Rheologic behaviour of clays in plastic state

Abstract

Clayey soils in plastic state exhibit viscoelastic behaviour, which imply simultaneous storing and dissipative mechanic characteristics and also strain-rate dependant properties. Such a response is partially captured by the so-called Atterberg plasticity limits. A more complete evaluation of the mechanic response of this type of soils in plastic state can be obtained by tests in a dynamic shear rheometer. Clayey soils composed by different proportions of kaolin and bentonite, as well as a variety of water contents, were subjected to dynamic shear testing to measure their rheological properties. All tests were performed at constant strain amplitude, fixed-range frequency sweep and constant temperature. Results show that the complex shear modulus has a relationship with the water content of soil and also with the plasticity index. In addition, the phase angle decreases as the frequency increases. Complex shear moduli by dynamic testing were compared against oedometric moduli estimated by correlations, showing satisfactory trends between them. The study helped to understand the sophisticated behaviour of soils in plastic state. Although the mechanic response of these materials is influenced by a number of factors, plasticity limits were consistent with complex moduli under analogue conditions of strain and frequency.