Soil classification — use of particle size distribution (gradation) curves For which materials are particle size distribution curves particularly useful for classification and engineering description?

Introduction / Context:
Gradation (particle size distribution) curves summarize how soil particles are distributed across sizes. They are essential for identification, classification, and predicting engineering behaviour such as permeability, compaction response, shear strength, and frost susceptibility.

Given Data / Assumptions:

• Soils may be coarse-grained (gravels/sands) or fine-grained (silts/clays).
• Gradation is derived from sieve analysis (coarse) and hydrometer/laser methods (fine).
• USCS/IS classifications refer to coefficients of uniformity/curvature and fines content.

Concept / Approach:
For coarse soils, the gradation curve directly informs whether a soil is well-graded or uniformly graded, influencing permeability and density. For fine soils, although plasticity indices and Atterberg limits dominate classification, the tail of the gradation and percentage finer than certain sizes (e.g., 0.075 mm) are still crucial for group symbols and behaviour predictions.

Step-by-Step Solution:
Prepare cumulative percent-finer vs particle size (log scale on x-axis).Extract D10, D30, D60 and compute Cu = D60/D10 and Cc = (D30^2)/(D10*D60) for coarse soils.Note fines percentage for classification crossover and for compaction/permeability trends.

Verification / Alternative check:
Classification systems (USCS/IS) explicitly combine gradation metrics with plasticity for a comprehensive description across coarse and fine ranges.

Why Other Options Are Wrong:
Limiting gradation curves to only fine or only coarse soils ignores standard practice; both benefit.
Silts/clays and peats need additional indices, but gradation still contributes to classification.

Common Pitfalls:

• Interpreting fine-soil behaviour from gradation alone; plasticity governs much of their engineering response.
• Reading percent-finer on arithmetic rather than logarithmic size scale, leading to misinterpretation.

Final Answer:
Both coarse-grained and fine-grained soils