Study properties of soft subgrade soil stabilized by sewage sludge/lime and nano-SiO2

The pozzolanic characteristics of a sludge incinerated into ash were determined in this study. Lime is commonly used as a stabilizer for the treatment of soils, whereas sewage sludge ash (SSA) is often applied with lime to improve soft subgrade soil. In this study, a cohesive soil categorized as A-4 (low-plasticity clay) by AASHTO classifications was mixed with SSA/lime with a 3:1 ratio. Nano-SiO₂ was also added to the soil. To identify changes in the workability, strength, permeability, and shear strength of the soft subgrade soil, basic soil tests were conducted, and the microstructure of the treated soil was analyzed. The results indicate that SSA/lime mixtures improve the properties of soft subgrade soil and transform the soil from \"poor subgrade soil\" to \"good to excellent subgrade soil\" with a CBR > 8. Additionally, the addition of 2% nano-SiO2 increases the unconfined compressive strength of soft subgrade soil treated with SSA/lime mixture by approximately 17 kPa. However, the swelling of the treated soil increased by approximately 0.1% after the addition of nano-SiO₂ and lime. Thus, soil swelling should be considered before lime and nano-SiO2 are applied to soft subgrade soil.