Use of secondary additives in fly ash based soil stabilization for soft subgrades

Expansive soils are widespread in many parts of the world. Due to its low strength, high compressibility, and massive volumetric changes, these soils are a potential origin of damage to roads, buildings, foundations and other geo-infrastructure. Extensive research has been conducted on the utilisation of fly ash to stabilize expansive soils. This paper describes how the efficiency of fly ash based soil stabilization can be improved using secondary additives. Class F fly ash, an industrial by-product, was used as the base additive. Lime, CSA cement, enzyme and polymers were utilized as secondary additives. A series of mechanical and microscopic tests (CBR, compaction test, SEM, XRD, FTIR and TGA) was carried out on different combinations of additives. The results indicate that secondary additives can be effectively used to improve the efficiency of fly ash based soil stabilization. Soil-fly ash-lime-enzyme was identified as an optimum combination to enhance bearing capacity while soil-fly ash-lime and soil-fly ash-enzyme also showed substantial improvements in subgrade performance. Findings from laboratory investigations were verified applying into 3-D numerical modelling to evaluate the pavement performance which revealed substantial benefits of pavement thickness reduction when fly ash stabilized weak soils are treated using secondary additives.