An investigation of tyre contact area on unbound aggregate

ABSTRACT: An important input parameter to the design of unpaved roads is the wheel load from vehicle traffic. Some design methods consider only the axle load regardless of tyre size whereas more advanced methods consider a tyre contact pressure and contact area at the unbound aggregate surface. They assume the contact pressure equals the tyre inflation pressure and the contact area is calculated accordingly – usually assumed to be a circular contact area. However, truck tyre pressures are often higher than typical aggregate bearing capacity meaning that the unbound aggregate must yield somewhat under such pressures while a proportion of tyre load must be transferred to the ground through the tyre carcass. A full-scale physical trial was undertaken by measuring the contact area between a dual tyre of different loads and tyre inflation pressures on a compacted unbound aggregate layer overlying a clay subgrade as well as on a concrete slab. The results confirmed that contact areas were significantly higher, and hence average contact pressures lower, than predicted by the assumption of tyre contact pressure equals the tyre inflation pressure. The error was as high as about 100% which has serious implications for unpaved roads design methods that adopt this assumption. An alternative method of estimating tyre contact pressure using Hertzian elastic contact theory was proposed. A low stiffness was adopted for the unbound aggregate to take account of yield while the tyre stiffness was adjusted for tyre inflation pressure. The proposed method provided a much improved prediction of the tyre contact areas and shapes in this physical study as well as other physical and numerical studies. If adopted in road design, it should improve the accuracy of designs across a wide range of axle loads and tyre sizes.