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Post-Katrina reconstruction included 350 miles of perimeter levees in New Orleans
By Chris Kelsey
Introduction
Following the extensive damage of Hurricane Katrina in August 2005, legislation authorized and funded the reconstruction and
improvement of the levee system in the New Orleans, La. (NOLA) region.
This notably ambitious engineering project established a goal to provide 100-year flood protection status for the NOLA area. Geosynthetic
materials are playing a key role in enabling these designs.
The levee referred to in contract documents as WBV-72 exemplifies this work. The roughly 2.8-mile-long levee, 2.4 miles of which were involved in this construction project, has been rebuilt to an elevation of 13.5ft with the support of three strengths of geotextile for base reinforcement and
separation.
Supporting St. Charles
WBV-72 is part is the West Bank & Vicinity (WBV) Hurricane Storm Damage and Risk Reduction System (HSDRRS). WBV-72 is a clay levee located in St. Charles Parish (pop. 53,000) and serves the system as a Lake Cataouatche Western Tie-In Levee (or East-West Levee). It runs parallel to U.S. Highway 90 and ties into WBV-73 on the west and WBV-17b.2 on the east.
Like similar projects in the region simultaneously directed by the New Orleans District Office of the U.S. Army Corps of Engineers (USACE), WBV-72 had an abbreviated timeline for construction that heightened the need for geosynthetic utilization.
Awarded in late March 2010, and with a strict finish date of June 2011, the levee could not wait years for soil consolidation. It was not purely a new levee. It added to the existing levee, but not directly on top. As in many of the other ongoing projects, the design called for off-setting the new alignment coming in behind the existing levee and building it taller and farther back.
The soils on which this construction would be founded were of varying degrees of weakness. But a multiyear soil-consolidation approach was not possible given the deadline.
With so much strength needed to be guaranteed on such a short construction timeline and with the number of fast-tracked projects needing to take place, both design and budget goals pointed to geosynthetic reinforcement as the best option.
Some of the region’s levees were rebuilt with geogrids. Levee 111, for example, which is situated along the sensitive Bayou Sauvage National Recreation Area, utilized a deep soil mixing technique in the creation of “soilcrete” columns that were reinforced with polyvinyl alcohol (PVA) geogrids. But in St. Charles Parish, geotextiles were found to be the best choice.
What lies beneath
Because of the varying soils along WBV-72’s 2.4-mile construction zone and the need to build certain levee zones higher, the
original design specified five types of geotextiles for base reinforcement. Each strength specified denoted a different 5% value of ultimate strength
in the product’ s machine direction.
As the project team of contractor WRS Compass (WRS Infrastructure Division) and the USACE consulted with the geotextile manufacturer, it was discovered that some of the requested strengths were close enough that only three types of geotextile strengths would be needed, especially when additional cost savings might be achieved through the quantity being acquired without sacrificing any design safety or installation efficiency.
The strengths used were:
- Type 1–490 kN/m (21,500yds2)
- Types 2 & 3–650 kN/m (187,403yds2)
- Types 4 & 5–830 kN/m (172,071yds2)
Primarily, these geotextiles were used as embankment reinforcement where they were installed at the base to help reinforce over the poor
foundation soils.
Because wick drains were not used in the construction process, settlement gauges were installed with the geotextile so that settlement could be
calculated as it happened and vetted against the original design.
One of the major cost advantages of this solution meant that the poor site soils did not need to be removed and replaced with something more
structural—an approach that would have been significantly more expensive. Instead, reinforcing over the top with the geotextile eliminated both differential settlement and slip plane failure concerns.
The woven geotextiles used on the project were manufactured with a high-modulus, low-creep polyester multifilament yarn that mobilizes
high-tensile forces at low strains, which makes it relevant for applications such as embankment reinforcement on soft ground, reinforced earth
structures, and stabilization of breakwater, jetties, and other water-abutting structures. What turned out to be ideal for the WBV-72 project was the
way the manufacturer could customize the geotextile rolls. This customization actually reduced project cost.
Panel sizes and installation arrangements matched up with the lengths of each roll shipped as close as possible to the design and site work flow. This maximized the site installation efficiency, saving time and reducing costly material waste.
The team had what it needed, not too much, and could timeline each stage of construction to the delivery of the rolls, which were shipped from the manufacturer straight through the Port of New Orleans to the site. The standard width of these geotextile panels was 5m (16.4ft). (This is the
standard width from the manufacturer.) But rolls were also produced in lengths of 80ft to 206ft. Though this approach may not work for every
project, in this case the efficiencies gained by roll length customization lowered cost and sped up construction.
Also, the geotextile strength was customized to the project specifications to further reduce cost. Not only did this allow the team to simplify from five types of geotextile strengths to three, it helped prevent overpaying for material, such as more expensive rolls of an unnecessarily higher strength. In time-sensitive projects the use “off-the-shelf” material might mean having to use simply the closest available qualifying strength. Depending on the available supply of material, that might mean something that is, for example, 100 kN/m above specification strength. That sort of product selection, while certainly safe, generally adds cost.
Here, the decision to customize strength actually reduced cost.
On target
The work on WBV-72 was wide-ranging, extending beyond just the levee work. A highway needed to be widened, a highway bridge and
crossing were constructed, dewatering took place, one pond’s levee was removed, floodwall gates were installed, and disturbed land was
reseeded. But the center of the work was the strengthening of the levee system—the 100-year flood protection.
The first geotextile rolls were shipped to the jobsite in April 2010. Through August, rolls were supplied in coordination with the contractor’ s
installation progress. Mostly, the three different strengths were matched to particular foundation soil needs, with the highest strength geotextile
being used on the worst soils and where the levee would be built highest. A couple areas did require two layers of geotextile reinforcement. Both
used the highest-strength geotextile at the base, select fill, and another geotextile layer (of a lesser strength than installed at the base) to ensure
the structural stability.
Late in 2010, some additional geotextile was supplied as needed (650 and 830 kN/m strengths), but all in all the project progressed as planned, on time and on budget.