Fabric reinforced embankment test section, plaquemine Parish, Louisiana, USA

Geotextiles and Geomembranes 6 (1987) 1-31

Fabric Reinforced Embankment Test Section, Plaquemine Parish, Louisiana, USA

Jack Fowler and Earl V. Edris Jr

Geotechnical Laboratory, Waterways Experiment Station, Corps of Engineers, PO Box 631, Vicksburg, Mississippi 39180-0631, USA

ABSTRACT

This paper describes the results of an instrumented high strength woven geotextile fabric reinforced levee test section that was designed and constructed utilizing some of the latest procedures practiced by the US Army Corps of Engineers. The levee test section was instrumented with settlement plates, slope inclinometers, and piezometers. Resistive potentiometers and strain gages were attached to the embedded fabric. The purpose of the test section was to determine the technical feasibility, construction practicality, and the economic feasibility of geotextile reinforced levees for future projects in the Corps of Engineers New Orleans District involving very soft, highly compressible foundations. The strains and loads measured on the fabric were within the range of values estimated during design. The first six months of the test section's performance has been successful. From the test section results, it is anticipated that significant cost savings will be realized when reinforced levees are utilized.

1 INTRODUCTION

This study was initiated as a result of Value Engineering Proposal 85-041 (VES) prepared by the New Orleans District Foundation and Materials Branch to investigate the potential use of high strength geotextiles for reinforcement of a hurricane protection levee to be constructed over a soft foundation. Several conventional construction techniques were considered and evaluated as to their constructability and economic feasibility. Environ- mental concerns and the time necessary to complete the levee on soft clay

1 Geotextiles and Geomembranes 0266-1144187/$03.50 © 1987, Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

2 Jack Fowler, Earl V. Edris Jr

deposits were a major consideration. It was felt that a fabric reinforced levee would have the least impact on the environment because it required the smallest area of wetland. Also, the reinforced levee would require the least amount of fill material for construction, and would be more economical to construct. Because this construction technique does not require staged construction it would also require the least amount of time to construct.

The location selected for the proposed 122 m (400 ft) long fabric reinforced test section was that portion of Reach A between Homeplace and Tropical Bend of the New Orleans to Venice, Louisiana, Hurricane Protec- tion Project, shown in Fig. 1. It was proposed to raise the existing levee cross section from elevation +2.4 m (+ 8.0 ft) NGVD (National Geodetic Vertical Datum) to a proposed elevation of +4-4 m (+ 14.5 ft) NGVD as shown in Fig. 2. The Hurricane Protection Project is very extensive, covering several parishes and hundreds of miles along the Louisiana coast and many tribu- taries leading into the Gulf of Mexico. Fabric reinforcement of levees is an important concept because the levee height requirement increases the closer the levees are constructed to the Gulf and the foundation conditions tend to get more difficult.

Reach A was selected for the fabric reinforced test section because to meet the stability criteria for levees constructed on soft foundations, large stability berms were required. Wetland environmental constraints would not allow large wide berms or displacement mud waves. It also presented an opportunity to resolve an impasse concerning existing construction techniques that were considered by some to be too expensive. This reach also presented an opportunity to conduct an instrumented test section. Finally, the project personnel had the desire and interest in new and innovative design and construction techniques which have been under consideration for a long time by District personnel.

This paper describes the design and construction methodology, and the instrument-recorded test results for a levee constructed using high performance geotextiles for reinforcement.

2 PRIOR CONSIDERATIONS

2.1 Conventional design and construction techniques

In the past, conventional design and construction techniques used tot soft soils in the New Orleans District have required the use of large berms, removal of the soft foundation soils and/or displacement sections. The topography in the New Orleans to Venice area is very fiat and swampy, and consists of very compressible peaty soils on the surface with soft clays at

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varying depths. Because the terrain is very flat, sources of suitable fill material for levee construction are limited.

Clay fill material is generally considered to be ideal for levee construction. When these materials are located they are very wet and soft, and generally have to be processed and dried out. Because these materials are limited and sparsely located, they are quite often barge- or truck-hauled long distances. Since these fill materials are very expensive, hydraulically dredged sand

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from the Mississippi River is used in the center core of the levee with a 1.2 m to 1.5 m (4 ft to 5 ft) layer of clay covering the unprotected surfaces.

Sands can be dredged from the Mississippi River by various means and hauled or pumped very long distances. They are much cheaper in place than the in place cost of fine grained soils. The use of the sand material is normally maximized to reduce the cost.

2.2 Value Engineering Study

The VES was intended to investigate the potential savings in cost and construction time for a high strength geotextile and/or geogrid reinforced levee versus a levee constructed by conventional methods. The use of high strength geotextiles or geogrids would provide the necessary tensile strength required to minimize lateral spreading, sliding and subsidence of the levee. They also reduce or eliminate the need for stability berms, therefore minimizing fill and borrow material requirements. These qualities are especially useful for new levees constructed on very soft and compressible foundations. 2 The use of the materials also reduces the environmental impact (i.e. wetland loss or damage) associated with wide berms, large displacement mud waves, large borrow, and haul fill operations. The details of the various proposals are presented in the VES study I and summarized by Fowler et al. 3

Some of the benefits or advantages of a fabric reinforced levee are that it could be completed in a period of 6 years or less instead of the planned 13.5 years. The environmental concern is reduced as only 40.5 ha (100 acres) of marsh would be destroyed by construction of dredged material containment areas instead of 1618 ha (4000 acres). Also, this allows the flexibility for raising of the levee to intermediate elevations over a period of time.

2.3 High performance geotextile case histories

In the late 1970s, the Waterways Experiment Station, US Army Corps of Engineers began experimenting with geotextiles placed over soft dredged soil to support dredged material containment dikes. 2 Pinto Pass at Mobile, Alabama 4 was one of the first projects designed and constructed by the US Army Corps of Engineers that used high performance geotextiles in this country. Ultimate fabric strengths were 158 kN/m-width (900 lb/in-width) in the fabric warp or machine or long direction and 44 kN/m-width (250 lb/ in-width) in the weft or cross machine direction. This project set a precedent for a number of embankments that were stabilized with the high performance geotextile warp direction oriented perpendicular to the longitudinal direction of the dike. Fabric rolls were placed the full length of the dike width without the use of seams. Seams were required in the weft


direction or along the selvedge edge of each fabric roll which corresponds to the minor or intermediate principal stress direction. Techniques for joining high performance geotextile fabrics in the weft direction are still in the initial stages of development. While seam strengths were about 35 kN/m-width (200 lb/in-width) at Pinto Pass, subsequent projects such as the Wilmington, Delaware dredged material disposal area required seam strengths of 140 kN/m-width (800 lb/in-width) in the weft direction, s Several dredged material disposal projects such as: Craney Island disposal area, Norfolk District; 6 Blakeney Island disposal areas and Greenwood disposal area, Mobile District; Brunswick disposal area, Savannah District; private industrial projects, and projects by other government agencies have followed the lead of these Corps of Engineers projects.

Some of the more recent projects have required stabilization of large areas of soft materials. Washington National Airport (Washington, DC) extension was one of the first in which the width was about equal to the length, 183 m by 213 m (600 ft by 700 ft). This project was constructed in water that varied in depth of about 0.6 m to 3.6 m (2 ft to 12 ft). 7 The Maryland Port Administration's Seagirt stabilization project 8 consisted of covering over 40.5 ha (100 acres) of very soft dredged material with a high performance geotextile fabric and driving over 0.9 million linear meters (3 million feet) of plastic strip drains through a 0-9 m (3 ft) deep sand covering and the fabric. Seams strengths have exceeded 140 kN/m-width (800 lb/in- width) in the field under very adverse conditions. There are unpublished reports from Atlantic Thread and Supply Company, Baltimore, Maryland, and from Great Britain of seam strengths exceeding 175 kN/m-width (1000 lb/in-width) for conditions where the fabrics require equal strengths in both directions or cases where the fabric is required to be launched from floating barges onto very soft marine sediments in tidal water depths of about 4.6 m (15 ft).

Although the geotextile projects tend to concentrate more on the fabric's ultimate strength and elongation at failure, the Corps of Engineers, Huntington District project at Mohicanville, Ohio, emphasized the importance and requirement of a high modulus for embankment reinforcement. 9 This value is normally expressed as the secant modulus at a specified strain value that corresponds to the approximate ultimate strain of the soil at failure which is generally about 3 to 5 percent. Therefore, most high performance geotextiles are specified at a strain value of about 4 percent. The Mohicanville project, however, required a strain value of about one percent which dictated the required modulus of the reinforcement. Lateral spreading of the embankment base was specified to be no less than about 30 to 61 cm (1 to 2 ft). Therefore, steel wire rather than the very extendable plastic fabrics was used to reinforce the embankment and to meet the


foundation design requirements that control excessive lateral spreading of the embankment base.

3 FOUNDATION INVESTIGATION

3.1 Site location and description

A narrow strip of land at a slightly higher elevation generally exists along the edge of the Mississippi River that is the result of a natural levee caused by flooding and deposition of coarse grain particles. In some areas this strip of land is very wide but the terrain generally consists of swales and small ridges. Historically these higher areas are developed (inhabited) and parallel levees are constructed to either side to protect them from potential flooding by the river and high tides, and tidal waves caused by hurricanes from the Gulf of Mexico.

The existing levee was constructed by incremental raising of the crest elevation as needed and when money became available. Borrow material for the original levee was excavated adjacent and parallel to the levee toe. This borrow ditch now serves as a ponding area during heavy rainfall which is collected and periodically pumped out. The borrow materials consisted of very soft wet organic clay and silt soils that were dragline cast in place during levee construction. Soil borings during this investigation indicated that these materials are still very soft and wet and have not appeared to have dried out or consolidated much with depth.

3.2 Foundation material properties

The New Orleans District conducted a subsurface foundation investigation in the area of the proposed test section. This investigation consisted of pushing two fixed piston samplers to depths of about 15 m (50 ft) to obtain Shelby tube undisturbed samples for laboratory testing and classification. One of these borings was drilled about 1.2 m to 1.5 m (4 to 5 ft) from the existing levee centerline toward the Gulf side. The other one was drilled about 19.2 m (63 ft) toward the Gulf side near the existing levee toe. This boring is close to the centerline of the new levee. Figure 3 shows a plan view of the location of the soil borings in the test section. Conventional laboratory triaxial shear strength tests were conducted on several samples from various depths.

Moisture contents for the silty and clayey materials near the surface varied from about 27 to 44 percent; below that depth, the water contents of these materials varied from about 50 to 107 percent. There were occasional

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thin layers of materials where the moisture contents were very low, varying f rom 25 to 34 percent and very wet layers that exhibited moisture contents beyond 100 and as high as 285 percent. Many of these layers had thin silt and sand strata or lenses. These layers were classified as highly plastic clays and low plasticity silt with organics on the surface. The consistency for most of these materials varied from medium to soft. There were some port ions of the borings where samples were so soft that they were lost.

Unconso l ida ted undrained and consolidated undrained triaxial shear tests conduc ted in the laboratory indicated that the undrained shear strength for materials under the proposed levee centerline from elevation +0.6 m to - 2 . 1 m (+2 ft to - 7 ft) was about 7-2 kPa (150 psf). The undrained shear s t rength for materials from -2 .1 m to - 7 . 6 m ( - 7 ft to - 2 5 ft) was about 14.4 kPa (300 psf) and for materials below this depth was about 21.6 kPa (450 psf). Shear strengths determined for materials located beneath the existing dike were considered to be about twice those found at the levee toe due to consolidat ion from the existing levee. Shear strength of materials in the existing dike was considered to be 19.2 kPa (400 psf) and for the materials in the newly constructed dike section, 9.6 kPa (200 psf). These shear s t rength values are depicted in Fig. 4.

4 F A B R I C R E I N F O R C E D TEST SECTION D E S I G N

4.1 Proposed geotextile reinforced levee test section

The p roposed geotextile reinforced levee design incorporated part of the existing levee into the final configuration. The existing levee crest was lowered to + 1.5 m (+5 ft) elevation. This material was temporarily stored on the inside face of the levee. The geotextile was placed on this cut surface and out into the marsh on the Gulf side. However , the test section was re located and the fabric was placed on an area with approximately half of the surface covered by marsh grass and the other half with ponded water that varied in dep th from 30 cm to 91 cm (1 ft to 3 ft). This relocation of the test section provided information as to the difficulty of placing fabric on these types of surface. The design also included a 1.2 m (4 ft) lift of sand over the geotexti le to minimize the amount of clay fill. Once a foot of sand was placed on the fabric, the fabric was folded back at the ends a min imum of 1.8 m (6 ft) at the inside and 2.4 m (8 ft) at the outside levee toe to provide an anchor to prevent fabric pullout. The 1.2 m (4 ft) sand layer was covered with clay to complete the levee cross section making sure that the outside toe had a m in imum of 1.5 m (5 ft) of cover to reduce underseepage. The fill mater ial was t rack-compacted with a dozer.

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4.2 Design and construction considerations

To successfully design a levee on very soft foundation, three potential failure modes must be investigated:

(a) Horizontal sliding, splitting, or spreading. (b) Rotational slope and/or foundation failure. (c) Excessive vertical foundation displacement.

The fabric must resist the unbalanced forces necessary for dike stability, and must develop high tensile forces at relatively low strains. Sufficient soil- fabric friction to prevent pullout must be available. The fabric's tensile forces resist the unbalanced forces, and its tensile modulus controls the vertical and horizontal displacement of the levee foundation. Adequate development of soil-fabric friction allows the transfer of dike load to the fabric. Use of the proper construction sequence to develop fabric tensile stresses at small fabric elongations or strains is essential. Figure 5 illustrates the potential fabric reinforced embankment failure modes.

Limit equilibrium analyses are recommended for design of geotextile reinforced embankments.~° The following stability calculations were performed for the proposed levee:

(a) Bearing capacity. (b) Foundation squeeze. (c) Circular and noncircular slope stability analyses. (d) Pullout analysis. (e) Lateral sliding, splitting and spreading analysis. (f) Fabric fill and seam strength requirements. (g) Fabric elongation from levee subsidence.

Even though this project is experimental, a localized failure could not be allowed to occur. Therefore, a factor of safety of 1.3 was chosen against rotational slope failure or wedge failure, 1.5 against spreading/splitting, 2.0 against sliding failure, and 1.3 against excessive rotational displacement for the geotextile fabric requirements.

4.3 Geotextile reinforced levee design

The overall bearing capacity safety factor calculations necessary for the geotextile reinforced levee in Reach A are described in the FHWA manual.~l Using the conventional Terzaghi equation for the ultimate bearing capacity, quit, of a strip footing on clay, with cohesion values of 9.6 kPa (200 psf) for a surface crust and 7.2 kPa (150 psf) without the crust, the quit values of 49.2 kPa (1028 psf) and 25.1 kPa (525 psf) were calculated.


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Fig. 5. Potential fabric reinforced embankment failure modes.

It has been found by experience that excessive mud wave formation is minimized when a dried crust or root mat is present on the ground surface. 4 With an embankment unit weight of 1.6 g/cm 3 (100 pcf) and a centerline height of 3.8 m (12.5 ft), the maximum applied stress under the levee centerline is 59.8 kPa (1250 psf). The factor of safety was determined for condit ions with and without crust or root mat on the surface. It was obvious that the bearing capacity was not sufficient for an unreinforced embank-


ment. For a geotextile reinforced embankment, the lower portion of its base will act like a mat foundation, thereby distributing the load somewhat uniformly over the entire embankment width. The average vertical applied stress of the 'mat' on the soft marsh soil was determined to be about 32.1 kPa (671 psf) based on a 15.2 m (50 ft) length of levee side slope and a 2.4 m (8 ft) levee crest width. Where enough dried crust or root mat exist to support a man, the factor of safety is about 1.2 but in areas where there is no crust the factor of safety is less than one (about 0.8). In these areas a certain degree of mud waving or foundation displacement can be expected during construction. However, in those areas where a crust or root mat is not available, bearing capacity may still not be a problem because of the wide embankment and the thin soft foundation clay layer at a shallow depth. It is unlikely that a complete bearing capacity failure will occur because this type of analysis assumes a uniform soil strength that extends to a depth of twice the embankment width.

A second bearing consideration is the possibility of the soft foundation material squeezing out to either side of the levee. A significant volume of soft material can move into the marsh or the drainage canal without any visible sign of disturbance. This disturbance may be detected if the surface areas adjacent to the levee toes are carefully surveyed. To analyze this situation, the active lateral forces at the levee toe generated by the embankment load are compared with the resisting passive forces and soil shear strength. 10

The cohesion required to prevent plastic flow or overall squeeze such as the squeeze between two plates is a function of the average vertical stress, the layer thickness, and the levee width (Jurgenson, 1934 as referenced by Ref. 11). The product of the average vertical stress and layer thickness are divided by the levee width to obtain the required cohesion. For the Reach A configuration, a cohesion of 1.8 kPa (37 psf) is required. The available cohesion is 7.2 kPa (150 psf) under the existing levee toe and 14.4 kPa (300 psf) under the existing levee crest, therefore this condition is satis- factory because it is greater than the 1-8 kPa (37 psf) required.

Foundat ion squeezing occurs when the passive resistance of the free field just beyond the levee toe is exceeded by the active earth pressure generated by the movements of the embankment. The difference between these two forces reduces to four times the cohesion minus the average vertical stress. H For Reach A, a shear strength of about 8.0 kPa (168 psf) is needed before the passive resistance begins to exceed the active pressure. The calculated values are - 3 . 4 kPa ( -71 psf) for the cohesion value without the surface crust and 6.2 kPa (129 psf) for the cohesion value with the surface crust. In areas where a crust or root mat exists (shear strength above 8.0 kPa), foundat ion squeeze is not a problem. However, in those areas where the


crust or root mat does not exist, varying degrees of foundation squeeze or displacement can be expected depending on the available foundation strength.

The drag force generated by the foundation squeeze under the fabric needs to be calculated. This force is determined by multiplying the distance from the levee toe on the marsh side to the existing levee by the largest amount that the passive resistance exceeds the active pressure. Using a distance of 15.2 m (50 ft) and a force difference of 3.4 kPa (71 psf), the drag force is calculated to be about 51-8 kN/m-width (3550 lb ft-width or 296 lb/in-width).

Slope stability analyses were performed as part of this study to determine the fabric tensile strength required to provide a factor of safety of 1.3 against rotational slope failure. The force equilibrium procedure with the Corps of Engineers modified Swedish side force assumption of parallel side forces and Spencer's procedure were used for these analyses. The side force inclination of 0 ° was used for the force equilibrium procedure. The Spencer method satisfies both force and moment equilibrium by assuming that the side force inclination is a constant in the safety factor calculations. The details of these procedures can be found in Refs 12, 13, and 14. Both circular and noncircular shear surfaces were evaluated in the analysis. Even though the rotational stability analysis assumes that ultimate fabric tensile strength will occur instantly to resist the active moment, some fabric strain, and consequently embankment displacement, will be necessary to develop tensile stress in the geotextile. The amount of movement required within the levee will be limited by the use of high tensile modulus fabric that exhibits good soil-fabric frictional properties. Conventional slope stability analysis assumes that the fabric reinforcement acts as a horizontal force to increase the resisting moment.

The levee cross section shown in Fig. 4 was used for this evaluation. To account for the higher shear strength under the existing levee, each soil layer was modeled as two soils with an increase in strength under the existing levee. This modeling technique results in a strength discontinuity at the vertical interface of the soil layers under the levee. Because only the cohesion values are varied, the effects on the critical shear surfaces of this modeling technique are minimized. The phreatic surface was assumed horizontal at elevation -0 .3 m ( - 1 ft). Shear surfaces contain an active and passive zone, however the geotextile is modeled as only carrying tension in the active zone.

The steps of the stability analysis are summarized herein with the details contained in Fowler e t al. 3 The first step in the analysis is to determine the critical circle without geotextile reinforcement. The offset distance value of the critical circle for this search is the boundary between the active and


passive zone. The safety factors from the searches without geotextile reinforcement ranged from 0.88 to 1.14 depending on the procedure used and the type of shear surface. The offset distance from the centerline of the old levee to the center point of the circular analysis was 2.4 m (8 ft) toward the canal side.

The next step was to add an assumed fabric reinforcement strength to the analysis. The computer program allows the reinforcement to be specified as straight line segments with a given tension. From the initial analysis and the distance required to develop the necessary pullout resistance, the zone of tension was established. The tension force in the reinforcement was varied until the safety factor for the critical shear surfaces was 1.3. The shear surfaces from the previous analysis were evaluated for three reinforcement tensions (140 kN/m-width (800 lbs/in-width), 210 kN/m-width (1200 lbs/in- width), and 298 kN/m-width (17001bs/in-width)). The 210 kN/m-width (1200 lbs/in-width) tension generated safety factors ranging from 1.35 to 1.58. The tension of 140 kN/m-width (800 Ibs/in-width) produced a safety factor range of 1.15 to 1.35. The safety factors for the 298 kN/m-width (1700 lbs/in-width) were all above 1.58. Both circular and wedge searches were performed using a tension of 210 kN/m-width (1200 lbs/in-width). The critical shear surfaces are shown in Fig. 4. A reinforcement tension force of 210 kN/m-width (14 400 lbs/ft-width, or 1200 lbs/in-width)was necessary to increase the safety factor to about 1.3 or above. For all the circular and noncircular critical shear surfaces generated by both analysis methods, several safety factors were slightly below 1.3 with the lowest being 1-27. While these are below the desired value, the significance of the values does not warrant more refined analyses with slightly increased fabric tensions.

Pullout resistance of the reinforcement at the intersection of the potential failure plane surface was determined by calculating the soil-fabric resistance and necessary fabric embedment length. 15 The intersection of the geotextile and the failure plane for the various shear surfaces varied from an offset distance of 7.3 to 13.4 m (24 to 44 ft) towards the gulf side. There are two components to fabric pullout resistance, one below and one above the fabric. Resistance below the fabric is based on the cohesion of the marsh material which for this example is 7.2 kPa (150 psf). Resistance above the fabric is determined by the average height of fill above the fabric in the affected areas. Calculating the resistance above and below the fabric as detailed in Koerner ~5 results in a value of 28.7 kN/m2-width (600 psf-width). For this calculation, an average height of sand fill above the fabric of 1.2 m (4 ft) was used, with a sand-fabric friction angle of two thirds of the sand's angle of internal friction (30°). There was no difference between the calculations with the marsh and sand, and those with existing clay levee and the new clay fill. With the fabric resistance and the ultimate tension required for


the fabric, the pullout length is determined to be 7-3 m (24 ft). Since the pullout resistance for both cases is about the same, the required embedment length is the same for fabric on the existing clay levee and on the marsh.

The degree of lateral sliding depends on the available soil-fabric friction angle, ~bsF. A factor of safety of 2.0 against sliding failure across the geotextile is required. Using lateral earth pressure theory, the required soil- fabric friction angle can be obtained for the geotextile by calculating the ratio of the resisting force to the driving forces. These forces described by Fowler 4 make use of the angle of shearing resistance between the fabric and the embankment soil. This angle is a function of the size of the embankment, and the coefficient of active earth pressure. Using the equations, the required soil-fabric friction for the clay fill is about 4.6 kN/m-width generating an angle of 26.5 ° . The same calculations for sand on the fabric generated an angle of 9.5 ° .

The required geotextile tensile strength (Tw) to provide a factor of safety of 1.5 against splitting in the fabric warp direction was determined as described by Fowler. 4 In this case, the safety factor is multiplied by the active pressure force as calculated by Fowler et al. 3 For clay fill with an active pressure of l l4kN/m-wid th (78131b/ft-width) the tensile strength is 171 kN/m-width (11 720 lb/ft-width or 977 lb/in-width). For sand fill with an active pressure of 41.7 kN/m-width (2861 lb/ft-width) the tensile strength is 62.2 kN/m-width (4292 lb/ft-width or 358 lb/in-width).

The tensile modulus (Ew) required to limit embankment average spreading and rotation to 5 percent fabric elongation is determined as described by Christopher et al. ~1 The tensile modulus is assumed to be twenty times the largest tensile strength, which for Reach A is 3442 kN/m- width (19 660 lb/in-width). From the slope stability analysis, the tensile modulus is 4200 kN/m-width (24 0001b/in-width). The likelihood of internal embankment slope failure is minimal on the sand because the factor of safety against failure was F S = tan 30°/tan 14 ° = 2.3 (where 0 = 30 ° is the angle of internal friction of the sand fill and 14 ° is the angle of the embankment slope).

Geotextile strength requirements in the weft or fill direction or across seams are generally dependent on the degree of softness of the foundation. Tensile strength requirements in this direction depend on the amount of squeezing out and dragging on the under side of the fabric and the amount of shoving or sliding that occurs during placement of the 1.2 m (4 ft) thick sand layer on the geotextile. To examine the forces introduced during this construction sequence, it is assumed that during placement of the fill material along the levee alignment, only about three of the 4.9 m (16 ft) wide rolls of geotextile are placed in advance of this operation. The placement loads in the fabric are then calculated for the fabric fill and seam strength require-


ments as the product of the number of rolls, the roll width, and the remolded shear strength of the foundation materials, x6 The tensile strength required is 70 kN/m-width (4800 lb/ft-width or 400 lb/in-width). Assuming a factor of safety of 1.5 for the geotextile in the fill direction or across seams, the strength requirement is 105 kN/m-width (600 lb/in-width). Fabric fill and seam tensile modulus is calculated assuming an elongation of no more than 5 percent. Based on a seam strength of 105 kN/m-width (600 lb/in-width), the tensile modulus is twenty times this strength or 2100 kN/m-width (12 000 lb/ in-width). To achieve seam strengths of this magnitude a parallel seam with double lock stitching made with a Union Special sewing machine or its equivalent is normally used.

The ultimate strength requirements depend on the fabric polymer that is specified and the required strength, T, of the fabric determined from the slope stability analysis. In order to avoid tertiary creep for fabrics that are loaded for long periods of time (10 to 50 years) the permissible load for polyester is of the order of 50 percent of the ultimate tensile strength, for polyamide it is of the order of 40 percent, for polypropylene and poly- ethylene it is recommended to remain below 25 percent. 16 For the prolonged application of geotextiles in this levee it was recommended that polyester fabrics be used. The ultimate fabric tensile strength was determined to be 420 kN/m-width (2400 lb/in-width).

Fabric elongation caused by the levee foundation subsiding from consolidation, general displacement failure, or squeezing out soft foundation materials will cause a subsequent elongation in the geotextile. The New Orleans District estimated that about 1 m (3.5 ft) of settlement would occur near the centerline of the new dike and about 1.4 m (4.5 ft) of settlement would occur about halfway down the slope on the marshside. Assuming the geotextile to be deflected in a parabolic shape, the characteristic equation of a parabola can be used to determine the percentage of fabric elongation at the maximum deflection point. For an embankment subsidence or deflection of 1.4 m (4.5 ft) and a base width of 18.3 m (60 ft) which is about twice the distance from the marshside toe to the maximum deflection point, the arc length was determined to be about 18.44 m (60.5 ft). The fabric elongation is less than one percent, which is well within the allowed maximum of 5 percent.

4.4 Geotextile fabric specifications

Geotextile fabric specifications were prepared from the results of an analysis for the test section that was conducted by the New Orleans District. A polyester fabric was specified and the required fabric tensile strength in the warp direction at 5 percent strain was 298 kN/m-width (1700 lb/in-width)


based on the requirements for a slope stability factor of safety of 1.3. The ultimate fabric strength in the warp direction was 508 kN/m-width (2900 lb/ in-width). The required tensile strength in the fill direction or across seams was specified at 105 kN/m-width (600 lb/in-width). A soil-fabric friction angle of 23 ° was specified for the sand cover materials and 8 ° for the clay levee materials. A 500 hour creep test at a constant load of 25 percent of the ultimate strength was specified and an elongation of not more than 4 percent was allowed. Seam strength was originally specified at 105 kN/m-width (6001b/in-width) but was subsequently reduced to 78.8kN/m-width (450 lb/in-width).

The minimum fabric requirements were determined by the Waterways Experiment Station from the analytical design procedure described in this paper. A woven polyester yarn fabric was chosen to provide the reinforcement. Based on Van Zanten, 16 the minimum strength is 2.0 times the required working tensile strength to compensate for creep. The fabric must have an ultimate" tensile strength in the fabric warp direction of 420 kN/m- width (2400 lb/in-width) (2.0 times the working strength, 210 kN/m-width). The ultimate tensile strength in the fabric fill and cross seam directions is 105 kN/m-width (600 lb/in-width). The secant tensile modulus at 5 percent fabric elongation in the fabric warp direction, E~ , is 4200 kN/m-width (24 000 lb/in-width), and in the fill and cross seam directions, EF, is 2100 kN/ m-width (12 000 lb/in-width). The soil-fabric friction, cf, is 30 kPa (625 psf) for clay fill on fabric and 9.5 ° for sand fill on fabric. General requirements about the equivalent opening sieve size, contractor survivability, and constructability are included in Ref. 10. Fabric specification must meet or exceed these requirements.

The fabric laboratory tests were performed by R. M . Koerner. 17 Initially only five geosynthetics were tested for potential use in stabilization and reinforcement of the Hurricane Protection Levee Reach A but after the test section was constructed the fabric actually used was tested. The six materials are shown in Table 1, along with a comparison of the laboratory fabric tests conducted and the design specification required by both the New Orleans District and the WES design methods.

Only three of the fabrics tested met the required ultimate tensile warp strength of 508 kN/m-width (2900 lb/in-width). To meet the required modulus, the manufacturer had to go to an ultimate strength of 664 kN/m- width (3793 lb/in-width). The fabric used in the test section did not meet the modulus requirement. Two of the fabrics met the tensile requirements in the fabric fill or seam directions. The grid strengths were low and if desired would require the use of multiple layers. Seam strengths were surprisingly low. Some polyester fabrics are capable of providing seam strengths in excess of 158 kN/m-width (900 lb/in-width). As a result of these tests,


TABLE 1 Test Results of Candidate Geosynthetics Compared with New Orleans District and WES

Design Specifications

Test Minimum Fabric Test Grid Strap type specification section SR-4

(NOD/WES) No. A No. 0--2 No. 37 500 fabric

Tensile warp kN/m-width 298/210 (lb/in-width) (1 700/1 200) at 5% Strain

Tensile warp kN/m-width 508/420 (lb/in-width) (2 900/2 400) at Ultimate

Tensile fill kN/m-width 105/105 (lb/in-width) (600/600) at Ultimate

Seam strength kN/m-width 105b/I05 (lb/in-width) (600b/600) at Ultimate

Creep percent elongation at 500 hours 4/4

Soil-fabric friction angle thSF (Degrees)

Levee/Levee 18/30 c Sand/Marsh 18/9.5

243 554 206 263 (1 388) (3 164) (1 177) (1 500)

406 664 519 508 80 101 (2 317) (3 793) (2965) (2900) (454) (577)

96 208 121 a 17 33 (549) (1 188) (694) (95) (185)

64 85 91 (363) (486) (518)

0.82 1.22 0.81 a 3.58 0.77

28 30 34 a 30 31 7 14 8 a 14 16

a Tests were not performed. b Requirement was reduced to 78.8 kN/m-width (450 lb/in-width). c Cohesion of fill.

r equ i red seam strength was reduced to 78.8 kN/m-width (450 lb/in-width). All o f the products met the creep specification.

The requ i red soil-fabric fidction angle of 18 ° was met with the levee soil on each side o f the fabric or grid, but not with the marsh soil and sand ad jacent to it. The grids showed a bet ter soil-fabric friction per formance than the fabrics because o f the larger s tr ike-through of the soil a f fo rded by their large ape r tu re size. Consider ing all of the test results versus the specification


values, it appears as though fabric No. 0-2 met or exceeded all requirements, except the friction angle with the sand/marsh combination. The marsh soil is of such high moisture content that no reinforcement inclusion stands much of a chance of generating much shear strength. The shear surface will always search out the weak plane which suggests the use of a granular soil layer on both sides of the reinforcement material. Pullout tests should have been planned for this project but were not. The direct shear tests that were performed did not simulate the sliding resistance of the materials above the fabric but only provided the resistance of the materials below the fabric. Pullout tests in a large scale 61 cm × 61 cm (2 ft × 2 ft) box are presently being planned for a similar project by the New Orleans District.

5 FABRIC REINFORCED TEST SECTION CONSTRUCTION

Prior to placement of the fabric reinforcement permission had to be obtained from the parish council to cut the existing levee from elevation +2.4 m (+8 ft) down to elevation +1.5 m (+5 ft) (see Fig. 2). It was necessary to reduce the levee height to provide the required fabric anchorage length in the levee cross section. Excess material removed from the levee crest was temporarily stored on the levee slope next to the canal. An at tempt was also made to separate the top soil for later use.

Reshaping the existing levee changed the conditions assumed in all the design procedures. Besides the working surface, a slide slope of about 25 percent exists along the marsh side. Thus, about 50 percent of the fabric was placed on existing levee material. In addition to the configuration dif- ferences, the existing levee introduces nonsymmetric foundation strengths. Because of strength increases due to consolidation under the existing levee, the enlarged levee will not behave as assumed in the design procedures. The nonuniform foundation material will influence the levee movement by causing more settlement and outward movement on the marsh side and less under the existing levee.

Presewn fabric panels for the 152 m (500 ft) long test section were delivered to the site in four sections, 38 m (125 ft) long and27 m (90 ft) wide. The fabric sections were unfolded and dragged into a sewing position with a t rack-mounted backhoe. The fabric was placed on the flat portion of the levee that was cut down to elevation 1.5 m (5 ft) and sewn together along the selvedged edge parallel to the longitudinal axis of the levee. After the sections were sewn together, the backhoe was used to drag the fabric into a position where the seams were oriented perpendicular to the longitudinal axis of the levee. Fabric near the levee toe was pulled out onto the marsh


surface by hand labor. This operation would have been much easier if a marsh buggy or some other type of low ground-pressure vehicle had been available to assist.

Once the fabric was placed, a 1.2 m (4 ft) lift of sand fill was placed by dump truck and track-compacted with a low ground-pressure dozer. During placement of the sand fill onto the fabric, the fabric was folded back a distance of about 3 m to 4.6 m (10 ft to 15 ft) at each levee toe onto a 61 cm (2 ft) layer of sand fill material to serve as an anchor for additional pullout resistance. The sand fill was maintained at this level for 12 days after which time the remaining clay fill below elevation 4.4 m (14.5 ft) was added over a period of 16 days. The clay fill material was truck-hauled from a nearby stockpiled borrow source and dumped on the previously placed sand fill. A wide-track low ground-pressure dozer spread the fill in 30 cm (1 ft) lifts. There were no quality control tests performed during placement of these materials.

6 INSTRUMENTATION INSTALLATION AND MONITORING

A total of six inclinometers located in two lines of three were installed in the test section. The locations of the inclinometers in the test section are shown in Fig. 3. Six readings were made of each inclinometer during the period from November 3, 1986 to March 26, 1987. Figures 6 and 7 show the movement versus depth of two inclinometers on the marsh side of the levee for all six readings. The inclinometers in the existing levee material showed no significant movement.

Inclinometers located near the levee crest indicated maximum horizontal movements of 18 cm to 30.5 cm (7 in to 12 in). The inclinometers located midway along the marshside slope indicated movements of 28 cm to 33 cm (11 in to 13 in). Nearly all movement occurred in the top 9.1 m (30 ft) of the foundation material. The inclinometer in the marshside slope had about two thirds of the total deflection by the second reading which occurred after the maximum fill height had been obtained. Along the centerline, about one half of the total deflection occurred by the second reading. Horizontal movement in the vicinity of the fabric was about 18 cm to 20 cm (7 in to 8 in) near the levee crest and about 25 cm to 33 cm (10 in to 13 in) midway along the marshside slope. This indicates that material was initially squeezed toward the marsh during placement.

Four settlement plates were installed on the fabric in the two profile lines shown in Fig. 3. These plates consisted of 61 cm (2 ft) square steel plates welded to a 5 cm (2 in) diameter pipe that is protected with a 7.6 cm (3 in) diameter floating pipe to prevent any drag friction on the 5 cm (2 in) riser


3.05

6 .10

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12.19

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18.29

2 1 5 4

24 .58

27 .43

30 .48

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[ . . . . . . NOTE: ELEVATION AT TOP OF PIPE - 6.04rn ( 19 .8 t FTI

ELEVATION OF FABRIC - Im (3.5 FT)

ELEVATION OF TOP OF GROUND - 4.42rn ( I 4.5 f iT) . . . . . . . . .

DEPTH OF PIPE = 1.62m (5.3 I.FT)

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pipe. The settlement plate readings were from October 22, 1986 to March 16, 1987. The results for one line of settlement plates are shown in Fig. 8. The New Orleans District predicted that about 1 m (3.5 ft) of settlement would occur under the centerline and about 1-4 m (4.5 ft) of settlement would occur about halfway down the slope on the marshside. Six months after construction, there was about 0.5 m (1.5 ft) of settlement at the centerline and just over 0-6 m (2.0 ft) of settlement halfway down the levee slope on the marshside. The settlement is less than the amount predicted prior to construction but it is anticipated that additional consolidation will occur.

The location of the two multipoint piezometers installed in the test section is shown in Fig. 3. Each piezometer set consists of four isolated tips at elevations close to - 1 . 5 , - 3 . 3 , - 6 .1 , and - 9 . 1 m ( - 5 , - 1 1 , - 2 0 , and - 3 0 ft). The results of the measurements taken from October 22, 1986 to August 24, 1987 are shown in Fig. 9. The maximum pore pressure increase


3.05

6.10

9.14

12,19

15.24

18.29

21.34

24.38

27.43

30.48

,C

(:3

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NOTE: ELEVATION AT TOP OF PIPE - 4.2rn (1:5.60 FT)

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DEPTH OF FABRIC = 2.2m (7. I FT)

2/ ! " " " -- ~ " h ' / I ] IINCLINOMETER NO. 15

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INITIAL

1 /26/86

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MOVEMENT TOWARD MARSH , cm

Fig. 7. Depth-deflection plots for Inclinometer 15.

recorded during construction was about 3 m (10 ft) and occurred at tip elevation - 3 . 3 m ( - 1 1 ft) after the final load was applied. This value was recorded six days before the second inclinometer reading and helps explain the reason for the large inclinometer deflections. This piezometric level reduced over several days to about 1.1 m (3.5 ft). The piezometric level in the deep clay, elevation - 9 . 1 m ( - 3 0 ft), increased about 2.3 m (7.5 ft) and has gradually reduced to about 1.7 m (5.5 ft) after the first eight months. The other piezometric tips indicated increases in pressure with a consistent reduction in pressure of about 0.6 m (2 ft) over ten to twenty days. Since then little pressure reduction has occurred. The variation in pore pressure with depth indicates that drainage is occurring at varying rates in the different layers.


15.2

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~ 45."/" I . - -

61.0

76.2

-1.51 I

- 2 -q I

-2 .5 t 0 30 60 90 120 150

ELAPSED T~ME, DAYS

Fig. 8. Time-history plots for settlement plates SI(~-) and $3(~').

Enka strain gages installed on the fabric by Dutch technicians were located in three rows near station 658+30 (station 658+29.3,658+33, and 658 + 37.2), The general location of the gages within the test section is shown in Fig. 3. The 10 cm (4 in) long metal foil strain gages were installed with three rows of transverse gages with the center row also containing longitudinal gages. A typical t ime-history plot of selected gages is shown in Fig. 10.

The results from all these gages were very consistent both with all the gages and the loading history. For the transverse gages the strain varied from about 0.5 percent to about 2.0 percent. The maximum transverse strain for each gage in a typical row is shown in Fig. 11, The gage nearest to the toe showed the largest strain, while the second nearest showed the lowest strain in two of the three sections. There is no systematic increase or decrease in strain across the section from the centerline to the toe on the marsh side. The gage closest to the toe is the only one located over the very soft marsh materials. The other gages are located over the existing dike materials. The longitudinal gages generated reasonable data. One gage indicated a negative strain for about 75 days. This indicates a longitudinal compression that eventually moves into tension. In general, the longitudinal strains are not as large as the transverse strains, ranging up to about 1 percent. Also


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there does not appear to be a relationship between the magnitude of the transverse and longitudinal strains. The data indicate a general increase of strain with time.

To evaluate the fabric load, Fig. 12 from laboratory work by Koerner 17 is used to relate load to strain. For a range of 1 to 2 percent strain, this figure indicates a fabric load of about 44 to 105 kN/m-width (250 to 600 lb/in- width).

Resistive potent iometer instrumentation was located on two fabric panels near stations 660+00 and 659+ 10. These gages did not generate consistent

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SYMBOL ELEV, N.GV.D.

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x CT2 I 3.5 4. I G' CT3 2. 1.5 6.6 # CT4 2'9.5 9.0

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510 L i 100 150

TIME, DAYS

Fig. 10. Time-history plots for Enka strain gages CTI to CT5.

data. Part of this inconsistency may have been caused by a flexible unidirectional device that was chosen to prevent transverse loads to the resistive potentiometer . Laboratory fabric load tests indicated that this device should have provided good data but data collected after field installation have been less than desirable. Of the 40 total gages installed, 17 did not indicate a change in reading after the initial 1-2 m (4 ft) of fill was applied. Of the remaining gages, some showed spikes in the data and did not immediately follow the fill height loading curve. It appears that about a fourth of the gages were useful. Of these gages, the strain was about 1 percent after initial jumps were subtracted.


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7 CONCLUSIONS AND RECOMMENDATIONS

The design and construction of the fabric reinforced levee test section was technically and economically successful and a practical construction alter- native to conventional construction methods. Environmentally, the reinforced levee section is very attractive because of the smaller levee size and the lower amount of land required or affected by construction. Cost savings of one-half to two-thirds the cost of conventional construction methods have been estimated for the fabric sections. 18

The range of fabric strains and loads measured in the test section were less than the values predicted during design. The measured fabric high strain locations were different than the required locations of the tension reinforcement from the slope stability analyses. This indicates that the reinforcement is preventing slope instability. The other investigated failure modes implied that some foundation squeezing could occur which the levee performance has verified. From evaluation of the instrumentation data, the test section appears to be rotating about a point along the existing levee. This is due to the unsymmetric foundation conditions. However, the geotextile reinforcement is probably minimizing the problem.

It is recommended that the continued use of a high performance geotextile be allowed in the remaining portions of the levee to be constructed. In fact the District has specified the fabric working strengths for the remaining levee designs at 217 kN/m-width (1240 lb/in-width).

ACKNOWLEDGEMENTS

Special thanks and acknowledgements are made to the New Orleans District Corps of Engineers for their financial and technical support; and in par- ticular to Messrs Jerry Satterlee and Frank Duarte for their keen interest in implementation of new innovative design and construction techniques for very soft foundations. Permission was granted by the Chief of Engineers to publish this information.

2.

REFERENCES

Value Engineering Proposal 85-04, Reinforcing Geotextiles, Reach A, US Army Corps of Engineers, New Orleans District (October 1985). Haliburton, T. A., Douglas, P. A. & Fowler, J., Feasibility of Pinto Island as a Long-Term Dredged Material Disposal Site, Miscellaneous Paper D-77-3, Dredged Material Research Program, US Army Engineer Waterways Experi- ment Station, CE, Vicksburg, Mississippi (December 1977).


3. Fowler, J., Edris, E. V. Jr & Duarte, F. M., Fabric Reinforced Embankment Test Section, Hurricane Protection Levee, Reach A, Plaquemine Parish, Empire, Louisiana, Miscellaneous Paper GL-87, US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi, December 1987 (in press).

4. Fowler, J., Analysis of Fabric Reinforced Embankment Test Section at Pinto Pass, Mobile, Alabama, PhD Thesis, Oklahoma State University (July 1979). Technical Report, EL-81-7, US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi (1981).

5. Department of the Army, US Army Engineer District, Philadelphia, Wilmington Harbor South Disposal Area Development, Wilmington Harbor, Christina River, Delaware, Design Memorandum, US Army Corps of Engineers Philadelphia District (June 1985).

6. Fowler, J. & Hanks, W. L., Fabric Reinforced Embankment Test Sections, Craney Island Disposal Area, Norfolk, Virginia, Miscellaneous Paper GL-87, US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi (December 1987) (in press).

7. Stevenson, P., Fabric Reinforced Embankment Provided Airport Runway Safety Modification, Geotechnical Fabrics Report, 1(4), (Spring 1984), pp. 4-8.

8. Koerner, R. M., Fowler, J. & Lawrence, C. A., Soft Soil Stabilization Studyjbr Wilmington Harbor South Dredged Material Disposal Area, Miscellaneous Paper GL-86-38, US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi (December 1986).

9. Fowler, J., Leach, R. E., Peters, J. F. & Horz, R. C., Mohicanville Reinforced Dike No. 2 Design Memorandum, Miscellaneous Paper GL-86-72, US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi (August 1986).

10. Office, Chief of Engineers, Department of the Army, Engineering Use of Geotextiles, Technical Manual 5-800-08, Washington, DC (in press).

11. Christopher, B. R., Holtz, R. D., & DiMaggio, J. A., Geotextile Engineering Manual, US Department of Transportation, FHWA Contract No. DTFH 61-80-C-00094 (February 1984).

12. Spencer, E., A Method of Analysis of the Stability of Embankments Assuming Parallel Inter-Slice Forces, Geotechnique, 17(1) (1967), pp. 11-26.

13. Office, Chief of Engineers, Department of the Army, Stability of Earth- and Rock-Fill Dams, Engineering Manual EM 1110-2-1902, Washington, DC (April 1970).

14. CAGE, G-CASE Task Group on Slope Stability, User's Guide." UTEXAS2 Slope Stability Package; Volume H Theory Section, Technical Report GL-86 (Draft), US Army Engineer Waterways Experiment Station, CE, Vicksburg, Mississippi.

15. Koerner, R. M., Designing with Geosynthetics, Prentice-Hall, Englewood Cliffs, New Jersey (1986).

16. Van Zanten, R. V., Geotextiles and Geomembranes in Civil Engineering, A. A. Balkema, Rotterdam, Boston, Massachusetts (1986).

17. Koerner, R. M., Report on Geosynthetic Testing for Hurricane Protection Levee, Reach A Test Section, Plaquemine Parish, Louisiana, Geosynthetic Research Institute, Drexel University, Philadelphia, Pennsylvania (1986).


18. Department of the Army, US Army Engineer District, New Orleans, Revised General Design Memorandum No. 5, January 1987, New Orleans to Venice, Louisiana, General Design Supplement No. 1, Revised October 1987, Reach A--Price City to Tropical Bend, Louisiana.

Documents

Fabric reinforced embankment test section, plaquemine Parish, Louisiana, USA