---
title: "USCS classification, 75 micron 4%"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:uscs-classify-sieve-four-percent"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-classify-sieve-four-percent
site: BCT Engineering
---

# USCS classification, 75 micron 4%

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2079 Asoj, 5 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-sieve-four-percent
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

The sieve analysis of a soil gave the following results:
% passing 75 micron sieve = 4
% retained on 4.75 mm sieve = 35
Coefficient of curvature = 2
Coefficient of uniformity = 5
Classify the soil according to the USCS system.

## Short answer

SP, poorly graded sand with gravel. Fines 4%, coarse 96%; gravel is 36% of coarse fraction, so sand dominates. Well-graded sand requires Cu>=6, but Cu=5 (Cc=2 ok), so poorly graded.

## Answer

#### Given
Passing 75 micron $= 4\%$; retained on 4.75 mm $= 35\%$; $C_c = 2$; $C_u = 5$.

#### Step 1: Coarse or fine?
Only 4% passes 75 micron, so more than 50% is retained: **coarse-grained**.

#### Step 2: Gravel or sand?
- Gravel (retained on 4.75 mm) $= 35\%$
- Sand $= 100 - 35 - 4 = 61\%$

Coarse fraction $= 96\%$; gravel is $35/96 = 36\%$ of it, less than half. So the soil is a **sand (S)**.

#### Step 3: Fines content
4% is less than 5%, so only the grading is needed (symbols SW or SP).

#### Step 4: Grading
For well-graded sand: $C_u \ge 6$ and $1 \le C_c \le 3$.
- $C_c = 2$ satisfies the condition.
- $C_u = 5 < 6$ fails.

The soil is not well graded.

**Answer: SP, poorly graded sand (with gravel).**

## Check yourself

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

1. With 4% passing 75 micron, the soil is
   A. coarse-grained
   B. fine-grained
   C. dual symbol
   D. organic
   Answer: A. Only 4% fines. (Key point: Coarse or fine)

2. Gravel 35%, fines 4%: sand is 61%. The soil is
   A. gravel since gravel is 35%
   B. sand since gravel is less than half the coarse fraction
   C. gravel since fines are low
   D. sand since Cc = 2
   Answer: B. 35/96 = 36% < 50%. (Key point: Gravel or sand)

3. With fines under 5%, the symbol depends on
   A. Atterberg limits only
   B. A-line position
   C. grading (Cu and Cc) only
   D. origin of the soil
   Answer: C. Choose W or P. (Key point: Fines < 5%)

4. With Cu=5 and Cc=2, sand is
   A. well graded (SW)
   B. SW-SM
   C. SC
   D. poorly graded (SP) since Cu < 6
   Answer: D. Cu must be >= 6 for SW. (Key point: Grading)

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