An approach to rheometry in soil mechanics - Structural changes in bentonite, clayey and silty soils

Rheology is regarded as the science of flow behaviour, where, based on isothermic equations, the deformation of fluids and plastic bodies subjected to external stresses may be described. Hooke's law of elasticity, Newton's law for ideal fluids (viscosity), Mohr-Coulomb's equation, and finally, Bingham's yielding are well known relationships and parameters in the field of rheology. Rheometry is a well established measurement technique to determine the specific rheological properties of fluid and plastic bodies. However, the application of this technique in soil mechanical investigations is yet relatively uncommon. In order to explain point contact processes and strength an extrapolation of such findings to data of triaxial, direct shear or oedometer tests is still missing. Thus, this paper aims to introduce rheometry as a suitable method to determine the mechanical behaviour of soils and mineral suspensions when subjected to external stresses. To do this a Na-bentonite, lbeco Seat-80 (IS-80) has been used as a testing material, and the suspensions were equilibrated with NaCl solutions in different concentrations in order to determine the ionic strength effects on interparticle strength and changes in mechanical properties. Furthermore a vertisol, a clayey oxisol, both from Brazil, and loess material from Israel, saturated with NaCl in several concentrations were analysed. A parallel-plate-rheometer MCR 300 (Modular Compact Rheometer, Paar Physica, Ostfildern, Germany) has been used to conduct oscillatory tests. From the stress-strain relationship parameters and specific characteristics as thixotropy, storage modulus G ' and loss modulus G '', viscosity eta, yield stress tau(y) and the linear viscoelastic deformation (LVE) range including a limiting value gamma(L) were determined and calculated, respectively. With aspect to salt effects, amplitude sweep tests on CaCO3 rich Avdat loess show an increasing stability due to higher NaCl concentrations. In a comparative test of Avdat loess and lbeco Seal-80, turbulent versus sliding shear behaviour could be illustrated. To demonstrate clay mineralogy effects or textural effects clay rich substrates from Brazil, a smectitic vertisol and a kaolinitic oxisol were compared, showing a higher level of stored elasticity (stability) in case of smectites and a lower value in regard to kaolinites. These preliminary results of amplitude sweep tests show that rheometry is a potential method of investigating microstructural characteristics of mineral suspensions and of clayey or silty soils. (c) 2006 Elsevier B.V. All rights reserved.