The 57-year-old Tallah bridge, which in many ways resembles the now fallen Majerhat bridge in design, bears signs of extreme stress, engineers said after an inspection on Saturday.

The concrete has peeled off in some parts of the underbelly of the bridge that takes about half of Calcutta’s truckload and the heavily corroded steel cable-ducts have been bared.

The bridge has been closed to heavy goods vehicles indefinitely, something that would be good news for commuters along BT Road who often complained about the congestion caused by the smoke belchers, but a graver possibility is that the entire bridge may have to be shut down after Puja fore a more detailed study of how bad the damage is.

“We will have to carry out a load-bearing test of the structure to understand the exact extent of the damage,” said a PWD engineer. “The bridge would have to be closed down for a few days for this load-test. That would be possibly after the Pujas.”

Engineers inspect the worn-out underbelly of the Tallah bridge on Saturday Picture by Bishwarup Dutta

Tallah bridge is a key structure connecting Shyambazar to large parts of northern fringes of the city, including Chiria More, Sinthee and Dunlop down BT Road. Apart from lorries, matadors and buses thousands of other vehicles, including cars, take this bridge to reach Calcutta from far away parts of Barrackpore and Sodepur.

Engineers said the bridge stands on 39 girders — Majerhat bridge stood on 38 similar girders — and each of them is around 25 metre long. Like Majerhat, a railway track runs beneath the Tallah bridge. In both these structures the girders are connected to each other with the help of pre-stressed steel cable ducts. Both Tallah and Majerhat bridges were built to carry 56 tonnes of vehicular weight on one flank of the bridge.

During Saturday’s inspection, which was carried out jointly by the railways and the PWD, engineers noticed that the cable-ducts in eight of the spans of Tallah bridge have started corroding. Out of these eight, two spans were located on the middle of the bridge, below which runs railway tracks. The concrete in these spans have started showing signs of cracks while in others, it has started peeling off.

The railway has decided to start repairing of the underbelly immediately. A basic plan of how to go about with the job has been drawn up. But before the actual repairing of the bridge begins, officers from the railway have asked the police to remove around 200 squatters from either side of the railway tracks to ensure safety.

The decision to start repair of the Tallah bridge was taken on Friday after Malay De, the chief secretary, convened a meeting with senior officials from the railways and the PWD. On Saturday, engineers brought in a mobile inspection unit to reach the belly of the bridge and noticed how parts of the concrete had peeled off exposing the steel cable-ducts. In at least two points, the cable-ducts had started jutting out of the concrete. Engineers took pictures before trying to understand the degree of stress on the cable ducts that led them to come out of the concrete.

“There appears to be seepage of water into the underbelly leading to corrosion,” said an engineer of the railway inspection team. “The ribbed concrete is clear sign of stress.”

Tallah bridge was among a clutch of bridges that PWD had identified as most vulnerable requiring urgent repairs back in September 2018. Since then only some of the expansion joints on the bridge’s surface has been replaced.