---
title: "USCS classification, 75 micron 39%"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:uscs-classify-75micron-39-percent"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-classify-75micron-39-percent
site: BCT Engineering
---

# USCS classification, 75 micron 39%

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2073 Bhadra, 2 marks
- 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-75micron-39-percent
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

Classify the following soil as per the unified soil classification system: soil passing from 75 µ sieve = 39%, liquid limit = 33%, plastic limit = 18%.

## Short answer

39% finer than 75µm makes it coarse-grained with >12% fines. The fines have PI=15% and plot above A-line for LL=33%, so clayey; assuming sand predominates, symbol SC.

## Answer

#### Given
Passing 75 micron $= 39\%$; $w_L = 33\%$; $w_P = 18\%$.

$$
I_p = 33 - 18 = 15\%
$$

- Only 39% is finer than 75 micron (less than 50%), so the soil is **coarse-grained**, with more than 12% fines.
- Fines classification: A-line at $w_L = 33$: $I_p = 0.73(33-20) = 9.5\%$. $I_p = 15$ lies above the A-line and is greater than 7, so the fines are **clayey (C)**.
- The gravel/sand split is not given. Assuming the coarse fraction is mainly sand (the usual case), the symbol is **SC**; if gravel dominates it would be GC.

**Answer: SC, clayey sand** (GC if gravel exceeds sand).

## Check yourself

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

1. 39% passing 75 micron means
   A. coarse-grained with > 12% fines
   B. fine-grained
   C. fines < 5%
   D. dual symbol
   Answer: A. Less than 50% passes. (Key point: Coarse or fine)

2. wL=33, Ip=15; A-line Ip = 9.5, so fines are
   A. silty (M)
   B. clayey (C)
   C. organic (O)
   D. gravelly
   Answer: B. Above A-line and Ip > 7. (Key point: Fines type)

3. Assuming sand dominates, the symbol is
   A. SM
   B. SP-SC
   C. SC
   D. CL
   Answer: C. GC if gravel dominates. (Key point: Symbol)

## Close variants set in other exams

- [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

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- [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
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