Investigating the effect of axle loads on the progression of potholing on bituminous road pavements in Uganda

Abstract

In Uganda Roads are the main mode of transportation of goods and people and government is putting in effort to construct more roads. National paved roads increased by 21.7% in the last five financial years while currently population is increasing and there are about 800,000 registered vehicles which increase by 2000 per year (UBOS, 2014). This increase in traffic has led to rapid deterioration of the roads partly because of the overloading of the heavy vehicles which necessitates the need to over value the axle load (the total weight felt by the roadway from all wheels connected to a given axle) during the design stage of a pavement. The increase in the axle loads on the roads makes us question the possible effects of increase in axle loads. The research was carried out purposely to investigate the effect of axle loads on progression of potholing on bituminous road surface. The traffic data for Sentema road and Ntinda - Nakawa road was collected by manual counts using Ministry of works and Transport (MoWT) tally sheets. This exercise was carried out for 2 days one weekend day and one weekday on each road. This traffic data showed changes in traffic volume and axle loads that were designed for the road. The surveys carried out established the road geometrics of the road and the extent of potholing, ravelling and cracking. The study determined the rate of pothole progression per million equivalent standard axle loads (MESALs) as adopted by the HOM III potholing progression model. With the changes in the traffic classes, differences in the axle loads were established. The study established the difference between potholing progression under two scenarios: designed loading and actual loading. On Sentema road, potholing progressed by 10.97% for any additional of 1 MESAL. This signifies that there is a direct / proportional relationship between the unit increase of axle loads and pothole progression. For any additional 1 MESAL, the potholing progressed by 19.57% for a pavement with a 20 mm surfacing. Whereas a pavement with 150 mm surfacing, potholing progresses by 2.29% for every additional I MESAL. Among the key recommendations, there is need to establish axle loads of a given road to be constructed prior to construction and also the concerned authorities should overlay and seal cracks to curtail emergency of potholes.