Estimating Soil Plasticity Index Using the Methylene Blue Test for Soil Classification at Selected Sites in Hilla City, Babylon Governorate, Iraq

Abstract

Grain size distribution and Atterberg limits, with particular emphasis on the plasticity index as a key quantitative indicator, are vital for characterizing soil mechanical behavior and assessing its deformability and stability under various loads. These are the standard foundations upon which soil classification systems used in geotechnical applications are built. An operator-dependent procedure may influence the plasticity index determination. This study aims to evaluate the applicability of the methylene blue test as an alternative indicator for estimating the plasticity index in fine-grained soil. It is relatively uncommon compared to the Atterberg limits. Fifteen soil samples, to a depth of 30 cm, were collected from various locations in the center of Hilla City, Babylon Governorate. They analyzed grain size distribution, determined the Atterberg limits and specific gravity, and measured the methylene blue value according to ASTM and NF standards. Therefore, this study sought to investigate the relationship between the plasticity index and methylene blue, determine the nature of their correlation, and assess the possibility of employing this relationship in geotechnical evaluation. The soils studied, characterized by high contents of silty clay and clayey silt, were classified as CL, ML, and CL-ML. The linear regression analysis between methylene blue and plasticity index yielded a moderate correlation coefficient (R² = 0.456). The methylene blue value ranged from 8 to 30. These results indicate that the methylene blue test can provide a practical, preliminary estimate of soil plasticity index and support soil classification when rapid assessment is required. Still, it is not a direct substitute for Atterberg limits. It provides a practical initial tool for the rapid assessment and engineering classification of fine-grained soils.