The direct simple shear (DSS) test is one of the most popular testing techniques for measuring the shear strength of soils and mine waste tailings. However, uncertainties remain regarding the suitable sample diameter and whether a DSS sample should be saturated or can be tested without flushing with water. Various designs and configurations of shearing caps are also incorporated in different DSS equipment with little information on their performance and comparison soil shearing behavior with different caps. This study examines the monotonic shearing behavior, static liquefaction and instability, and post-liquefaction strength of a coarse oil tailings sand in extensive series of monotonic DSS tests on two different specimen diameters of 50 mm and 70 mm. Moist-tamped samples are reconstituted with and without flushing with water and sheared using top and bottom caps with concentric wedges and projecting pins. These are examined across a wide range of consolidation vertical stress, and for three different stress paths corresponding to undrained (CV), drained (CVS), and constant-shear unloading (CSU) shearing paths. Static liquefaction and instability were triggered in the CV and the CSU tests at the emergences of undrained strength reduction and volumetric collapse, respectively. The results show little effects of sample flushing and diameter on the static liquefaction triggering and post-liquefaction shear strengths of the tailings sand. The effect of sample diameter was primarily observed on the one-dimensional compressibility and volumetric strain of samples. The smaller diameter specimens underwent smaller volume changes during one-dimensional compression and drained shearing compared with the larger D = 70 specimens.

The direct simple shear (DSS) test serves as a vital method in geotechnics, allowing the measurement of peak and post-liquefaction shear strengths, along with the critical state friction angle of soils. Additionally, the simple shearing mode applied in a DSS test is the predominant failure mode in many geotechnical engineering problems. Although the DSS test is widely used to determine soil strength, a significant challenge with the DSS device is the non-uniformity of stress and strain distributions at the specimen boundaries. This non-uniformity depends on not only the specimen size but also the size of soil particles. The influence of specimen size on boundary effects is typically evaluated using the ratio of specimen diameter (D) to height (H). The median particle diameter (D50), as an indicator of a soil's particle size, could be another influential factor affecting the non-uniformities of stress and strain on specimen boundaries in a DSS test. Through three-dimensional discrete element method (DEM) simulations, this research explores these factors. Specimens were generated with a particle size distribution (PSD) scaled from a coarse sand sample. Laboratory monotonic DSS testing results on the coarse sand were employed to calibrate the DEM model and ascertain the modeling parameters. Boundary displacements were regulated to maintain a constant-volume condition which represents undrained shearing behavior. Various specimen diameters were simulated with identical void ratios to investigate the influence of D/H on stress path, peak and post-peak shear strengths, and critical state behavior. DEM simulations allowed the generation of several particle size distributions through different scaling factors applied to the sand gradation to determine the combined effect D50 and D/H. Limiting D/H and D50/D ratios are subsequently proposed to mitigate specimen boundary effects.