---
title: "Classification for subgrade suitability"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:subgrade-suitability-classification"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/subgrade-suitability-classification
site: BCT Engineering
---

# Classification for subgrade suitability

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2074 Bhadra, 2 marks
- Video: [Classify Soils using AASHTO Soil Classification System|Group Index](https://www.youtube.com/watch?v=OlXZNqYwamQ) — Master Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/subgrade-suitability-classification
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

For finding the suitability of soils as subgrade for highways, which soil classification is generally used? Write down the name of each group according to that classification. Show the general rating of those groups as a suitability of subgrade.

## Short answer

AASHTO is used. Groups: A-1-a, A-1-b, A-3, A-2-4 to A-2-7 granular; A-4 to A-7 silt-clay. Rating: A-1 and A-3 excellent-good, A-2 excellent-good (A-2-6/7 fair), A-4 to A-7 fair-poor.

## Answer

The **AASHTO classification system** (American Association of State Highway and Transportation Officials) is generally used for rating soil as a highway subgrade.

#### Groups
**Granular materials (35% or less passing 75 micron):**
- A-1-a: mainly stone fragments or gravel, with or without fines
- A-1-b: mainly coarse sand, with or without fines
- A-3: fine sand
- A-2-4, A-2-5, A-2-6, A-2-7: silty or clayey gravel and sand

**Silt-clay materials (more than 35% passing 75 micron):**
- A-4: silty soils
- A-5: elastic (micaceous or diatomaceous) silty soils
- A-6: clayey soils
- A-7-5, A-7-6: elastic clayey soils (A-7-5 has a moderate plasticity index but may be elastic and subject to large volume change; A-7-6 is highly plastic with very high volume change)

#### General rating as subgrade
| Group | Rating |
|---|---|
| A-1-a, A-1-b | Excellent to good |
| A-3 | Excellent to good |
| A-2 | Excellent to good (A-2-6, A-2-7 fair) |
| A-4, A-5 | Fair to poor |
| A-6 | Fair to poor |
| A-7-5, A-7-6 | Fair to poor (poor to very poor when swelling) |

Within each group, a lower **group index** (0 for the best soils) indicates a better subgrade.

## Check yourself

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

1. Subgrade suitability is generally judged using
   A. AASHTO classification
   B. MIT classification
   C. USDA triangle
   D. Pedological classification
   Answer: A. AASHTO is the highway system. (Key point: System used)

2. Granular materials in AASHTO have
   A. more than 35% passing
   B. 35% or less passing 75 micron
   C. more than 50% passing
   D. less than 5% passing
   Answer: B. A-1 to A-3 and A-2 are granular. (Key point: Granular materials)

3. Which groups are rated fair to poor?
   A. A-1 and A-3
   B. A-1-a only
   C. A-4 to A-7
   D. A-2-4
   Answer: C. Silt-clay groups are fair to poor. (Key point: Ratings)

## Related in Soil Mechanics

- [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
- [AASHTO classification of inorganic soil](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aaashto-classify-inorganic-soil) — Soil Mechanics, chapter 3
- [Soil classification systems](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acommon-classification-systems) — Soil Mechanics, chapter 3, asked 3×
- [Purpose of soil classification](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Apurpose-of-soil-classification) — Soil Mechanics, chapter 3, asked 3×
- [Field identification of fine-grained soils](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afield-identification-clay-silt) — Soil Mechanics, chapter 3, asked 4×
- [How soils are formed](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asoil-formation-process) — Soil Mechanics, chapter 1, asked 2×

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