Critical shear stress and its relationship with cohesion for sand-kaolinite mixtures

Detachment and mobilization of fine particles are important considerations both in soil erosion studies and in studies related to the fate and transport of colloidal contaminants. Previous studies reported in the literature expressed particle detachment in terms of critical shear stress, which indicates the shear stress required to be generated on pore walls through pore-fluid flow, to initiate particle detachment. In this paper, the relationship between critical shear stress and cohesion is explored using both theoretical and experimental investigations. A spherical particle model is used to relate the two quantities for mixtures of sand and kaolinite. The model represents clusters of clay platelets as particles similar to spherical sand grains and expresses cohesion in terms of hydrodynamic forces associated with critical shear stress. Experiments using a compaction permeameter to facilitate an indirect evaluation of critical shear stress for four different mixtures of sand and kaolinite are reported. A time-dependent change in critical shear stress is found to be associated with a corresponding increase in cohesion; for mixtures with low percentages of kaolinite, the spherical particle model captured the variation in cohesion in terms of critical shear stress.