The paper presents an experimental study on the residual shear strength of different clayey soils at varying totalsuctions. In a ?rst part of the paper some insight into the behaviour of residual shear strength of a ?lled rock discontinuityis presented to bring into light the sensitivity to hydraulic states of a clayey gouge material subjected to shearing along largedisplacements. An adapted Bromhead ring shear apparatus, in which the sample is enclosed in a controlled relative humiditychamber, has been used to control total suction during shearing. Afterwards, results on different types of clayey materials arepresented to discuss the importance of the plasticity of the clay on the residual friction angle changes. These changes showan important increase in the residual friction angle with imposed total suction that may reach 15?for medium plastic clay andwithout cohesion component. The reasons for this increase have been explained by the more granular character of the dry clay asa result of aggregate densi?cation and desaturation during strong drying. The changes have been interpreted using a simple modelinspired in both macroscopic and microstructural observations, which is based on the stiffness of the soil (or aggregates) anddepends on the product of the total suction applied and the degree of saturation. A microstructural model, already developed totake into account microstructural aspects on water retention and following an equivalent behavioural response to the macroscopicshrinkage curve, has been used to estimate the degree of saturation at the microstructural level (inside aggregates). To highlightits applicability in geotechnical practice, the different parameters used in the water retention and residual shear strength modelshave been shown to depend on the plasticity index of the different clayey soils studied.