NEW SHARON — A day after the old iron bridge crossing the Sandy River failed to fall after dynamite charges went off, a project manager explained what happened.

Steel on the 1916 iron bridge was cast into the concrete abutment closest to the Farmington end, which prevented the bridge from initially dropping into the Sandy River on Thursday, according to Andy McPherson, project manager with CPM Constructors of Freeport. That casting was not shown on the old bridge plans, he said.

The concrete was poured around a piece of bridge steel, he said. Instead of sitting on the concrete abutment, he said, the steel was set in the concrete to secure the bridge, which prevented it from falling.

There were four dynamite charges positioned in two abutments that went off at approximately 1:30 p.m. Thursday. The idea had been for the abutments holding up the bridge to be blasted out from underneath it and for the bridge to tip over into the river.

The beams of the bridge floor were rotted, and it was not safe for pedestrian or vehicle use, McPherson said.

Contractors decided to put in a few extra hours Thursday to get the bridge down rather than trying to do it another day. They used a giant jackhammer on an excavator to lower the bridge, and then used hydraulic sheers on another excavator to make a couple of cuts in the steel, he said.

The bridge, which weighed approximately 250 tons and was 262 feet long, dropped straight down into the river just after 5 p.m.

In the end, the bridge’s position worked out better for dismantling because the excavator with the shears can now drive on the bridge to cut up the steel instead of driving in the river, he said.

The company swapped out the hydraulic oil in the excavator for a special kind of vegetable oil to prevent possible river pollution, he said. The machine was also pressure washed prior to doing the project for the same reason.

It is expected to take three days to remove all the steel.

It will be taken to a salvage yard. The town will get 200 granite blocks and the rest will be used to shore up the embankments, he said.

“Everything will be recycled. Nothing will go to waste on this one,” he said.

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