Abstract:
— Most of the roads constructed in Ethiopia fail prematurely before serving the design life due to
various causes arising from many factors. One of the roads that failed before reaching its design lifetime is the
Bako to Nekemte Trunk Road. This road was constructed and opened to traffic in 2013. The design pavement life
was estimated at 20 years. However; pavement failures have manifested since 2014 after it was opened for traffic.
The main objective of this research was to investigate the cause for the failure of flexible pavement on the Bako to
Nekemte road segment. To achieve this objective, the study was followed by the experimental research type and
purposive sampling method. Also, field observation and pavement condition survey methods for data collection
were used. The asphalt, base course, subgrade, and sub-base layer material sampled from the road segment are the
material used for this study. Different pavement failures such as rutting, pothole, alligator cracking, raveling, edge
cracking, depression, and corrugation were observed along the road section. For estimations of pavement condition
index, the road was divided into five different sections based on distress densities, which were measured during
the pavement condition survey. From those five sections, four sections were selected for the determination of the
Pavement Condition Index (PCI). Using systematic random sampling 159 sample units were evaluated for
Pavement Condition Rating (PCR) and the result shows 1.89% excellent, 8.18% very good, 20.13% good, 31.45%
fair, 18.87 % poor, 18.24% very poor, and 1.26% failed. Based on PCI value, samples of pavement layer were
taken from failed surface condition (1), poor surface condition (2), and none distressed area (1) for comparison
purposes. Different quality tests like Atterberg limit, wash gradation, soil classification, compaction test, California
Bearing Ratio (CBR), Los Angeles Abrasion (LAA), Flakiness Index ( FI), Aggregate Crushing Value (ACV), Ten
percent fines value (TFV), bitumen content, and gradation of asphalt were carried out. The results of these tests
were compared with ERA specifications to identify the probable cause of pavement failure. The study found that
the cause for failure of flexible pavement in the case of Bako to Nekemte was: insufficient and absence of side
drainage structure, traffic loading, poor gradation of base course and sub-base material, and poor quality of
subgrade soil. The study also recommended that the road urgently needed routine maintenance as a treatment option
to reduce further deterioration and extend its service time.
