Abstract:
The main objective of this paper is to observe the effect on the swelling behavior of subgrade of HumboArba Minch road when stabilized by using different percentage of fly ash (10%-40%). The swelling
properties of the subgrade soils was evaluated before and after adding four different percentages of fly ash
(10%, 20%, 30% and 40%) by weight of expansive soil with curing periods of 7,14 and 21 days. The
results show that the fly ash provides better index properties in terms of liquid limit, plastic limit,
plasticity index and improve swell shrink properties. Generally the most notable decrease in the swelling
pressure occurs within the first 7 days when 30% of fly ash used as stabilizer.
The finding of this thesis outlined that the sub-grade soil of untreated sample was classified as A-7-5 by
the AASHTO and CH as per USCS systems which is consider as poor subgrade quality. These soils have
very low load bearing capacity and are highly expansive of the sub-grade soil.
The addition of fly ash to expansive sub grade soil (CH soil) increases the optimum moisture content from
35% at 0% fly ash to 48% at 30% fly ash. And it declines to 44% at 40% fly ash. The MDD is reduced
from 2g/cm
3
to 1.13g/cm
3
at 40% fly ash for the same compactive effort. Addition of 30% fly ash and 7
days curing period will increase the moisture content from 48% to 50% and reduce the MDD from
1.34g/cm
3
to 1.12g/cm
3
.
The swelling pressure of the virgin soil is 95Kpa. Its value is reduced to 8.5 Kpa at 30% addition of fly
ash. Further improvement can be achieved by curing the mixture for 14 days which results the swelling
pressure of 7Kpa. Generally, 30% addition of the fly ash can be taken as the optimum dosage. And curing
the mixture will result in better ground improvement.
Therefore, it can be concluded that fly ash can improve the swelling behavior of the sub grade soil in
Humbo-Arba Minch road. Practically, the effective fly ash content should be 30% by weight of the soil
mass which provides the least plasticity index, the least swell and shrinkage parameters, the least swelling
pressure and provides high quality of subgrade pavement rating.
