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Classification for subgrade suitability

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.

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.

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

GroupRating
A-1-a, A-1-bExcellent to good
A-3Excellent to good
A-2Excellent to good (A-2-6, A-2-7 fair)
A-4, A-5Fair to poor
A-6Fair to poor
A-7-5, A-7-6Fair to poor (poor to very poor when swelling)

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

✓ Answer checked by Claude Opus 5.5 (high effort)

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