SCHOOL OF GRADUATE STUDIES FACULTY OF TECHNOLOGY ...

For the study area the CEC is determined for all the samples and the values are compared
to the ranges given in Table 8. This shows that except one sample which is taken from
Kebele 15 all the rest showed CEC greater than 40.
Another test is also conducted to determine the Na+ concentration in the soil of the study
area. Na + , Ma 2+ and Ca 2+ ions have their own role in the shrink-swell property of
expansive soil. These cations balance the negative surface charge of clay particles.
Especially the cations are held close to the surface in air-dry soil. But when water is
available the hydration energy opposes the inter particle force which held these cations to
the clay surface. As the amount of water increases in the soil, the cations start to enlarge
and they tend to diffuse to the more dilute solution. But at the same time the negative
surface force of the clay particle increases and this phenomenon attracts more cations
towards the surface and cations accumulate around the clay particle. This accumulation
of cations forms a layer, which is called diffused double layer (DDL). The DDL formed
around every other clay particles overlaps and this overlapping produces repulsion force
which is expressed as swelling pressure. The thicker the DDL the more is the swelling
potential. The thickness of the DDL is governed by the presence of cations with low
valance. Thus more swelling would occur in a soil having exchangeable sodium (NA + )
cations than same soil with calcium (Ca 2+ ) or magnesium (mg 2+ ) cations [14].
For this reason the Na + concentration of the soil of the study area is determined through a
laboratory analysis and the result is given in Table 11
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