BACKGROUND

Existing AASHTO specifications for construction of mechanically stabilized earth (MSE) retaining walls require the use of high-quality, free-draining, granular backfill. In many areas, the availability of high-quality structural backfill has been decreasing. It is anticipated that as backfill supplies decrease, costs will most likely increase. Research and practice indicates that many soils other than those classified as AASHTO A-1-a soils have high strength (friction angles higher than 34 degrees) and have been shown to be suitable as backfill within the reinforced zone of MSE retaining walls.

An FHWA Report ("Mechanically Stabilized Earth Walls and Reinforced Soil Slopes: Design and Construction Guidelines," FHWA-SA-96-071) and a paper by Keller (Gordon Keller, "Experiences with Mechanically Stabilized Structures and Native Soil Backfill," Transportation Research Record No. 1474, Mechanically Stabilized Backfill and Properties of Geosynthetics and Geocomposites, pp. 30-38, TRB, Washington, DC, November 1995) indicate that backfill materials, outside the specified gradation and plasticity requirements for AASHTO A-1-a soil, have shown satisfactory performance for reinforced soil slopes and MSE retaining walls. In addition, the National Concrete Masonry Association approves the use of backfill other than A-1-a soils in MSE retaining walls. However, strict adherence to AASHTO requirements for plasticity and percentage of material passing sieve #200 preclude the use of soils other than those classified as AASHTO A-1-a for use as backfill in the reinforced soil zone for MSE walls. Given the successful performance of reinforced slopes and retaining walls with backfill other than A-1-a soils, there is a need to investigate the use of a wider range of soil types for backfill within the reinforced zone of MSE retaining walls.

OBJECTIVE

The objective of this research is to develop selection guidelines, soil parameters, testing methods, and construction specifications that will allow the use of a wider range of backfill materials within the reinforced zone of mechanically stabilized earth (MSE) retaining walls.

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

TASKS

PHASE I (1.) Conduct a literature search to determine the use and performance of soils used as backfill in the construction of MSE retaining walls. Synthesize current practice for design, specification, and construction of MSE retaining-wall backfill. Identify standard and nonstandard laboratory and field-test procedures used to determine the characteristics of the compacted reinforced soil composite within the reinforced zone of MSE retaining walls. Determine the types of numerical analyses used for MSE retaining-wall analysis and their success in predicting retaining-wall performance. Identify the limitations of the numerical analyses. Identify the minimum laboratory or field testing programs required to adequately evaluate retaining-wall backfill. (2.) Survey proprietary MSE retaining-wall manufacturers and transportation agencies within the United States, Europe, Canada, and Asia and:

• Determine current design and construction specifications and practices associated with MSE retaining-wall backfill. Include design details such as drainage elements, compaction techniques, and zone fills.
• Determine existing material property and laboratory data (physical, strength, and electro-chemical) for MSE retaining-wall backfill. Incorporate these data into an accessible database.
• Determine a transportation agency's economic incentive for relaxing material specifications for MSE retaining-wall backfill.
• Identify MSE retaining walls that did not perform as designed because of backfill quality and/or construction issues. Document the reason for lack of performance with specific emphasis on MSE retaining-wall backfill. Include case histories where lower-quality backfill was used successfully.
• Summarize measures used to enhance the performance of backfill.

• Develop draft representative soil parameters for backfill for MSE retaining walls that can be expected to offer acceptable performance levels.
• Select the type or types of MSE retaining walls for full-scale field-testing that offer the highest probability of performing satisfactorily with the lowest-quality backfill soils.
• Update the work plan to describe in detail the research team's plan for completing the research. The work plan shall identify the type of MSE wall(s) and backfill soils to be used for laboratory testing, numerical analysis, and full-scale testing.

(4.) Within 10 months of the contract award, submit an interim report documenting the results of Tasks 1 through 3. Provide an updated work plan for completing Phases II and III. If after the completion of Phase I, the research team concludes there is a need for full-scale testing, the interim report should include a discussion and justification of the full-scale testing to be conducted in Phase II. Meet with the NCHRP technical oversight panel to discuss the interim report and updated work plan. Work on Phase II will not begin without prior approval of the NCHRP.

PHASE II (5.) Conduct laboratory testing of selected MSE retaining-wall backfill materials to develop input parameters for numerical analyses. Conduct numerical analyses to predict the performance of retaining-wall systems selected for full-scale testing. (6.) Construct and load a full-scale instrumented MSE test wall(s) to validate the performance of different MSE retaining-wall backfill soils not meeting AASHTO A-1-a soil requirements. The full-scale test shall include a control section designed and built to current AASHTO design and construction requirements. The monitoring period for the loaded test wall shall be at least 12 months so as to allow the assessment of time dependent deformation. Moisture shall be introduced into the backfill material during the monitoring period. Using the results of the full-scale field tests, validate the Task 5 numerical analyses. (7.) Develop draft selection guidelines, representative soil parameters, appropriate testing methods, and construction specifications for a wide range of soil types suitable for backfill within the reinforced zone of MSE retaining walls. Submit the draft material for review by the technical oversight panel. Meet with the panel to discuss the draft material. Work on Phase III shall not begin without prior approval from NCHRP.

PHASE III (8.) Based on the panel's guidance from the Task 7 meeting, finalize the selection guidelines, representative soil parameters, appropriate testing methods, and construction specifications in a format suitable for adoption by AASHTO.
(9.) Submit a final report documenting the entire research effort. Include the selection guidelines, representative soil parameters, appropriate testing methods, and construction specifications for a wide variety of backfill materials in a stand-alone appendices as appropriate.


Product Availability: An uncorrected draft, as submitted by the Contractor is posted here.