Shear Strength of Poorly Graded Granular Material in Multi-Stage Direct Shear Test

Shear strength of soils plays an important role in geotechnical stability design. A growing number of studies have been carried out on the assessment of the shear strength of granular material using direct shear tests. However, for the same soil sample, different testing procedures may create different shear strengths causing difficulties in selecting the suitable design parameters. In this study, single-stage and multi-stage reversal direct shear tests on granular material were investigated under the drained condition as the dried sample state. Tested samples were glass beads. The drained shear strength of granular material was investigated under various normal stresses of 50, 100, and 150 kPa and three shearing rates of 0.01, 0.1, and 1.0 mm/min corresponding to slow, intermediate, and rapid speeds. The study results indicated that the shear stress of samples obtained from the different testing approaches tended to increase as the reversals increased in both the single-stage method and the multi-stage method. The strength envelopes revealed a good agreement between the fourth reversal in the single-stage test and the multi-stage direct shear test. In addition, the friction angle of glass beads was independent of shear rates under drained conditions.