Geosynthetics in Reinforcement and Hydraulic Applications

This paper presents results from a study undertaken to quantitatively evaluate a fibrous medium containing both voids and man-made filaments. The evaluation was conducted using optical images captured from surfaces of coupons cut from resin-impregnated specimens of a needle punched nonwoven (NPNW) geotextile under various boundary load conditions. The sizes of voids enclosed by adjacent filaments were measured using optical image analysis and expressed in terms of the largest inscribing opening size (LIOS). In contrast to the dimensionless parameters of global or local void ratio, the largest inscribing opening size is useful for directly measuring the actual void size which affects properties such as the hydraulic conductivity, soil retention/migration, compressibility, and tensile strength. LIOS was particularly suitable for analyzing images containing linear or curved line segments with relatively low values of void ratio. The results showed how the LIOS distribution varies as a function of normal load and shear displacement.

Prefabricated vertical drains (PVDs), also known as wick drains, are typically used to increase the rate of consolidation. In addition, PVDs have been used to remediate contaminated soils by serving as mini extraction and injection wells. One of the major factors impacting the design of PVDs is the quantification of the effect of smear (the disturbance of soil surrounding the PVD during installation). A device, dubbed the PVD Permeameter, that measures the in-situ values of hydraulic conductivity and horizontal coefficient of consolidation has been under investigation and development. The PVD Permeameter consists of a short length of PVD material placed in the ground to the desired position using an actual PVD installation rig. The major benefit of this device is that it provides a cost-effective means of determining in situ parameters that already include smear; thus removing the uncertainty of the effects of smear from design. A field study indicated that more refinement was needed in the design of the PVD Permeameter. Specifically, laboratory studies were undertaken to investigate the impact of the length to diameter ratio and to confirm that the equation used to determine the horizontal hydraulic conductivity from the field trial was appropriate. All of the experiments were performed using a sand/kaolinite mixture. The laboratory experiments utilized scaled-down versions of the PVD Permeameter used in the field trial. The laboratory work indicates that the equation used to calculate the horizontal hydraulic conductivity is appropriate and that a larger length to diameter ratio does improve results. This work is an ongoing collaborative effort between researchers at Villanova and The Nilex Corporation, LLC; the laboratory program was originally presented as part of GRI-18.

This study focuses on the impact of soil density on the hydraulic interaction between unsaturated, low plasticity clay and geosynthetic drainage layers. The hydraulic interaction was evaluated using the system hydraulic conductivity, moisture retention curves, and moisture and suction profiles obtained during transient infiltration tests. The transient infiltration tests were performed on soil-geosynthetic profiles constructed within 203-mm diameter columns. The results of the infiltration tests indicate that the soil-geosynthetic interface is affected by the capillary break effect. In particular, the results indicate that loosely compacted clays have capillary breakthrough at smaller suctions than highly compacted clays. Also, the increase in moisture storage due to the capillary break was more pronounced for looser clays.

The bubblepoint test (BPT) (ASTM D6767) has the potential to characterize geotextile pore size more completely and accurately than current methods such as the AOS test (ASTM D4751). A relatively inexpensive BPT apparatus has been constructed, which has been shown to be consistent in tests of geotextiles and accurate in characterizing the openings of wire screens. Accurate measurement of the contact angle between the geotextile and the wetting fluid used in the BPT is shown to be important for correct data reduction. Different wetting fluids used according to ASTM D6767 were shown to produce different pore size distributions for a nonwoven geoxtile.

The Beartooth Highway crosses the Beartooth Mountains on the Montana-Wyoming border and is a principal access to Yellowstone National Park. On May 20, 2005, snow melt and a severe rainstorm on the 3,345 meter pass triggered large debris flows that damaged the roadway in 13 locations. As part of the emergency repair, two mechanically stabilized earth (MSE) retaining walls up to 7.9 m tall and more than 46 m long were constructed to restore the roadway. The foundation below one wall was prepared by replacing 4 vertical meters of highly fractured rock in the center of the debris flow chute with reinforced concrete and by installing rock dowels to reinforce adversely dipping rock joints. To span highly weathered rock and a fault within another debris flow chute, the central 15 m of the second MSE wall was founded on a 1 m thick, micropile-supported, tied-back, reinforced concrete slab. Soil nail reinforcement with a temporary wire mesh facing was used to retain the excavation backslope during foundation slab and MSE wall construction. This paper discusses MSE wall design and construction, including, access and equipment limitations, foundation preparation, temporary soil nail excavation support system, rock foundation stability, micropile design and construction, and MSE wall reinforcement.

Mechanically stabilized earth (MSE) walls are frequently constructed to accommodate widening of existing roads or construction of new roadways in steep terrain. Because substantial excavation must be performed to establish a flat bench upon which to construct the MSE wall, unshored construction may not be feasible. Therefore, a shoring wall is often constructed to stabilize the backslope to facilitate MSE wall construction. Full-scale testing was conducted to examine two hypotheses with regard to shored mechanically stabilized earth (SMSE) walls: (1) construction of an MSE wall in front of a rigid backslope (i.e., shoring system) results in reduced external lateral loading on the MSE wall compared to a conventional MSE wall; and (2) full connection of the MSE reinforcements to the shoring wall provides limited benefit. The full-scale test model was constructed using short geosynthetic reinforcements, ranging in length from 25 to 39 percent of the wall height. The test model was equipped with strain gages, pressure cells (horizontal and vertical), linear variable displacement transducers (LVDT), potentiometers, and inclinometers, as well as surveyed using optical methods. The test wall was extremely stable, even under unrealistically high footing surcharge loads, i.e. 10 times the normal traffic loading. Interpretation of the instrumentation data yielded significant results regarding potential arching effects, internal wall pressures, and reinforcement strains.

The National Concrete Masonry Association (NCMA) published the First Edition of the Design Manual for Segmental Retaining Walls (DMSRW) in 1993 to provide a standardized engineering approach for the analysis and design of conventional and reinforced soil segmental retaining walls (SRWs). The Second Edition of the DMSRW was published in 1997 and the companion NCMA Segmental Retaining Wall Drainage Manual was published in 2002; the latter provided general guidelines for incorporating surface and subsurface drainage details and systems into SRW design and construction to minimize the potential for hydrostatic forces to develop and to provide guidance for including hydrostatic forces when drainage alone will not eliminate the potential for water to load the SRW. The Third Edition of the DMSRW, completed in 2006, incorporates significant changes in the state-of-the-practice regarding MSEW and SRW topics. This paper summarizes these changes and discusses the rational for each. This paper will serve as a useful guide for practioners, particularly those designing with the aid of software based upon the earlier Second Edition of the DMSRW. The paper is directed towards designers and specifiers of the materials and methods used for SRWs.

The construction, maintenance and replacement of crossing structures within North America are significant aspects of the transportation infrastructure. Cost effective means of constructing, maintaining and replacing crossings are vital to maintaining the transportation system. Historically, economical crossings were constructed with locally available materials. Where ideal construction materials were not readily available, construction techniques were developed to utilize the local materials. By utilizing the knowledge of the behaviour of reinforced soils with the understanding of soil arching, reinforced soil arches can be constructed using local mineral soil fill reinforced with economical geotextile fabrics. The construction of a 33 m long by 2.4 m wide reinforced soil arch on an industrial log haul road on the north end of Vancouver Island, British Columbia, Canada, demonstrated that significant cost savings can be realized by constructing crossings with reinforced soil arches.

This paper presents the results of a comparative study on the design approaches for geosynthetic reinforced modular block walls in a tiered configuration. Four field walls were analyzed based on the current design approaches and the discrepancies between the two approaches were identified. Finite element analyses were additionally conducted using a calibrated finite element model on a hypothetical wall having various offset distances. The results of the FE analysis were examined in terms of the wall deformation and the reinforcement load to study the effect of offset distance on the wall behavior. The results indicated among other things that the FHWA design approach yields rather conservative results, i.e, lower factors of safety, than the NCMA design approach. Also found that reinforcement lengths should be determined based on the global stability analysis for cases with the intermediate offset distance rather than the external stability calculations.

Presented are the results of five large-scale Geosynthetic Reinforcement Soil (GRS) experiments referred to as Mini Piers (MP). The MPs were constructed to evaluate the effect of reinforcement spacing and reinforcement strength on the performance of a GRS mass. The experiments were performed at the Federal Highway Administration (FHWA) Turner-Fairbank Highway Research Center (T-FHRC) in McLean, VA in 1997. The results indicate a more pronounced improvement in the bearing capacity for close-spaced reinforced soil systems and that the performance of a GRS mass is more dependent on the spacing of the reinforcement and not necessarily the strength of the reinforcement. Test results are also correlated to a full-scale experiment to suggest the use of a laboratory test to predict the performance of a GRS mass. Innovators have successfully applied Segmental Retaining Wall (SRW) technology to a variety of applications by modifying the design of GRS to include the benefit of reinforcement spacing. However, a problem with implementing the technology into the main stream is that many experts assert the need for a revised design procedure for more closely spaced reinforcement systems because the current design does not include the benefit of reinforcement spacing created by soil-geosynthetic interaction. The purpose of this investigation was to provide general observations about GRS behavior in terms of reinforcement spacing and the improvement reinforcement spacing has on the performance of a GRS mass. It is hopeful that the results of these experiments will encourage the users of GRS technology to rethink design assumptions in current Mechanically Stabilized Earth (MSE) wall policy related to the frequency of reinforcement spacing. Currently the design of MSE walls does not account for the benefit of soil — geosynthetic interaction. The authors acknowledge that current design may be valid for certain MSE systems, but assert the need for a different design method for closely spaced (less than 16") GRS systems.

The long-term design strength of a geosynthetic reinforcement employed in current design methods is determined either by performing laboratory tests on the geosynthetic without considering soil-geosynthetic interaction or by imposing a very large safety factor to the tensile strength of the geosynthetic without any regard to the effect of backfill. This paper reviews long-term creep behavior of soil-geosynthetic systems and proposed revisions to the current design methods. Based on the authors' experiences and judgments, a cumulative long-term reduction factor, k, for geosynthetic reinforcement is recommended. The value of k is a function of the backfill gradation and index properties, reinforcement spacing, and geosynthetic polymer type. In addition, a rational procedure to account for soil-geosynthetic interactive creep behavior based on the SGIP test is presented.

This paper will discuss the design, construction, and performance of a geosynthetic—reinforced soil (GRS) integrated abutment used for Bowman Road Bridge, built in Defiance County, Ohio, during the fall of 2005. The project was part of the Federal Highway Administration's (FHWA) "Bridge of the Future Program" to develop technologies to build more efficient, durable 70 to 90 foot single-span bridges. One of the most significant aspects of this project was the evaluation of the "integrated abutment" concept; this simple method of combining the substructure with the superstructure is different from the "integral abutment" method of construction. The bridge consists of prestressed concrete box beams supported on GRS abutments without the use of a deep foundation to support the superstructure. The GRS abutments were built on a Reinforced Soil Foundation (RSF) over the clay subsoil. The bridge has no cast-in-place concrete. The bridge also does not have an approach slab; the intent was to allow the bridge and the adjacent road to settle together, providing a bump free, smooth ride for drivers traveling over the bridge. The cost to construct this bridge was about 20 percent less than the quoted price of a bridge supported on pile-capped abutments with 2:1 slopes. The bridge was instrumented and surveyed to evaluate performance and to refine the "integrated abutment" design concept. To date, the performance of the bridge is excellent and the angular distortion of the superstructure is well within AASHTO criteria for simple supported bridges.

The design objective of geosynthetic reinforced earth structures is to determine safe and economical layout and strength of reinforcement. The two companion papers outline two complementary approaches that satisfy this objective. The approaches are based on limit equilibrium analysis. The analysis considers the interaction of soil-geosynthetic and the interaction among reinforcement layers. The end result ensures sufficient length and strength of reinforcement. It also considers the required strength at its front end at the connection to the facing units. This paper shows an approach that produces the required tensile resistance of the geosynthetics so as to render the same factor of safety everywhere. This approach yields the "demand" aspect in design and therefore is considered fundamental in the context of soil reinforcement. It is an ideal approach that is providing a baseline solution. The second approach, presented in the companion paper, considers the "supply" or the existing strength of reinforcement and renders the resulting safety factor everywhere. This approach is the practical one as it matches the supply with the demand via safety factors. While the second approach is not strictly novel, the companion paper demonstrates how it can be used effectively and objectively utilizing color-coded safety maps to optimize the reinforcement length and strength.

The design objective of geosynthetic reinforced earth structures is to determine safe and economical layout and strength of reinforcement. The two companion papers outline two complementary approaches that satisfy this objective. The approaches are based on limit equilibrium analysis. The analysis considers the interaction of soil-geosynthetic and the interaction among reinforcement layers. The end result ensures sufficient length and strength of reinforcement. It also considers the required strength at its front end at the connection to the facing units. The first companion paper shows an approach that produces the required tensile resistance of the geosynthetics so as to render the same factor of safety everywhere. This approach yields the "demand" aspect in design and therefore is considered fundamental in the context of soil reinforcement. It is an ideal approach that is providing a baseline solution. The second approach, presented in this paper, considers the "supply" or the existing strength of reinforcement and renders the resulting safety factor everywhere. This approach is the practical one as it matches the supply with the demand via safety factors. While the second approach is not strictly novel, it demonstrates how it can be used effectively and objectively utilizing color-coded safety maps to optimize the reinforcement length and strength.

This paper describes the shift of paradigm in earth retaining structures, from the Mechanically Stabilized Earth (MSE) structures to the Geosynthetic-Reinforced Soil (GRS) structures. MSE structures rely on the use of widely spaced metallic or high strength geosynthetics as tie-backs or "quasi-tiebacks" and heavy concrete blocks or panels as facing to stabilize the backfill; while the GRS structures makes use of closely spaced geosynthetic reinforcements and well compacted granular fill with minimal facing elements to create an inherently strong reinforced soil composite. GRS is superior to MSE in resisting both static and dynamic loads. Negative batter walls are an excellent illustration of the differences in design philosophies between MSE and GRS. A number of case histories of negative batter walls for a variety of applications are discussed. In addition, the paper points out deficiencies of the current guidelines for design and construction of MSE structures and addresses the future of the reinforced soil technology.

Current design methods "borrowed" from the design concepts for externally stabilized retaining walls and adopted the Rankine or Coulomb earth pressure theories for design of segmental Geosynthetic-Reinforced Soil (GRS) walls. The earth pressure is used to determine the minimum reinforcement strength, minimum reinforcement length, and required connection strength between facing blocks. There is a myth that the connection strength of the wall facing needs to be sufficiently high in order to resist the Rankine or Coulomb lateral earth pressure. Such a design procedure has encouraged the use of heavy facing blocks and the use of pins and lips to increase resistance to lateral earth pressure on the wall face. This paper addresses the lateral earth pressure against the facing. Case studies on the subject of facing lateral earth pressure are presented and discussed. The "bin pressure" concept is introduced and recommended for evaluation of facing connection strength requirements in design.

Limit equilibrium analyses and 2-dimensional numerical simulations are performed to estimate the performance of a geosynthetic-reinforced embankment supporting a bridge abutment. Geogrid reinforcement was evaluated as a means of improving the stability of the embankment slope and reducing deformation under design loads. Results indicated a reduction in total and differential deformation due to the inclusion of reinforcement. The deformation of the embankment slope was correlated to the global factor of safety. Vertical and lateral movements for the reinforced case were computed to be 65% and 78%, respectively, of those of the unreinforced embankment case. The load carrying capacity of the reinforced embankment was estimated to be 5 times larger than the unreinforced case while providing the same limit equilibrium factor of safety.