A mobilised bearing capacity approach to the performance-based design of unpaved roads

Unpaved roads are used on low traffic or temporary roads such as for materials transport across construction sites or mines. Performance is usually expressed as a rut depth and if it becomes too large, the passage of vehicles may be hindered and repairs needed. A new design method to calculate the required aggregate thickness to avoid excessive rutting both at the surface and at subgrade level is proposed. Tyre size is an input parameter allowing application of the method to a wide range of cases from heavy haul roads to lightly trafficked local roads. A wide range of rut depths and axle passes may be specified, even as low as values typically required in proof roll testing. The proposed method takes a mobilised strength approach to predict permanent deformation on the first pass by means of newly derived hyperbolic relationships. The permanent deformation on the first pass is then coupled with a separately derived deformation accumulation (logistic) function to predict rut depths following the first pass. The adaptation of well-established bearing capacity design methods with fundamental soil strength parameters has provided a framework for the method’s application to a wide range of aggregate and subgrade soil types, including low-quality or recycled aggregates as well as those mechanically stabilised by geogrid.