By Shilpa Kanade, Deputy Chief Engineer, and Laxmikant Vaidya, Executive Engineer, Brihanmumbai Municipal Corporation (Water Supply Projects Department)
The Water Supply Projects Department of the Brihanmumbai Municipal Corporation (BMC) was established in 1970. This department undertakes the conception, planning, design and execution of projects related to water augmentation and source development. These projects include dams, tunnels, pipelines, water treatment plants (WTPs), desalination plants, recycling and reuse of tertiary treated sewage water, pumping stations, service reservoirs, and master balancing reservoirs. The department is also responsible for the replacement and rehabilitation of the existing water supply network through the construction of underground tunnels and the re-engineering of WTPs and reservoirs. After construction and commissioning, the completed projects are handed over to the Hydraulic Engineer Department for operations and maintenance.
Till date, BMC has constructed a total tunnel length of over 100 km. Additionally, it receives water from seven dams. Of these, two are owned by the state government. In a recent development, the Manori desalination plant with a capacity of 200 million litres per day (mld) – expandable to 400 mld – was awarded, with works currently under way.
Key water supply sources in Mumbai
Mumbai’s water supply system draws water from multiple reservoirs and sources, including Bhatsa, Vaitarna, Middle Vaitarna, Upper Vaitarna, Tansa, Tulsi and Vihar. The collected water is conveyed through an extensive network of conveyance systems, treatment plants, reservoirs and pumping stations before being distributed to consumers. These sources constitute a cumulative water supply of around 4,170 mld to Mumbai city.
The system relies on major WTPs such as the Bhandup Complex WTP and Panjrapur WTP to ensure that water is adequately treated prior to distribution. The integrated network of raw water conveyance, treatment infrastructure and transmission facilities plays a crucial role in providing continuous and reliable water supply to Mumbai and its adjoining areas.
Consequently, with growing urbanisation, there is no space to lay new pipelines. Hence, BMC has switched to underground tunnels as the primary method for water conveyance. By transferring the network to deep tunnels, water theft and contamination are eliminated. The tunnels also offer maintenance-free, reliable, tamper-proof water conveyance alternatives. The rising hydraulic gradient is another benefit. Many residential areas are located in somewhat elevated areas; hence the tunnels help by supplying water to elevated areas of the city.
Innovative technologies in use
BMC pioneered the use of tunnel boring machines (TBMs), introducing them for water supply tunnels in 1984. Since this milestone, all of BMC’s water supply tunnels have been bored using TBMs. These tunnels are typically constructed at a depth ranging from 50 metres to 150 metres. Due to the high altitude of the master balancing reservoirs and the deep alignment of the tunnels, the internal water pressure ranges from 8 bars to 20 bars (around 290 PSI). To withstand this pressure and prevent a bursting effect, these tunnels are provided with a cast-in-situ reinforced cement concrete (RCC) lining specifically designed for hoop stress.
At very weak geological stretches, steel liners of I.S. 2002 grade (boiler quality) with concrete backpacking are installed. Furthermore, to check high amounts of water ingress, various advanced grouting methods are deployed, including cement, silica gel, polyurethane-based single/double component grouting, and sodium silicate and cement mixtures. For larger operations where the tunnel bore diameters exceed 5 metres, horizontal and vertical conveyors are utilised to ensure the efficient transportation of muck from the tunnel face directly to the surface.
Key challenges faced
Conducting geotechnical investigations is the biggest challenge, as the water supply tunnels must be constructed in a straight line at great depths directly beneath highly urbanised areas. Owing to the immense depth of some of the tunnels, engineers occasionally encounter massive water ingress reaching up to 30,000 litres per minute. Managing this situation becomes difficult, as this enormous quantum of water must be both actively pumped out and successfully arrested at such depths.
Moreover, installing the RCC lining reinforcement and casting the concrete in-situ lining is inherently time-consuming. Since this cast-in-situ lining process can only commence after tunnel boring is fully completed, it significantly impacts project timelines. Specifically, while boring progresses at about 375 metres per month and the RCC lining follows at around 325 metres per month, the resulting average overall tunnelling progress drops to as low as 180-190 metres per month.
Most importantly, unauthorised bore wells puncture the water supply tunnels. In such cases, the tunnels have to be repaired from inside, for which the entire tunnel has to be dewatered, ventilated, repaired, filled back, flushed and chlorinated. These repairs are highly expensive and time-consuming, resulting in an outage period of three to four months. During this downtime, maintaining a continuous water supply to the citizens residing in the affected command area becomes an immense challenge.