A Dual Interface Apparatus for Testing Unrestricted Friction of Soil Along Solid Surfaces

A dual interface shear apparatus (simple or direct) was developed to evaluate the distribution and magnitude of friction between granular materials and solid inextensible surfaces. Ideal and natural granular materials were sheared along controlled and random solid surface geometries. The test results were evaluated through a model describing the interface friction mechanism based on a micromechanical approach. The influence of boundary conditions and interfacial shear modes was examined and compared to the results obtained in a modified direct shear box. The test results show that the grain shape and the surface roughness, quantified with respect to the grain size, are the primary parameters controlling the interfacial shear strength at a given normal stress level. Three zones of normalized roughness associated with different shear strength levels were identified. These zones can be categorized into two possible shear mechanisms: interfacial and internal. The use of a small-size direct shear box (modified) to evaluate interfacial friction seems markedly influenced by the boundary conditions. The resulting interface friction angles exceed those that would develop along an unrestricted interface of soil and a solid surface, especially when shear takes place along the solid surface (e.g., smooth and intermediate normalized roughness). Erratum to this paper appears in 18(4).