Three-dimensional visco-elasto-plastic stress-strain-properties at small strains of stiff geomaterials

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Elasto-plastic stress-strain properties, as well as viscous properties (time-dependent properties), of stiff geomaterials under three-dimensional stress conditions were investigated by performing special tests using advanced triaxial and plane strain apparatuses. Specimens of stiff clays, sands, gravels, silty sand and sedimentary soft rocks were subjected to tests in which different constant strain rates were maintained, the constant axial strain rate was changed stepwise or the rate of strain rate was kept constant at several stages. For quasi-elastic stress-strain properties at strains less than about 0.001%, a hypo-elasticity model was proposed, for which the elastic constants are specific functions of instantaneous stresses. As to the viscous properties, the isotach model was found relevant to soft clays and a silty sand. A simple isotach model is proposed, in which the current stress is a unique function of the instantaneous irreversible strain and irreversible strain rate, and the current stress increment is a unique function of the instantaneous elastic strain increment. It was found that with clean sands, the isotach model is not relevant, and the shear stress and shear strain relationships are essentially independent of constant strain rates differing by a factor up to 500. It was also found that the stress level noticeably and temporarily increased and decreased when the strain rate increased and decreased stepwise (i.e., when a time history of strain acceleration and deceleration was given). Similarly, with a time history of strain deceleration, the sands exhibited noticeable creep deformation and stress relaxation.