The National Academies

NCHRP 12-59 [Completed]

Design and Construction of Geosynthetic-Reinforced Soil (GRS) Abutments for Bridge Support

  Project Data
Funds: $299,998
Research Agency: University of Colorado-Denver
Principal Investigator: Jonathan Wu
Effective Date: 8/1/2001
Completion Date: 12/31/2004
Comments: NCHRP Report 556: Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing

The findings of research conducted under this project are presented in NCHRP Report 556 and NCHRP Web-Only Document 81.

The use of geosynthetic-reinforced soil (GRS) systems as the foundation for or as integral structural components of bridge abutments and piers is receiving increased attention and interest. The soil mass of these systems is reinforced in layers with a polymeric geosynthetic (e.g., strips, grids, or sheets) and, in most typically used systems, the layered reinforcement is fixed to facing elements that constitute the outer wall. Because the facing elements are not rigidly attached to each other, the wall has some limited flexibility. Various materials, including stone, block, or timber, may be used for the flexible facing.

GRS bridge abutments and piers are more forgiving to differential foundation settlement, more adaptable to low-quality backfill, easier to construct, and more economical than their conventional counterparts. GRS bridge abutments and piers can be put into service quickly and can be built by maintenance personnel. This system may have considerable advantages for pedestrian structures, especially where access by heavy equipment is not available. It is economical for (a) temporary pier use because of its easy demolition and the recyclable nature of its components; (b) emergency work because of reduced lead time and lower equipment needs and skills; and (c) massive-looking piers that are desired for aesthetic reasons.

Full-scale tests conducted by the FHWA and by the Colorado Department of Transportation (DOT) in Denver on GRS bridge abutments and piers with segmental modular block facing have demonstrated excellent performance characteristics and very high load-carrying capacity. In these tests, the bridge was supported directly on the GRS mass.

In addition, the Colorado DOT recently designed and constructed a GRS abutment to support both bridge and approach roadway structures. The project included the construction of a shallow-strip-spread foundation that supports the bridge superstructure and is placed directly on the GRS wall. The design of this bridge is overly conservative because its performance could not be tested under actual service conditions and because of the critical nature of the structure (which support six lanes over Interstate 25). After 18 months in service, the structure has shown excellent performance. The monitored movements are much smaller than those expected in the design and allowed for by the performance requirements, and there is no indication of the development of the "bridge bump problem" or of any structural damage. Movements developed after opening the bridge to traffic are negligible.

The technology of GRS bridge abutments and piers with flexible facing has not been adopted in routine highway bridge construction. The primary obstacles are threefold. The first obstacle is the lack of a rational and reliable design method for such bridge-supporting structures. For example, although the vertical spacing of the reinforcement has been found to have a very strong effect on the performance of the structure, current design methods fail to reflect this important fact. Also, field-measured strains are known to be drastically smaller than those predicted by current design methods. Clearly, the current design methods are not sufficient. The second obstacle is the lack of well-developed guidelines and specifications for construction of the structures. These are critical to the successful application of this technology. The third obstacle is the perceived suspicion that polymeric geosynthetics may not be sufficiently strong and stable during the design life of the bridge structures because of their relatively high service loads. The research conducted under NCHRP Project 12-59 was limited to bridge abutments and approaches and did not address bridge piers.

The research conducted under NCHRP Project 12-59 resulted in the development of rational, reliable design and construction guidelines for geosynthetic-reinforced soil (GRS) bridge abutments and approaches with flexible facing elements.

Status: Research is complete.

Product Availability: NCHRP Report 556 and accompanying NCHRP Web-Only Document 81 (Appendix C of the final report) are available.

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