Abstract:
In Ethiopia, roads often fail before the end of the design period or, in some cases, a
few months after completion. Failures can have many different causes, but are
typically caused by poor construction practices, poor workmanship, inadequate
knowledge, and the use of inferior construction materials. One of the road sections
that failed before the design period was the road from Dejen to Felegebirhan. The
purpose of this study is to report the results of field and laboratory investigations,
determine the cause of the failure, and consider possible remedies. From the field
investigation, samples were taken from each of the pavement layers, the current
condition of the existing pavement was assessed, and in-situ tests were carried out in
test pits dug at staggered locations. According to the test results, the liquid limits for
subgrade, fill, sub base and base course are 57% to 74%, 48% to 62%, 24% to 58%
and NP to 41% respectively. The plastic index ranges from 25% to 33%, 18% to 27%,
5% to 21% and NP to 16% for subgrade, backfill, sub base and base course
respectively. For CBR, subgrade, fill, sub base and base course the ranges are 1% to
4%, 3% to 28%, 28% to 132% and 58% to 125% respectively. For natural subgrade,
backfill, sub base and base course, the swelling values range from 2.6% to 6.4%,
1.27% to 3.24%, 0.52% to 2.0% and 0.18% to 1.08% respectively. In addition, the
DCP-CBR values of the subgrade range from 12% to 50% and the field density test
results for the natural subgrade, fill, sub base and base course range from 0.86
gm/cm3 to 1.16 gm/cm3, 1.17 gm/cm3 to 1.67 gm/cm3, 1.34 gm/cm3 to 3.07 gm/cm3
and 1.77 gm/cm3 to 2.35 gm/cm3 respectively. The study area also suffers from poor
drainage and the expansiveness of the sub grade soil varies from high to very high.
Therefore, a number of factors including traffic loads, expansive subgrade, inadequate
side drains, poor workmanship and the use of inferior construction materials
contribute to pavement failure.
Keywords: CBR values, disturbance, design specification, natural sub grade, several
problems and Poor drainage.