Geotextiles in levees (Part 2 of 2)

History, performance, and design of geotextiles in levees: A report from New Orleans

By Jody Dendurent and Mark Woodward

Editor’s note: Part 1 of this 2-part series ran in the April/May issue of Geosynthetics. Following the Abstract and Introduction below, Part 2 concludes with sections 3, 4, and 5 of this manuscript.

Abstract

Geotextiles are a key element in building levees that will survive catastrophic storms. The geotextile-reinforced earthen levees in New Orleans performed remarkably well during Hurricane Katrina in August 2005. With the current focus on updating its levee system to protect New Orleans from a 100-year storm event, the U.S. Army Corps of Engineers (USACE) may rely heavily on geotextiles in its designs.

In addition to reinforcing levees and allowing existing levees to be built taller and more robust, geotextiles help reduce construction costs and reduce the size of or even eliminate stability berms. Geotextiles are also used in innovative ways for recovery and immediate repair of damaged levees.

This article focuses on the history, performance, and future use of geotextiles in levees. It will include a review of literature and design methods, an examination of the performance of the geotextile-reinforced levees that survived Hurricane Katrina, and USACE design improvements.

Introduction

Most of the engineering focus in the aftermath of Hurricane Katrina has been on the failed levees, the causes of failure, and how to improve levee design so we do not experience similar calamities in the future.

Little attention from the media, technical investigators, and the general public has been focused on the levees that successfully withstood the similar storm surges and conditions as the levees that failed. For example, 9mi. (14.5km) of the St. Charles Levee and 7mi. (11.3km) of the Jefferson Lakefront levee on the south shore of Lake Pontchartrain are reinforced with geosynthetics (Figure 1).

In a press release from the Industrial Fabrics Association International (IFAI) in April 2008, the USACE-New Orleans District (NOD) stated that “both the St. Charles and Jefferson levees were loaded (filled by the storm) during Katrina and performed exceptionally. They were stable and the geosynthetic was inherent to their strength” (Aho, 2008a).

USACE engineers have been among the pioneers in levee design who saw the merits of using high-strength geotextiles to improve the stability of the levees and in the process save billions of dollars in construction costs, thousands of acres in land, and years of construction time. USACE has continuously been at the forefront of innovation in the design of geosynthetic-reinforced embankments and levees on soft soils. As the USACE works to update the New Orleans levee system to withstand future storm events, it will continue using geotextiles as part of the design methods, and it is still partnering with industry leaders to improve public safety and to refine the design of reinforced levees.

3. Performance of geotextile-reinforcedlevees in Hurricane Katrina

As stated in the introduction, the geotextile-reinforced levees all performed exceptionally well through Hurricane Katrina

Both the Jefferson Parish Lakefront and the St. Charles Parish levees were inundated with storm surges but were not breached, while other parts of the greater New Orleans storm and damage reduction system failed. The only breach in a geotextile-reinforced levee was in the Plaquemines Parish “Reach A” levee due to erosion from overtopping and close proximity of a perpendicular canal. The failure was not attributed to a bearing capacity, slope stability, or geotextile failure, and it is possible that the presence of the geotextile layer prevented deeper scour.

3.1 Plaquemines Parish Levee

3.1.1 Design

The levee section known as Reach A begins south of the city of New orleans at the Buras Levee District MR&T Mainline Levee near City Price, La. It extends to the B-1 hurricane levee in the vicinity of Tropical Bend (Figure 2).

The levee height ranges from elevation 11.0ft (3.4m) North American Vertical Datum (NAVD) to 14.5ft (4.4m) NAVD. The levee was built in two lifts on top of soils with cohesions as low as 150psf. The typical levee enlargement cross section for Reach A consisted of a marsh side embankment with a wave berm. Floodwalls were constructed at the pumping stations.

The base of the levee was constructed on one or two separated layers of geotextiles of varying strengths that were anchored into the existing levee. A sand blanket was placed on the geotextile and covered with at least 2ft (0.6m) of clay. The additional layer of geotextile was used at structural locations, such as pipelines, where a 1.5 global stability factor of safety was required by USACE-NOD. The geotextile tensile strengths (at 5% strain) ranged from 1,070 lbs/in. (187 kN/m) to 2,420 lbs/in. (424 kN/m) on the protected side and 140 lbs/in. (25 kN/m) to 1,860 lbs/in. (326 kN/m) on the flood side of the levee. Figure 3 shows a typical geotextile-reinforced earthen levee section for Reach A (USACE, 1987a).

3.1.2 Performance in Hurricane Katrina

During Hurricane Katrina along Reach A, there was movement of the transition walls between the reinforced levee and the Hayes and Gainard Woods pump stations and a breach at Nairn, La., at a floodwall that allowed a pipeline penetration. The only area where USACE- NOD had to replace a section of geotextile was at Homeplace, La.

The hurricane protection levee at Homeplace was significantly scoured during Hurricane Katrina and parish workers tenuously reconstructed the section shortly afterward. The levee section was damaged further after Hurricane Rita and the levee crown and slopes were replaced using material from the adjacent levee crown.

The as-built, geotextile-reinforced levee system was damaged in the area of the scour. In addition, the compaction and moisture control during the initial repairs were questionable and therefore the stability of the section was of concern. The permanent repair of the levee section entailed degrading the levee section on either side of the scoured area to the elevation of the geotextile. The geotextile was then replaced for that entire degraded section and the full levee embankment was reconstructed to include stability and wave berms.

It is the opinion of USACE-NOD that the geotextile did not fail to serve its purpose. The scour down to the geotextile area (which was overtopped) was adjacent to a flood-side perpendicular canal. It is possible that the geotextile prevented scour beneath the base of the levee.

3.2 St. Charles Parish Levee north of Airline Highway

3.2.1 Design (USACE, 1989)

The St. Charles Parish Levee north of Airline Highway levee is located in St. Charles Parish on the east bank of the Mississippi River (Figure 1). It separates approximately 26,000 acres (10,522 ha) of wetlands from the developed areas of St. Charles Parish.

The geotextile-reinforced levee was constructed of semicompacted haul clay fill on a sand bed and reinforced by one layer of high strength geotextile (varying from 300 lbs/in. ( 53 kN/m) to 700 lbs/in. (123 kN/m) at 5% strain). The net levee grade elevation varies from 12.0ft (3.7m) NAVD to 13.0ft (4.0m) NAVD. The levee was built in two lifts over 15 years.

The design of the St. Charles Levee utilized the results and advancements of the USACE-NOD test sections. The original levee design was revised to reflect knowledge gained from the Bonnet Carre test section, which demonstrated that the initial design was conservative. The new design accounted for foundation shear strength gain during construction. Shear strength testing after subsequent lift construction of the mainline levee validated assumed shear strength gains. Figure 4 shows a typical design section of this levee.

3.2.2 Performance in Hurricane Katrina

The St. Charles Parish Levee experienced a storm surge during Hurricane Katrina lower than the surges along the south shore of Lake Pontchartrain where other types of levees failed. The St. Charles Parish Levee was not breached or damaged during Hurricane Katrina.

3.2.3 Future

The St. Charles Parish Levee is currently being raised to elevation +14ft (4.3m) NAVD. A straddle enlargement is planned to raise the protection to elevation +16.5ft (5.0m) NAVD mainly through the use of stability berms. The tensile strength of the existing geotextile is typically not high to provide additional benefit in the enlarged section or failure planes circumvent the existing geotextile.

3.3 Jefferson Parish Lakefront Levee

3.3.1 Design (USACE, 1987b)

The Jefferson Parish Lakefront Levee is located on the east bank of the Mississippi River. It is approximately 10.4 miles (16.4 km) in length and is bounded on the north by Lake Pontchartrain.

The geotextile-reinforced portion of the levee is an enlargement of 9.4 miles (15.1 km) of the existing earthen levee to raise it to elevation +17 ft (5.2 m) NAVD. The high-strength geotextile (1,000 lbs/in (175 kN/m) to 2,010 lbs/in (352 kN/m) at 5% strain) was used to reinforce the soil foundation so that the levee could be brought to grade and section using the minimum amount of fill and no additional rights-of-way.

The existing levee was degraded, the geotextile was installed, and the levee was built up to final grade. Figure 5 shows a typical section of the Jefferson Parish Lakefront Levee.

3.3.2 Performance in Hurricane Katrina

The Jefferson Parish Lakefront levee experienced a surge during Hurricane Katrina similar to the surges along the eastern south shore of Lake Pontchartrain where other types of levees did fail. The Jefferson Parish Lakefront levee was not breached or damaged during Hurricane Katrina.

3.3.3 Future

The Jefferson Parish Lakefront levee is currently being analyzed hoping that the existing geotextile will still provide some benefit and the levee can be raised using a straddle enlargement. An alternative under consideration is to move the levee centerline slightly towards Lake Pontchartrain.

3.4 Performances in hurricanes Gustav and Ike

Damage assessments made after hurricanes Gustav and Ike revealed no damage to the Reach A levee even though both storms loaded the entire 13-mile (4.0-km) stretch of levee.

Damage assessments of both the St. Charles Parish and Jefferson Lakefront geosynthetic reinforced levees also revealed no damage to either levee subsequent to loading from both hurricanes Gustav and Ike.

4. Future design and use of geotextiles in levees

In the USACE-NOD’s efforts to bring the Greater New Orleans Hurricane and Storm Damage Risk Reduction System to design storm event flood elevations, geotextiles may be incorporated into many of the enlarged and new levees to strengthen designs, reduce berm widths, save on fill material and subsequent lifts, reduce vertical settlement, and reduce the amount of real estate needed for the levee

4.1 Existing design methods

The design methodology used by the USACE-NOD for geotextile-reinforced levees analyzes several modes of failure including inadequate bearing capacity, slope stability failure, inadequate embedment/anchorage length of the geotextile, lateral embankment sliding/spreading, creep of geotextile, service life of geotextile, and inadequate seam strength and/or field overlap requirement of the geotextile.

The design methodology also outlines potential for foundation strength gains and use of noncircular slope stability failure surfaces using limit equilibrium methods. These design improvements have not yet been incorporated into published USACE or UFC manuals. Numerical analysis of geotextile-reinforced sections and strength reduction over the time and life of geotextiles are being researched for future inclusion into design criteria.

4.2 Raising levees already reinforced with geotextiles

The basic alternatives for enlarging existing geotextile-reinforced levees include:

• degrading the existing levee down to its base, installing a new geotextile, and rebuilding the levee to the design elevation. Due to construction durations and a June to December hurricane season, USACE-NOD prefers not to degrade long stretches of existing levee protection, thus limiting this option.
• building directly on top of the existing levee with long berms for stability in lieu of a new geotextile .
• building a new levee on a different alignment.

4.3 Raising unreinforced levees with geotextiles

There are two viable scenarios for raising the elevation of existing unreinforced earthen levees using geotextiles without degrading the existing levee.

Both alternatives are new reinforced levees constructed on the protected side of the existing levees, one behind an existing unreinforced earthen levee and the other behind an I-wall levee (Figure 6).

4.4 Design improvements

As discussed throughout this article, USACE-NOD has instituted the following design improvements for geotextile-reinforced levees:

• setting higher factors of safety.
• incorporating (or quantifying) the foundation shear strength gain during construction.
• evaluation of more complex failure surfaces than previously used wedge or circular failure surfaces.
• designing the geotextile for the ultimate elevation of that levee and the loading required in the year 2057.
• setting an upper limit on the permissible geotextile tensile strength consistent with standard geotextile products

4.5 Construction innovations

There have not been any major construction innovations on geotextile reinforcement placement as a result of Hurricane Katrina. USACE-NOD continues to specify and enforce strict construction standards to ensure that the benefits of the geotextiles are fully realized.

4.6 Research

The USACE and the geosynthetic industry have identified the following areas of research and testing as priorities that are relevant to geotextile-reinforced levee design and performance:

• long-term design properties of geotextiles used in levees.
• measurement of settlement of geotextile-reinforced levees.
• measurement of lateral spreading of the base of geotextile-reinforced levees.
• a design methodology accounting for foundation shear strength gain during construction in geotextile-reinforced levees.
• numerical analysis and field verification of the actual failure modes and failure surfaces of geotextile-reinforced levees.
• the use of geotextiles for prevention of piping damage.
• the use of geotextiles for scour protection.
• the use of geotextile tubes in levees.

Language authorizing the USACE Engineering Research and Development Center (ERDC) to conduct studies, testing, and demonstration in some of these areas has been submitted in the Water Resources Development Act 2008 in the United States Congress (Aho, 2008b).

5. Conclusion

Sprague et al. (1993) summarize the evolution of basic soft-soil embankment design and significant projects that demonstrated the acceptance of and major lessons learned from the use of high strength, high modulus geotextiles in this application to that time.

Since then, the fundamental design approach has not changed significantly, but USACE-NOD has refined and advanced the design and state-of-practice of geotextile-reinforced levees considerably. These advancements could be beneficial to other entities (public and private) involved in levee design and embankment over soft-soil foundation applications.

The substantial influence of the USACE-NOD test section monitoring results analysis and the performance of the geotextile-reinforced levees through hurricanes Katrina, Gustav, and Ike, as well as the lessons learned from Katrina, provide further evidence that geotextiles have greatly contributed in sustaining our infrastructure and protecting the public.

Jody L. Dendurent, P.E., TenCate Geosynthetics, Wichita, Kansas and Mark L. Woodward, P.E., New Orleans District, U.S. Army Corps of Engineers

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