Grain-Size Parameters – Acceptable Range of Coefficient of Curvature (C_c) for Well-Graded Soils For a soil to be considered well graded (in addition to meeting the uniformity requirement), the coefficient of curvature C_c should typically fall within which range?

Introduction / Context:
Grain-size distribution parameters help classify soils for filter design, compaction, and drainage. Besides the uniformity coefficient C_u, the coefficient of curvature C_c gauges the shape of the gradation curve near the median size and helps identify well-graded materials.

Given Data / Assumptions:

• C_c is defined as C_c = (D30^2) / (D10 * D60).
• Conventional thresholds: well-graded sands/gravels require both adequate C_u and appropriate C_c range.

Concept / Approach:

Standard practice considers a soil well graded if its coefficient of curvature lies roughly between 1 and 3 along with sufficient uniformity (e.g., C_u > ~6 for sands, > ~4 for gravels, depending on the standard). Values far outside this range often indicate humps or gaps in the gradation curve (poor gradation or gap gradation).

Step-by-Step Solution:

Verification / Alternative check:

Classification manuals and filter design guides consistently use this band in conjunction with a suitable C_u threshold.

Why Other Options Are Wrong:

Ranges below 1 or above 3 usually reflect kinked or gap-graded curves; very high values suggest an overly pronounced hump.

Common Pitfalls:

Judging well-graded solely on C_c without checking C_u; misreading D-sizes from the cumulative percent passing chart.

Final Answer:

1.0 to 3.0