Advanced Experimental Approaches in Conducting Direct Shear Tests for Soil Mechanics

This study examines the impact of shearing direction on the soil shear strength using advanced direct shear testing. Cubic samples were tested in the vertical direction and parallelepiped samples in the horizontal. The hypothesis was that unlike the normal stress σ₁ in the case of the classic direct shear testing in horizontal shearing, where σ₁=σ₃≠0 but σ₂=0; in the vertical shearing condition σ₂≠0 . Jaky’s formula and Hooke’s law used by Terzaghi were used in the determination of the σ₂. Jaky’s formula emphasized frictional resistance using φ from the results of the horizontal shearing, while Hooke’s law accounted for elastic deformation using the Poisson’s ratio. Results showed that cubic samples yielded higher cohesion and internal friction angles than parallelepipedic samples, with Jaky’s formula often overestimating strength. Hooke’s law provided more moderate and realistic values. These findings highlight the importance of shearing direction in evaluating slope stability, particularly in overconsolidated, collapsible soils, which can exhibit high strength followed by potential failure under loading. The study concludes that while cubic samples improve accuracy in slope stability assessments, careful model selection is essential to avoid overly conservative or excessive design parameters.