Which parameters are used to identify well-graded soils?

• A. Plasticity index and liquid limit
• B. Coefficient of Uniformity (Cu) and Coefficient of Curvature (Cc)
• C. Grain size and moisture content
• D. Density and porosity

Answer: (B)

Identifying well-graded soils relies on understanding the distribution and arrangement of various particle sizes within the soil matrix. The Coefficient of Uniformity (Cu) and Coefficient of Curvature (Cc) are critical parameters used for this purpose because they provide insight into the gradation of the soil.

The Coefficient of Uniformity (Cu) is defined as the ratio of the diameter of the particle at the 60th percentile to the diameter at the 10th percentile (D60/D10). A well-graded soil, which contains a wide range of particle sizes, will demonstrate a higher Cu value, indicating that the soil is capable of filling voids effectively and displaying good compaction characteristics.

The Coefficient of Curvature (Cc) is calculated using the 30th percentile, comparing it to the 10th and 60th percentiles (Cc = D30² / (D10 * D60)). A well-graded soil typically exhibits a Cc value between 1 and 3, indicating a continuous gradation with no significant gaps in particle sizes.

Together, these coefficients allow geotechnical engineers to assess how well the soil can support structures and facilitate proper drainage, which are essential qualities in construction and soil stability assessments.