A machine called jaw crusher will be used to tear down the 57-year-old Tallah bridge to minimise the amount of dust released in the air and the vibration caused by demolition, PWD engineers said.

The government has decided to start pulling down the north Calcutta bridge, which has lost much of its load-bearing capacity and is in danger of collapse, from the first week of January.

“A jaw crusher works like a shark opening its jaws to gobble up concrete and steel,” an engineer said. “The machine produces much less vibration compared with more common crushing machines and releases negligible amount of dust in the air.”

Another reason why the government wants to deploy the machine is that it will help speed up the demolition.

“The government wants to start work on the new bridge at the earliest. The faster the bridge is demolished, the earlier we will be able to start work on the new structure,” the engineer said.

The demolition blueprint drawn up by the government states that the Tallah bridge would be close to traffic from the night of January 4.

The agency that has been awarded the contract to pull down the structure will need a few days after that to move in with men and machines.

The first to go will be the approach roads. “The approach roads will be fenced and the jaw crusher will be placed on one of them,” the engineer said.

The machine will work in combination with a diamond cutter and an excavator. The machines working in tandem would take a month to raze the bridge.

“We have spelt out in the tender document that the agency would have to dismantle the two approach roads to the Tallah bridge within 15 days,” an engineer said.

“The part of the bridge above the railway tracks is maintained by the railways. Another fortnight has been kept for bringing down that part.”

The diamond-cutter machine will be used to rip apart concrete slabs and cut them into smaller blocks. These pieces will then be fed into the jaw crusher.

A jaw crusher has two jaws — one moveable and the other fixed (see picture). Once a concrete chunk is scooped out, it is placed on the fixed jaw plate.

The moveable jaw is connected to a moving wheel. As soon as a concrete chunk falls, the moveable jaw starts exerting force against the fixed jaw and the chunk is reduced to rubble. The small particles are then sucked out through a pipe.

“We will be able to address the environment concerns because the jaw crusher will ensure that a far lesser volume of dust is released in the air, compared with other demolition gadgets. This methodology is accepted worldwide,” the engineer said.

Engineers tasked with supervising the demolition of the Tallah bridge said jaw crushers were much in use in parts of Europe such as Sweden, Italy and France, and in the US.