Intra-laboratory and inter-laboratory testing was conducted to assess the bias, repeatability, and reproducibility of the shear strength of compacted granular backfill materials tested in direct shear using AASHTO T 236. Inter-laboratory testing was conducted by ten laboratories using four granular backfill materials. Triaxial compression tests were also conducted on the granular backfill materials to establish a reference friction angle (ϕ′) for computing bias. Comparison of failure envelopes and friction angles from the intra-laboratory study showed that the test method is highly repeatable (ϕ′±0.1°) when conducted in a single laboratory by a single operator using the same equipment. In contrast, data from the inter-laboratory study showed high variability in the failure envelopes and friction angles, with ϕ′ varying by as much as 18.2° for a given backfill. Analysis of the data from the inter-laboratory study showed that the reproducibility of direct shear tests on granular backfill is 8.8°. The bias in ϕ′ is −2.5° when area corrections are not applied and −1.5° when area corrections are applied.
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