Abstract:
The geotechnical performance evaluation of subgrade soils of Humbo-Gesuba gravel
Road, Southern Ethiopia, is investigated to evaluate their performance as subgrade.
Here, Humbo-Gesuba road is a road for which a thin layer of gravel was found spread
and compacted directly on the subgrade without any subgrade investigation. The road is
with some parts having no defined road alignment and road formation width. A total of
26 subgrade samples along the road is collected and subjected to index properties test,
compaction test, California Bearing Ratio (CBR), and CBR swell test under ASTM and
AASHTO standards for soil testing. Soil index properties revealed that the subgrade soils
composed mainly of fine contents and percentage passing 0.075mm sieve ranged from
56.07% to 94.55%. The maximum dry density ranged between 1.31 g/cc and 1.62 g/cc
while their optimum moisture contents ranged from 20.5% to 36.5%. The liquid limit
ranged between 34% and 91%; the plastic limit ranged between 13% and 46% while the
plasticity index ranged from 8% to 47%. The three-point CBR (96 hours - soaked) at 95%
MDD ranged between 2.2% and 8.7%. The study concluded from the results of the
compaction test, Atterberg limits test, and CBR test that the subgrade soil samples of
chainages 8+000-16+000, about 35% of the total, perform poorly to use as either
subgrade or road construction materials. Most Humbo-Gesuba road subgrade soils rate
poor to fair as subgrade materials. The study has established the need for the upgrade of
the performance of these unsuitable materials for use as the subgrade. The two options
that are fit for the upgrade are the over-excavation and stabilization options. The over
excavation method of enhancement of the performance of the subgrade materials that
have been studied will require the excavation of the subgrade soil to the depth of 1m,
1.3m, and 1.7m and the subsequent replacement with higher quality materials for
subgrade soils of plasticity index of 20%-30%, 30%-40%, and 40%-50% respectively.
The stabilization method prescribed will require the addition of asphalt as the binder. The
choice of the method adopted shall strictly be guided by the cost implication of the
implementation of the method.
