Geosynthetics Research and Development in Progress

A new test method for clogging potential of geotextile filters by finer grained soils has been developed. The test, called the flexible wall gradient ratio (FWGR) test, is a combination of a flexible wall permeameter and the standard Gradient Ratio test. Because the test allows for backpressure saturation, the time required for stabilization during a clogging test is greatly reduced. Trials were performed using a broadly-graded glacial till with a hydraulic conductivity of about 10^–6 m/s; a variety of geotextiles were used to obtain the range of likely behaviours from piping to clogging. Results showed that the FWGR was able to obtain accurate results within a week, compared with times of two months for the same trials with the conventional gradient ratio test. The FWGR results were consistent and showed good repeatability. The FWGR seems to be an improvement over current test methods for performance testing of geotextile filters in fine-grained soils.

The 1990s was a decade of a quiet revolution in geosynthetics. This group of construction materials evolved from performing basic functions of separation, reinforcement, containment, filtration and drainage to serving as a passive element (conductive geomembranes) locating post-installation liner system damage, to actively inducing physical and chemical changes within a soil mass. This paper explores the phenomena of electro osmosis and use of conductive polymers in prefabricated vertical drains (PVDs) to induce excess pore pressures and facilitate consolidation and strengthening of a soil mass.

Rigid concrete pavements are subject to cracking, due to traffic load, fluctuation of temperature, and particularly moisture. An alternative for improved pavement drainage is to incorporate a geocomposite drainage layer underneath the concrete pavement. The geocomposite has advantage over natural drainage materials in maintaining its flow capacity and compressive stiffness under the construction and service loads. This paper presents the ongoing study of concrete pavements over triplanar drainage geocomposites in West Virginia University. The frictional study of concrete blocks cast over the geocomposite demonstrated that geocomposite and concrete come to a very strong bond where the governing friction behavior is that between the geocomposite and subgrade. It was observed that initial drying shrinkage within the concrete specimens has a greater rate than that of concrete specimens over geocomposite, and average concrete cast on geocomposite has 9% less drying shrinkage than that of concrete cast with standard mold. The water seepage that occurs before the concrete has set has no affect on the overall compressive strength of the concrete. The forthcoming research will address the subgrade reaction with the geocomposite above the subgrade, concrete beam fatigue behavior, and shear transfer across the joint or crack.

Results from an experimental program to evaluate the behavior of fiber-reinforced soils under drained and undrained loading conditions are presented. The results indicate that mobilization of resistance in reinforcing fibers is heavily strain dependent, which necessitates that failure models include consideration of acceptable levels of strain for a particular application. Furthermore, analyses of the results suggest that the reinforcing fibers produce both hydrostatic (or mean) and deviatoric (or shear) stress changes that combine to produce the overall improvement observed for the fiber-reinforced soil composite. The hydrostatic contribution to stress was observed to produce consistent, but unusual pore pressure and volume change responses under undrained and drained loading conditions, respectively. A simple, "strain-based" model is also presented that accounts for both the hydrostatic and deviatoric contributions of the fibers as well as initial stresses induced in the fibers due to compaction, extrusion, and consolidation of the specimens.

The use of biaxial geogrids has rapidly increased in recent years. Their ability to resist biaxially applied loads and strains is critical to their success. However, standard test procedures are essentially restricted to uniaxial testing and conventional analytical techniques do not easily allow the load-strain characteristics of the materials in two orthogonal directions to be mathematically combined. In this paper testing techniques are described which allow characterisation of the tensile ribs and junctions forming the geogrids and allow biaxial testing of the overall structures. It is suggested that the combination of these testing techniques and new analytical methods provides the means to more appropriately specify and represent the operational behaviour of biaxial geogrids in design.

Green roofs have entire roof tops covered with vegetation as opposed to shingles, bitumen, cement or gravel in conventional roofs. During the previous four years, the number of extensive green roofs (which are characterized by a relatively shallow depth of the growing medium) in the United States has grown at the rate of about 50% each year. Even then the total coverage area for green roofs in the US remains insignificant as compared with Germany, where green roofs have grown steadily over the past 25 years. The future of green roofs in the US depends not only on right incentives, which are more or less non-existent at this time, but also on the establishment of national performance standards, research facilities and test methods. The need for test standards and performance criteria is especially urgent for synthetic materials such as waterproofing membranes, root barrier layers, geocomposites and geotextiles, where many competing products can quickly flood the market, making it almost impossible to distinguish between what works and what does not. Given the right conditions, it is not unlikely that the size of green roof market in the US could far exceed that in Germany in eight years, and perhaps sooner.

This paper presents the development and commissioning of a laboratory test equipment to investigate the performance of geosynthetic-reinforced clay subjected to seasonal wetting-drying cycles. This work is a part of an on-going research project that will develop embankment stabilization techniques using geosynthetic reinforcement for highway interchanges and bridge sites. It focuses on how to improve the reliability of the design of geosynthetic-reinforced soil structures using marginal or non-select fills (clays and silts) by incorporating environmentally-induced loading (wetting-drying in clays and freezing-thawing in silts).

In this paper, the structural benefit of using a single layer of geogrid in base reinforcement as measured in the confinement test is related to Layered Elastic Theory. While derivation of a Traffic Benefit Ratio (TBR) and/or Base Course Reduction (BCR) is still a work-in-progress, a specification strategy is suggested based on the confinement factor and associated index properties. Further analytical and field research is needed to demonstrate a direct relationship between elastic theory and confinement testing.

In the Fall of 2000, the first of two mechanically stabilized earth (MSE) berms was completed at the G.R.O.W.S. Landfill located in Morrisville, Pennsylvania as part of the Southwest Expansion construction. A second MSE berm was subsequently constructed as part of the Northeast Expansion and was completed during the Winter of 2001. Both of these berms have been monitored for wall movement and deflection prior to waste placement and while the adjacent landfill cells were being filled. Slope inclinometer casings were installed in the berms and monitored periodically over the past four years during the course of landfill operations. This paper will discuss the performance of the MSE berms in terms of movement monitored by the slope inclinometers.

Reliability analyses are becoming more common place. They play an important role in geosynthetic design because they provide for a rational and defensible means of managing uncertainty in field performance. Residual shear strengths that are less than the peak strength are also common place with many geosynthetic materials. Conventional reliability analyses do not explicitly incorporate residual strengths; they are based on the mean and standard deviation for the strength. The residual strength is only considered implicitly as a data point in establishing the mean or standard deviation. However, a residual strength is more than just a data point in the distribution of possible strengths. It represents the lowest possible strength that the material will have. From a reliability standpoint, this lowest possible strength plays a significant role. A slope cannot fail if the applied stress is less than the residual strength; that is, the probability of failure is zero in this case. This paper describes ongoing work at The University of Texas at Austin to establish residual strengths for different geosynthetic materials, to develop tools to include residual strengths in reliability analyses, and to assemble practical methodologies so that residual strengths can rationally be included in design.

Localized strains due to production defects, seams and punctured zones significantly affect mechanical performance of geosynthetic materials. Accurate determination of localized strains becomes particularly important for QC/QA evaluation of these materials and plays a critical role in design problems. A number of geosynthetic specimens were tested in an accredited laboratory to determine strain distributions under wide-with tensile loading using image analysis techniques. Specimens were tested using both roller and pneumatic grips to identify the effects of clamping. The tensile load, cross head extensions and image frames at a specified rate were recorded during testing. The testing plan included nonwoven, low strength and high strength woven geotextiles each sewn with flat, butterfly and J-type seam to determine the effect of seaming on strain distributions. Punctured specimens were also used to simulate localized failures in field applications. Average strains were calculated at peak strength values to compare the performance of different geosynthetic specimens. The results indicated that digital image analysis is highly effective in determining the localized strains and their distributions within specimens tested with two different grips. Furthermore, the image-based strains can clearly identify the performance of different seam types and effect of puncture on specimen performance.

In recent years, geosynthetics have been used in combination with pile or column systems to support embankments over soft foundations. A number of research activities on this application have been performed in the past several years. This paper reviews some of these activities and focuses on preliminary results of current research being conducted by the authors.

GSI-Taiwan became as a GSI National member on July 2000. GSI-Taiwan was established based upon the Geosynthetic Laboratory of the Department of Civil Engineering, National Pingtung University of Science and Technology, Taiwan. This laboratory was established as a teaching and research laboratory in the early of 1994. Later, this laboratory was also served as a third-party laboratory to provide testing service for government agencies and industries in Taiwan. In addition, this laboratory also conducted research studies for industry to improve their products and to establish engineering database for their products. To ensure the quality of the service, this laboratory was recognized as a certified laboratory by the Chinese National Laboratory Accreditation (CNLA) and the Geosynthetic Accreditation Institute (GAI) in 1999 and 2000, respectively. At present, GSI-Taiwan laboratory has been accredited for more than seventy and thirty test methods by the CNLA and GAI-LAP, respectively. In addition to serve as a teaching laboratory and to provide third-party testing service, to conduct research study is another main objective of this laboratory. More than 40 research projects were conducted since the laboratory was established. In addition to the mechanical properties of geosynthetics, the durability and endurance properties of geotextiles, geogrids, and geopipes are also the main research areas. Details of on going research topics are discussed in the following paragraphs.

The objective of this study is to evaluate a proposed bench-scale performance test for determining the best location for geosynthetic placement for base reinforcement applications. The ASTM D1196 standard test method was incorporated in the proposed bench-scale test. A standard servo controlled universal compression test machine was used as the loading device. A series of performance test program was conducted. The test soil was compacted in a steel test tank designed to fit inside the test machine. Three uniform graded granular soils, rounded white quartz sand, a brownie gray rounded riverbed medium gravel and a large white rounded marble gravel, were selected. Geosynthetic reinforcement materials included a high-strength polypropylene geotextile, two types of uni-axial geogrids, and one biaxial geogrid. Two steel circular plates with diameters of 150 mm and 300 mm were used to simulate the footprint of various sized tires. Geosynthetic specimens were placed at depths of 5cm, 10cm, and 15cm for each type of test soil and geosynthetic reinforcement. The proposed bench-scale performance test found is a simple, economical, and effective approach to evaluate the effect of geosynthetic for base reinforcement application. The results from this investigation indicated that the geosynthetic reinforcement placed at a depth of 5 cm showed the best reinforcement effect for the various test soils. Placing the geosynthetic at a depth of 15cm or below showed almost no reinforcement effect. This result was applicable for all of the different test soils and various load plate sizes. Using biaxial geogrids with apertures equal to D₅₀ of the reinforced soil provided the best reinforcement effect for coarse granular materials. A 10cm overburden over the reinforcement materials provided sufficient pressure to fix the reinforcement in place for the base reinforcement application. Boundary effect can be significant reduced as conducting the test with 300 mm load plate by placing a layer of soft padding material around the inside perimeter of the test tank.

The characteristics of the impinging flow over drainage layers that include a geocomposite are being investigated at the University of Texas at Austin. The focus of this study is on applications where a drainage system consists of geocomposite overlain by a sand layer. An experimental testing program is underway, which involves infiltration tests through a sand-geocomposite system constructed to quantify the impinging flow from the sand into the geocomposite drainage layer under unsaturated conditions.

Perhaps it is time to bring recycled tires into the family of geosynthetics. In particular, recycled tire bales are being proposed for applications that are traditionally accomplished using various soil reinforcement applications (e.g. MSE walls and steep slopes). There are growing environmental interests in the utilization of recycled tire bales for civil engineering applications, triggered partly by the significant volumes of tires that could be disposed of in transportation projects. In particular, tire bales have been used to stabilize steep slopes due to a combination, of their lightweight and reinforcement mechanisms. Tire bales are manufactured by compressing approximately 100 waste auto and light truck tires into a 2 cubic yard, 1-ton bales. Each bale is fastened with galvanized or stainless steel baling wire. However, only limited quantification of the mechanical properties of tire bales has been conducted so far. Accordingly, an experimental testing program was initiated by the Texas Department of Transportation at the University of Texas at Austin to quantify the interface shear strength between tire bales stacked in a brick fashion. This paper presents preliminary results of large-scale direct shear tests, undertaken to evaluate the interface shear strength between 1-ton tire bales.

Failures of MSE wall and slope structures are frequently attributed to poor drainage and the presence of water within or behind the reinforced zone. Such failures are likely to increase as the use of low permeability backfill material is increased due to perceived (i.e., neglecting the cost of failure) economics. Prefabricated composite sheet drains can provide a solution enabling rapid drainage while keeping costs down. This research should produce a computerized design to enable a rational incorporation of such drains in MSE structures.

Fiber-reinforcement is a promising solution to applications such as reinforcement of thin soil veneers and localized repair of failed slopes. However, fiber-reinforced structures have been conventionally designed using homogenized approaches. This has possibly compromised a rational basis for design and, consequently, the use of fibers in geotechnical practice. A new discrete approach was recently developed that allows the design of fiber-reinforced soil by independent characterization of soil specimens and of fiber specimens (i.e. similar as in the design of conventional planar-reinforced soil). An experimental testing program involving triaxial testing of unreinforced and fiber-reinforced specimens was undertaken to validate the discrete methodology. The predicted results were found to agree well with the experimental results. Additionally, this paper discusses using peak or residual shear strength of unreinforced soil in the design of fiber-reinforced soil based on the strain compatibility consideration.

This paper describes research being performed under National Cooperative Highway Research Project (NCHRP) HR 24-22. The objective of the project is to develop selection guidelines, soil parameters, testing methods, and construction specifications that will allow the use of a wider range of reinforced fill materials within the reinforced zone of mechanically stabilized earth (MSE) walls than is currently practiced. The results of a literature search on the use and performance of different soil types for reinforced fill in MSE walls is reported. State transportation agency and private industry responses to a "survey of the current practice" for MSE backfill are discussed. New guidelines for MSE backfill are recommended, and economic incentive to relax current MSE wall reinforced fill specifications is presented. Numerical analyses used in the design of MSE walls are described. A full-scale field test will be conducted in Phase II to establish properties for "high fines" reinforced fill and associated design controls that give acceptable performance, and details are described.

As part of a larger geosynthetic research project involving the construction of full-scale flexible pavement test sections instrumented with foil strain gages, a study is being completed to investigate various methods of protecting those strain gages to improve quality control in the field. Two trial test sections were constructed (as part of this study) and reinforced with instrumented geotextile and geogrid materials. Each geosynthetic had five separate gage areas, and each gage area had a different protection alternative. A total of 30 gages were installed. The base course aggregate was placed and compacted using the same full-scale construction loading conditions expected on site, and the compaction phase was followed by additional truck traffic. Gage resistances were monitored as a function of time and loading scheme so that the preferred method of gage protection could be identified for use during field construction.

Thirteen heavily instrumented test sections will be installed along a newly constructed frontage road located in northeast Arkansas. Two woven geotextiles, one nonwoven geotextile, and one geogrid will be installed at the subgrade-base course interface for two different base course thicknesses. Structural, environmental, and quality control data will be obtained remotely for approximately one year. The objective of this research study is to evaluate the governing geosynthetic for this application, establish the primary performance mechanism of improvement, and determine the cost/benefit ratio of this technology. Recommendations and construction procedures for geosynthetic-reinforced flexible pavements will be submitted to the Arkansas Highway and Transportation Department (AHTD) at the completion of this research.

This paper discusses development and fabrication of an experimental device for assessing geosynthetic behavior under confined or unconfined conditions in uniaxial, biaxial, or axisymetric tension. The device is capable of performing both monotonic and cyclic tests over a range of strain rates (creep to near-blast rates) and under loading conditions that closely replicate those experienced over a variety of in-service conditions. The design concept is described and details of the experimental device are presented.

Prefabricated vertical drains (PVDs) are often used to improve soils, by speeding up the rate of consolidation, and are increasingly used for remediation of contaminated soils. When PVDs are installed in the ground there is always a zone of disturbed soil surrounding the PVDs. The extent and the impact of this smear zone is difficult to quantify and this uncertainty complicates the use of laboratory values in design. Consequently, obtaining in-situ values for the hydraulic conductivity and horizontal coefficient of consolidation that include the effects of smear would be of great use to engineers using PVDs in design. A method that utilizes PVDs to obtain in-situ measurements of these parameters is described. The primary advantages of this method are its cost effectiveness and that smear is included in the results. A trial test was conducted on San Francisco Bay Mud. This promising method yields values that are logical and compare well with published values for the Bay Mud. Additional studies are planned to investigate the effects of different end treatments and the impact of the length to diameter ratio. This work is a collaborative effort between researchers at Villanova University and the Nilex Corporation, LLC.

Mechanically stabilized earth walls have now experienced widespread and successful use for a number of years. This use has been accompanied by the development of well-established and widely used design procedures, including those by the Federal Highway Administration, American Association of State Highway and Transportation Officials and the National Concrete Masonry Association. As the use and confidence in MSE walls has grown, multi-tiered walls are now being constructed which involve several levels of separate walls. However, the design procedures for such multi-tiered walls are not well established. An examination of designs for a number of multi-tiered walls was conducted, which reveals that widely differing design approaches have been used. In response, efforts have been underway for the past two years at The University of Texas at Austin to evaluate and develop design procedures for multi-tiered walls.

A test embankment of relatively soft cohesive soil with three different geogrids to reinforce the slopes was constructed in 1986 at Devon, Alberta. The embankment was fully instrumented to measure geogrid strains, soil deformations in the embankment as well as the foundation and pore pressures in both the embankment and foundation soils. Monitoring of the embankment continued to 1990 at which time major soil deformations had slowed down. Monitoring has been renewed in 2003 and 2004 to evaluate the long-term performance of this reinforced test embankment.

The use of geotechnical centrifuges for modeling of geosynthetic systems can be advantageous. Various scenarios, such as failures, can be modeled. Large cost savings can be realized, both for research and specific projects. Virtually all centrifuge centers encourage outside users to use their facilities. In general, centrifuge modeling has been underutilized for geosynthetic studies, and potential users should investigate the possibilities and feasibility of centrifuge modeling. An example of a centrifuge modeling project dealing with a rapid and innovative slope stability scheme using high strength geocomposites is discussed.

This short paper presents revised information on the creep reduction factors used in the widely accepted method for geotextile mass per unit area design to prevent geomembrane puncture. The laboratory tests have been ongoing for 6-years and have not failed to date, leading to the tentative conclusion that published values of RF_CR are quite conservative.

There are many applications where geomembranes are purposely left exposed to sunlight and the high temperatures associated with such exposure. Many of these applications are hydraulic-related; such as masonry and concrete dam waterproofing, canal liners, reservoir liners and covers, and agriculture waste liners and covers. Lifetime prediction of these geomembranes is an obvious quest, and it is the goal of this project. Equipment is currently in place and geomembrane samples are incubating. While it is too early for quantitative results, the experimental design and goals of the study will be presented.

The design of geotextile containers for dewatering applications typically requires good retention of the confined fill material solids. This becomes difficult to achieve when the fill material is fine grained and the pore openings are large. Furthermore, the proper retention becomes critical if the confined material is contaminated. Fly ash is one of the fine-grained geomaterials that exhibit behavior that is different from regular soils because of differences in its particle structure and chemical composition. A laboratory testing program was conducted to evaluate the retention performance of geotextiles with fly ash slurries. The results indicated that fly ash could be successfully retained by a variety of woven geotextiles and nonwoven/woven combinations. Results also showed that use of a double-layer geotextile system, rather than a single woven geotextile, significantly increases the retention capacity.

This paper provides recommendations for clamping/gripping of geosynthetic clay liners (GCLs) when performing ASTM D6243, Test Method for Determining the Internal and Interface Shear Resistance of Geosynthetic Clay Liners by the Direct Shear Method. Currently, there is significant variability between laboratories in shear testing procedures for GCLs; with the primary difference being related to specimen clamping/gripping. The recommendations presented in this paper are intended to provide suggestions for consistent test results both within and between laboratories.

While there are significant economic reasons for relaxing the currently stringent specifications for backfill material in reinforced soil structures, it is also true that failure of these structures have often involved inadequate drainage and use of low quality, fine-grained soils. However, geosynthetic inclusions can be used within fine-grained soils if they can provide not only reinforcement, but also lateral drainage. While using reinforcement with in-plane drainage capability is conceptually enticing, transmissivity requirements for such application have not been properly quantified. This study presents preliminary results of an ongoing experimental testing program involving geogrid pullout tests conducted in wet, fine-grained soils. Pairs of tests were conducted using geogrids with the same tensile strength but with and without in-plane drainage channels. The beneficial effect of lateral drainage is being quantified.

Prefabricated vertical drains (PVDs), also known as wick or strip drains, commonly used to accelerate consolidation of fine-grained soils and sludges, have been successfully demonstrated for enhancing in situ remediation of contaminated soils and groundwater. Over the last decade, the technology basis was developed for using PVDs to accelerate in situ flushing, i.e., removal of contaminants below the groundwater table. During the last five years, the basis for using PVDs to enhance soil vapor extraction systems, i.e., removal of contaminated soil gas in the vadose zone was developed. Both of these geosynthetic applications have been field-demonstrated on a limited basis. The first at an abandoned TCE evaporation pond at a U.S. Department of Energy processing plant and the second at a municipal solid waste landfill experiencing subsurface lateral migration of methane gas. The PVD-enhanced remediation technologies have yet to find wide acceptance and utilization. Perhaps if several remaining issues are resolved and additional, well-instrumented field demonstrations are performed with results implemented in a design manual, consultants, remediation contractors and regulatory agencies will begin to include and utilize these technologies among their remediation options. In this paper we present an overview of the PVD-remediations technologies, a brief description of some past field demonstrations and our thoughts on research/development issues necessary to further the use of this technology.

This paper describes preliminary results of a novel construction method developed to reduce the vertical stresses on buried pipes. The method, named Geovala (vala means trench in Portuguese), involves the use of geosynthetic reinforcement layers placed within the soil mass, above the crown of the buried pipe. The main objective is to redistribute the vertical stresses acting on top of the pipe to the lateral surrounding soil. The construction methodology involves not only trench installation but also of construction of protecting structures. An experimental testing program was conducted to evaluate this system. The results obtained from tests conducted in both small- and large-scale facilities show that vertical stresses acting on top of the pipe can be significantly reduced by using geosynthetic reinforcement. The variables governing the design include the layout of the geosynthetics (length and position of the reinforcement in relation to the top of the structure), the soil type, the soil density, and the stiffness of the geosynthetic inclusion. It is expected that implementation of this construction technique will allow the use of more flexible pipes as well as their placement at more shallow depths. This can lead to significant cost savings in projects involving pipes in civil and environmental applications.

Penetrations of synthetic liner waste containment systems during or after filling can be a disaster for a landfill owner or operator, particularly if the liner penetration is caused during drilling of gas wells. Waste thickness over the liner at the time of gas well drilling could be 50 feet or more, making repair a difficult and costly operation. Methods for repair of synthetic liner systems at depth have been limited. In most cases, the waste has been excavated to expose the liner for a manual repair, a very costly and time consuming method for waste depths in excess of 50 feet. In some cases, the repair has consisted of a bentonite plug to seal the penetration. While less costly, the assurance of an effective, permanent seal is greatly reduced. Further, this method does not address regulatory assertion that compliance with Subtitle D requires a synthetic liner. Through the aid of a case history, an alternate means of repairing liners under significant waste thickness will be demonstrated. The use of polyurea, a fluid two-part synthetic material that cures under liquid, will be discussed. Applied remotely under tightly controlled conditions, polyurea has been successfully used to repair a synthetic liner system under 60 feet of waste.

Multi-layer liner systems employed in waste containment applications, require an effective and economic anchorage system to secure the installed geosynthetics against applied short-term and long-term loading conditions. Observations of constructed facilities indicate that the anchorage capacity determined during design is often significantly different from actual (field) anchor capacities. This paper will present research currently being performed to evaluate the anchorage capacity through experimental and numerical modeling. Results obtained from the analyses can be integrated to develop modifications to existing analytical models for practical design applications.

A research program is described for measurement of the shear behavior of geosynthetic clay liners (GCLs) and GCL interfaces under dynamic loading conditions. A state-of-the-art, large-scale dynamic direct shear machine is currently being constructed for the project that has the capability to test rectangular GCL specimens measuring 0.3 x 1.1 m (12 x 42 in.). The device has a maximum shear displacement of 254 mm (10 in.), a maximum normal stress of 2,400 kPa (50,000 psf), a maximum shear stress of 750 kPa (15,700 psf), and the capability to impart sinusoidal shearing with a displacement amplitude of 25 mm (1.0 in.) at a frequency of 5 Hz. This paper describes the objectives of the research program, the equipment under construction, and the dynamic shearing tests to be conducted.

An issue of continuing environmental concern is the migration of potentially hazardous metals in waste containment facilities to the subsurface. Geosynthetic clay liners (GCLs) are now well recognised as an alternative to traditional compacted clay liners. Past research has established the use of clays for attenuating metallic contaminants; however the attenuation capacity of GCLs for metals remains in question. This paper describes research aimed at characterizing GCL behaviour in the presence of metal-rich permeants and assess their potential for metal attenuation in order to evaluate their potential use in different waste containment scenarios. To date, the migration of select metals (Al, Fe, Mn, Ni, Pb, Cd, Cu, Zn) have been investigated using continuous flow permeameters and diffusion testing in the presence of a municipal solid waste leachate and an acid mine drainage solution. The ability of the GCL to attenuate metals was measured by monitoring aqueous solutions before and after the experiment and by bulk analysis of the GCL itself. In predicting the long-term performance of the GCL, it is important to examine the mechanism of metal attenuation such as the role of adsorption onto bentonite, sorption onto Fe and Mn hydroxides and the effect of reducing conditions on the long-term stability of these hydroxides. Delayed breakthrough times and material analysis have shown evidence of metal attenuation and insights into adsorption mechanisms have been made. Material analysis to date has included constituent analysis of digested GCL soil, soil pore water analysis and X-ray diffraction. Future research will use material analysis methods of micro-imaging (electron microprobe, scanning electron microscopy) to further explain retention mechanisms.

The expansion of geosynthetic use has followed discovery of applications where they provide new, better or alternative solutions to infrastructure problems. This paper describes ongoing research to employ the phenomena that geotextiles host microorganism growth to the protection of drinking water sources and stream habitats. The basic concept is to filter organic pollutants from polluted waters before discharge, and use a biomass in the internal porosity to degrade the intercepted solids to stable end products. Bench scale experiments show that municipal wastewater can be treated to produce an effluent that exceeds the secondary treatment standards intended to prevent oxygen depletion in receiving streams. The project is now focused on developing new "Best Management Practices" (BMPs) for intermittent wet weather flows such as urban runoff and combined sewer overflows. The target pollutants are suspended solids and pathogens. Most current BMPs require large surface area and significant new pipe infrastructure. This is frequently impractical for application at urban sites with limited surface space and an existing pipe network and outfall. To treat wet weather discharges with minimal local disruption; compact, subsurface, end-of-pipe retrofit treatment units are needed. All work to date has employed available products. The goal is to quantify the influence of measurable geotextile parameters (permittivity, porosity, etc.) on hydraulic and treatment efficiency. The ultimate result anticipated is a template for manufacture of new products with optimized internal structures for this new application.

A series of large-scale direct shear strength tests were conducted to evaluate the effect of shear displacement rate on the internal shear strength of reinforced GCLs. Specifically, the tests were conducted using shear displacement rates ranging from 0.0015 to 1.0 mm/min and using normal stresses ranging from 50 to 520 kPa. The peak internal shear strength was found to increase with decreasing shear displacement rates for high normal stresses, while the opposite trend was observed for low normal stresses. Interpretation of these results, involving indirect evaluation of shear-induced pore water pressures, is proposed to explain this apparently counterintuitive trend. In addition, a decreasing large-displacement shear strength was noted with increasing shear displacement rate for tests at all normal stresses. Future research is geared toward the verification of the trends in peak and large-displacement for other GCLs over a wider range of shear displacement rates.

Golder Associates Inc. was asked to simulate and compare the drainage characteristics of two basement wall foundation systems. The first system was a glass fiber insulation board with a strip drain (Geosynthetic System); the second system was a standard corrugated slotted drain tile (pipe) and gravel wrapped with a geotextile (Aggregate System). Our client was interested in offering an alternative to the Aggregate System with a Geosynthetic System to analyze any similarities and/or differences between the two systems. Two different types of soils were used as backfill material in the tests to simulate drainage differences that could be expected in different areas of the country. Test results indicate that the Geosynthetic System is an effective replacement for the traditional Aggregate System.

This paper discusses the ongoing efforts of SL Limitada in regards to product certification as it relates to GRI-GM13 specified geomembranes. The paper presents two data sets; one for smooth and one for textured geomembranes (both 1.5 mm thick) that have been through the GCI-PCP certification process. These two materials have been tested by three different GAI-LAP accredited laboratories. Results are repeatable as well as reproducible. Most interesting are the endurance test results. The two geomembranes tested had different formulations and were produced on different lines in the factory. In addition, a new endurance test challenging antioxidant mobility (i.e., high temperature incubation in water) was also investigated.

Recent cracking failures in flexible polypropylene (fPP) geomembranes and floating covers are reviewed together with analytical data on antioxidation and ultraviolet (UV) resistance additive components in new and degraded material. It is evident that stress has a significant effect on the UV degradation rate and a much smaller effect on thermal degradation. The objective of the research is to generate an adequate durability specification for fPP geomembranes in different applications

Separation-in-plane (SIP) is a phenomenon that occurs during peel testing of geomembrane seams, sometimes in flexible polypropylene and less frequently in high density polyethylene. Its cause has never been experimentally verified, yet it is considered to be an acceptable form of material delamination. It is hypothesized that it is fundamentally a consequence of using incompatible carrier resins for additives and is aggravated by some geomembrane processing characteristics. Research to date is summarized and additional work is proposed. It is concluded that it is preferable to avoid it rather than to accept it.

The maximum apparent opening size (AOS) for a nonwoven needlepunched geotextile is limited to approximately 0.2 mm. While adequate for most designs, a larger opening size may be desirable for certain geotextile applications. A series of manufacturing trials were conducted to assess the feasibility of increasing the opening size of nonwoven needlepunched geotextiles. These trials can be broadly grouped into two categories: i) fiber denier trials, and ii) post-finish perforation trials. The first type of trial entailed increasing the geotextile fiber denier to fifteen from the current maximum of eight. The geotextile AOS increased as a result, while mechanical properties decreased. In the second group of trials, a calendared nonwoven needlepunched (NW-NP) geotextile was perforated using two different processes. In the first case, a laboratory scale needle loom was used to needlepunch the geotextile. Both standard (barbed) as well as barbless needles were used in the trials. It was observed that fibers tended to shift with time and block the openings soon after the needling process. As a result, AOS tests showed little or no improvement. In the second method, needles manufactured specially for these trials were mounted over a rotating drum where they were continuously heated using a gas flame. The drum was then manually rotated against the geotextile, thus penetrating the hot needles through the geotextile. Perforations on the order of 0.5 mm were achieved with this method. Product development is ongoing as of the time of writing this paper.

When a geomembrane seam specimen is tested in the peel mode, the material adjacent to the weld is bent at the same time as a tensile force is applied to it. As a result of bending, large and atypical stresses are imposed on the bent material; particularly at the outer fibers which is dependant on the thickness of the geomembrane. Initiation of failure at these outer fibers on relatively thick material may be anticipated. Clearly, the tensile strength with a material containing a 90° bend is less than a straight material with no bend. Also, when such failures occur total elongation of the specimen is low and debate continues as to the significance of low elongation values. Often the term "brittleness" of the undefined heat affected material adjacent to the weld is blamed. Using a custom-made 90° clamp, tensile tests have been conducted on unwelded geomembrane specimens. Results are then presented and compared with traditional peel test results. The correlation of a slightly offset 90° sheet tensile strength with a seam tested in peel (also 90°) is very interesting.

Waste Management Inc. and the USEPA are jointly studying landfill bioreactors through a Cooperative Research and Development Agreement (CRADA) at the Outer Loop landfill in Louisville, KY. The study is evaluating two types of landfill bioreactor technology; aerobic-anaerobic and facultative. The aerobic-anaerobic landfill bioreactor (AALB) process being evaluated involves treating the waste aerobically for a short period followed by anaerobic operation. Duplicate six-acre AALB cells are being studied as they are being filled with waste. The waste density, measured as mass placed over volume consumed, has increased between 14 and 27 percent. Initial densities of 1400 to 1550 lbs/yd³ have been increased to 1840 lbs/yd³ in one case and 1560 lb/yd³ in the other. The facultative landfill bioreactor (FLB) is designed to remove nitrogen from the landfill by external leachate nitrification followed by in situ denitrification of the treated leachate in the landfil. In 2000, a 5-year-old subtitle D landfill was retrofitted to evaluate the FLB process. Since liquid injection began in 2001, the waste density in the FLB has increased by 120 lbs/yd³ to 1620 lbs/yd³. Verification of changes in the municipal solid waste (MSW) has been performed by collecting and analyzing samples of the waste mass over the life of the project. Changes in the cellulose, volatile solids, biochemical methane potential and lignin content of the waste are consistent with the observed increase in density. While the study has at least two more years before the research agreement is complete, this initial work supports predictions that significant gains in airspace are attainable with bioreactor technology. The solid waste industry has projected airspace gains of as much as 30 percent over the life of a bioreactor project.

The waterproofing of dams, canals, and tunnels often has the associated geomembrane submerged in water. When this is the case, air incubation in ovens, and/or under ultraviolet exposure may not adequately address the in-situ aging process. For example, with polyolefins (HDPE, LLDPE and fPP) the depletion of antioxidants proceeds faster in water than in air, yet the oxygen concentration is lower in water than in air. For PVC, the migration of the plasticizers is governed by diffusion and desorption from the surface. Thus, water immersion must be investigated and compared to the more usual ways of incubation of using forced air ovens, and ultraviolet weatherometers. This project addresses the various aging process and presents preliminary information in regard to water immersion.

A major issue in the use of all geomembranes including high-density polyethylene (HDPE) is to estimate the material's lifetime to resist degradation. For polyolefins, this includes three stages: antioxidant depletion (Stage "A"), induction time to oxidation onset (Stage "B"), and the time to reach half-life of relevant engineering properties (Stage "C"). Utilizing temperature alone to accelerate oxidation degradation requires a very long incubation time. The focus of this study is to apply high pressure in addition to elevated temperature. With this pressure-temperature incubation method, the incubation time to reach Stage "C" is expected to be shortened significantly. This research-in-progress effort includes two types of HDPE geomembranes: unstabilized, and stabilized with an antioxidant package. All samples were incubated at temperatures of 45, 55, and 65°C, and pressures of 1.3, 2.8, 3.5 and 6.3 MPa (pure oxygen). For comparison purposes, the unstabilized geomembrane was also incubated in regular oven-aging (ASTM D5721), and in landfill simulating incubation devices. Both of the latter two incubations applied temperature alone as the acceleration factor. The temperatures were maintained at 55, 65, 75, and 85°C. All three types of incubation are currently ongoing.

Adequate stress crack resistance (SCR) is critical to the long-term behavior of high density polyethylene (HDPE) materials. In the current notched constant ligament stress (NCLS) test (ASTM F 2136), the evaluation of SCR of corrugated drainage HDPE pipe employs remolded plaques in cutting the appropriate test specimens. Thus, the residual stresses embedded in the final product by the manufacturing process are excluded from the SCR assessment. Since the residual stresses may have a profound effect on SCR, specimens cut directly from the manufactured pipes are being adopted for the test. Still at an early stage, the results included in this paper represent an ongoing study.

Oxidation is generally recognized as the key degradation mechanism regarding the long-term durability of high density polyethylene (HDPE) geomembranes. For protection against oxidation during its service lifetime, antioxidants are added to the formulation. In this study, a comparison of long-term behavior between the oxidative properties of two HDPE resins and two types of compounded HDPE geomembrane sheets (manufactured by flat die and circular die) are investigated. These four different materials were subjected to air oven aging, UV exposure, and buffer solution aging. Before and after exposure, specimens were evaluated for density, melt index (MI), carbon black content (CBC), standard oxidative induction time (STD-OIT), and high pressure oxidative induction time (HP-OIT). In addition, each set of incubations was maintained at three different temperatures so that the set of results could be subsequently used to calculate predicted lifetime using the Arrhenius equation.

A long-standing installation practice is that light weight equipment (like ATVs) can be driven on previously placed geomembranes to position and deploy additional geomembranes or other geosynthetics. This practice has been recently challenged by installers who currently have larger equipment with lower tire pressures than ATVs, and by regulators who will not allow equipment on the previously placed geomembrane whatever the type or tire pressure. The paper describes a field oriented set of experiments and the preliminary steps to be taken when the project begins this summer.

As the use of corrugated high density polyethylene (HDPE) pipes increases in transportation applications, the requirements on their long-term properties pipes becomes more rigorous. Recently, the Florida Department of Transportation required all drainage pipes in critical applications to have a design life of 100 years. Thus, a research project was initiated to assess the long-term properties of corrugated HDPE pipes. This paper presents the stress crack resistance (SCR) part of the project. The evaluation of SCR in this study is focused on the finished pipe and not the HDPE resins that are the focus of the AASHTO M 294 specification. As a result, manufacturing stresses are included in the results. SCR tests were developed to replicate pipe locations that have shown cracking in the field. Results from the preliminary test confirmed that the junction and longitudinal profiles of the corrugated HDPE pipes are most susceptible to stress cracking. An interim specification is established using an incubation environment of 80°C in water under an applied stress of 1.72 mPa (250 psi) for a minimum duration of 155 days. The testing protocol for the full specification is on-going to confirm and refine the extrapolation methods.

A laboratory investigation as to the hydrolysis behavior of polyethylene terephthalate (PET) yarns is the subject of this ongoing study. The project stems from an earlier specification which was developed based on the results of an independent durability study. The specification requires PET yams that are used for reinforcement applications to have a maximum carboxyl end group (CEG) value of 30 mmol/kg, and a minimum molecular weight of 25,000 kg/mol. In this study, eight PET yarns with various CEG and molecular weight values were evaluated for their hydrolysis property under pH = 7 water at temperatures 45, 55 and 65°C. The duration of the study was 4 years. The hydrolysis behavior was monitored using tensile strength properties which were found to be quite sensitive to the material changes. The decrease of tensile strength increases with incubation temperatures. For PET yarns with CEG values of 30 mmol/kg, a 60% drop in tensile strength resulted after 4 years at 65°C. Using this data, the extrapolated strength loss at 20°C is 11% after 100 years. In terms of a long term chemical reduction factor for design purposes this is equivalent to a value of 1.12.

In this paper, the compressive creep behavior of an expanded polystyrene (EPS) geofoam was investigated by an accelerated creep test called the stepped isothermal method (SIM). In addition, the temperature effect on the compressive strength was evaluated. The data showed that at 44°C the slope of strength versus temperature changed rather abruptly. The results of such change led to a significant increase in the creep rate. Based on the current data, the applicability of SIM on EPS geofoam is uncertain and further investigation is recommended.

Geotextile tubes have potentially a major application for the decontamination of pollutant-bearing sediments and sludges. Two major technical issues that may cause ineffectiveness of the remedial operations are (i) the inability to retain organic and inorganic pollutants that are dissolved in the water phase, and (ii) the slow dewatering process due to rapid formation of a filter cake by colloidal particles on the inside of the fabric. In this study, we are adding various reactive materials to the sediments and sludges so as to transfer soluble pollutants to the solid phase. The reactive materials we have selected are activated carbon and charcoal for immobilizing organic pollutants, and phosphoric rock for precipitating heavy metals. Our preliminary design indicates that adding a small fraction (0.5 wt%) of charcoal can cause an order of magnitude reduction of aqueous phase concentrations for many EPA-prioritized organic pollutants, and that a small quantity of phosphoric rock added can reduce aqueous phase concentrations of lead, zinc, copper, etc., to their respective EPA drinking water standards. This paper is a research-in-progress report. Detailed experimental work for optimizing engineering design of such amendments is currently being undertaken and the results will be presented shortly.

This short paper presents updated information on GSI's highway geotextile separation study. In the transportation industry one of the major uses of geotextiles is a separator between fine grained subgrade soils and the overlying drainable base course materials in a pavement system. The purpose being to prevent the intrusion of the fine grained soils into the drainable base materials and vice versa, the gravel base course from protruding into the soil subgrade. This current study, under the auspices of the Geosynthetic Institute, is to evaluate the performance of geotextiles as separators under various roadway and environmental conditions. There are currently 13 sites, widely distributed in the U.S., under investigation. Geotextiles as separators in pavement cross sections are significantly under-utilized. One of the impediments is the unknown cost-to-benefit ratio of geotextile systems as compared to conventional, nongeotextile, ones. While geotextiles have been used as separators between soil subgrade and stone aggregate for over 25 years, their life-cycle improvement of pavement systems has only been qualitatively assessed. Thus, a nationwide study is underway to quantify geotextile performance in this application. The focus is to access the benefits of a geotextile separator used in paved roads underlain by stiff and firm subgrades. This study is not focused on stabilization or reinforcement of highways on soft subgrades, an area in which the benefits of geotextiles have long ago been demonstrated. It is felt that a large database is needed to assess the performance with and without geotextile separators at the same location, and then a quantitative assessment of the performance of the respective sections over the lifetime of the roadway is necessary. Even further, this assessment should be national in scope. In this way, all types of traffic, subgrade, climatic and environmental conditions can be included in the assessment.

The technology of SRWs using geosynthetic reinforcement is arguably the fastest growing segment of geosynthetics. Due to their good aesthetics, costs, constructability and performance, these structures are seen throughout the world. However, some of these systems are being challenged for having very high alkalinity which is unsuitable for some PET geosynthetics. The focus of this paper is to share preliminary data from a SRW at GSI which was built with three different block types. The wall was built as a test wall, but it is full size and serves a dual purpose of replacing a failed timber wall. Alkalinity (as measured by pH) measurements are being taken directly between the block surfaces at the contact of the geogrids to the SRW block units. Data for the first year of monitoring is presented.

The authors have performed an economic analysis of performing geoelectric leak location surveys on heap leach pads for gold and copper mining. Most likely average values for the input variables yielded benefit-to-cost ratios of 6.2 for gold mining and 5.6 for copper mining. The probability that the benefit-to-cost ratio for gold heap leaching would be greater than one was 97%, while the probability for copper was 96%. The benefit-to-cost ratios were determined by calculating the liner leakage rate and equivalent value of the lost solution versus the cost of performing a leak: detection survey to find the leaks. Due to the large fluctuations in operating parameters for each heap leach site, a site-specific analysis would have to be performed to most accurately determine the benefit-to-cost ratio for a particular site, especially when leaching low grade ore, placing the liner on low permeability subgrade, or operating with low head. All monetary values presented herein are in US dollars.

This paper suggests that the dimensional stability of geosynthetic clay liners (GCLs) should be a significant design consideration worthy of special attention in specifications and testing. This issue, which has been all but ignored in the design profession to date, is being raised as a result of reported field observations.

Closure design of the Ordot Dump is being completed in preparation for its ultimate closure in 2007 after a sixty year operational history marred by health, safety, and emissions concerns. Design of the closure will be completed with substantial application of geosynthetics and leading edge technology for containment, erosion control, and long term integrity of the cap.

The overall goal of this project is to contribute towards the use of geosynthetics in the design of dams and other hydraulic systems. More specifically, this project aims at quantifying the leakage through geomembrane and composite liners under conditions representative of dams. Although experimental validation of leakage equations has been conducted, the relatively small head (e.g., 0.3 m) allowed over liners for environmental applications has restricted such validation to a narrow range of governing variables. Instead, the use of geomembrane and composite liners for hydraulic systems such as dams involves heads several hundreds larger than those in environmental applications, as well as vertical or near-vertical liner orientations. Accordingly, experimental data is needed to refine the available leakage equations for use in the design of hydraulic systems. An experimental testing program involving quantification of leakage under high heads through geomembranes, soils, concrete and combinations of various other geosynthetics is under way at the University of Texas at Austin. The experimental setup, preliminary results and the scope of additional studies are presented in this paper.

Past practice for the disposal of municipal solid waste (MSW) has been to entomb it for as long a time as possible; a practice which can be called dry landfilling. A more recent liquids management strategy is to purposely add leachate and off-site liquids to the waste mass for rapid degradation of the organic components. This is called wet landfilling. There are at least three types of wet landfilling; leachate recirculation, anaerobic bioreactor, and aerobic bioreactor. The particular landfill which is the subject of this paper is of the anaerobic bioreactor type. The landfill cell being evaluated is 3 ha (5 acre) in size and is 60 m (200 ft) high. Wells have been installed to various depths such that the following properties of the degrading MSW can be quantified; temperature, moisture, liquid properties (COD, pH, TDS and TS), and gas properties (CH₄, CO₂, O₂ and N₂). This research-in-progress paper presents the data obtained over a 3-year period and provides some tentative remarks. The study will continue as long as degradation is occurring, perhaps for an additional 5 to 10-years.

The issue of adequate drainage of leachate collection and removal systems at municipal solid waste landfills has been a long-standing concern among all involved. As the organic components of the waste degrade the residue is high in moisture content, dissolved and suspended solids, and microorganism content as well. The situation is greatly heightened with wet leachate management practices, as in anaerobic bioreactor landfills. This field project is focused on examining the long term flow rates of fully degraded MSW placed over different drainage media (gravels and sands, with and without geotextile filters). The experimental setup follows ASTM D1987 arid uses leachate from an adjacent landfill. Initial results from this ongoing project are presented.

This paper contrasts the field performance of three geotextile tube case histories with results from hanging bag tests and pressure filtration tests. The geotextile tubes were used in three different applications; (i) shoreline protection using a sand infill, (ii) dewatering of dredged harbor sediment, and (iii) dewatering of lagooned industrial ash. The "Hanging Bag Test" (or HBT) and "Pressure Filtration Test" (or PFT) were used as comparative methods. Each of the site soils were tested with four different geotextiles commonly used in geotextile tube fabrication. Results indicate that both the HBT and the PFT are good predictors of field performance. The HBT is a relatively large field performance test and is the current standard, whereas the PFT is a small lab test that appears to be manageable and versatile.

An important aspect of understanding a polymer material's long term performance is knowing the environmental conditions that it will experience during its service life. A key environmental condition for a geomembrane (as well as other geosynthetics) is its insitu temperature. Using thermocouples attached to geomembrane liners and covers at two landfills containing municipal solid waste, long-term temperature data has been obtained. Furthermore, the monitoring has been conducted at both dry (conventional) and wet (bioreactor) landfills. Data indicates that liner temperatures beneath the solid waste at dry landfills are lower than at wet landfills. Cover temperatures, however, are controlled by ambient temperatures at the site and show a pronounced annual cyclic behavior. Temperature monitoring under all four conditions is ongoing.

Measuring the core thickness of textured geomembrane is conventionally done with a dead weight micrometer equipped with tapered presser feet. In a complimentary manner, the height of the textured asperities are micrometer measured with a thin stylis. However, both methods are problematic and have led to numerous conflicts between manufacturer and specifier. This paper introduces an ultrasonic test as an alternative for both core thickness and asperity height. Findings presented in this paper show that not only the core thickness but the asperity height can potentially be obtained ultrasonically. This work is in its beginning stage and testing is currently not accurate enough to replace conventional equipment. However, it is hoped that such work will lead to the development of a standard test method that can be used by manufacturers and specifiers in the future as the precision of the method approves.

Tubes formed from high strength geotextiles, filled with dredged material or other local sediments, have been used for shore protection in several different configurations. Individual tubes are on the order of 30 feet in circumference and about 6 feet high. Larger or smaller tubes can also be manufactured or the tubes can be stacked to provide higher structures. The tubes are placed either perpendicular to the shoreline as a coastal groin or parallel to the shoreline to protect upland infrastructure against wave action and erosion. Often they are used on the upper beach as the core of a sand dune to provide a last line of defense for upland infrastructure in the event the dune is destroyed by a storm. Tubes have also been used on exposed beaches as emergency shore protection since they can be deployed rapidly by experienced contractors. The location of a shore-parallel tube structure on the beach profile determines its function and the level of wave attack it can experience. Unfortunately, limited data are available to designers on tube stability under wave attack. This paper seeks to develop some of that information for shore-parallel geotextile tube structures located above the still water level (SWL) and subjected to some overtopping. Since toe scour plays a significant role in a tube's stability, laboratory tests and field evaluations of tube performance are needed. This paper presents analyses preliminary to such studies.

A bioreactor landfill demonstration pilot project has been proposed at a county-owned landfill in the County of St. Clair, Michigan. Two identical bioreactor cells will be constructed and independently monitored for two different liquid injections (landfill leachate vs. septage) to evaluate the effectiveness of bioreactor approach and analyze the specific impact of septage addition on the waste decomposition processes. Instrumentation will be implemented for data collection and evaluation of bioreactor performances. Results from this demonstration project will advance our understanding regarding the impact of septage addition in the decomposition of municipal solid waste (MSW), and in particular, in bioreactor landfills. Quantified benefits (accelerated waste decomposition, airspace recovery, increased LFG generation rate, improved leachate quality, etc.) as well as adequate operational guidelines established from the projects experience can be applied to future bioreactor landfill projects.

An experimental testing program is being conducted to evaluate the failure mechanisms of pipelines subjected to a localized loss of support. The program involves a series of centrifuge tests conducted to evaluate the response of pipes embedded in sand backfill and placed over a rigid trapdoor base that is released during testing. The tests involve a pipe with prototype diameter of 1.1 m and a soil cover height of approximately 5 times the pipe diameter. Failure patterns are observed along longitudinal and transverse cross-sections of the pipe-soil system. The influence of pipe stiffness and backfill density on the behavior of the system is preliminarily assessed in this paper. The transverse sections showed the development of well-defined slip surfaces initiating at the edge of the void. Slip surfaces did not develop along the longitudinal cross sections. The mode of failure of flexible pipes involved buckling in the shoulder of the pipe and a reversal of curvature at the invert due to excessive deflection. Preliminary test results indicate that the density of the surrounding soil has a significant effect on the damage experienced by flexible pipes.