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Variability in the Engineering Properties of Open-Graded Aggregate Backfills: A Large-Scale Direct Shear Round-Robin Study
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2023-07-01
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[PDF-30.70 MB]
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Edition:Final Report
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Abstract:Open-graded aggregates (OGAs) are a common type of structural backfill used to build a variety of transportation earthworks. Therefore, the Federal Highway Administration (FHWA) has made a concerted effort over the past decade to better understand and quantify the strength-deformation characteristics of OGAs through large-scale direct shear (LSDS) testing performed on a wide variety of stone sizes. However, all of this testing was performed using a single type of LSDS device, primarily conducted by a single laboratory technician, with OGAs generally obtained from a single quarry. To address these limitations and better understand the variability expected for OGAs across the country, FHWA initiated a round-robin study that included sieve, maximum and minimum density, and LSDS tests. In the study, six laboratories tested three stone size classifications (No. 57, No. 68, and No. 8) from five quarries with different mineralogy, meaning 90 OGAs were evaluated. Ottawa 20-30 sand was also included as standard soil for comparison. A statistical analysis of all the data for key geotechnical design parameters was then performed to determine mean values, standard deviations, coefficients of variation, and suggested statistical distribution types. This report details the testing program; results from all the laboratory tests performed and from the statistical analyses, including the sensitivity of engineering properties to different factors; and the impact on design, with suggested values for preliminary design provided.
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