Characterisation of permanent deformation behaviour of unbound granular materials under repeated triaxial loading

The present paper reports the results of a laboratory experiment that aimed to investigate the permanent deformation behaviour of two unbound granular materials for road subbase layers under repeated triaxial loading. In the first part of research the shakedown concept was used to classify the mechanical response of the granular mixtures. The obtained results confirmed the potential usefulness of this theoretical approach for ranking granular materials with regard to their rutting potential. The second part was entirely dedicated to the development of an analytical model to describe the long-term permanent deformation behaviour of these materials. The proposed model allowed permanent strain accumulation as function of the number of load applications and applied stress level to be described. The results, besides being consistent with the shakedown concept, showed the ability of the model to reflect the change in mechanical behaviour due to the different nature of materials, under specific stress and moisture condition. In addition, the model allowed the permanent deformation accumulation resistance of the material to be properly investigated through specific strain rate envelope curves defined on a Mohr–Coulomb diagram. Therefore, this study could propose an innovative and practical design approach for permanent deformation behaviour assessment of unbound granular material and consequently for evaluating its suitability in the pavement in order to avoid undesirable response.