---
title: "USCS classification, 75 micron 8%"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:uscs-classify-75micron-8-percent"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-classify-75micron-8-percent
site: BCT Engineering
---

# USCS classification, 75 micron 8%

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2079 Jestha, 6 marks
- Video: [Unit:3 | Soil Identification and Classification | Numerical of USCS | Prashant YT | Soil Mechanics |](https://www.youtube.com/watch?v=nKsQqC25_Uo) — Prashant YT
- 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-8-percent
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

A soil sample on laboratory test gives the following results. Classify the soil and give its symbol as per the USCS classification system.
Passing through 75-micron sieve = 8%
Passing through 4.75 mm sieve = 42%
Coefficient of uniformity = 6
Coefficient of curvature = 4
Plasticity index = 4

## Short answer

GP-GM, poorly graded gravel with silt and sand. Fines 8% require dual symbol; gravel is 63% of coarse fraction. Gravel grading fails Cc=4>3 despite Cu=6, and fines with Ip=4 are silt.

## Answer

#### Given
Passing 75 micron $= 8\%$; passing 4.75 mm $= 42\%$; $C_u = 6$; $C_c = 4$; $I_p = 4$.

#### Step 1: Coarse or fine?
92% is retained on the 75 micron sieve, so the soil is **coarse-grained**.

#### Step 2: Gravel or sand?
- Retained on 4.75 mm (gravel) $= 100 - 42 = 58\%$
- Sand $= 42 - 8 = 34\%$

Gravel is more than half of the coarse fraction ($58/92 = 63\%$), so the soil is **gravel (G)**.

#### Step 3: Fines content
8% is between 5% and 12%, so a **dual symbol** is required.

#### Step 4: Grading
For well-graded gravel: $C_u \ge 4$ and $1 \le C_c \le 3$.
- $C_u = 6 \ge 4$: OK
- $C_c = 4 > 3$: fails

So the gravel is **poorly graded (GP)**.

#### Step 5: Nature of fines
$I_p = 4$ is only at the lower limit of the 4 - 7 band, and the liquid limit is not given. With $I_p$ not above 4, the fines are classed as silt, so they are taken as **silty (M)**.

**Answer: GP-GM, poorly graded gravel with silt and sand.**

## Check yourself

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

1. Passing 4.75 mm 42%, passing 75 micron 8%: gravel fraction is
   A. 58%, more than half of coarse fraction, so gravel
   B. 42%, so sand
   C. 34%, so gravel
   D. 8%, so fine
   Answer: A. Retained on 4.75 = 58%; 58/92 = 63%. (Key point: Gravel or sand)

2. Why a dual symbol?
   A. Fines exceed 12%
   B. Fines of 8% lie between 5% and 12%
   C. Fines are below 5%
   D. Ip equals 4
   Answer: B. 5-12% needs dual symbol. (Key point: Dual symbol)

3. Cu=6, Cc=4 for gravel gives
   A. well graded since Cu >= 4
   B. well graded since Cc >= 1
   C. poorly graded since Cc > 3
   D. sand
   Answer: C. Cc must be 1-3. (Key point: Grading)

4. Final symbol is
   A. GW-GC
   B. GP-GC
   C. SP-SM
   D. GP-GM
   Answer: D. Poorly graded gravel with silty fines (Ip = 4). (Key point: Final symbol)

## Related in Soil Mechanics

- [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, 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, coarse fraction 62%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-coarse-fraction-62) — 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
- [USCS classification of soils A and B](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-soils-a-b) — Soil Mechanics, chapter 3

## Related in other subjects

- [Chi-square uniformity test, 40 numbers](https://www.bctengineer.com/question.md?key=semester_8%2Fsimulation_and_modeling%3Achi-square-uniformity-table-2074) — Simulation and Modeling, chapter 6