Polynomial relationship of compaction properties of silicate-based RHA modified expansive soil for pavement subgrade purposes

The effect of varying proportions of rice husk ash (RHA) on the compaction behaviour of modified soil has been investigated under laboratory conditions. Problematic soils exhibit undesirable characteristics that make them unsuitable for use as foundation materials due to their swell shrink properties. Clay dominant in montmorillonite and illite belongs to such group of soil due to the net negative cations at the surface when exposed to moisture. For this reason, such soils are modified in a stabilization process to improve their mechanical properties. In this research series of preliminary studies were carried out and it was discovered that the studied soil has 73.2% passing number 200 sieve, has liquid limit of 48% and plasticity index of 19%. This helped to classify the soil as A-7-6 soil according to AASHTO classification method. The soil was also classified as poorly graded soil and highly plastic. The dominant mineral is montmorillonite observed by scanning electron microscopy (SEM) method and due to its high affinity with moisture due to cation exchange, the soil swells and shrinks. The soil was treated with 2% to 30% by weight of solid with RHA and the results were observed. The results showed a steady decrease in the maximum dry density (MDD) of the RHA modified soil. The MDD did not reduce beyond the minimum value for clay soil, which is 1.20 g/cm3. The decrease recorded in MDD was due to smaller specific gravity compared to the soil. Conversely, the optimum moisture content increased due to moulding moisture demand of the isomorphic net negative cation exchange. Though the cementing property of the RHA was due to the silicate-based aluminosilicates that provided bonding of the treated material, the blend will need a filler material to achieve a more appropriate densification. Finally, mathematical relationships of a polynomial form were proposed that summarized the total compaction behaviour of the RHA modified soil.