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Mumbai marvel: tunnelling work sets new standards in Asia

Engineers in Mumbai are building India’s deepest and longest water tunnel

Meeting the water needs of any growing city is a challenge but in the Indian city of Mumbai the problem has been reaching crisis point. Daily supply capacity is currently 3,350M.l, but demand from the city’s 12.5M population runs at around 4,200M.l.

Add in 29% population growth forecasts for the next 10 years and the impact of the monsoon season contaminating the existing supply, and the situation becomes more acute.

Both the daily and monthly progress we have seen are records for TBM tunnelling in India

Work to provide a solution has just reached a major milestone with completion of tunnelling on a new £110M pipeline to increase supply capacity and safeguard the quality from seasonal impacts. The effect of the tunnel will be life-changing for residents of Mumbai but it is also a significant achievement.

Engineers celebrate the TBM breakthrough on the Mumbai water tunnel scheme

Engineers celebrate the TBM breakthrough on the Mumbai water tunnel scheme

The project, which is being funded by the Municipal Corporation of Greater Mumbai, is set to deliver one of the longest and deepest water tunnels ever built in Asia and record-breaking ­tunnelling rates.

The 8.3km long, 6.25m diameter tunnel between Kapurbawdi and Bhandup will supply water from reservoirs in Modaksagar, Gargai and Pinjal to a water filtration plant at Bhandup, which is the largest in Asia.

Joint venture contractors Unity Infraprojects and IVCRL were appointed to undertake the tunnelling work for the scheme in November 2009 and played a key role is the decision to use a tunnel boring machine (TBM) for the work. The depth of the tunnel - 109m at Kapurbawdi and 125m below ground level at Bhandup - meant that the scheme was always going to be challenging so the first use of a TBM in the Mumbai region might seem like a gamble.

Nonetheless, the joint venture has just celebrated breakthrough of the Robbins main beam hard rock TBM, which also helped cut the tunnelling programme from five years to just 21 months.

Despite challenging ground conditions, the TBM bored through the basalt rock to deliver production rates of 870m a month and up to 58m per day. “Both the daily and monthly progress we have seen are records for TBM tunnelling in India,” says Unity general project manager Pravin Titare.

Difficult geology

Although the TBM has now completed its drive, the work was not straightforward, with difficult ground encountered that included fractured rock and water inflows throughout the tunnel. IVRCL deputy general manager Bipin Arey says: “The geology was particularly bad at times and our team took all precautionary measures and advanced slowly.”

These conditions were anticipated, and the Robbins TBM was equipped with 482mm cutters and a probe drill to allow grouting to be carried out ahead of advancement in the worst conditions.

According to Titare, ground support was also critical to progress. “The TBM was very safe in the adverse geological conditions,” he says. “The rock support system and ring beam erector were helpful to reduce downtime and allowed us more production time,” he says.

Robbins assisted the contractors with the sequential launch of the TBM

Robbins assisted the contractors with the sequential launch of the TBM

In addition to supplying the TBM, Robbins also provided the conveyor system for the muck away as well as servicestaff on site to monitor the equipment and assist with daily upkeep and inspection of the TBM.

The launch of the TBM was done sequentially via the 109m deep shaft at Kapurbawdi. An initial start-up excavation of 50m started in June 2012, with vital back-up decks connected to the TBM cutter head using cables. With the start-up drive completed, the decks were lowered and a continuous conveyor system was installed for muck haulage.

Work is now underway to connect the tunnel to the reservoir systems at Kapurbawdi and the filtration plant in Bhandup in a bid to have the connection in place ahead of next year’s monsoon season.

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