mining strata. These are used where the scale of blasting operation is large and substantial quantities have to be charged per day.

Steel is the second most important raw material to be used in tunnel construction. Structural steel is primarily used for steel rib supports, lattice girders and mesh reinforcement. Reinforcement steel is used for tunnel reinforcement, rock bolting/anchoring, self-drilling anchors and water-expandable friction anchors. All sizes and sections of reinforcement and structural steel are available in India.

High-tensile steel in the form of ribs and steel fibre reinforced shotcrete with rock bolts is being used as support systems. The primary support system includes rock anchors/bolts with shotcrete, steel ribs with backfill, lattice girders, anchors, wire mesh and shotcrete.

Providing steel ribs is the first-stage support for rock tunnels. The advantages are increased flexibility in changing rib spacing along with the ability to handle squeezing ground by resetting the ribs after remining. Lattice girders are supporting elements in tunnels and normally consist of steel bars laced together in a triangular pattern. They are made to suit the shape of the tunnel and are encased in shotcrete. They are easy to install and thus reduce construction time.

Further, rock bolts/anchors made of reinforcement steel coupled with nuts, washers and bearing plates of specific sizes as per the design, are inserted in a drilled hole to strengthen the tunnel. Besides, grouting is done to arrest water seepage from the excavated portion. This could be done at two stages of tunnel construction – pre-excavation grouting and post-grouting. The former is done when the probe hole result shows water seepage of over 25 litres per minute. Post-grouting is done using three types of grout materials – cement grout with non-shrinking admixture, polyurethane foam and epoxy grout.

The quality of concrete, another key raw material used in tunnel construction, has evolved over the years. At present, various additives are used to improve the durability and strength of concrete. For example, for the tunnel in the Tala hydropower project (jointly undertaken by India and Bhutan) concrete was made with Portland slag cement (PSC) and silica fume as lining to make it abrasion- resistant. Other types of cement available in India are ordinary Portland cement, fly ash-based Portland pozzolana cement, calcined-based Portland pozzolana cement, sulphur-resistant cement, etc. Rapid hardening cement has pozzolana material added to it, either before clinkering or to the ground clinker. Lining of tunnels is mostly carried out in three stages covering 12.5 metres, known as blocks, where each block requires 140-150 cubic metres of concrete.

Generally, a good shotcrete accelerator is added to the concrete at spray points to allow quick setting.  It needs three minutes of initial setting with 10 minutes of final setting. The accelerator is an essential part of shotcrete. All these products are available in India and their effective cost has reduced over the past decade.

Of late, steel fibre-reinforced concrete is also being used in the tunnelling industry. The most recent example is the Rohtang pass tunnel project where the material is being used as tunnel lining. It provides ductility in tension and compression as well as high resistance against the spalling of concrete. It is durable and has flexural strength in all four directions. The material is also resistant to cracking and deflection.

Fibres used with concrete can be microfibres, macro-synthetic fibres, hybrid fibres (a blend of macro-synthetic and glass fibres) and steel fibres. While they are being manufactured by a number of Indian producers, some established European brands have also commenced production in India.

Microfibres are 6-20 mm long, tens of microns in diameter and dosing at 1-2 kg per cubic metre. They are useful for control of plastic shrinkage, passive fibre protection and anti-spalling. However, as these do not provide any structural benefits to concrete, they are not much in use now.

Macro-synthetic and hybrid fibres are manufactured using synthetic materials with polyolefin-based fibres yielding the best results. These are cheap compared to steel fibres and have long-term durability as they are corrosion-free. They have a post-crack load capacity equivalent to SL82 mesh at regular dose rates. They improve concrete ductility, shrinkage and temperature crack control, eliminate bending, cutting and placement of the steel mesh, and increase efficiency. These are also safer and lighter than steel.

Other materials include aggregates, admixtures (water reducers, retarders and super-plasticisers), silica fumes, reinforcement steel for lining, geomembranes, etc.

Conclusion

Huge investments in infrastructure, escalating costs and dwindling surface land will spur the number of tunnel projects in India. With this, the type of raw materials used for tunnel construction will evolve to speed up tunnel completion in an efficient, cost-effective and safe manner. Though the materials used currently are still traditional, innovations are beginning to be seen on the ground, and technology is expected to evolve further.