Effect of Rice Husk Ash on the Compaction and Strength Properties of a Non-Plastic Borrow-Pit Soil from Okada, Edo State, Nigeria

This study evaluated the effects of rice husk ash (RHA) on the compaction and strength properties of a non-plastic borrow-pit soil collected from Okada, Edo State, Nigeria. Soil was sampled from a trial pit at 1.2 m depth and mixed with 0%, 5%, 10% and 15% RHA by dry soil mass. Laboratory testing comprised index-property determination, British Standard Light compaction, soaked California Bearing Ratio (CBR), unconfined compressive strength (UCS), and X-ray fluorescence with loss-on-ignition characterisation of the ash. The soil was classified as AASHTO A-1-b with a Group Index of 0. Increasing RHA content reduced maximum dry density from 1,932 to 1,705 kg/m³ and increased optimum moisture content from 11.28% to 14.40%. The selected CBR rose from 32.13% for untreated soil to 45.73% at 5% RHA, then declined at higher contents. Mean UCS increased from 134.28 to 191.16 kN/m² at 5% RHA before decreasing at 10% and 15%. Differences in UCS among treatment groups were statistically significant, F(3, 8) = 50.742, p < .001. The 5% mixture gave the highest measured strength response among the tested mixtures. Because CBR was based on one specimen per mixture and the soil came from one sampling location, replicated testing and field validation are required before broader application.