Jonathan T. H. Wu

GRS bridge abutments—an effective means to alleviate bridge approach settlement

Abstract Field tests of segmental block-faced geosynthetic-reinforced soil (GRS) bridge abutments and piers have demonstrated excellent performance characteristics and very high load carrying capacity. One important feature of GRS abutment is that it can potentially eliminate the use of piling when situated over a weak foundation. This will not only reduce the costs but also reduce “bridge bumps” often experienced at the ends of a bridge resting on a pile-supported abutment. This study was undertaken to investigate the potential of GRS bridge abutments to alleviate bridge approach settlements. The study was conducted by the finite element method of analysis using the computer program DACSAR. The program was first calibrated by comparing its results with the measured data of the Founders/Meadows bridge abutment recently constructed by the Colorado Department of Transportation. A parametric study was then conducted to examine the effects of different foundation soils, ranging from loose sand to stiff clay, on the performance of a GRS abutment. Special attention was placed on the maximum vertical and horizontal movements of the abutment as well as the approach settlement characteristics. The study indicated that the finite element computer code DACSAR is a reliable analytical tool for analyzing the performance of GRS bridge abutments and that the GRS abutment is an effective means to reduce differential settlements between the abutment and the approach embankment.

Load-Carrying Capacity and Required Reinforcement Strength of Closely Spaced Soil-Geosynthetic Composites

AbstractIn current design methods for reinforced soil walls, it has been tacitly assumed that reinforcement strength and reinforcement spacing play an equal role. This fundamental design assumption has led to the use of larger reinforcement spacing (0.3–1.0 m) in conjunction with stronger reinforcement to reduce construction time. Recent studies, however, have clearly indicated that the role of reinforcement spacing is much more significant than that of reinforcement strength. With closely spaced (reinforcement spacing ≤0.3 m) reinforcement, the beneficial effects of geosynthetic inclusion is significantly enhanced, and the load-deformation behavior can be characterized as that of a composite material. A reinforced soil mass with closely spaced geosynthetic reinforcement is referred to as geosynthetic-reinforced soil (GRS). In this study, an analytical model is developed for predicting the ultimate load-carrying capacity and required reinforcement strength of a GRS mass. The model was developed based on a...

Effects of backfill on the performance of GRS retaining walls

Abstract In this study, a finite element program was validated by comparing its analytical results with the results of a well-instrumented large-scale laboratory test conducted on a geosynthetic reinforced soil (GRS) retaining wall under well-controlled test conditions. The validated computer program was then used to investigate the effects of backfill type on the behavior of GRS retaining walls. Three different geosynthetic reinforcements and sixteen different backfills were implemented in the analysis of three different wall configurations to produce 144 analysis combinations. It was shown that the type of backfill had the most profound effect on the behavior of the GRS retaining wall. It was also shown that the stiffness of the geosynthetic reinforcement had a considerable effect on the behavior of the GRS retaining wall when the backfill was of lower stiffness and shear strength. A parametric study was performed on GRS retaining walls based on the finite element analyses to assist the design engineer in choosing the appropriate backfill and the appropriate geosynthetic reinforcement for GRS retaining walls in order to satisfy the prescribed requirements of maximum lateral displacement, maximum axial strain in the reinforcements, and/or average safety factors.

SHORT-TERM STRENGTH AND DEFORMATION CHARACTERISTICS OF GEOTEXTILES UNDER TYPICAL OPERATIONAL CONDITIONS

Abstract An apparatus capable of measuring the strength and deformation properties of geotextiles under unconfined conditions and under the confinement of a membrane or a soil was developed. The appratus differed from conventional in-soil test apparatuses in that during the soil-confinement test the soil was allowed to deform with the geotextile while being confined by a prescribed pressure — simulating the predominant operational condition of geotextiles in reinforced soil structures. Three non-woven geotextiles manufactured in different materials and by different bonding processes were used in this study, and their stress-confinement effects were studied. It was shown that the stress-confinement effect existed in the spun-bonded and needle-punched geotextiles but not in the heat-bonded geotextile. The effect of using different materials (membrane and soil) for the confinement was also studied. Under otherwise identical conditions, the results were very similar between the in-membrane and in-soil tests. It was concluded that the in-membrane test is sufficient for evaluating the load-deformation properties of geotextile. Mathematical models were used to represent the measured load-deformation relationships of the geotextiles, and their accuracy was discussed.

Hydraulic conductivity of geotextiles under typical operational conditions

Abstract It is well known that granular backfill can account for more than 50% of the total construction cost for typical geosynthetic-reinforced soil structures. It is therefore desirable to investigate the possibility of using low-quality onsite soil, which may be cohesive and near saturated, as backfill. A geosynthetic that possesses adequate drainage capability in addition to having high tensile stiffness and strength would be highly suitable for this purpose. This study was conducted to investigate the cross-plane and inplane hydraulic conductivities of such geotextiles under typical operational conditions. Two types of geotextile, namely, a nonwoven and a woven-nonwoven composite geotextile, were tested by using different methods of confinement in their virgin state. Samples of the geotextiles retrieved from the field were also tested, and the results were compared with the hydraulic conductivity of virgin specimens. An equation is proposed to include the effect of confining stresses on the hydraulic conductivity of geotextiles. A reduction factor, termed the degree of retention (DOR), is introduced to express the long-term reduction in hydraulic conductivity due to soil-particle retention. In addition, a simple performance test is proposed for investigating the flow behavior of a soil-geotextile composite under its typical operational conditions.

REINFORCED SOIL FOR BRIDGE SUPPORT APPLICATIONS ON LOW-VOLUME ROADS

A quick and simple method of bridge substructure construction using geosynthetic reinforced soil (GRS) is illustrated. GRS is used to build abutments and pier foundations for simple bridges. It is a refinement of existing reinforced soil technology used during the last 20 years. The interaction of a closely spaced geosynthetic reinforced soil system and the reasons why conventional design methods are not appropriate for these closely spaced systems are explained. This method is not recommended for all bridge building assignments; for example, it is not suitable for construction of permanent bridges in scour zones. The technique is ideal for remote locations, inaccessible to use of concrete and other traditional materials. A generic style of GRS construction is explained to ensure performance and internal stability. Construction is rapid with conventional equipment. The materials are common, inexpensive, and generally available. An overview of recent full-scale research is provided. The results of two full-scale prototype tests are presented to demonstrate performance and limitations and to confirm the design of such systems. A case history is presented that shows the versatility of the technology in a bridge support application. A procedure for prestraining or preloading the reinforced soil to enhance performance is provided. For bridge support applications, preloading of the GRS has the benefit of limiting postconstruction creep settlement. Preloading also proof-tests the structure and verifies the quality of construction. Additional sketches are included to show its potential for common applications. A brief discussion about design considerations to limit potential problems is offered.

Measuring Inherent Load-Extension Properties of Geotextiles for Design of Reinforced Structures

Existing test methods for measuring the load-extension properties, including the stiffness and the ultimate strength, of geotextiles as they are subjected to confining pressures are reviewed and critically evaluated. In addition, a new test method which surmounts the drawbacks of the existing methods is presented. The new method has three distinct characteristics: (1) it is an “element” test, thus the load-extension properties determined from the test are the inherent properties of the geotextile; (2) the test measures the confined stiffness and strength of geotextiles without inducing soil-geotextile interface adhesion, thereby simulating the predominant operational condition in typical geotextile-reinforced soil structures; and (3) the stiffness and strength obtained from the test are conservative values if soil-geotextile interface slippage does occur in a reinforced structure. The new test method offers a unified and more rational method for determining the load-extension properties of geotextiles in the design and specification of geotextile-reinforced soil structures.

Mini Pier Experiments: Geosynthetic Reinforcement Spacing and Strength as Related to Performance

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.

FEASIBILITY OF GEOSYNTHETIC INCLUSION FOR REDUCING SWELLING OF EXPANSIVE SOILS

The feasibility of geosynthetic inclusion for reducing swelling of expansive soils was studied by performing laboratory soil-geosynthetic swell tests on an expansive soil. The test specimen measures 12 × 12 × 12 in., with a sheet of geosynthetic embedded horizontally at the midheight of the soil. To prepare the test specimen, the soil was first compacted, in 1-in. lifts, inside a wooden mold to the prescribed density and moisture content. The soil was then allowed to swell subject to wetting by soil suction. The vertical and lateral deformations of the specimen were monitored throughout the test. To assess the effect of geosynthetic inclusion, a test without geosynthetic inclusion was performed in otherwise identical conditions for comparison purposes. The test method and test results are described. On the basis of the test results, the feasibility of geosynthetic inclusion for reducing swelling of expansive soils in practical applications is addressed.

A Modified Soil-Geosynthetic Interactive Performance Test for Evaluating Deformation Behavior of GRS Structures

A modified Soil-Geosynthetic Interactive Performance (SGIP) test apparatus for evaluating short- and long-term deformation behavior of soil-geosynthetic composites was developed. In the test, a specimen of soil-geosynthetic composite, with dimensions of 305 mm wide, 605 mm long, and 305 mm high, was subjected to a vertical sustained load under plane strain condition. The applied load was transferred from the soil to the geosynthetic, and it allowed the soil and geosynthetic to deform in an interactive manner. Lateral and vertical displacements of the test specimen and strains in the reinforcement were measured. A series of the soil-geosynthetic performance tests were conducted to examine test repeatability, failure mode, and deformation behavior of different soil-geosynthetic composites. Test results and discussion of test results are presented. The applicability of the performance test to actual GRS structures was examined by comparing test results with measured behavior of a 5.4-m high GRS pier.