Accurate determination of clay contents in Shanghai soils

The nine typical Shanghai soils are usually silty clay or clay, which appears inconsistent with their low clay content in the relevant publications. The literature review shows that the documented clay content of Shanghai soils ranges from 0% to 30.8% by weight. This inconsistency may stem from two factors: (1) the Shanghai soil classification system relies solely on the plasticity index for soil naming; and (2) the conventional steel sieving method cannot separate the clay from the fine soils (clay and silt mixtures). This paper aims to accurately determine the clay content in Shanghai soils. It uses nylon cloth sieves with apertures ranging from 0.063 mm to 0.0008 mm and completely separates the clay particles from the fine soils. The nine typical Shanghai soils are tested and sieved into distinct subgroups of clay, silt, sand, and gravel particles. Results demonstrate clay content ranges from 18.99% to 79.33%, substantially higher than literature values and consistent with their names of either silty clay or clay. Macro, micro, and scanning electron microscope (SEM) images reveal effective separation of clay, silt, sand, and gravel particles. The clay exhibits cohesive properties, while the silt, sand, and gravel comprise clean, non-cohesive individual particles. The clay and silt fractions are confirmed to be within their respective sieving limits by SEM-based particle size measurements. Additionally, Atterberg limits testing highlights the high plasticity of the clay particles and the non-plastic nature of the silt particles.