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.