Design of a 720-mm Square Direct Shear Box and Investigation of the Impact of Boundary Conditions on Large-Scale Measured Strength

Large-scale direct-shear equipment is described and experimental results are presented showing the influence of test boundary conditions on direct shear strength. The equipment was constructed at the University of Newcastle in 2013 for testing square samples up to 720 mm and applying vertical loads up to 4 MPa. Its boundary conditions were modified in 2017 to better comply with the testing philosophy in standards. The main modifications introduced include full compensation for normal load eccentricity induced in the original arrangement and the introduction of a system that would allow the installation of a seamless gap between shear box halves before shearing. Additional modifications considered the restriction of unwanted carriage spinning during lateral displacement and reduction of frictional force between the specimen and the vertical walls of the box. After the modifications were introduced, implementing a careful method for sample construction, repeatability of the friction angle at large displacement in the range of 1° was achieved among single tests. Further equipment improvement strategies were identified.