The Thermomechanical Model for Structured Soil Based on Energy
Dissipation
Abstract: The ideas of thermomechanics, the first and second laws of thermodynamics, is introduced to describe the energy dissipation of structured soil deformation. The energy dissipation due to deformation and cementation bondsrsquo; damage are distinguished explicitly. Not all energy is dissipated by elasto-plastic behavior, but some are for damage dissipative of cementation bonds between soil particles. Based on the thermomechanical model proposed by Collins, its shortage of considering soil structure is improved by proposing a new energy dissipation potential function. Once the forms of the free energy function and dissipation potential function have been specified，the corresponding yield surface, flow rule, hardening rule as well as the elasticity law are deduced in a systematic manner. The model automatically satisfies the laws of thermodynamics, and there is no need to invoke any other postulations.
Key words: thermomechanics; constitutive equations; structured soil; elastoplasticity; the cementation bond
There are two methods to construct the constitutive model of soil elastoplastic constitutive model.One method is the experimental curve fitting method, which can get the empirical model.
For example, the Lade-Duncan model; Another approach is to assume from some theoretical assumptions.Through strict mathematical derivation, the concrete elastic law, yield condition, plastic potential function and hardening rule are given, namely the theoretical model.
Like the Cambridge model. The establishment of theoretical model is based on the simulation of macro and micro concepts of soil.Therefore, some essential characteristics of soil can be deeply described.However, these functions and their parameters tend to be independent, which is not rigorous in theory, and there is no strict discussion on whether the laws of thermodynamics are satisfied. Therefore, there are unreasonable aspects[1,2].
In the last twenty years, great progress has been made in modern thermodynamic theory.
It provides a unified framework for the derivation of constitutive equations of various materials including soil and other granular materials[3].Recently, this theory has been systematically applied to the simulation of soil constitutive model and made a breakthrough[4-6].The thermodynamic method of simulating the constitutive properties of rock and soil has abandoned the unreasonable practices of traditional elastoplastic modeling.Directly from two thermodynamic potential function, namely, free energy function and dissipation of the incremental function, using the Legendre transformation, the elastic-plastic theory must be derived directly yield condition, flow rule and hardening law and law of elasticity. This method not only has a compact mathematical structure, but also eliminates many artificial assumptions in traditional methods and automatically meets the laws of thermodynamics. Generalized thermodynamics applied to constitutive model is an important branch of contemporary soil mechanics research[7,8].Collins,Houlsby, Puzrin, Hilder and others[9,10] gave the theoretical framework for constructing the soil mechanics model without considering the time effect.This theory is applied to the study of elastic-plastic constitutive model. But so far, there is no thermodynamic model can be used for simulation of natural undisturbed soil stress-strain relationship, the reason for this is that the existing thermodynamic modeling idea and method of dissipation parameters according to the form of incremental function, is remoulded, failed to take into consideration the natural soil structural. For this shortage, this article is based on the principle of energy dissipation, the traditional thermodynamic modeling method is improved, the concepts of structural bond damage energy dissipation, it expanded into a unified thermodynamic modeling method can reflect soil structure.
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