Soil with particle size less than __ mm is called fine grained soil (silt or clay).
Try it first: pick an option, then read the explanation.
Detailed Explanation
The single most important dividing line in this entire topic — separating coarse-grained soil (sand and gravel) from fine-grained soil (silt and clay) — is a specific IS sieve size.
Why "0.075" Is Correct
Particles smaller than 0.075 mm (75 microns) are classified as fine-grained soil — silt and clay. This is the same 75-micron boundary that defines the fine edge of sand (see Q2) and the coarse edge of silt.
Why the Other Options Are Wrong
- 4.750 mm: this is the gravel/sand boundary (see Q2), not the coarse/fine-grained boundary — far too large a size to separate fine-grained soil from sand.
- 0.020 mm: not a standard IS classification boundary at all — it falls within the silt range, not at any recognized dividing line.
- 0.015 mm: also not a standard IS boundary — similarly falls within the silt range rather than marking any recognized classification limit.
Key Point
0.075 mm is the single most-tested boundary in this whole topic — it's both the coarse/fine-grained dividing line (this question) and the sand/silt boundary (Q2) simultaneously, since these are the same boundary viewed from two different classification angles.
Key Concepts for Students
- Coarse-grained soil = gravel + sand (particles > 0.075 mm). Fine-grained soil = silt + clay (particles < 0.075 mm).
- This same 0.075 mm figure is used as the 50%-passing threshold in the full USCS classification sequence (see Q3, Step 1).
- 0.075 mm is also the practical limit of sieve analysis — see Q15 for why particles finer than this need sedimentation (hydrometer) analysis instead.
- This boundary is identical across both the sand/silt particle-size classification (Q2) and the coarse/fine-grained engineering classification split (this question) — one number, two uses.
Going Deeper: Coarse-Grained vs. Fine-Grained: Why This Split Matters
The coarse/fine-grained distinction isn't just a labeling convenience — it determines which entire branch of the USCS classification procedure applies to a soil sample.
| Category | Particles | Classified Primarily By |
|---|---|---|
| Coarse-grained soil (> 50% coarse) | Gravel + Sand (> 0.075 mm) | Grain-size distribution (Cu, Cc) — see Q1, Q7, Q22-26 |
| Fine-grained soil (> 50% fine) | Silt + Clay (< 0.075 mm) | Plasticity chart / A-line (Liquid limit, Plasticity index) — see Q11, Q12 |
This is exactly the Step 1 decision in Q3's full classification walkthrough: whichever fraction (coarse or fine) makes up more than 50% of the sample by weight determines whether the rest of the classification proceeds via grain-size gradation criteria, or via the liquid-limit/plasticity-index plasticity chart. Getting this first split right is the single most consequential step in the entire USCS classification process — every subsequent decision depends on it.