An alternative philosophy on the deterioration and design of low volume roads

Abstract

The performance of paved roads is generally assessed in terms of riding quality and rut depth with failure being assumed at various terminal conditions depending on the category of road. The extent of the road affected by this terminal condition also depends on the road category. On this basis, the road will be deemed to have failed when the rut depth over 20 or 50% (Category C and D respectively) of the road exceeds 20 mm and/or the riding quality exceeds 4.6 or 5.1 m/km respectively. Two considerations come into play when assessing the performance of low volume roads near their terminal condition. Firstly, as long as the pavement surfacing is still intact (i.e. not too many un-repaired potholes), the road will always provide a better service/riding quality and lower vehicle operating cost than the equivalent unpaved road. Secondly, the mode of failure of conventional roads is the cumulative permanent deformation (rutting) of the road under heavy axle loads. Low volume roads usually have few heavy loads, the majority of which often occur during the dry season, and traffic induced rutting is minimal – the majority of rutting is a result of early compaction of the layers due to lower densities achieved in the pavement layers during construction. Of greater consequence with these roads is the effect of a few overloaded vehicles (particularly during wet weather or when drainage maintenance has been neglected) causing large strains (or even isolated shear failures) in the layers. These roads, however, are usually still perfectly fit-for-purpose (after localised repairs) and often do not deteriorate further with time, even with minimal surfacing maintenance. Conventional mechanistic empirical analyses carried out on these roads either severely over- or underestimate their structural capacity, with many still providing adequate surface after 20 or more years. More use of the in situ shear strength in designing such roads is thus proposed. This paper discusses different philosophies in designing and predicting the carrying capacity of such low volume roads