COLUMBIA — When a 1,000-foot cable snapped on the Wando River bridge, the explosion was so violent concrete grout shot out of its plastic covering like shrapnel, Department of Transportation commissioners were told Thursday.
"That grout exploded up and down, a hundred feet away from the location," DOT Deputy Secretary Leland Colvin said after briefing the members on last month's closure and re-opening of the westbound bridge linking Mount Pleasant and Daniel Island.
DOT officials closed the James B. Edwards Bridge on May 14, causing several weeks of traffic headaches for the Charleston area, after finding the snapped "tendon," which is actually 19 individual strands of cable wrapped in a plastic tube and filled with grout, during a weekly inspection.
Eight such external "tendons," or main cables, span 1,000 feet under the middle sections of each bridge. Their purpose is to tie segments together to withstand bridge traffic; 84 additional tendons embedded in the concrete keep the bridge standing.
National experts later determined the culprit was corrosion due to water seeping through the bridge's concrete deck above. Yet testing just two feet from the explosive failure about four months earlier — among 84,000 feet of cable tested after a damaged cable was found in the same area two years ago — found no corrosion.
That's because the break occurred within a 7-foot-thick concrete wall. Testing cables through those sections is nearly impossible without drilling, something that essentially makes "Swiss cheese" out of the bridge and would cause more water intrusion, Colvin said.
"No matter what people want to say about it," said DOT Chairman Ben Davis, "no matter if you’d monitored the bridge every five minutes, you wouldn’t have known it until it broke."
He and other commissioners applauded the DOT's work.
Since the bridge reopened June 2, more than a week ahead of schedule, a two-person crew has inspected the bridge daily. They walk together in the hollow concrete box that hangs below the traffic deck of both the eastbound and westbound bridges in a 3.4-mile round trip that takes about five hours. And an acoustic monitoring system is being put in place to immediately detect any breaks, Colvin said.
"We'll actually put microphones on the cables. If there’s any type of rupture, it will send a signal back to various folks letting us know," he said.
The monitoring system of cameras and microphones will eventually span both bridges. But that requires running electricity the entire length of each bridge, which could take six months. In the meantime, phase one of the monitoring, where the break occurred, is powered by generators.
Once all the "kinks are worked out" with that system, daily walk-throughs will end, Colvin said.
Repairs involved removing, repouring and sealing concrete on the bridge decking, and running additional cables for redundancy.
Eventually, both bridges' middle sections will have two additional main cables, bringing the total count of the external cables to 10 each. That work, as well as additional sealing, could cause lane closures, but not a bridge closure again, Colvin said.
The cost of the repairs is still unknown. Bills are still coming in on the work already done.
Building the crossover lanes, which allowed two-way traffic on the eastbound bridge to relieve some of the congestion while the other bridge was out, cost $673,000, DOT officials said.
While the bridge is the only one of its kind in South Carolina and there have been corrosion problems since 2010, 19 years after it opened, the bridge won't need to be rebuilt anytime soon, Colvin assured commissioners.
"It's about 30 years into a typical 50-year design, but we expect to get many, many more years of service" with the planned maintenance, he said.