---
title: "Describe one classification system, limits"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:describe-classification-system-limits"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/describe-classification-system-limits
site: BCT Engineering
---

# Describe one classification system, limits

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2078 Poush, 3 marks
- Video: [Soil Classification](https://www.youtube.com/watch?v=idC_gwg8cdI) — UL Civil Eng
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/describe-classification-system-limits
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

Describe in detail one important engineering soil classification system, clearly bringing out its limitations.

## Answer

#### Unified Soil Classification System (USCS)
Proposed by A. Casagrande (1948) and adopted by the US Army Corps of Engineers and Bureau of Reclamation; it is the most widely used engineering system. Soil is classified by **grain size distribution and plasticity**.

#### Procedure
1. **Coarse-grained** if more than 50% is retained on the 75 micron (No. 200) sieve; **fine-grained** if 50% or more passes.
2. **Coarse-grained:** G (gravel) if more than half of the coarse fraction is retained on 4.75 mm sieve, else S (sand).
   - Fines < 5%: **GW, GP, SW, SP**. Well graded if $C_u \ge 4$ (gravel) or $\ge 6$ (sand) and $1 \le C_c \le 3$.
   - Fines > 12%: **GM, GC, SM, SC** using Atterberg limits (A-line).
   - Fines 5 - 12%: dual symbols (e.g. GW-GC).
3. **Fine-grained:** use the plasticity chart. Below A-line: **M** (silt); above: **C** (clay); organic: **O**; $w_L < 50\%$ gives **L**, $w_L \ge 50\%$ gives **H**: ML, CL, OL, MH, CH, OH. **Pt** is peat.

| Major division | Groups |
|---|---|
| Gravels | GW, GP, GM, GC |
| Sands | SW, SP, SM, SC |
| Silts and clays, $w_L < 50$ | ML, CL, OL |
| Silts and clays, $w_L > 50$ | MH, CH, OH |
| Highly organic | Pt |

#### Limitations
- Only the fraction smaller than 75 mm is considered; cobbles and boulders are not classified.
- Gives no direct measure of engineering properties such as strength or compressibility.
- Dual symbols and borderline cases cause ambiguity.
- Particle shape, density and structure (in-situ condition) are not considered.
- Single boundary at $w_L = 50\%$ gives no medium plasticity class (the IS system corrects this).
- Based on the remoulded soil, so natural structure effects are ignored.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In USCS, a soil is coarse-grained if
   A. more than 50% is retained on 75 micron sieve
   B. more than 35% is retained
   C. more than 50% passes 75 micron
   D. more than 50% retained on 4.75 mm
   Answer: A. Fine-grained if 50% or more passes 75 micron. (Key point: Coarse vs fine)

2. Dual symbols (e.g. GW-GC) are used when fines are
   A. below 5%
   B. 5 to 12%
   C. above 12%
   D. above 50%
   Answer: B. Below 5% grading only; above 12% fines type. (Key point: Dual symbols)

3. Well-graded sand needs
   A. Cu >= 4 and Cc > 3
   B. Cu >= 4 and Cc < 1
   C. Cu >= 6 and 1 <= Cc <= 3
   D. Cu < 6 and Cc = 2
   Answer: C. Gravel needs Cu >= 4; sand Cu >= 6. (Key point: Well graded sand)

4. One limitation of USCS is
   A. it ignores grain size
   B. it ignores Atterberg limits
   C. it classifies boulders
   D. it gives no direct measure of strength or compressibility
   Answer: D. It does not directly measure engineering properties. (Key point: A limitation)

## Related in Soil Mechanics

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- [AASHTO classification of inorganic soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aaashto-classify-inorganic-soil) — Soil Mechanics, chapter 3
- [Purpose of soil classification](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Apurpose-of-soil-classification) — Soil Mechanics, chapter 3, asked 3×
- [Define soil; soil engineering problems](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Adefine-soil-engineering-problems) — Soil Mechanics, chapter 1
- [MIT classification system](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amit-classification-grain-size) — Soil Mechanics, chapter 3, asked 2×
- [Limitations of Mohr-Coulomb theory](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Amohr-coulomb-limitations) — Soil Mechanics, chapter 10

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