---
title: "USCS classification of soils A and B"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:uscs-classify-soils-a-b"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-classify-soils-a-b
site: BCT Engineering
---

# USCS classification of soils A and B

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2078 Chaitra, 5 marks
- Video: [Master USCS Soil Classification: Step-by-Step Examples for Engineering](https://www.youtube.com/watch?v=VQNxhNTslX0) — Soil Mechanics and Engineering Geology
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-classify-soils-a-b
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

Classify the soils A and B with the properties shown below according to the unified soil classification system.

| Soil | $w_L$ (%) | $I_p$ (%) | % passing through 4.75 mm sieve | % passing through 75 µ sieve |
|---|---|---|---|---|
| A | 45 | 29 | 100 | 59 |
| B | 55 | 15 | 100 | 85 |

## Short answer

Soil A is CL, sandy lean clay; Soil B is MH, inorganic elastic silt. Both pass 75 micron more than 50%, so fine-grained. A has Ip=29 above A-line 18.25 and wL=45<50; B has Ip=15 below A-line 25.55 and wL=55>50.

## Answer

Soils A and B have 59% and 85% passing the 75 micron sieve (more than 50%), so both are **fine-grained**. Use the plasticity chart with the A-line $I_p = 0.73(w_L - 20)$.

#### Soil A: $w_L = 45\%$, $I_p = 29\%$
$$
I_{p,A\text{-line}} = 0.73(45 - 20) = 18.25\%
$$
$I_p = 29 > 18.25$, so the soil lies **above the A-line** (clay), and $w_L = 45 < 50$ (low plasticity).

**Soil A: CL, inorganic clay of low plasticity (lean clay).** It is also sandy, since 41% is coarse.

#### Soil B: $w_L = 55\%$, $I_p = 15\%$
$$
I_{p,A\text{-line}} = 0.73(55 - 20) = 25.55\%
$$
$I_p = 15 < 25.55$, so the soil lies **below the A-line** (silt), and $w_L = 55 > 50$ (high plasticity).

**Soil B: MH, inorganic silt of high plasticity (elastic silt).**

(Both are taken as inorganic, since no data on organic content is given.)

## Check yourself

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

1. Soils with 59% and 85% passing 75 micron are
   A. both fine-grained
   B. both coarse-grained
   C. A coarse, B fine
   D. dual symbol soils
   Answer: A. Over 50% passes. (Key point: Coarse or fine)

2. A-line Ip at wL=45 is
   A. 18.25%
   B. 29%
   C. 25.55%
   D. 32.9%
   Answer: A. 0.73x25 = 18.25. (Key point: A-line for A)

3. Soil A (wL=45, Ip=29) is
   A. CH
   B. CL
   C. ML
   D. MH
   Answer: B. Above A-line, wL < 50. (Key point: Soil A)

4. Soil B (wL=55, Ip=15) with A-line Ip=25.55 is
   A. CH
   B. CL
   C. MH
   D. ML
   Answer: C. Below A-line and wL > 50. (Key point: Soil B)

## Related in Soil Mechanics

- [USCS classification, 75 micron 8%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-75micron-8-percent) — Soil Mechanics, chapter 3
- [USCS classification, 75 micron 4%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-sieve-four-percent) — Soil Mechanics, chapter 3
- [USCS classification from PSD curves](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-psd-curves-a-b) — Soil Mechanics, chapter 3
- [USCS classification, 75 micron 62%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-75micron-62-percent) — Soil Mechanics, chapter 3
- [USCS classification, 75 micron 39%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-75micron-39-percent) — Soil Mechanics, chapter 3
- [USCS classification using D10, D30, D60](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-d-values) — Soil Mechanics, chapter 3

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