THE SOFTENING CONSTITUTIVE MODEL OF LOESS Major-. Geotechnical Engineering Name: Xia Wangmin

In this paper the stress-strain and deformation characteristics of loess of the fifth tunnel of Luohuiqu were analyzed by means of conventional tri-axial tests and the softening constitutive models were established. The models provide not only the basic and the necessary calculation conditions to analyze the mechanics response and construction optimization of simple shield tunnel method on the fifth tunnel of Luohuiqu, but also a new method in researching the softening constitutive models of loess. Some useful results were found after those.Firstly, the softening and dilatancy characteristics of Qi loess stress-strain were found by means of conventional tri-axial tests. In order to establish the nonlinear elastic model of Qi loess, the hump curve was used to stimulate both the softening stress-strain curve and the volume deformation curve. After the streamlines and potential lines were plotted, the potential line of the hardening and softening parts were taken as the same ellipse form. In order to establish the softening elasto-plastic model of Qi loess, the plastic potential function used is in a ellipse form and the plastic work function was used as its hardening parameter.Based on thermodynamics laws, inner-variable theory and analysis of conventional tn-axial tests data and breakage characteristics of loess, this paper proposed that the loess would produce elastic, plastic and nonreversible damage deformation during loading. Supposed the two nonreversible deformations both obey the Drucker law, the plastic loading function was established based on experimental data and the plastic work was used as hardening parameter Supposed loess damage is mostly due to shear and the damage potential function has the same form of the equivalent plastic strain linein the plastic part, the damage potential function was gained by fitting the test data. Then the elastic-plastic damage constitutive model was established according to damage theory.Lastly, by comparison between calculation results of the three models and the test results, it has been found that the three models can all stimulate the softening and dilatancy characteristics of loess. Nonlinear elastic model can stimulate the hardening part before peak value, but the stimulation to softening part is not satisfying. Elasto-plastic model can basically stimulate the full course of loess deformation, but the stimulation to softening part is not better than that of hardening part. Elasto-plastic damage model is the best model of stimulating loess deformation, and the breakage degree during the loess deformation can be estimated based on magnitude and variety of the damage.