Rutting is the load-induced permanent deformation of a flexible pavement. Depending on the magnitude of the load and the relative strength of the pavement layers, permanent deformation can occur in the subgrade, the base, or the upper hot-mix asphalt (HMA) layers. Appropriate remedial action for existing rutting requires a knowledge of the relative contributions of the layers (subgrade, base, and HMA) to the total permanent deformation in the pavement.

The technical literature suggests that the layer or layers primarily responsible for the rutting of a flexible pavement can be identified from an analysis of the shape of the pavement's transverse surface profile. Measuring a transverse surface profile is easier and less hazardous than cutting a transverse trench to examine underlying layers.

The objectives of this project were to (1) investigate whether the relative contributions of the layers to rutting in a flexible pavement can be determined from an analysis of its transverse surface profile and (2) prepare a method for estimating the relative contributions of pavement layers to total rutting.

The research team first conducted extensive computer analyses using the finite element method to simulate rutting failures in HMA surface mixtures, base courses, and subgrades. Transverse surface profile characteristics indicative of failure within specific structural layers were identified in the form of distortion parameters; specific criteria for these parameters were developed and validated through comparison with trench studies conducted on accelerated pavement test and full-scale field sections. A method to estimate the contributions of individual pavement layers to the total rutting was developed in the form of a recommended test method in AASHTO standard format.

The final report has been published as NCHRP Report 468.