Direct shear testing of a marginal material using a large shear box

In Australia, Q181C Test Method of Direct Shear Testing to estimate the Effective Angle of Internal Friction at Constant Volume Conditions for Granular (Coarse Grained) Materials is commonly applied to assess the suitability of backfills for reinforced earth walls. This paper presents a comparison between the results of three series of shear box tests on a typical ripped rock material, of marginal quality, that might be considered as a possible backfill material for a reinforced earth wall. Tests performed used 300 mm and 60 mm shear boxes, soil samples prepared to sub-19 mm and sub-4.75 mm sizes, and a range of shearing rates. The effect of pre-testing samples was also considered. The results show that accurate effective friction parameter measurements for coarse grained, granular backfill soils require the use of fresh soil specimens for each shearing test; the use of a large shear box that can accommodate soils with relatively large particles; and careful selection of shearing rates so that shearing takes place under drained conditions. If the above requirements are compromised, the measured effective friction angle is likely to be a significant under-estimate of the true effective friction angle, increasing the likelihood of the material failing to meet prescribed material quality standards.