Michael T. Adams

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.

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.

A generic soil-geosynthetic composite test

Abstract Geosynthetic-Reinforced Soil (GRS) mass, comprising soil and layers of geosynthetic reinforcement, is not a uniform mass. To examine the behavior of a GRS mass by a laboratory test, a sufficiently large-size specimen of soil and reinforcement is needed to produce a representative soil-geosynthetic composite. This paper presents a generic test, referred to as the Soil-Geosynthetic Composite (SGC) test, for investigating stress-deformation behavior of soil-geosynthetic composites in a plane strain condition. The specimen dimensions, 2.0 m high and 1.4 m wide in a plane strain configuration, were determined by the finite element method of analysis. The configuration, specimen dimensions, test conditions, and procedure of the SGC test are described. In addition, the results of a SGC test with nine sheets of reinforcement, as well as those of an unreinforced soil test conducted in otherwise identical conditions, are presented. In the test, the soil mass was subject to a prescribed value of confining pressure, applied by vacuum through latex membrane covering the entire surface area of the mass in an air-tight condition. Vertical loads were applied on the top surface of the soil mass until a failure condition was reached. The behaviors of the soil masses, including vertical displacements, lateral movement, and strains in the geosynthetic reinforcement, were carefully monitored. The measured data allow the behavior of reinforced and unreinforced soils to be compared directly, provide a better understanding of soil-geosynthetic composite behavior, and serve as the basis for verification of numerical models to investigate the performance of GRS structures.

Required minimum reinforcement stiffness and strength in geosynthetic-reinforced soil (GRS) walls and abutments

Abstract In current design methods for geosynthetic-reinforced soil (GRS) walls and abutments, there is a fundamental design assumption that the reinforcement strength and spacing play an equal role in the performance of a GRS wall/abutment, i.e., larger reinforcement spacing can be fully compensated by using proportionally stronger reinforcement and lead to the same performance. This has encouraged designers to use larger reinforcement spacing in conjunction with stronger reinforcement for reduction in construction time. Recent studies, however, has indicated that reinforcement spacing plays a much more significant role in the performance of a GRS wall/abutment than reinforcement strength. In this paper, an analytical model that is capable of reflecting more accurately the roles of reinforcement spacing and reinforcement strength is presented. Using available data from large-scale experiments, it is shown that the analytical model provides a much improved tool for predicting reinforcement forces at failure than the current design equation. Based on the analytical model, a protocol for determination of required minimum reinforcement stiffness and strength in design is presented.

Mini-Pier Testing To Estimate Performance of Full-Scale Geosynthetic Reinforced Soil Bridge Abutments

The geosynthetic reinforced soil (GRS) performance test (PT), also called a mini-pier experiment, was developed by the Federal Highway Administration (FHWA) to evaluate the material strength properties of GRS composites built with a unique combination of reinforcement, compacted fill, and facing elements. The PT consists of constructing a 1.4-m square column of alternating layers of compacted granular fill and geosynthetic reinforcement with a facing element that is frictionally connected up to a height of 2 m, then axially loading the GRS mass while measuring deformation to monitor performance. The results can be directly used in the design of GRS abutments and integrated bridge systems. Considering that the geometry of the PT is square in plan, the equivalency of the results to a bridge application, which more resembles a plane strain condition, is evaluated and presented in this paper. The analysis indicates that the PT closely approximates the bearing resistance, or capacity, of a typical GRS abutment, and is a conservative estimate when predicting stiffness. These results indicate that the PT can be used as a design tool for GRS abutments at both the strength and service limit states.

Case Study: Condition Assessment of a 36-Year-Old Mechanically Stabilized Earth Wall in Virginia

AbstractIn 2012, a mechanically stabilized earth (MSE) wall was demolished in Virginia due to road realignment activities, providing an opportunity to assess the condition of the steel-bar mat rein...

Composite properties from instrumented load tests on mini-piers reinforced with geotextiles

Four pairs of large-scale instrumented geosynthetic reinforced soil (GRS) square columns were load tested to study the effects of varying reinforcement strength to spacing ratio, to discern the lateral pressures during construction and during load testing, and to derive shear strength parameters of the GRS composite. Each pair was identical in every respect, except one was loaded with a dry-stacked concrete masonry unit (CMU) facing in place and the other without. Lateral pressures during construction were found to be small for the facing type used in this study. Also, based on the derived GRS composite shear strength parameters, it was found that (1) the GRS composite Mohr-Coulomb envelopes are not parallel to those for the unreinforced soil; (2) the reinforcement increased the composite cohesion compared to the unreinforced soil (cohesion increases with decreasing spacing and increasing reinforcement strength); (3) the composite friction angle is less than that of the unreinforced soil (friction angle increases with decreasing reinforcement strength and increasing spacing); (4) as the composite friction angle increases, the active lateral earth pressure coefficient decreases; and (5) the benefits of reinforcing a soil become increasingly significant as the reinforcement spacing decreases.