Long-term performance of pavement structures is significantly impacted by the stability of
the underlying soils. Roads have to be supported on sound sub-surface and base material.
However, in many Kenyan regions, the local sub-grade materials are poor in strength and
other properties, for instance red soils may not provide suitable sub-grade requirements.
In situ subgrades often do not provide the support required to achieve acceptable
performance under traffic loading and environmental demands. The strength and stiffness of
a soil layer can be improved through the use of additives to permit a reduction in design
thickness of the stabilized material compared with an un-stabilized or unbound material. The
design thickness strength, stability, and durability requirements of a base or sub-base course
can be reduced if further analysis indicates suitability
Although stabilization is an effective alternative for improving soil properties, the
engineering properties derived from stabilization vary widely due to heterogeneity in soil
composition and due to differences in physical and chemical interactions between the soil

composition and due to differences in physical and chemical interactions between the soil
and candidate stabilizers. Information on grain size distribution and Atterberg limits must be
known to initiate the selection process of a stabilizer.
These variations necessitate the consideration of site-specific treatment options which must
be validated through testing of soil-stabilizer mixtures. This project addresses soil treatment
with the traditional calcium-based stabilizers. Soil for use in constructing pavement layers in
road construction normally requires some improvement of the different characteristics of
that particular soil in order to meet certain engineering properties. This improvement is
normally achieved by the use of a stabilizing agent, normally cement and lime.
The possibility of including pozzolanic ash in the process of soil improvement/stabilization
was carried out during this study with results supporting the same. Results from this study
indicate that pozzolanic ash can be used as an additive to soil when improving the
engineering properties of the soil used with an impact of reducing the cost of the

improvement while still achieving the set standards.
The results from this study have shown that natural pozzolanic ash has an improving
characteristic in the strength of soil used as sub grade.


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