Over the past few years, tunnel construction has picked up pace in India. Tunnels for the supply of water have gained uptake with the launch of a large number of government programmes. The majority of tunnelling opportunities in the water and sewerage sector are in Mumbai. Water tunnels are one of the key components of the Mumbai water supply system. The city has around 83 km of water tunnels and a huge water supply network. At present, the city has seven sources of water – Vihar Lake, Tulsi Lake, Tansa dam, Vaitarna (Modak) dam, upper Vaitarna dam, Bhatsa dam and the recently constructed middle Vaitarna dam. The net flow of water supply to the city is over 3,750 million litres per day (mld). The city has three water treatment plants, at Bhandup (2,810 mld), Panjarapur (1,365 mld), and Vehar and Tulsi (108 mld).
Salient features of Mumbai water tunnels
Over the years, tunnels have been constructed for the augmentation of water supply in the city. Some tunnels have also been constructed for improving the distribution system by replacing old and damaged water mains. These water tunnels are 50-100 metres below ground level and are constructed with a full face tunnel boring machine in hard rock, as compared to transport tunnels, which are constructed at shallow depths. Some tunnels pass below the runway of the Mumbai International Airport while some pass below the seabed and creek for a length of 1 km.
Challenges and issues
The city of Mumbai has faced many challenges in tunnel construction. The high water table in the city results in the heavy ingress of subsoil water, and dewatering and restriction on the drainage of pumped water due to the density of population. Further, the city faces challenges such as drilling and blasting restriction for shafts and tail and assembly tunnels due to the proximity of high-rise commercial and residential establishments as well as dilapidated structures. De-mucking is the primary challenge due to the non-availability of space for dumping in the city as well as surroundings. Some of the other challenges are transportation of the muck up to the designated disposal sites in line with the environmental norms and traffic regulations, projects delays and cost overruns, restricted working hours during the night to prevent noise disturbance etc.
In sum
The tunnelling industry in India is witnessing a high-growth
trajectory. Water and sewerage tunnel infrastructure is expected to expand significantly with several projects coming up in the sector. As a result, opportunities for raw material suppliers and equipment providers will increase. However, construction and completion challenges faced by most infrastructure projects along with issues such as delays in projects and cost overruns, restricted working hours during the night, drilling and blasting restrictions may require greater attention in the near term. w
With inputs from a presentation by Vasant Gaikwad, Chief Engineer, Water Supply Project Department, Municipal Corporation of Greater Mumbai, at a recent Indian Infrastructure conference