Thermodynamics Based Constitutive Modeling For Overconsolidated Clays

With the boosting of the economic power and national strength, the constructions of infrastructures, such as highways, railways, airports and dyke, appear everywhere in China. Over-consolidated (OC) clays are widely distributed in the coastal area of eastern China. Accurately predicting the deformation and strength of foundations that built over the OC clays is a rather challenging task but remains an important subject in geotechnical engineering area.This research aims at establishing a thermodynamics based constitutive model for isotropic and anisotropic OC clays under both drained and undrained conditions. A new dissipative function is assumed by modifying the dissipative functions of modified Cam-Clay model and a model, and together with a reasonably selected free energy function, the yield function in the true stress space is deduced. Abundant experimental evidence has consistently shown that highly OC clays have produced plastic strain from the very beginning of the shearing, but models based on conventional elastoplastic theory fail to capture such phenomenon, as a purely elastic response inside the initial yield surface associated with the maximum consolidation stress is assumed. To overcome this, a bounding surface framework is employed and an appropriate mapping rule is proposed in the modeling. To simulate the behavior of the anisotropic consolidated clays, the concept of the rotational hardening is revisited from the perspective of thermodynamics and critical state theory. A novel rotational hardening rule is proposed which satisfies the requirements of both thermodynamics principles and critical state theory. The capability of the model is verified by triaxial compression tests of different types of clays under varying test conditions.The main outcome of this research can be summarized as follows:1) A careful examination on the experimental data indicates that at critical state the ratio of mean normal stress p and the size of yield surface p0c ranges is not always equal to 0.5. Therefore, a α-β modified yield surface is proposed and the new model parameter β is used to adjust the intersection between the critical state line and yield surface in the p-q plane; 3) The bounding surface model is employed to simulate the plastic deformation at the beginning of shearing. Adopting a movable mapping center on the p-axis and working together with the dilatancy guarantees the dilative response at the’dry’side, while contractive response at the ’wet’ side; 3) Because the free energy cannot be accumulated infinitely, the rotated bounding surface must turn back to align on the hydrostatic axis symmetrically at critical state. An innovative rotational hardening rule for anisotropic OC clays is formulated to meet this requirement; 4) The initial inclination angle 90 of the bounding surface is linearly related to the initial stress ratio η0 of the soils. The original size of the bounding surface no longer equals to the mean normal stress for normally consolidated clays, and should be determined by the intersection of the NCL 90 and URL at η0 in the e-lnp plane; 5) Using this model to simulate the responses of different types of clay under different conditions, the results show that the proposed model can predict the strength and deformation of OC clay well.