Soil classification by particle size Among the listed soils, which contains the finest (smallest) grain-size fraction by standard geotechnical classification?

Introduction / Context:
Grain-size distribution is foundational in soil classification (e.g., IS, ASTM, Unified Soil Classification System). Recognizing which fraction is finest helps in predicting plasticity, compressibility, and permeability of soils used in foundations and earthworks.

Given Data / Assumptions:

• Typical size ranges: gravel > sand > silt > clay.
• Clay exhibits clay-sized particles with significant surface activity.
• Silt is finer than sand but coarser than clay.

Concept / Approach:

By definition, clay particles are the smallest mineral particles in common geotechnical gradations, generally < 0.002 mm in size, leading to high plasticity and cohesion.

Step-by-Step Solution:

Verification / Alternative check:

Standards (IS:1498, ASTM D2487) consistently place clay as the finest fraction.

Why Other Options Are Wrong:

Coarse/fine sand and silt are all coarser than clay; gravel is much coarser still.

Common Pitfalls:

Confusing silt with clay due to similar appearance when wet; relying on feel rather than size and plasticity tests.

Final Answer:

Clay