Exploring Box Fixity and Platen Texture in Large-Scale Direct Shear Testing

The Federal Highway Administration (FHWA) recently completed an interlaboratory round-robin study that explored the shear strength variability of open-graded aggregates (OGAs) in large-scale direct shear (LSDS) devices. The study found that many participating labs had significantly different device configurations and test procedures. Unlike standard direct shear devices (box width ≤100 mm), there are no unique industry standards for LSDS device design (box width ≥300 mm) or test procedures. Thus, LSDS testing remains the Wild West compared to more established standard direct shear test practices. To explore this frontier, FHWA’s geotechnical laboratory is experimentally and numerically investigating key variables of LSDS device design. This paper presents results from FHWA’s experimental investigation into the influence of upper shear box fixity and the texture of a hinged load platen on the deformation behavior and strength of a selected OGA. Results indicate that box fixity has a significant influence, with mobile boxes producing peak shear stresses approximately half those of fixed boxes. In contrast, platen texture had negligible influence. Ultimately, this paper seeks to begin an in-depth discussion of the questions prompted by the round-robin study—which device configurations produce the most consistent results, and which configurations best represent an aggregate’s true behavior and strength?