Massive steel chunks of the Silas N. Pearman Bridge became entangled as they collapsed into the Town Creek Channel after 2,250 lbs. of strategically placed explosives meant to slice the bridge into 11 sections, failed to completely discharge.
The misfire has complicated the clean-up process for prime contractors Jay Cashman Inc. and Testa Corporation.
The Oct. 11 blast was meant to demolish a 500-ft. portion of the Pearman Bridge that crosses the Town Creek in Charleston, SC. This was a test run to the larger portions of the Pearman Bridge and the John P. Grace Memorial Bridge.
According to Lincoln Blake, U.S. Army Corps of Engineers project manager of navigation in the Charleston District, the bridge over the Town Creek Channel was chosen for the trial demolition because the channel is primarily used by private vessels, and a limited amount of barge traffic, but is not a major commercial shipping route. Also the nearby Cooper River main channel is available to allow vessels an alternate way to travel upstream of the site.
“In addition, the Town Creek Channel is representative of conditions in the main Cooper channel and therefore thought to be a good test of the methodology for the demolition and also to prove that retrieval of the 40- and 55-ton bridge segments from the water could be done in 24 hours,” Blake explained.
Extensive planning went into preparing for the demolition.
According to Lt. Jr. Grade Calvin Summers, chief of the container facility and explosive load branch of the U.S. Coast Guard, the Coast Guard, along with private, state, local and federal agencies spent months devising a game plan where all parties had an equal voice in the demolition process.
“The Coast Guard is responsible for controlling marine traffic and all water security and worked with all agencies involved in this project, including the South Carolina Department of Transportation (SCDOT) and private contractors, to ensure the demolition would proceed smoothly and safely,” Summers said.
“Forty-five minutes before the blast, the Town Creek Channel was closed to all traffic — commercial and private — and a 1,500-ft. safety blast radius zone was established.”
The first step in the demolition involved removing the concrete decks of the bridge.
According to Leland Colvin, project director of SCDOT, after the concrete decks were removed, crews put explosive materials along sections of the steel trusses.
“The explosions involved a series of blasts that attempted to break the bridge’s steel trusses into 11 sections —small enough so cranes could lift them out of the channel bottom,” Colvin stated.
Explosions Fail to Sever Trusses
Blake said the process of knocking down the bridge went well, but within the first few hours after the explosions, crews noticed problems with the recovery of the truss segments were upon them.
“We observed blasts at all the locations that explosives had been placed,” Blake said. “The steel trusses dropped into the channel in partial view above the water, but failed to completely separate into the 11 sections and became entangled in one and other.”
Recovery efforts will be challenging and time consuming. SCDOT Spokesman Pete Poore said seven of the 11 sections have been removed from the water as of Oct. 17.
According to Blake, the Corps is responsible for confirming that all the bridge debris is removed from channel bottom.
“Once all portions of the steel trusses are accounted for, the Corps will inform the Coast Guard it is clear to reopen the channel,” Blake said.
Charles Dwyer, SCDOT project manager, said he could not estimate when the channel will reopen.
“The divers must locate all the segments of the fallen bridge,” Dwyer said. “Then using underwater torches cut them into the planned 11 pieces and attach cables so they can be hoisted out of the channel.”
A Manitowoc 2250 crane mounted on a barge is being used to pick the pre-rigged segments out of the channel.
“The uncertainty of retrieval time is due largely to the low visibility and the strong currents in the channel making the job of the divers very difficult,” Dwyer said. “In addition the safety of the divers is of major concern and we must proceed with extreme caution.”
Dwyer said the contractors will not be penalized for the delay in opening the channel because Town Creek is not a major shipping channel.
“Future plans to demolish the remaining portion of the Pearman Bridge and Grace Bridge will be re-examined after a complete analysis of the complications associated with this demolition are determined. The Oct. 25 demolition of the second half of the Pearman Bridge has been cancelled,” Colvin said.
Demolition contractors for both the Pearman and Grace bridges are two Boston-based companies — Jay Cashman Inc. and Testa Corporation. Demolition Dynamics was subcontracted to blast the steel trusses of the bridges and Collins Engineering of Charleston, SC, was subcontracted to locate all the pieces of the bridges in the channel bottom and assist in their retrieval. The divers were subcontracted by the prime contractors.
This demolition is part of a $59.6-million project to remove both the Pearman and Grace bridges, which were replaced in July by the $632-million eight-lane, 1,546-foot (471 m) Arthur Ravenel Jr. Bridge crossing the Cooper River in Charleston — the longest cable-stay span in North America.
According to Colvin, the 18-month contract to demolish the two bridges began Aug. 3, however work didn’t actually start until Aug. 8. CEG