Critical state friction angle of sands

The friction angle is the most important parameter used for analysing the response of sands to loading. However, its variation with stress level, fabric and particle damage has been debated. This study examines the yield and critical state friction angles of three sands using triaxial compression and ring shear tests. Only contractive responses were used to define the yield friction angles and the critical state friction angles from the triaxial tests. However, both contractive and dilative (through particle damage) specimens reached a critical state in the ring shear tests, and therefore critical state friction angles were defined from both dense and loose specimens. The yield friction angle was affected by the initial sand fabric, decreasing as the pre-shear void ratio increased. In contrast, the critical state friction angle from the ring shear tests was independent of stress paths analysed in this paper, independent of initial sand fabric, and decreased only slightly with stress level, becoming essentially constant at stresses larger than about 200 kPa. Its value depended primarily on particle mineralogy and shape (angularity). Particle damage induced in the ring shear tests increased the critical state friction angle by a few degrees, as a wider range of particle sizes and more angular particles were produced.