BACKGROUND

It is estimated that 10 million square feet of mechanically stabilized earth (MSE) retaining wall is constructed annually in the United States. Most MSE retaining walls used in highway fill applications are constructed with a roadside barrier system consisting of a traffic barrier placed on a continuous footing (structural slab). The footing is intended to reduce the influence of barrier impact loads on the retaining wall system by distributing the load over a wide area. The proper design of the roadside barrier, the structural slab, and the MSE wall system requires a good understanding of how barrier impact loads are transferred through the structural slab and into the wall system.

The move from the AASHTO Standard Specifications for Highway Bridges (ASD) to the AASHTO LRFD Bridge Design Specifications has made evident the need for a rational, reliable procedure for designing roadside barrier systems placed on MSE retaining walls. Current procedures and design details for roadside barriers placed on retaining walls vary widely among state highway agencies. Most designs currently used are overly conservative because of inadequate understanding of how barrier impact loads are distributed through the barrier and transferred to the structural slab and the wall system. There is a need to develop procedures for use by state highway agencies in designing roadside barrier systems placed on MSE retaining walls.

OBJECTIVE

The objective of this research is to develop, in a format suitable for adoption by AASHTO, procedures for designing roadside barrier systems placed on mechanically stabilized earth retaining structures.

Accomplishment of the project objective will require at least the following tasks.

TASKS

PHASE I (1.) Analyze, describe, and critique pertinent domestic and international research, on the basis of applicability, conclusiveness of findings, and usefulness in developing a methodology for the design of roadside barrier systems placed on MSE retaining walls. Critically review the AASHTO Standard Specifications for Highway Bridges and the AASHTO LRFD Bridge Design Specifications and relevant interim updates. (2.) Survey state highway agencies, federal agencies, crash testing facilities, and retaining wall manufacturers to determine existing methodologies and details used in designing roadside barrier systems placed on MSE retaining walls. Identify the barrier impact loads and crash-test level (as defined in NCHRP Report 350) associated with particular barrier systems. Find and document lessons learned from known failures of existing systems. (3.) Summarize the state of practice for roadside barrier systems placed on MSE retaining walls. Identify successful design procedures and details that warrant further investigation. In addition, identify promising design procedures and details for further investigation that are not necessarily used in current practice. (4.) Based on the information developed in Tasks 1, 2, and 3:

• Propose design procedures for further investigation that address barrier impact loads and the transfer of these loads to the MSE retaining wall.
• Prepare a detailed plan for developing a recommended design procedure. The plan shall include proposed analytical methods, component testing, and full-scale crash testing needed for development and validation of the design procedure.
• Update and elaborate on the work plan for completing Phases II and III of the research.

(5.) Within 6 months of contract award, submit an interim report documenting the results of Phase I. The interim report shall include as a separate appendix the updated work plan for completing Phases II and III. Meet with the NCHRP panel to discuss the interim report and the updated work plan. Work on Phase II will not begin without prior approval of the NCHRP.

PHASE II (6.) Develop preliminary procedures for the design of roadside barrier systems placed on mechanically stabilized earth retaining structures. Evaluate the preliminary procedures using the analytical methods and component testing agreed on during the Task 5 interim meeting. Modify and re-evaluate the preliminary procedures as necessary.
(7.) Develop the justification for and the details of a full-scale crash testing plan for validating the preliminary procedures. The testing plan shall clearly list those full-scale tests that can be conducted within the existing project budget as well as additional tests required (if any) that are outside the existing budget. The testing plan shall include a detailed description of the instrumentation to be used in the barrier, the load transfer system, and the MSE retaining wall system. (8.) Submit the procedures, modeling results, and full-scale crash test plan for review by the NCHRP panel. Meet with the NCHRP panel to discuss the procedures and crash testing plan. Work on Phase III will not begin without prior approval of the NCHRP.

PHASE III (9.) Validate the procedures by executing the full-scale crash testing plan agreed on during the Task 8 interim meeting. Modify the procedures as necessary. (10.) Submit a final report documenting the entire research effort. Include the procedures in a stand-alone appendix, in a format suitable for adoption by AASHTO.

Status: The project has been completed and the report published.

Product Availability: NCHRP Report 663: Design of Roadside Barrier Systems Placed on MSE Retaining Walls