---
title: "USCS versus AASHTO"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:uscs-vs-aashto"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-vs-aashto
site: BCT Engineering
---

# USCS versus AASHTO

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2078 Poush, 2 marks
- Video: [(Soil Mechanics and Foundations) Complete Comparative Analysis of the Unified Soil Classification...](https://www.youtube.com/watch?v=wiYkpTzaTr4) — 구조기술사의 현장로그
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/uscs-vs-aashto
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

Point out similarities and differences between the USCS system and the AASHTO system of soil classification.

## Short answer

Both use grain size and Atterberg limits. USCS uses 50% passing 75 micron boundary and letter symbols; AASHTO uses 35% boundary and A-1 to A-7 with group index. USCS uses plasticity chart; AASHTO does not and rates subgrade quality.

## Answer

Both classify soil using grain size distribution and Atterberg limits.

#### Similarities
- Both use sieve analysis and liquid and plastic limits.
- Both are based on the fraction of soil finer than 75 mm.
- Both separate coarse-grained from fine-grained soil using the 75 micron sieve, though at different percentages.
- Both are used widely in engineering practice.

#### Differences
| Basis | USCS | AASHTO |
|---|---|---|
| Purpose | General engineering | Highway subgrade |
| Coarse/fine boundary | 50% passing 75 micron | 35% passing 75 micron |
| Symbols | Letters (GW, SC, CH) | A-1 to A-7 with group index |
| Plasticity chart | Used (A-line) | Not used |
| Organic soil | Separate OL, OH, Pt | No separate group |
| Group index | Not used | Used to rate subgrade |
| Rating | Order of groups not numerical | Subgrade quality worsens from A-1 to A-7 |

## Check yourself

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

1. Coarse/fine boundary in AASHTO is
   A. 35% passing 75 micron
   B. 50% passing 75 micron
   C. 50% passing 4.75 mm
   D. 12% passing 75 micron
   Answer: A. USCS uses 50%. (Key point: Coarse/fine boundary)

2. Group index is used in
   A. USCS to rate dams
   B. AASHTO to rate subgrade
   C. both equally
   D. neither
   Answer: B. AASHTO only. (Key point: Group index)

3. Which uses the plasticity chart (A-line)?
   A. AASHTO
   B. both
   C. USCS
   D. neither
   Answer: C. AASHTO doesn't use it. (Key point: Plasticity chart)

## Related in Soil Mechanics

- [AASHTO classification of inorganic soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aaashto-classify-inorganic-soil) — Soil Mechanics, chapter 3
- [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 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 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 4%](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Auscs-classify-sieve-four-percent) — 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

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