By Angelle Bergeron
Geosynthetic materials are playing a critical role in providing New Orleans with its best flood protections ever.
“We couldn’t have delivered this work by 2011 without geosynthetics,” said Richard Varuso, deputy chief of the geotechnical branch of the U.S. Army Corps of Engineers’ New Orleans District (USACE/NOD). “Geosynthetics save construction time.”
When Hurricane Katrina hit New Orleans in August 2005, the storm surge caused widespread overtopping and some critical failures of the city’s flood protection levees. As a result, Congress authorized $14.6 billion and charged the Corps of Engineers with bringing the Greater New Orleans Hurricane and Storm Damage Risk Reduction System to 100-year levels of protection by June 2011.
New Orleans’ hurricane protections had been cobbled together ever since Hurricane Betsy battered the city in 1965. Trickles of civil works funding, subsidence, improper maintenance, politics, and even limited understanding of levee conditions resulted in New Orleans having what many have referred to as a “system” in name only.
Post-Katrina forensic analysis by the Interagency Performance Evaluation Task Force (IPET) informed updated design standards.
Armed with those new design standards and a fully funded program, the Corps embarked on a mission to give New Orleans its first comprehensive protection system, and one that provides greater risk reduction than ever before. The new system has bigger and more complex components, including taller, more robust earthen levees, better-designed concrete floodwalls, drainage pump stations, and huge surge barriers.
To deliver the tremendous amount of improvements in the tight time frame, the Corps, and its contractors and engineering consultants, employed new designs, contracting, and updated engineering, construction methods, and materials.
Geosynthetics have been used liberally throughout the 350-mile perimeter system. The Corps buried geotextile tubes in sand to harden dunes and protect coastal beaches. Geotextile fabric is woven into miles of earthen levees to increase strength.
Wick drains have been installed to achieve rapid soil consolidation in foundations of structures and in earthen levees. In one project, a dramatic combination of multiple geofabrics and wick drains were used to gain soil consolidation in just 60 days in a reach of levees that range from +18ft to +25ft in height and are 320–360ft wide.
The Corps and geosynthetics
It’s not as if the Corps had never used geosynthetic materials before.
“The first use of woven fabric as embankment reinforcement was 1975,” said Brian Baillie, an engineering manager for Huesker Inc.’s U.S. division. Since that time, the Corps has routinely used geosynthetics for “internal stabilization to help with failure planes and uniform settlement,” he said.
The Corps’ New Orleans District has been using geosynthetic materials to build earthen levees since the 1980s, said Walter Baumy, chief of the USACE/NOD engineering division. “We use geos to add tensile capacity to weak planes in soft soils. Using geos can mean a smaller footprint for the levee. That means less impact to infrastructure, less environmental impact, ability to work within existing rights of way, and less quantity of material. That results in substantial cost and time savings. That also means we can start construction sooner because it impacts surroundings less. Environmentals go quicker and less borrow material is needed to actually build that levee.”
Within the 350 miles of enhanced or newly constructed levees, “There are more jobs with geos than without,” Baumy said.
Prior to Katrina, geotextile fabrics were routinely considered for large levees, Varuso said. “If that levee was so big that there would be too much environmental impact, or right-of-way problems, then we would look at the use of geo.”
That process has not changed since Katrina, Varuso added: “The difference, after Katrina, is that the levees we are building are so much higher. Now that I’m going 20ft or more tall, and the footprints are getting so big, we are getting a larger benefit using geos.”
A 7.54-mile reach of levee in eastern New Orleans, known as LPV 109, includes probably the most fascinating concentration of geosynthetics—9 million linear feet of wick drains and 1 million yards of geotextiles—of any project in the system.
Geotechnical design engineers from San Francisco-based URS Corp. designed the layers of rock, sand blankets, geotextiles, and wick drains that evacuated moisture and helped the earthen levee gain strength and achieve desired consolidation within an unprecedented 45-60 days. “We were able to get 2,000 pounds-per-inch strength from those fabrics,” Varuso said.
The levee was built with a base of geotextile separator fabric, topped with a +2ft to +3ft elevation sand blanket. Wick drains—more accurately known as prefabricated vertical drains (PVD)—were installed next, then more separation fabric, an 8in.-thick layer of gravel, more separation fabric, and clay.
U.S. Wick Drain of Leland, N.C., used five stitchers—wick drain installing rigs—to punch almost 300,000 holes for the 9 million feet of PVD, said Mark Palmatier, owner and president. “Usually, machines can get about 20,000ft in a 12-hour day, but we had production days where we put in 100,000ft per day,” Palmatier said. “We were averaging about 80,000ft per day, with 65,000 being probably our lowest day.” The specialty contractor began May 2010 and finished in October 2010.
At the time, LPV 109 was the largest project for square footage and amount of wick holes ever done on a job in America, Palmatier said. (Now U.S. Wick Drain has moved on to an even larger job in Virginia.) Palmatier said that success on the $2.5 million New Orleans levee project put the company in position to pursue the $6 million Virginia job.
LPV 109 also called for three different types of geotextiles, said Andres Ramos, Archer Western’s assistant project manager.
The separator geotextile that was placed right on top of the stone layer is a high-strength, woven polyester fabric that Ramos described as “like a filter fabric, maybe a little sturdier.” The contractor placed nearly 1.5 million yds2 of this material, which was sold in 300ft-long bolts, 12ft wide. The rolls of separator fabric were light enough that two men could manage them.
Contractor Archer Western also used a thicker, high-strength, woven, polyester fabric made in Germany. “We placed that right on top of the stone, along the centerline of the levee,” Ramos said. “We used 231,341 square yards. The bolts were 16 feet by 328 feet.”
A third geotextile, also a woven polyester, was placed at elevation +7ft. This geofabric also came in bolts of 16ft × 328ft and Archer Western used 315,303 square yards of it.
For the heavier fabrics, design specs called for placement of 16ft × 80ft panels. “It was really difficult to cut, so we went through a lot of razor blades,” Ramos said.
Because these rolls weighed about 600lbs apiece, the contractor attached a spreader bar to a skid steer to place the fabric. “We would slide the bar down the core of it, pick it up, and two guys would hold one end while the skid steer would back up,” he said.
But a standard roll core is thick cardboard. “When you try to lift it up with a fork lift, it breaks, and trying to insert a spreader bar is nearly impossible,” Ramos said. “If I had to do it over again, I would order an upgraded core of PVC or steel. The cardboard cores appear to save money up front, but it [can] slow you down.”
Use of geosynthetics
URS had anticipated achieving soil consolidation and strength gain in 60-90 days. But by June 2011, instrumentation readings indicated the requisite 3-4ft of settlement was achieved in only 45-60 days.
“We expect to get another 1 to 1.5ft of settlement in the next nine years, which means we’ll still have the required flood protection height in 10 years,” said John Volk, URS’ lead geotechnical engineer for LPV 109.
Other than the huge amount of wick drain, no single element of the LPV 109 project was far afield from anything geosynthetics have proven to accomplish in many other projects throughout New Orleans and elsewhere. “They used a combination of well-known technologies to accomplish the goal,” said Baillie, the Huesker engineering manager. However, the project did call attention to the benefits of geosynthetics in embankment structures.
Additionally, projects in New Orleans demonstrated to the Corps the increased capabilities and strengths of geosynthetics. “These high-strength geosynthetics weren’t around in the ’60s (when the original hurricane system was being constructed),” Baillie said. “They didn’t gain popularity until the ’70s or ’80s, and those were typically lightweight, nonwoven materials. And today, due to research and development, there is more design guidance available to engineers who want to include these materials.”
USACE’s Varuso agrees that the material of yesteryear could not deliver the 2,000 pounds-per-inch strength that was achieved at LPV 109. “Manufacturing of these products is becoming state of the art and they are advancing the technology almost on a monthly basis,” he said. “Geos are becoming more popular and better understood in the geotech community.”
Further, at about $3,700 per linear foot, the cost of delivering LPV 109 with fabrics and wick drains was considerably less than other processes used to build huge levees in New Orleans fast, such as deep soil mixing ($12,000) and concrete T-walls ($10,000–$15,000), Volk said.
Geosynthetics are being considered as the Corps explores options to raise a 32-mile stretch of non-federal levees in New Orleans’ Plaquemines Parish. “We are trying to make sure Plaquemines is getting the biggest bang for their buck, so we are going to implement as many techniques as possible,” Varuso said.
Other Corps districts from across the country have visited New Orleans to see the materials and methods deployed there during the past five years, Baumy said. “We’ve learned a lot over the years. Designs and specifications have improved. Using materials like [geosynthetics] is becoming common knowledge.”
Applying that knowledge moving forward will help the Corps and the geosynthetics industry reduce risk for the people of New Orleans and elsewhere around the world.