Improving the consolidation behavior of soft clay through chemical stabilization: Selection of the optimum mixture, physical model assessment, and SEM analysis

Soft clay is characterized by high compressibility and may experience considerable consolidation settlement under sustained loading. This study evaluates the effectiveness of chemical stabilization using ordinary Portland cement (OPC), silica fume (SF), and SikaControl®-3705 in reducing the consolidation settlement of soft clay collected from Al-Bawia, Diyala Governorate, Iraq. Thirteen mixture conditions, comprising untreated soil, individual stabilizers, binary blends, and ternary combinations, were prepared and cured for 28-days. One-dimensional consolidation tests were performed on replicate specimens (n = 3) to evaluate settlement, compression index (Cc), initial void ratio (e0), coefficient of volume compressibility (mv), and coefficient of consolidation (Cv). Using a two-stage quantitative criterion based on a minimum 50% settlement reduction followed by the Improvement-to-Dosage Ratio (IDR), the practical optimum was identified as 10% OPC + 5% SF + 1% SikaControl®-3705, which achieved a 55.9% settlement reduction and an IDR of 3.49. The mixture with the highest technical performance, containing 13% OPC + 7% SF + 1.5% SikaControl®-3705, reduced settlement to 2.60 mm (61.8%). The selected mixture was subsequently assessed in a physical model under incremental loading up to 100 kg over a 28-day observation period. The final settlement decreased from 11.20 mm for untreated soil to 2.82 mm for the selected mixture, representing a 74.8% reduction. Scanning electron microscopy (SEM) revealed a transition from a relatively open structure in the untreated soil to a denser and better-bonded structure in the stabilized specimen. Under the conditions investigated, the combined use of OPC and SF with SikaControl®-3705 effectively reduced the consolidation settlement of the tested soft clay.