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Contractor Cuts Schedule by Decade on Super-Fast Superlevee

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The U.S. Army Corps of Engineers and its contractors are setting a record for achieving the fastest gain in consolidation and strength on earthen levees—a mere 60 to 90 days, compared with 10 to 11 years—by using an intricate design that layers a sand blanket, geotextile fabrics, rock and wick drains to evacuate moisture from marshy soils.

PPhoto: Angelle Bergeron
Stitchers' Wick-drain contractor 'stabs' 250,000 fabric-tape wicks through sand bed to draw moisture from the soil, speeding consolidation.
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“Something of this major import, of this scope, is rare, and we are using unique and unusual means to achieve those goals,” says Al Naomi, program manager, URS Corp., San Francisco. URS performed geotechnical design and developed plans and specifications for the levee reach, known as LPV 109.

Archer Western Contractors Ltd., Atlanta, holds the $114.9-million contract to deliver the roughly 7.54-mile stretch of what will be some of the most massive earthen levees in the Greater New Orleans Hurricane and Storm Damage Risk Reduction System. Built on soft, marshy soils in eastern New Orleans, the levees are, from toe to toe, 320 ft to 360 ft wide, with elevations varying from +18 ft to +25 ft.

“When you put up to 30 ft of fill height on those soft soils, it would no doubt cause a slope failure,” says Dwayne Smith, program geotechnical manager. “This method was chosen because right-of-way was available to allow for wider construction and [staging].” A massive deep-soil mixing design was used in an adjacent reach with a constrained right-of-way.

The URS design calls for an initial placement of a geotextile separator fabric and a +2-ft to +3-ft-elevation sand blanket that forms the base of the levee. “The separation fabric is basically to keep marsh soils down below from contaminating sands in the blanket,” Smith says.

Bertucci Industrial Services LLC, Jefferson, La., in July completed that $25.6-million initial contract, which required 1 million cu yd of sand.

The remainder of the design includes layers that vary by width and depth along different areas of the reach, depending on underlying sand strata and soil conditions, Smith says. The layers above the sand blanket include the wick drains and instrumentation, followed by separation fabric, 8-in.-thick gravel and then more separation fabric and clay in 1-ft lifts, installed no more than 3 ft per week.

As clay goes down at different elevations, higher-strength geotextile fabric will be installed in some areas, Smith says. The fabric strength is about 20,000 lbs per sq ft. “The fabric adds load on the column and provides extra protection against slope-shearing failure,” he says.

“The fabric is very complicated,” adds Lance Nowacki, project scheduler for Archer Western. “There is overlapping everywhere and different types of fabric.” The design calls for a 0.40-mil-thick, 27-ft-wide geomembrane for the new levee’s entire length. “The geo is tucked behind the stone and pulled out 47 ft to keep seepage from the flood side,” Nowacki says. The project calls for three types of separator fabric at 1 million yd total.

Cone penetrometer testing was performed to determine the wick-tip depth. Wicks pierce through a 280-ft-wide drain bed in a 5-ft triangular configuration. Depths vary from -8 ft to -35 ft, staying roughly 6 ft above the underlying sand strata to provide a barrier against seepage in a hurricane event, Smith says.

Archer Western contracted US. Wick Drain, Leland, N.C., to install about 9 million linear ft of wick drain. The firm is stabbing about 5,000 wicks, or 65,000 linear ft per day, says Martin Pospisil, Archer Western project manager.

“It’s the largest project for square footage and amount of holes that’s ever been done in the United States,” says Mark Palmatier, U.S. Wick Drain president and owner. Palmitier says he is aware of a project in California that required 11 million linear feet of wick, but says the wicks were deeper and there were fewer holes. “Only 100,000 holes,” he says, “Here, we are installing 250,000.”

The contractor is using five “stitchers,” or wick drain rigs—the most Palmatier has ever used on a single project. A plate attached to the tip of the wick is pushed by a mandrel through the soft soil to anchor it at the desired depth.

U.S. Wick Drain makes a wick that meets Corps specifications, but it opted to use German-manufactured “Colbond CX 1,000” to keep pace with the schedule. The tensile strength is much higher than conventional wick because the fabric is thermally bonded to the plastic, Palmatier says, adding that, by the end of August, installation was ahead of schedule, at 55% complete. “The only time we shut down is for lightning,” he says.

Thompson Engineering, Mobile, Ala., and Applied Geomechanics Inc., San Francisco, are installing monitoring instruments, including multipoint bore-hole extensometers—also known as MPBX, or “spider,” magnetometers—to track settlement;...

 

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