Final answer:
The soil is considered well-graded, as indicated by its Coefficient of Uniformity (Cu) value of 20, which is above the usual threshold, and its Coefficient of Curvature (Cc) value of 2.5, which is within the ideal range of 1 to 3.
Step-by-step explanation:
When analyzing soil gradation using sieve analysis, civil engineers often rely on two important coefficients: the Coefficient of Uniformity (Cu) and the Coefficient of Curvature (Cc). These coefficients help in classifying the soil's gradation characteristics, which in turn affects its suitability for various engineering purposes. The Coefficient of Uniformity is calculated as the ratio between the particle size for 60% finer (D60) to the size for 10% finer (D10), Cu = D60 / D10. A Cu value greater than 4 for gravels, or 6 for sands, indicates a well-graded soil with a wide range of particle sizes. The Coefficient of Curvature falls between 1 and 3 for well-graded soils and is calculated using the formula Cc = (D30^2) / (D10 * D60).
In the given scenario, with a Cu value of 20 and a Cc value of 2.5, the soil is considered well-graded because the Cu is significantly higher than the threshold values and the Cc is within the range of 1 to 3. This implies a good distribution of various particle sizes from coarse to fine within the soil, indicating a soil that has the potential for good compaction and stability, desirable characteristics in construction and foundation work.