Abstract:
Expansive soil present in its natural state at the construction site may not have suitable strength.
The Construction of pavement on expansive subgrade soil is highly risky because such soil is
susceptible to differential settlements, poor shear strength, high compressibility due to their
swell-shrink behavior, and volume change with changes in moisture content which induces
cracking, and overlying causing major structural and geotechnical challenges worldwide. These
problems are extensively occurring in Ethiopia. The main aim of this study was to investigate the
suitability of Cow Dung Ash (CDA) and Lime mix for stabilization of expansive subgrade soil of
Robe town. To achieve this objective disturbed soil samples were collected from three test pits
from robe town for different laboratory tests using the purposive sampling method at a 1.5m
depth of pit from the ground. Accordingly, expansive soil was stabilized with the mix of cow dung
ash by proportioning 0%-15% by an increment of 3% by dry weight to get the optimum
percentages of cow dung ash and then activate the mixture by lime in the proportion of (0%-
12%) by increments of 3%. Laboratory tests were conducted to determine moisture content,
specific gravity, grain size analysis, Atterberg limits, proctor compaction test, free swell test,
California Bearing Ratio (CBR), and CBR swell, tests. The test procedures were based on,
American Association of State Highway and Transportation Officials (AASHTO) and American
Society for Testing and Materials (ASTM) laboratory test standards. From the study all soil
sample at their natural state shows expansive soil, as they all range in A-7-5 according to
AASHTO soil classification and CH according to unified soil classification system (USCS) and
they having low CBR value and high CBR swell. The result of chemical composition of cow dung
ash (CDA) indicates the combined percent composition of main oxides (SiO2 + Al2O3 + Fe2O3)
was 71.27% which is above the minimum of (70%) specified by ASTM (C618). The result of
treated soil shows that decrease in plasticity index, free swelling index, specific gravity, MDD
and CBR swell while increase in OMC and CBR value. From this study (6%CDA + 9%Lime)
was an optimum ratio which achieved by improving most of the geotechnical properties of soils
of Robe town. It is recommended to test additional parameter like unconfined compressive
strength and study in different parts of the country by mixing Cow dung ash with other different
stabilizers by other researchers.
