Reproducibility of direct shear tests conducted on granular backfill materials

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 (phi') for computing bias. Comparison of failure envelopes and friction angles from the intra-laboratory study showed that the test method is highly repeatable (phi' +/-0.1 degrees) 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 phi' varying by as much as 18.2 degrees 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 degrees. The bias in phi' is -2.5 degrees when area corrections are not applied and -1.5 degrees when area corrections are applied.