Although the movement of traffic is on, a bottle-neck has been created atop the overbridge forcing vehicles to squeeze into one single lane from the otherwise two-lane stretch.

On Thursday the Road Construction Department (RCD) engineers started repair work on the overbridge, which was constructed in 1984 at cost of Rs four crore.

Road department chief engineer R D Ram said, "There is no structural problem. We have examined the bridge and all the load-bearing beams are fine. The bridge is not falling down".

He, however, agreed that it could cause serious accidents specially during the night and directed his officers to place reflectors all around the crater to warn motorists.

An engineer said fat rats existed at the genesis of the problem. "This area is rat-infested as a vegetable market operates right below the bridge.

Rats have eaten away the bitumen layer on the road at many places and have caused many holes on the road surface," he said.

Through these holes rain water had entered and seeped through the sand filling that forms the base of the carriage way. The water then drained out through "weep holes" (openings made in brick walls) and carried out sand with it, he said.

"As sand drained out, a vacuum pocket formed inside the sand-fill area and the force of this vacuum sucked in a big portion of the road," he explained.

Ram said extensive repairs would be carried out to plug the problem permanently. "We plan to push in metal and stone chips through the weep holes, into the sand-fill area to prevent rats from coming in. We also want to dig from the top of the flyover and get to the vacuum pocket," he said.

Once the vacuum pocket is found, it will be filled in with metal and sand. "We will inspect the entire bridge to see if any other portion is affected. I have also asked the engineer in-charge to explain why this problem was not detected on time," Ram said.