Influences on Permanent Deformation Behavior of Unbound Granular Materials

Abstract

The results of a research program aimed at characterizing the permanent deformation behaviors of different types of unbound aggregates under repeated loading are summarized. Tests were performed in a triaxial or hollow-cylinder apparatus, depending on the grading of the materials. The scope of the research was to assess the relationship between total permanent axial strain and both the number of load applications and the stress level. The test results were used to verify a model recently developed in France in which attempts are made to predict the long-term behaviors of granular materials. The French model is examined with particular reference to its validity on the effect of stress level on permanent deformation of unbound granular materials. The results of the study indicate that the French model is generally successful in predicting the permanent strain for a given number of load applications. For one of the materials, however, the model did not seem to fit the data well. The French model also suggests that the variation in total permanent strain with stress level is related to the static failure line and could be determined by comparing the maximum shear stress ratio with the slope of the estimated failure line. This is discussed and shown to be questionable because it results in either unreasonable failure parameters or a very low level of correlation with the observations. An attempt was made to modify the French model, but that also proved to be unsuccessful. The concept of relating the permanent deformation behavior to the static failure condition of the material is therefore questioned.