Mumbai-Pune expressway’s ‘missing link’ to bridge the two cities closer

The monsoon has posed the biggest challenge to engineers working on the cable-stayed bridge with heavy rain, which occurs almost daily, hampering work. This portion falls in the ghat section, which sees 50- to 65-mm rain daily, even more, during the monsoon.

“Only 50 per cent of the time we are able to work on this portion on rainy days. When the rainfall is around 50 mm, work goes on albeit at a slower pace. However, if the rainfall exceeds 65 mm, we are unable to carry out any work,” said Rajesh Patil, Chief Engineer, MSRDC.

The project

On Mumbai side, the Rs 6,600-crore ‘missing link’ starts at Khopoli, bypasses the expressway through an 840-metres-long viaduct (60 metres height), enters a 1.75-km-long tunnel, then goes over the 640-metres-long cable-stayed bridge (100 ft height) and thereafter enters another 8.9-km-long tunnel. A part of the tunnel lies 170 feet below the ground with the Lonavala lake above it, emerging near the Sinhgad Institute.

While the distance between these two end points — Khopoli and Sinhgad Institute — along the existing Mumbai-Pune expressway is 19 km, comprising steep slopes and the ghat section putting restriction on speed limit of vehicles, the ‘missing link’ will cut the distance by 5.7 km, saving 20-30 minutes of commute time. More importantly, motorists won’t have to drive through the ghats and would be able to drive faster at up to 120 kmph.

With the ‘missing link’, the MSRDC also aims to solve an annual worry during the monsoon — traffic gridlocks at Khandala due to falling boulders and morum from hillocks that flank the road.

The bridge goes over the Sahyadri Valley and if we had planned a traditional bridge, the number of pylons and piers would have been four times more” — Rajesh Patil Chief Engineer, MSRDC

Engineering feat

The cable-stayed bridge is designed to withstand windspeeds up to 70 metres per second or 252 kmph. Usually carried out for aircraft and large bridge designs, before the design of the cable-stayed bridge was finalised, a prototype was subjected to wind tunnel testing at Force Institute in Denmark.

According to engineers, it is important that the bridge is able to sustain extreme windspeeds with several low pressures, often developing into full-blown cyclones — Cyclone Nisarga, Cyclone Tauktae and Cyclone Biparjoy to name a few — forming in the Arabian Sea in recent years.

Cable-stayed bridge

The bridge’s maximum span length is 305 metres (mid-span), while the length of the end-spans on either side is 165 metres. The maximum height of the pylons — tower-type structures that support the cables — is 170 metres.

“The bridge goes over the Sahyadri Valley and if we had planned a traditional bridge, the number of pylons and piers would have been four times more. That could have been troublesome especially in the valley because every pier has to be 100 metres high. However, as it’s a cable-stayed bridge, only four pylons and eight piers are required. In a traditional bridge, the number of pylons and piers would have been four times that number posing engineering challenges, taken longer time to complete the project, required more resources and money,” said Patil.

He further said that if the span length would have been more than 500-700 metres then in that case a suspension bridge could have been an option. But a notch below, the cable-stayed bridge is a suitable option, he added.

After work started, this is the second monsoon. In the previous monsoon, foundation work was taken up.

With the ‘missing link’, the MSRDC also aims to solve an annual worry during the monsoon — traffic gridlocks at Khandala due to falling boulders and morum from hillocks that flank the road.

Currently, pylon construction is under way and have reached an average height of 65 metres. There are four pylons. Once the pylons reach a height of 100 metres then construction of the deck slab will begin. For laymen, deck slab is the surface that supports moving traffic.

Patil explained that once a 9-metre length of the deck slab is ready, it will be tied with cables to the pylon. As the mid-span length is 305 metres, cables will be tied to the deck slab at 152 metres from either side.

The MSRDC is targeting to begin work on deck slabs and complete at least one pylon out of four by August this year. While viaduct work is 70 per cent complete, span load test will be done in July on the Pune-bound corridor.

There are two viaducts, one for the Pune-bound corridor and other for the Mumbai-bound corridor. The Pune corridor is nearly complete. Work on the Mumbai-bound corridor is also on track.

Patil said the contractor will ply vehicles carrying morum and other heavy load on the stretch of the completed viaduct to check span load as per guidelines of Indian Road Congress.

Officials said the MSRDC also gathered information on the bridge collapse over River Ganga in Bhagalpur in Bihar and measures have been adopted to not repeat the mistakes at the construction stage on the cable-stayed bridge.

Deadline

By September this year, the MSRDC plans to finish substantial work and only work on the cable-stayed bridge will remain, to be finished as early as possible. Although the entire project deadline is December 2023, in case of delay, officials say they would be able to complete it in early 2024.

Tunnels

Excavation work for the two tunnels covering a length of over 10.5 km has been completed and finishing work including surfacing and plumbing is in progress. The underground tunnel of the ‘missing link’ is Asia’s widest road tunnel with a width of 23.75 metres and will have eight lanes (4+4 lanes) along with 2.5-metre-wide emergency lanes on either side.