KOLKATA: Jamming of expansion joints with bitumen and concrete while re-laying the surface has become dangerous for most bridges across the city. An investigation into a sudden subsidence on Karunamoyee bridge in Tollygunge on Sunday has raised alarms for other city bridges as well.Though Kolkata Metropolitan Development Authority (KMDA) engineers have played down the seriousness of the damage, experts said that unless the gaps are restored, the expansion at the joint will cause stress and further subsidence. “The repair job on the bridge might be a minor one, but that does not lighten the gravity of the issue,” said an expert.KMDA will carry out repairs on the bridge after Sunday midnight, when it is closed for traffic, and will likely take less than two hours. However, sources said, officials will be looking into similar jamming of expansion joints in other city bridges as well. Experts said such jamming of joints was quite common. “We often fail to gauge the stress it causes on the beams and the surface because of hindered free-expansion,” one of them said.The bridge expansion joint system (BEJS) is a very important feature. Expansion and contraction of a bridge, caused by heat and the cold respectively, make uniform gaps at the joints necessary. However, in majority of city bridges, like Karunamoyee bridge at Tollygunge, the room for expansion has disappeared under multiple layers of bitumen.Full-fledged repairing and re-laying of the surface does not cause such problems. But, in most cases, agencies carry out stop-gap repairs without taking into account the advice of bridge experts. Apart from raising the laden weight of the bridge, laying and re-laying of the surface chokes the gap and damages the rubber-buffer within. The expansion due to the heat generated by friction of tyres during the movement of the vehicles then causes stress on the beams and surface. This had led to the subsidence on Karunamoyee bridge, an expert said.“One of the major problems with the use of elatomeric cushion seals is the failure of the anchoring device and subsequent loss of the joint during stop-gap repairs. The voided area gets filled with bituminous materials,” admitted a KMDA official.

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