VICKSBURG — Two concrete walls blasted into the Mississippi River bank below the old U.S. Highway 80 bridge are passing a tough test during a rainy start to 2013, bridge officials said, as the latest study on the Interstate 20 bridge shows piers moved at the same pace last year as its older neighbor.
In October, barriers 18 feet high and about 8 inches thick were built into the steep bank below the bridge’s administrative office. They replaced steel bulkheads that had given up about 6 feet of bank each heavy rainstorm in recent years, superintendent Herman Smith said.
“We were constantly having washouts,” Smith said. “It was washing more and more. We knew at one point or another, it would give.”
Storm water drains over rocks below the superstructure as before, but flows slower since contractors moved about 80 tons of dirt to reshape the bank to build the “shotcrete,” Smith said.
The mottled-surfaced barriers have held the bank during a wet winter — rainfall since Jan. 1 in Vicksburg is nearly a foot above normal, according to the National Weather Service.
“It’ll keep the water slow,” Smith said, adding about 80 tons of dirt moved to construct the new protection. “The less speed water has, the lesser chance you have of erosion. Hopefully, we’ve cured this problem for a long time.”
The work, performed by Georgia-based Hayward Baker, came with a $549,440 price tag.
A third wall, lower on the bank and closer to the water’s edge, was nixed from the original concept to keep costs down.
About $165,000 is left in the Vicksburg Bridge Commission’s $1.2 million budget this year for bank stabilization work.
Construction of a third wall would require more money than that, Smith said, mainly for feasibility studies and other preliminary work.
Multiple studies have noted movement below both Mississippi River bridges in Vicksburg.
Pier 2, the first large support below the old bridge from the Mississippi side, moved 1½ inches west in 2012, according to the bridge’s annual stability report from Baton Rouge-based G.E.C. Previous reports have noted 24 to 30 inches of westerly movement since its construction in 1930.
In 2006, a study directed by the Louisiana Department of Transportation and Development indicated no danger for motorists who use the I-20 crossing, completed in 1973, but revealed “significant movement” in a support pier on the Mississippi side directly across from pier 2 on the old bridge.
Movement below the highway crossing continued last year and drainage issues persist on the Mississippi side, according to consultants working with LDODT for more than a year on bridge stability and related issues.
“When the river drops low, that’s when the bank tends to be less stable,” said Robert Werner of Florida-based Ardaman & Associates Inc. during a progress report last week before the five-member commission, which requested the update because of potential effects of the study’s findings on the old bridge.
“In a sense, the I-20 bridge piers have pinned down the strip of land that the bridge sits on,” he said.
Last fall, Werner said, markers placed below the I-20 bridge and on the river bank moved west between 1 and 2 inches after experts checked them after Tropical Storm Isaac’s remnants moved across Vicksburg. The storm hit south-central Louisiana on Aug. 29, the same day the river at Vicksburg’s river bridges dipped to minus 1 foot, its lowest mark for 2012.
Slides showed one near pier F below I-20, across from pier 3 on the old bridge, moved 1-1/2 inches, while markers placed north and south of Ameristar Casino moved more than 10 inches west.
A 100-foot scour hole just south of the interstate bridge has been filled periodically with riprap, Werner said.
Cracks in the bank developed in 1929 and 1939 just north and south of the present-day bridges, an area where earth movements “tend to be large,” Werner said.
Stormwater drains to the river over the grown-over fissures from the Washington Street/Warrenton Road interchange, which Werner called “fundamentally, not a great idea” but solvable through extra backfill on the bank if cracks ever re-open.
Werner and Bill Marcuson III, a civil engineer and a lead expert on the study, advised more communication between bridge officials and Vicksburg’s riverside casinos on stability issues.
“The river is a dynamic situation,” Marcuson said. “It’s constantly eating at the bottom of all the properties down there.”
LDODT expects the study to conclude later this year. The Mississippi Department of Environmental Quality’s Office of Geology has assisted the effort.