In-situ assessment of the stress-dependent stiffness of unbound aggregate bases: application in inverted base pavements

Unbound aggregate bases are the primary structural components in many flexible pavements. The response of the unbound aggregate base is critical to the overall performance of the pavement, particularly in inverted base pavements given the proximity of the base to the traffic loads. The behaviour of granular materials such as unbound aggregate bases is inherently nonlinear and anisotropic. An experimental methodology is developed to assess the in-situ stress-dependent small-strain stiffness of unbound aggregate bases under controlled load using wave propagation techniques. CODA wave analysis is used to detect small changes in travel time. The methodology is applied in two distinct case histories of inverted base pavements. Results show that field-compacted granular bases exhibit higher stiffness, lower stress sensitivity and more pronounced anisotropy than laboratory-compacted specimens. The discrepancy in stiffness observed among the two field case histories is primarily attributed to traffic preconditioning sustained by the older pavement. Additional results show that the effect of suction on the stiffness of coarse-grained granular bases is insignificant.