Investigation of the effect of shoulder properties, axle load, and tyre pressure on edge failure of thin surfaced granular pavements
New Zealand has a tradition of building granular pavements surfaced with chipseals. This form of construction is used on most rural roads and can perform well under traffic volumes of over 10,000 vehicles/lane/day. Because of New Zealand mountainous topography many highways have very narrow shoulders. Due to this lack of lateral support and the intrusion of heavy axle loads onto the edge of pavement, edge failure has been encountered on many of these roads. This has more recently been exacerbated by the growth in forestry which has seen large truck and trailer units transporting logs out of the forests. The main objective of the research work reported here is to investigate how the pavement shoulder properties and load parameters affect this type of distress. A three dimensional finite elements model and a half fractional factorial experimental design were made to study five factors, viz., shoulder width, shoulder stiffness, axle load, tyre pressure and pavement thickness. Each factor was studied at two levels to simulate extremely low and high conditions. The results of the finite elements model have been validated to ensure accurate predictions. The multilayer elastic solution was carried out using Circly and Everstress software. Linear and nonlinear analyses using isotropic and crossanisotropic material properties were compared to the actual strain measurements carried out at Transit New Zealand accelerated test track facility (CAPTIF). None of the analytical solutions perfectly matched the measured strains. In addition, the results of the multilayer and finite elements analyses were compared together at different locations and depths from loaded area. The multilayer and three dimensional finite elements solutions were reasonably close for stresses, strains and deflections. Only very slight differences were observed for stresses and strains near the surface because of the difference between the circular and rectangular shape of loading areas in the mulilayer and finite elements analyses. The statistical experimental factorial analysis showed that the order of importance of the different factors affecting pavement response relies on the type of response and the location of that response. Three pavement responses were studied, maximum deflection between dual tyres, maximum deflection under the outer wheel load, and compressive stain at the top of the subgrade near the shoulder (300 mm from the centre of the dual). The axle load was the most significant factor affecting the deflection and strain at all locations. The interaction of shoulder stiffness, width and thickness has been shown to significantly affect the pavement response. It is proposed that the results of this research can be used by pavement designers to obtain an indication of the effect of lack of shoulder support on pavement performance.