Experimental tests on residual shear strength of clays using “high suction control system” have shown that the friction angle significantly increases at high suction, and that such an increase is more significant as the plasticity index increases (Vaunat J. et al. 2006; Meca 2007; Vaunat J. et al. 2007). These results are not in agreement with the classical results on saturated soils where the residual friction angle reduces as plasticity index increases (Bishop 1971; Lupini J.F. et al. 1981; Skempton 1985). This paper intends to explain these results by taking into consideration the microstructure of the tested materials. In particular, direct observations after tests with Environmental Scanning Electron Micrographs (ESEM) and Microstructure Intrusion Porosimetry (MIP) techniques were used. The analysis shows that samples undergo microstructural changes due to drying: micro-cracking, stiffening and developing of aggregate arrangements and changes in microporosity are the main observed features. These microstructural changes seem to develop a stiffer material which indeed behaves in a “granular” way in terms of strength.

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