Research On Soft Clays' Constitutive Relations Based On Hyper-plasticity

Hyper-plasticity is a new method to construct models of soils, which is basedon the first and second law of thermodynamics, and the stress-strain relations can bedetermined by free energy function and dissipation function. This new method hasbroken the framework of classical plasticity theory. Some physical models have beenused to interpret the internal variables, and the hypothesis of taking plastic strain asthe internal state parameter is analyzed in this paper. Ziegler's orthogonalitypostulate is proved to be equal to the maximum rate of dissipation criterion withLegendre transformation and Euler's theory. The role of free energy function anddissipation function has been analyzed, and the series of Mises models have beenused to explain the method of hyper-plasticity.Part of plastic work can be interlocked by plastic deformation in the form offree-energy. This point has led to the difference of maximum plastic work theory andmaximum entropy product theory. The relations between the two theories have beenproposed in this paper. Elastic-plastic coupling and dissipative coupling are mainreasons to make the associated flow rule unsatisfied for geomaterials. Dissipativeuncoupling is involved in Drucker postulate as the the choice of initial stress isarbitrary. Yield surface will move with stress state when Elastic-plastic coupling isconsidered, and the shape of yield surface will change when transformed fromdissipative stress space to true stress space when dissipative coupling is considered.The spacing ratio is defined to be the ratio between volumetric stresses of NCLand CSL on the same unloading line, which has been chosen as 2.72 in OriginalCam-Clay model and 2 in Modified Cam-Clay model. In fact, the spacing ratio is aparameter of soil properties, which has much influence on soil behavior. Based onhyper-plasticity theory, a model has been proposed to take account of the change ofthe spacing ratio, and the influence of spacing ratio on constitutive relations has beenanalyzed.Bases on the analysis of 13 typical intact clays' compression line, conclusioncan be drawn that the consolidation line of intact clays can be divided into two linearparts in double logarithm space. The compresson index and unloading index havebeen analyzed in both single-logarithm and double-logrithm space. It is proposedthat the spacing ratio can be taken as a constant parameter based on experimentalresult of some typical intact clays.As failure is the last phase of deformation, there are close relations betweensoil's constitutive model and strength. Anisotropy is an important feature of naturalsoils, it is proved that the inclined ellipsoid is the simplest yield surface shape thatcan take account of anisotropy of soft clays based on hyper-plasticity theory.Companied with critical state soil mechanics, formulas of undrained strength under K₀ consolidation have been proposed, these formulas can be used in triaxial stressconditions and plane strain conditions. Based on a large number of experimental data,the formulas are validated to be true. If we take other parameters as constant, a linearrelation can be got between the undrained strength ratio and the effective frictionalangle. Simplized formulas have been proposed based on this linear relation forengineering application.A unified model for clays has been proposed within the framework ofhyper-plasticity theory. A rotational hardening rule has been proposed. The physicalmeaning of model parameters and the method to determine them have been putforward.