On July 6, Tampa, FL’s, Crosstown Expressway experienced its second failure in the last four months, when column No. 99 –– two columns to the west of No. 97, which collapsed April 13 –– sunk into the ground.
Column, or pier, No. 99 settled 1.3 in. (3.3 cm), a third of an inch more than the project’s tolerance.
The failure, which was not as severe as the first, according to the St. Petersburg Times, forced more delays and tests to check the safety of the other supports.
“Settlement happens during construction,” said Pat McCue, executive director of the Tampa-Hillsborough County Expressway Authority. “Settlement up to 1 inch is acceptable. When you put all that weight on top of a pier, it’s going to settle a little.”
He noted that other piers have settled .13 to .75 in. (.33 to 1.9 cm).
The Times reported on July 15 that seismic testing around No. 99 found that while it is anchored in hard limestone, it is two-thirds surrounded by muck, which does not provide adequate support.
Both columns sank under similar circumstances: a 175-ton construction girder, or truss, along with roadbed was placed on top. Officials have said the weight was far more than what the columns would have to support once the road opens to traffic.
Unlike the April collapse, No. 99 sunk gradually.
“We actually stopped work to watch it,” McCue said. “Once the weight is taken off, we expect it to spring back a little, though maybe not to where it was.”
McCue noted that the recent experiences may cause the state standard for boring to be revised.
According to the Times, additional soil borings done at the site show there is solid rock all around No. 99, but it is 12 to 15 ft. (3.7 to 4.5 m) below the bottom of the columns’ foundation.
Crews will sink two additional foundations into the good rock below and to either side of No. 99 in an effort to save the structure. The column will then be attached to its “sister” foundations.
The Times also reported that a similar solution is in the works for No. 97. Still, officials are unsure what caused that collapse and crews have just recently cleared away much of the debris from that failure. It will cost nearly $9 million to repair column No. 97.
Officials of the Tampa-Hillsborough County Expressway Authority, initially blamed that 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 McCue. “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. 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,” McCue said in April.
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.