Investigation On The Causes Of Failure In Unpaved Roads And Recommended Remedial Measures (The Case Of Jimma –Meteso Road Section)

Abstract

The pavement is the uppermost layer of the road that carries the vehicle load. A road pavement can be flexible pavement, rigid pavement, or unpaved road materials such as gravel wearing course, stabilized gravel, or earth road. Gravel roads are composed of the gravel wearing course, a subgrade layer, and an optional sub-base layer. The most common failures on gravel roads are raveling, rutting, potholes, erosion, and gravel loss, and these failures are associated with a number of factors. This research tried to identify the causes of failures on unpaved gravel roads by considering the Jimma Meteso road project as a case study. From the road, sections that are severely damaged were identified the most common types of failures observed are potholes, rutting, raveling, corrugation and gravel loss and erosion. Based on a visual inspection, to determine the field density of the subgrade sand replacement method was used and soil samples were collected and tested in the laboratory. The tests included sieve analysis, Atterberg’s limit, gradation, Californian Bearing Ratio (CBR), and compaction was conducted on the wearing course material as well as the subgrade. The traffic loading on the site has also been analyzed from the traffic count data. From the comparison of the laboratory test results with the Ethiopian Roads Authority (ERA) gravel and low volume road manual, indicated that there is a variation for most of the tests. The gravel wearing course has more amounts of fines than required and has higher plasticity index. Whereas, the subgrade has a high plasticity index and is not strong enough to sustain the axle loads. From secondary data obtained from ERA traffic analysis, it has been observed the actual traffic on the road is greater than the design traffic AADT; thus, the failure of the road has been attributed to poor construction material and heavy traffic. Finally, for each type of failure observed, remedial actions such as spot regravelling, installing proper drainage, blading the gravel and blending with better material were recommended.