Which of the following represents the smallest size (mm) of fine aggregate (sand)?
Try it first: pick an option, then read the explanation.
Detailed Explanation: MIT Grain-Size Classification
Soil particles span an enormous range of sizes, from boulders visible to the naked eye down to clay particles far too small to see even under an ordinary microscope. To communicate about soil consistently, engineers divide this range into named size classes. One of the earliest and most influential grain-size classification systems was developed by the Massachusetts Institute of Technology (MIT), which defines four principal particle-size groups:
| Soil Fraction | Particle Size Range (mm) |
|---|---|
| Clay | < 0.002 |
| Silt | 0.002 – 0.06 |
| Sand | 0.06 – 2 |
| Gravel | > 2 |
Reading the Sand Range
Under the MIT system, the sand fraction spans particle sizes from 0.06 mm (its lower/finer boundary, where it meets silt) up to 2 mm (its upper/coarser boundary, where it meets gravel). The question asks for the smallest size within the sand range, which corresponds to its lower boundary of 0.06 mm — making option (b) correct.
Option (a), 0.01 mm, actually falls within the silt range (0.002–0.06 mm), not sand. Option (c), 1.5 mm, and option (d), 2 mm, both fall correctly within the sand range but are not its smallest value — 2 mm in fact marks the sand-to-gravel boundary, the largest size sand can be, not the smallest.
Key Concepts for Students
- MIT classification boundaries (memorize these): Clay < 0.002 mm, Silt 0.002–0.06 mm, Sand 0.06–2 mm, Gravel > 2 mm.
- Different classification systems use slightly different boundaries: the Indian Standard (IS) classification and the Unified Soil Classification System (USCS) use marginally different cut-off values, so always note which system a question refers to.
- 0.06 mm is a critical boundary value: it separates silt from sand under the MIT system, and questions frequently test whether students know this exact transition point.
- Grain size alone doesn't capture everything: particle shape, mineralogy, and plasticity also matter, especially for fine-grained soils like clay and silt, which behave very differently despite sometimes overlapping sizes.
- This classification underlies later topics: permeability, compaction behaviour, and shear strength are all strongly influenced by which grain-size fraction dominates a given soil.