In this paper clear evidence based on experimental results is given regarding the possibility of representing the behaviour of clays in shear within the framework of the so called "stress-dilatancy theory". As with well-established interpretations of the behaviour of granular material, the frictional response of clays in triaxial testing appears determined by the dilatancy that the soil exhibits under the specific testing conditions in terms of density and stresses. As for granular soils, dilatancy is shown to be a function of the distance, both in terms of density and stress ratio, of the actual state from the critical state, where shearing occurs at constant volume. To represent these distances, use is made of the state parameter ψ as defined by Been & Jefferies (1985) and of the difference between the mobilized friction and the friction at critical state. The relationship used to link dilatancy to the soil actual state, that is the flow rule, is of the same type as those valid for granular soils. Values of the relevant parameters for clays are obtained from test results and compared with those available from the literature that so far refer to sand samples only.
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