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1927-29 The Willingdon Bridge which spans the River Hooghly at Bally was by far the most expensive and the most difficult of the railway bridges to be constructed in India up to that time. The bridge was constructed at a total cost of Rs 1,14,67,000.
At the point where this spectacular structure crosses the river is 2,520 feet wide. The viaduct at Bally consists of 22 spans of 30 feet girders built of masonry piers, whose foundations have been piled with reinforced concrete piles 40 to 50 feet long. The bridge itself consists of seven 350 feet main spans and two 80 feet land spans. The eight main piers in the river are founded on octagonal steel caissons, 70 feet by 37 feet, having two dredging holes each 19 feet in diameter. The caissons were all floated into position and founded by loading with concrete, sustaining the load on compressed air buoyancy and releasing the air on a suitable falling tide.
A detail in the erection, unusual in bridge building, was the method of putting camber into the girders. Instead of building the bottom boom with the required camber in the first instance, as is usually done, the bottom boom of the Bally bridge girders was erected level on the cross girders, and then the ends of the boom were dropped four inches. Another extraordinary feature of the bridge which makes it virtually unique in the world is that the caissons (70 feet by 37 feet) have been sunk in a tidal river in four feet of water, with a current at times six miles an hour. It may be mentioned that at periodic intervals the Hooghly River is visited by tidal bores which are known to overturn heavy boats, tear steamers from their anchors, and even pull away floating docks from their moorings. They have been the nightmare of all bridge builders and of everyone connected with navigational work in the Hooghly River. Provision had to be made for these tidal bores.