Enhancing soft subgrade soil with a sewage sludge ash/cement mixture and nano-silicon dioxide

In this study, a mixture of sewage sludge ash (SSA) and cement was applied to improve the properties of soft cohesive soil. In addition, nano-SiO₂ was applied as an additive to the SSA/cement-treated soil. Four different mix ratios of SSA/cement (1:1, 2:1, 3:1, and 4:1) were used as stabilizers to improve the strength of the soft cohesive subgrade soil, and the total amount of SSA/cement added was fixed at 15 % of the soil weight. The effect of 2 % nano-SiO₂ was also investigated. Experiments, including Atterberg limit measurements, compaction tests, unconfined compressive strength (UCS) tests, and scanning electron microscopy, were performed. The test results indicated that a mixture of SSA and cement improved the engineering properties of soft subgrade soil, generating a UCS that was 6.5–8.4 times better over 28 days and a CBR value that was 61–97 higher. Furthermore, the addition of nano-SiO₂ enhanced the engineering properties after adding various SSA/cement ratios, increasing the UCSs that were 30–47 kPa higher over 28 days and generating CBR values that were 3–9 higher. This study suggests that to best develop the additive efficiency, nano-SiO₂ should be added to soil treated with an SSA/cement ratio of 3:1.