Changes in geometrical standards to minimize deterioration of gravel roads in Sri Lanka

Abstract

Majority of Sri Lanka's road network belongs to the categories of either earth roads or gravel roads. However, these unpaved roads are the backbone of Sri Lanka's rural economy and they provide the mobility for rural population. Unfortunately, most of these unpaved roads are not in good condition and the local authorities are spending a lot of money to convert these to paved roads due to public pressure. This has resulted in even higher spending in maintenance in the long run. This paper looks into the problems associated with gravel roads in Sri Lanka and studies the possibility of using properly built gravel roads as a solution to the higher cost in road maintenance in rural areas. Traffic and road condition surveys were conducted to identify the present traffic volumes and identify the primary reasons for road failures. A road condition survey is focused to identify geometrical parameters such as longitudinal and cross slopes, radii of curvature and carriageway width etc. that have direct influence on the road condition depending on the environmental factors. These observations are used to develop suitable geometrical design standards for gravel roads to accommodate non-motorized or alternate modes of transport as well. A comprehensive cost analysis is done to identify the construction/ maintenance costs for paved and gravel roads. Traffic surveys revealed that the traffic volumes on these gravel roads are very much less than 150 vehicles per day and the percentage of heavy vehicles is negligible. It is found that lack of drainage facilities, surface run off, and lack of proper geometric standards are the main reasons for road failures. It is recommended to use carraigeway width of 3.5 m with sufficient shoulders to minimize the effects due to surface run off Further, it is recommended to use a combination of maximum and average longitudinal gradient. If the average gradient limitation cannot be met, it is cost effective to use the maximum gradient at curves and pave these small sections to prevent surface erosion.