A comparison of design and actual traffic levels and its likely impact on the pavement premature failure : case study of Gulu – Atiak – Nimule Highway

Abstract

The Gulu-Atiak-Nimule highway was constructed and completed to a flexible pavement standard in the year 2015 and opened to traffic that traverses this highway from mainly Kenya, Tanzania via Uganda to South Sudan. The anticipated increase in traffic volumes and especially heavy axle load trucks might have brought the road pavement to a premature failure. The road is currently short of its structural capacity, highly distressed and showing signs of failure. This is affecting the flow of traffic and increasing operational costs coupled with the fact that a number of accidents have been reported resulting from a number of distresses such as potholes, cracks, edge shoulder drops, rutting, ravelling, and failed sections of the road. This research study involved carrying out traffic surveys, axle load surveys and road condition assessment surveys. The traffic surveys involved manual classified seven consecutive day count for 12 hours and 24 hours. The 24-hour count was carried out for 2 days, with 1 day within the week and 1 other day within the weekend. The axle load survey was done at the fixed weighbridge at Elegu near the South Sudan boarder. The condition assessment was undertaken using the ASTM D6433 method that involved visual inspections and recording of distresses observed on the pavement surface and qualifying them. Results from the study indicated that the current Average Daily Traffic (ADT) and Average Annual Daily Traffic (AADT) are 2604 and 2939 Vehicles per day respectively. Heavy traffic alone consisting of mainly truck trailers and semi-trailers constituted 29% of the total traffic volume. The Cumulative Equivalent Million Standard Axle for the year 2024 was calculated and obtained as 1.285million ESA which is a T3 Traffic Design Class. The comparison of these volumes to that considered at design reflected a traffic growth rate of 14% and yet that at design, the growth rate was taken at 6% per year for the first 10years. The axle load survey results indicated that the average weigh of heavy traffic was 43.025tons and this generated a damaging factor to the pavement of 2.973. The pavement condition evaluation carried out, indicated that the Pavement Condition Index and International Roughness Index for the Gulu–Ataik sectional link was 38 and 4.0m/km respectively; while that of Ataik –Nimule section registered PCI and IRI of 64 and 3.5m/Km respectively. This study has revealed that the current traffic volume is high totalling to 2604vpd, with 29% of this being attributed to heavy traffic and therefore increased million standard axles. The section from Gulu–Ataik was in a poor condition and hence required rehabilitation to further extend the pavement life; while that of Atiak-Nimule was in a fair to good condition and required routine maintenance.