The Tampa, FL, weather report for the morning of Tuesday, April 13, warned of oncoming storms late in the day.
So at 7:15 a.m., as drivers began streaming into the morning’s rush hour traffic, the sound of clapping thunder came as an early surprise to those traveling in and out of downtown on the Lee Roy Selmon Crosstown Expressway.
Yet, it wasn’t thunder causing the earth to tremble in the early morning hours. It was the sound of massive slabs of concrete folding into a V-shaped crease in the center of the expressway. Chunks of concrete rained down onto the roadway, landing on four vehicles and sending two construction workers to the hospital with minor injuries.
A concrete pier sank 20 ft. (6 m), bringing down the two spans of the newly constructed elevated road deck.
Officials of the Tampa-Hillsborough County Expressway Authority, the agency that oversees the expressway and construction of the elevated, reversible road, blamed the collapse on a sinkhole that opened beneath the pier, sucking in 15 ft. (4.6 m) of a support column. The road buckled, falling onto heavy construction equipment and assisting in opening the sinkhole.
“As far as we know, it wasn’t a design problem or a construction problem,” said Pat McCue, the authority’s executive director. “It was a natural occurrence that was impossible to predict.”
Sinkholes usually occur in areas where the rock below the land surface is limestone, carbonate rock, salt beds or rocks that can naturally be dissolved by circulating ground water.
According to the U.S. Geological Survey’s Web site, as the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces grow too large.
If there is not enough support for the land above the spaces, the site notes, a sudden collapse of the land surface can occur.
Workers drilled 73 ft. (22.3 m) to test the soil before building the 33-ft. (10 m) pier, said McCue. The pier’s foundation is buried more than 60 ft. (18.3 m) below ground.
“We found nothing there but limestone and support material that could support the bridge,” he said.
Similar tests were done for each of the 212 piers that will support the 9-mi. (14.5 km) road. Each test bored at least 10 ft. (3 m) beneath the foundation.
“When you have a sinkhole as deep as this, it is virtually undetectable.”
McCue said that experts believed the sinkhole opened under the weight of a 350,000-lb. (158.8 kg) truss, or erection girder, which is used to connect segments of the elevated expressway.
Using an overhead crane, crews lower segments of the roadway onto the truss where they are pulled together with steel cables inside the bridge and the erection girder is launched into the next span. The underslung self-launching erection truss is capable of carrying in excess of 1,200 tons of bridge segments.
Rush hour traffic would only be a fraction of the weight of the truss.
Traffic on the span will always move in the same direction. During morning rush hour, the reversible bridge will have three lanes carrying traffic –– in one direction –– from Brandon and I-75 into downtown Tampa. Later in the day, the same three lanes of traffic will be reversed, and traffic will move away from downtown Tampa into the suburbs connecting to I-75.
The main accesses on the reversible span are being constructed at each end of the elevated roadway. The reversible lanes bridge will be the only facility of its kind in the entire Southeast.
PCL Constructors Inc. is constructing most of the elevated portion of the roadway including a section passing over I-75.
Once complete, the reversible bridge will include 3,032 reinforced concrete segments sitting upon piers that are 6 ft. (1.8 m) wide at the base. Typically, segments are 9 ft., 4 in. (2.8 m) long and cost approximately $10,000 each.
The company has crews using Caisson drills to drill shafts to depths of nearly 70-ft. (21.3 m). Unlike the segments, the 220 vertical concrete piers are cast in place.
Crews currently are putting up bridge segments in multiple locations.
Construction on the expressway began in February 2003 and is anticipated to be complete in July 2005.
Because the work is ahead of schedule, McCue does not expect the recent collapse to affect the 2005 completion date.
He also said he had no doubts about the safety of the elevated roadway –– but shied away from predicting Mother Nature.
“There are sinkholes throughout Florida,” he said. “There is no such thing as an assurance that it won’t happen again.”
AP Photo: On April 13, massive slabs of concrete folded into a V-shaped crease in the center of the Lee Roy Selmon expressway outside of downtown Tampa, FL. Authorities determined it was a sinkhole that caused the early morning collapse.