Permanent deformation behaviour of unbound granular materials

TLDR: In this article, the authors investigated the relationship between permanent deformation in unbound granular materials and the number of load cycles and the impact of applied stresses in road pavement design.

Abstract: The purpose of the study is to increase the understanding of plastic behaviour of unbound granular materials through a literature survey and laboratory testing. Reviews of previous research show that permanent deformation in granular materials is influenced by several factors. Generally, modelling of plastic behaviour is based on the gradual accumulation of permanent strain with number of load cycles and the impact of applied stresses. During the last two decades, attention has been paid to the shakedown theory and its potential for developing true analytical models in road pavement design. The experimental part of the investigation deals with the permanent deformation behaviour of five different aggregates under repetitive loading. Triaxial and hollow cylinder tests are used to assess the relationship between permanent strain and both number of load applications and stress level. The test results are used to verify some of the existing models found in the literature, with greater emphasis on the newly developed Paute model. The general concept suggested by the Paute model regarding the effect of stresses is discussed and shown to be questionable, as it leads to incorrect conclusions about material behaviour. However, the Paute model is found generally suitable for predicting the accumulation of permanent strain with number of load applications. A model is introduced expressing the accumulation of permanent axial strain, at any given number of cycles, as a function of applied stresses, taking into account the maximum shear stress ratio and the length of stress path in p-q space. The results of the analysis indicate similarities between the model and the concept of the shakedown theory. At low levels of shear stress ratio, the growth of permanent strain appears to level off, resulting in an ultimate equilibrium condition characterised by an elastic material behaviour. (A)