TIVERTON — “Fire in the hole!”

At just after 1:20 p.m., a moment after a member of a demolition crew yelled the warning, an explosion erupted in the middle of the Sakonnet River.

Two piers that once supported the Sakonnet River Railroad Bridge between Portsmouth and Tiverton blew apart from the force of the detonation. Chunks of concrete flew into the air and crashed into the water, sending waves rolling into the Tiverton shore as clouds of smoke drifted above.

The state Department of Transportation yesterday used 1,950 pounds of explosives to blow up the concrete piers, the last remnants of the bridge, which in its heyday would swing in and out to allow boats to pass through.

It all seemed to go according to plan, except, that is, for the timing of the demolition.

The piers were set to explode precisely at 12:30 p.m., but the DOT couldn’t have predicted the untimely appearance of a seal in the waters around the piers.

Workers with Advanced Blasting, the demolition contractor, waited nearly an hour in the bitter cold before taking down the piers so the seal could swim out of harm’s way.

“Everything went well besides waiting for that seal to leave,” said Will Hernandez, district managing engineer with the DOT. “Sometimes it takes five minutes. Sometimes it takes five hours. But you have to wait.”

The demolition was the culmination of a $1.6-million project to remove the old railroad bridge that lay on the north side of the much taller Sakonnet River Bridge.

The bridge was built in 1899 by the Pennsylvania Steel Co. and for nearly a century carried passenger and freight trains on the Old Colony & Newport Railroad.

It was closed in 1980 after an overweight train loaded with military equipment caused heavy damage. The bridge, one of the last remaining swing spans in Rhode Island, was left in the open position to allow boat traffic along the river, but in 1988, a barge ran into it.

The Coast Guard deemed the bridge a safety hazard and an obstruction to navigation. According to the DOT, the Coast Guard supported the removal of the bridge.

Over the past several months, crews with Testa Corp., which was hired by the DOT, removed the steel portions of the bridge, 700 tons in all, clearing the way for the concrete piers to be blown up.

The demolition temporarily closed Route 24 along the Sakonnet River Bridge to traffic. A 500-foot safety zone was maintained around the piers, which closed a section of the river and portions of Evans Avenue and Riverside Drive in Tiverton.

After the explosion, DOT engineers inspected the work. Hernandez said the piers, which were 40-feet deep, had been substantially broken up.

An excavator will be taken to the area on a barge next week to lift an estimated 1,500 cubic yards of concrete out of the river.

The demolition of the railroad bridge is unrelated to the long-planned replacement of the Sakonnet River Bridge. The new span is set to be built on the south side of the current bridge. The state is putting in test pilings for the new bridge, said Hernandez.

Although the railroad bridge is gone, one piece of it has been preserved.

A two-foot wide phosphor-bronze center bearing, upon which the 251-foot swing span turned, was saved. It will be given to the Portsmouth Historical Society and eventually put on display.

akuffner@projo.com