Resilient behaviour of unbound granular materials through repeated load triaxial test: influence of the conditioning stress

The present paper reports the results of a laboratory investigation aimed to investigate the resilient behaviour of unbound granular materials (UGMs) for pavement unbound layers under repeated triaxial loading. The first part of research was focused on the effects of stress history, in terms of loading conditioning phase and stress paths, on the resilient response of the investigated UGM. The obtained results highlighted that the samples that experienced several stress paths after an initial stress conditioning showed reduced resilient moduli as well as higher stress dependency of the resilient properties as compared with samples not subjected to any specific stress history. The second part of research aimed to evaluate the influence of the conditioning stress level on the resilient modulus. The results showed poorer resilient characteristics along the several stress paths as the selected conditioning stress increases. Whereas, a sort of "loading memory effect", resulting in a similar mechanical behaviour, can be recognised as a similar stress state between conditioning phase and subsequent stress path is considered. The study confirms the high dependency of the resilient response of UGM on the stress condition and in particular, the main role of conditioning confining pressure on the stable mechanical behaviour of these materials is highlighted. Finally, a simplified approach that would allow the intrinsic resilient properties of UGM to be predicted for a specific combination of conditioning stresses and stress paths is proposed.