During construction a magnet reader passed through the tube can locate the spiders, which slide down the tube as the wicks draw water from the soil and the weight of the new lifts compress and strengthen it. “They are accurate to about 1/8 of an inch,” Volk says.
The inclinometers are made with specially designed PVC tubes installed vertically through the levee and into the sand layer as well. Each cluster has one installed at the levee “hinge” on the protected side, where the 4:1 slope meets the long, low- slope mass of the stability berm. Another is placed near the land side toe.
Lateral movements of the soil bow the center of the inclinometer tubes out of plumb. The distortion is measured with a 30-in.-long wheeled sensor that rides groved orientation tracks inside the tube when drawn through it. The rate of change in the deflection slows as the soil hardens and moisture is drawn away.
Vibrating wire piezometers also are placed under the highest pressure point below the levee, below the crest, Volk says. Data cables with plenty of slack to allow for soil movement and compression lead back through a rock and sand drainage layer to a terminal box at the protected-side toe. As the levee material is piled on, the pressure increases for every foot added, but then drops as the wicks draw the water away.
Finally, each cluster has a 6-in.-dia., 60-to-70-ft-long benchmark monument at the protected-side toe that is drilled and grouted into the sand layer as well.
URS will continue data collection through October as the contractor puts final touches on the levee before turning monitoring over to the Corps. “We expect to get another 1 to 1.5 ft of settlement in the next nine years—which means we'll still have the required flood protection height in 10 years,” Volk says.
The Corps is using geotextile fabrics and wick drains liberally on New Orleans' 350-mile perimeter system. It had used both materials prior to this program, but “When they were doing this before, it was for 2- to 3-ft levee lifts,” Volk says. “This was a concentrated effort to build a complete levee on soft grounds.”
“The use of wick drains will give a lot of consolidation in areas with virgin ground,” adds Varuso. “We are going to implement as many techniques as possible to give Plaquemines the biggest bang for the buck.”
Volk says LPV 109 cost $3,700 per linear ft, compared with deep soil mixing, also used to speed flood protection building on soft soil, which costs about $12,000 per linear ft. Concrete T-walls cost $10,000 to $15,000 per linear ft.
“This is the cheapest way to build over soft soils,” Volk says. “At 30 cents a lineal ft, wick drains are probably the most economical thing on a construction site.”
The methods deployed on LPV 109 are valuable anywhere a large earthen embankment is required and property isn't at a premium, says Dwayne Smith, URS program geotechnical manager. “When we put together our final report, the results for this levee may very well become factors the Corps uses in the future,” Smith says.
“I believe this report may end up in textbooks,” Smith adds.