India is gradually adopting new tunnelling methods, but high cost and a skill shortage are coming in the way of widespread deployment.
There´s no doubt about it - opportunities for tunnelling specialists are growing. ´With the Central Government focusing on improving infrastructure, especially in the less connected north and northeast to pave the way for more industrial activity, we expect opportunities for tunnelling to grow,´ says Prabhat Mittal, Associate Vice President, Sandvik Mining. ´Until a few years ago we were seeing demand primarily for hydro projects; recently, we have seen substantial demand for road, railways and defence projects,´ says Soma Chakrabarty, Director, Eurostar Engineering, which offers Cifa shotcreting equipment in India.
With most tunnels in India designed from the outset to be constructed using the drill-and-blast method, it is the ´expansion of metro systems in Bengaluru, Chennai and Delhi that offer the best short to medium-term opportunities for tunnel boring machine (TBM) deployment´, according to Manoj Garg, Managing Director, Herrenknecht India. ´In the medium to long term, India´s ambitious plans to build an extensive high-speed rail network offer opportunities.
Early reports indicate that the high-speed route between Mumbai and Ahmedabad alone might require a 20-km-long subsea tunnel.´
Towards eco-friendly tunnelling
India has conventionally adopted drilling and blasting for tunnelling, ´which increases the chances of environmental damage because it affects the surrounding rock strata; blasting also raises the chances of something going wrong,´ says Mittal.
´In contrast, mechanical cutting blast-free technologies such as those behind TBMs or road headers do not affect the surrounding rock strata, creating stronger tunnels that are longer lasting.´
Mechanical cutting technologies are also more productive and faster by about 40 per cent, reckons Mittal. Drilling and blasting is slower because it is a cyclical operation involving different teams for drilling, blasting, mucking and bolting. Mechanical cutting is a continuous process wherein the mucking happens automatically through a conveyor built into the main machine; the elimination of blasting minimises the cost and time involved in bolting to support the roof.
Between the two, mechanical cutting equipment road headers and TBMs, road headers are more compact and productive as well as more flexible for offering the ability to tweak the cutting profile (shape). Notwithstanding the environmental advantages of TBMs, these machines have seen limited deployment in India. ´TBMs have largely proven unsuccessful in the Himalayas, young mountains with different strata of rock ranging between soft to moderate rock. Even drilling and blasting often does not progress as expected because of the diverse rock strata,´ opines AK Aggarwal, Projects Director, TICIL Division, KCT Group.
´In southern India, where tunnelling involves cutting through hard granite, TBMs are well suited to the terrain.
But hydro projects down south are usually too short (<1 km) to justify the use of a TBM.´
With regard to shotcreting equipment, Chakraborty speaks of greater demand for self-propelled, carrier-mounted machines as against stationary machines that need to be towed. ´Behind this preference is growing recognition and desire for the greater productivity and operational flexibility of the self-propelled models; for instance it is easier to work in larger diameter or inclined tunnels, especially with the Cifa CSS-3/CST shotcreting machines because of their low feeding height for hopper or articulation and all wheel drive, less turning radii and 4x4 drive,´ she says. ´Cost-wise, the self-propelled models are up to four times more expensive than stationary models.´
In methodology, ´nowadays it has become the norm to use environment-friendly alkali-free chemicals for shotcreting (Cifa uses a tested influx system) while this was optional until a few years ago,´ she adds.
´Shotcreting has improved with the availability of more skilled workers and better imported and partially indigenous (Schwing Stetter in collaboration with Greaves) equipment,´ agrees Aggarwal. ´We have used models with robotic arms and the Normet machine, which is well suited to steel fibre-reinforced shotcreting. Reinforced shotcreting can eliminate the need for concreting in certain approach and access tunnels.´
Tunnelling for cleaner cities
Tunnelling is a good futuristic choice for Indian cities, explains Manoj Garg, Managing Director, Herrenknecht India.
Given the diminishing air quality in Indian cities and the potential of rail-based transport to minimise air pollution, it would be prudent for planning authorities to push for further development in this sector. Each car, bus or truck and even two and three-wheeler taken off the road is a small contribution to better air quality, to a better environment. But there is more to environment degradation than just air pollution. On large-scale metro extensions, tunnels constructed by TBMs compete with elevated structures. An elevated viaduct costs a third of a twin metro tunnel of the same length; however, it also has a negative visual impact on the environment. But as construction costs are easier to reckon and compare, elevated viaducts are usually given preference where viable. Gotthard Base Tunnel, part of the New Rail Link through the Alps (see box), which will open for regular traffic in December 2016, was built with the environment in mind. It aims to protect the Alps from road traffic by taking roughly 40 million tonne of goods per annum off the road. Road tunnels are another good example. In Spain, the Madrid Calle 30 refurbishment project, which began in 2004 and was completed in 2007, rerouted a significant part of the road through tunnels, allowing surface areas to be redeveloped to improve people´s lives. In doing so, the project also improved air quality in the area û the tunnel ventilation systems are designed to filter out particles. The project also cut journey times and distances, further cutting emissions. City council estimates pegged the reduction in emission as a result of the project at 35,000 tonne per annum in 2007, increasing to 64,800 tonne per annum in 2037.
World´s longest railway tunnel opened
Name: Gotthard Base Tunnel
Date of completion: June 1, 2016 (will open for regular traffic in December 2016)
Project duration: 17 years
Project value: CHF 12.2 billion
Length: 57.1 km
Design: Twin-tube tunnel
Cross passages: Every 325 m
Diameter: Approximately 9 m
Jobs provided: 2,600 (at peak)
Tunnelling method: Eighty per cent of tunnelling was undertaken by four Herrenknecht gripper TBMs.
Machine specification: 3,500 kW main drive, length per TBM including backup approximately 410 m, weight per TBM including backup approximately 3,000 tonne. Challenges: Gotthard Base Tunnel is not only the longest rail tunnel in the world, surpassing Seikan Tunnel in Japan (53.9 km) and the Eurotunnel between UK and France (50.5 km), but also the deepest with a maximum overburden of 2,300 m (1.4 miles). During construction, miners had to deal with radial squeezing of up to 800 mm, rock bursts and extreme rock temperatures.
Timetabled daily traffic: 260 high-capacity freight trains at a maximum speed of 160 km per hour and 65 passenger trains at a maximum speed of 250 km per hour.
How HCC overcomes tunnelling challenges
Covering the company´s nine-decade-long experience in tunnelling, spanning the elementary drill-and-blast method, the cut-and-cover method, the New Austrian Tunnelling Method, shields and tunnel boring machines (TBMs) and special methods, Ambuj Jain, COO, HCC, shares some key challenges the company has faced and how these have been overcome:
Tunnelling through the fragile, weak and jointed rock masses of the Himalayas: The leading challenges are high overburden, thickly vegetated and inaccessible terrain, varied rock formations, presence of small and big shear zones or thrusts and associated hydrological challenges. Potential unforeseen problems include face collapse, chimney formation, water inrush, hot water springs, gas explosion, squeezing, etc. Despite these challenges, HCC created a world record for the first successful TBM operation in the Himalayan region by using a double-shield TBM for its Kishanganga hydropower project, thus also creating a national record for the highest monthly progress in tunnelling, 816 m, in October 2013. As the technology used to drill a tunnel depends completely on the strata of the rock overburden and other geotechnical parameters in that particular region, HCC has relied on detailed geological explorations, state-of-the-art designs, adaptable tunnelling practices, in-depth knowledge of monitoring and feedback systems and mechanisms with an inbuilt culture for safety.
Worst geology encountered so far: During the construction of the Dagachhu Hydro Power Project in Bhutan, HCC engineers discovered that the ground strata was not adequate to hold the structure as there were several water courses beneath the surface and the land was marshy. Inclinometers and geotechnical equipment were used to measure the underground movements or deformations, which were found to be significant. So, the alignment of the head race channel was changed and soil stabilisation was used to improve the foundations with additional shotcrete and special anchoring. At the head race tunnel, various support systems such as rock bolts, wire mesh with shotcrete, steel ribs, lattice girders and winches were used in varying quantities.
Choosing a tunnelling method for the Delhi Metro and Kolkata Metro: Tunnelling in urban grounds for metros is complex and challenging, often owing to construction in soft soils, existing urban infrastructure, high-rise buildings, existing public utilities and, most important, safety for human life. Choosing between the cut-and-cover method, shield tunnelling method, TBMs, etc, for underground sections depends on an understanding of tunnelling-induced ground loss mechanisms and the associated displacements, and the risks they pose to adjacent buildings, structures and utilities. Advancing a TBM through sedimentary rock with some groundwater: Advancing the TBM during the construction of a 19.2-km-long tunnel for the Pula Subbaiah Veligonda irrigation channel in Andhra Pradesh was challenging because the sedimentary rock had a number of faults and folds and some groundwater. HCC got a probe drill mounted on the Robbins 10-m diameter double-shield TBM to verify the geology 30 m ahead of the machine. A strict programme of probe drilling combined with adequate ground stabilisation allowed the TBM to advance.
- Charu Bahri