Defining the Boundary Conditions for Composite Behavior of Geosynthetic Reinforced Soil (GRS) Structures

Geosynthetic mechanically stabilized earth (GMSE) and geosynthetic reinforced soil (GRS) have been successfully used for retaining wall support since the 1970's. While both GMSE and GRS incorporate layers of compacted granular fill material and geosynthetic reinforcement connected to a facing element, GRS consists of closer reinforcement spacing, typically 8 inches, whereas GMSE consists of larger spacing. Researchers and practitioners have noted that the observed performance (e.g., vertical and lateral deformation, thrust against the face, capacity, etc.) at both the strength and service limit states for closely spaced GRS is considerably different than that of larger spaced GMSE for a variety of applications, including retaining walls and bridge abutments. The current American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications do not distinguish between GRS and GMSE and recommend the same design methodology (i.e., the Simplified Method) for both technologies. The Simplified Method models the reinforcement as tie-back elements whereby the purpose of the geosynthetic is to provide tensile resistance to the driving forces; however, in GRS, where reinforcement spacing is close, the geosynthetic serves multiple purposes (e.g., to increase confinement, to reduce lateral deformation, to suppress dilation, etc.). The impact of these added functions are not accounted for by the Simplified Method or other proposed design models for GMSE, which can lead to overly conservative and costly GRS designs. The Federal Highway Administration (FHWA) has taken the first steps in differentiating between GMSE and GRS design and construction with separate guidance for each technology; the Simplified Method for GMSE and the Composite Method for GRS. The boundary between the design models is based on reinforcement spacing; however, the composite behavior of reinforced soil likely depends on more than the reinforcement spacing, including reinforced backfill properties, reinforcement properties, facing rigidity, and loading conditions. Before distinguishing the two technologies and implementing changes to the AASHTO LRFD Bridge Design Specifications, the AASHTO Bridge Committee's T-15 technical committee has expressed interest in further defining the boundary of the composite nature of geosynthetic reinforced soil. Research is needed to evaluate the impact of the various related factors on the boundary between GMSE and GRS that result in a difference in performance. Recommendations on when to use a tie-back design model versus a composite design model for various applications should also be developed. Such research should propose modifications to the AASHTO specifications that enable recognition of a separate design model for closely spaced GRS different from that currently used for GMSE design, thus allowing for broader application of GRS. The objectives of this research are to: (1) determine the boundary conditions for composite behavior of GRS abutments as a function of, but not limited to, vertical spacing of soil reinforcements, reinforcement type (geotextiles and geogrids) and characteristics, and backfill properties; and (2) provide detailed proposed revisions to the AASHTO LRFD specifications, incorporating the results of the composite behavior portion of this research.


  • English


  • Status: Active
  • Funding: $500000.00
  • Contract Numbers:

    Project 24-41

  • Sponsor Organizations:

    Federal Highway Administration

    1200 New Jersey Avenue, SE
    Washington, DC  United States  20590

    American Association of State Highway & Transportation Officials (AASHTO)

    444 North Capitol Street, NW, Suite 225
    Washington, DC  United States  20001

    National Cooperative Highway Research Program

    Transportation Research Board
    500 Fifth Street, NW
    Washington, DC  United States  20001
  • Project Managers:

    Reynaud, David

  • Performing Organizations:

    University of Texas, Austin

    Austin, TX  United States  78712
  • Principal Investigators:

    Zornberg, Jorge

  • Start Date: 20140901
  • Expected Completion Date: 0
  • Actual Completion Date: 20170901
  • Source Data: RiP Project 37694

Subject/Index Terms

Filing Info

  • Accession Number: 01543537
  • Record Type: Research project
  • Source Agency: Transportation Research Board
  • Contract Numbers: Project 24-41
  • Files: TRB, RiP
  • Created Date: Nov 18 2014 1:01AM