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.
