Friction Corrections for Small-Scale Direct Shear, Residual Shear, and Interface Shear Box Tests

ABSTRACT ERRATUM: This paper published online on November 26, 2024, with the authors’ affiliations incorrectly listed as “Geotechnical Laboratory.” The authors’ affiliations should have been listed as “Innovative Geotechnology, Geotechnical Laboratory.” The affiliations were corrected, and the updated paper was posted online on April 30, 2025. The publisher apologizes for this error. Direct shear devices have been used for drained direct shear, residual shear, and interface shear box tests. Direct shear devices have many sources of friction that affect the shear resistance readings from the soil. The sources of the friction include the bearing under the shear device, the spring-equipped deformation indicator (linear variable differential transformer), and the gap screws (top half of shear box). The contribution of the system friction becomes significant when testing soft soils, as seen in interface shear box tests, as well as when the shearing process involves repeated forward and backward displacements, as in residual shear strength tests. The bearing under the reservoir generates normal-stress-dependent friction (i.e., the higher the normal stress, the higher the friction). In a conventional small-scale (63.5 mm in diameter) direct shear test, the top half of the shear box is raised prior to the shear to create a gap between the two halves of the shear box. The gap screws can be allowed to contact the bottom half of the shear box during the shear, and the friction from the gap screws can be easily corrected. Because of the spring force from the horizontal deformation indicator and hysteresis of the load cell, asymmetric resistances between the forward and backward shearing are common in a residual strength test. In this article, we demonstrate the significance of the system friction in a direct shear device and propose a procedure to measure and correct the system friction components. The forces acting on the soil in the top half of the shear box, depending on the test device setup, get rather complicated. The practical and theoretical aspects of the gap between the halves of the shear box are discussed. By performing a separate measurement for the system friction, these frictional components can be corrected for the shear force measurements.