Design of granular pavements

The use of high-quality quarried crushed rock aggregates is generally required to comply with current specifications for unbound granular materials (UGMs) in pavements. The source of these high-quality materials can be a long distance from the site, resulting in high transportation costs. The use of more local sources of marginal materials or the use of secondary aggregates is not allowed if they do not fully comply with existing specifications. These materials can, however, be assessed for their suitability for use in a pavement by considering performance criteria such as resistance to permanent deformation and degradation instead of relying on compliance with inflexible specifications. The final thickness of the asphalt cover and the pavement depth are governed by conventional pavement design methods, which consider the number of vehicle passes, subgrade strength, and some material property, commonly the California bearing ratio or resilient modulus. A pavement design method that includes as a design criterion an assessment of the resistance to deformation of a UGM in a pavement structure at a particular stress state is proposed. The particular stress state at which the aggregate is to perform in an acceptable way is related to the in situ stress, that is, the stress that the aggregate is anticipated to experience at a particular depth in the pavement. Because the stresses are more severe closer to the pavement surface, the aggregates should be better able to resist these stresses the closer they are laid to the surface in the pavement. This method was applied to two Northern Ireland aggregates of different quality (NI Good and NI Poor). The results showed that the NI Poor aggregate performed at an acceptable level with respect to permanent deformation, provided that a minimum of 70 mm of asphalt cover was provided. It was predicted that the NI Good material would require 60 mm of asphalt cover.