Soil-soil and pile-soil interface friction angles are important design parameters for driven or drilled and grouted pile foundations in clayed soils and fine-grained soft rocks. Peak and ultimate/ residual friction angles are routinely determined by direct shear box or ring shear tests and correlations are often explored by linking the angles to soils’ index properties. However, significant variabilities are commonly observed with experimental data, and the exact reasons are not always clear. Deviations seen with empirical correlations often lead to stark choices of representative design values, which can impact foundation requirements substantially. This paper reviews first how clay mineralogy and activity affect the residual friction angle (φres) of pure minerals and soils. A database of φres values from high-quality published experiments is then assembled and presented. Factors contributing to the variations in φres are clarified. Trends for φres against clay size fraction, clay mineral content, liquid limit and plasticity index are identified. Further insights are provided into the interface residual friction angles (δres) for piles driven into fine-grained soft rocks, noting that the driving operations may break down large aggregates into clay particles and lead to potentially low δres, especially when they contain high activity platy clay minerals, such as montmorillonite. Recommendations are given of project-specific laboratory interface shear testing to determine representative δres and reduce design uncertainties. The findings of this work help to (1) facilitate the estimation of φres and δres based on minerology, clay content and other index properties, when project-specific testing is not available, and (2) provide guidance for developing tailored laboratory testing programme for acquiring φres and δres values
Zhou et al. (Mon,) studied this question.