By Angelle Bergeron
Geosynthetic materials have figured prominently in providing New Orleans with the best flood protection the city has ever had.
“We couldn’t have delivered this work by 2011 without geosynthetics,” says Richard Varuso, deputy chief of the geotechnical branch of the U.S. Army Corps of Engineers’ New Orleans District (USACE/NOD).
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 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 these 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 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, and 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 18–25ft in height and are 320–360 feet 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 in 1975,” says 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 says.
The Corps’ New Orleans District has been using geosynthetic materials to build earthen levees since the 1980s, says 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 says.
Prior to Katrina, geotextile fabrics were routinely considered for large levees, Varuso says. “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 20 feet or more tall, and the footprints are getting so big, we are getting a larger benefit using geos.”
Millions of feet of geotextiles
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 says.
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, says Mark Palmatier, owner and president. “Usually, machines can get about 20,000 feet in a 12-hour day, but we had production days where we put in 100,000 feet per day,” Palmatier says. At the time, LPV 109 was the largest project for square footage and number of wick holes ever done on a job in the U.S., according to Palmatier.
LPV 109 also called for three different types of geotextiles, says 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 square yards of this material; the rolls were light enough that two men could manage them.
Contractor Archer Western also used 231,341 square yards of a thicker, high-strength, woven polyester fabric, and a third geotextile—also a woven polyester—was placed at elevation +7 feet.
For the heavier fabrics, design specs called for placement of 16-foot-by-80-foot panels. Because these rolls weighed about 600 pounds apiece, the contractor attached a spreader bar to a skid steer to place the fabric.
Benefits of geosynthetics
URS had anticipated achieving soil consolidation and strength gain in 60–90 days, but by June 2011, instrumentation readings indicated the requisite 3–4 feet of settlement was achieved in only 45–60 days.
“We expect to get another 1–1.5 feet of settlement in the next nine years, which means we’ll still have the required flood protection height in 10 years,” says 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,” says 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 says. “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 yesterday’s material 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 says. “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 says.
Geosynthetics are being considered as the Corps explores options to raise a 32-mile stretch of non-federal levees in New Orleans’ Plaquemines Parish, and 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 says. “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.