Soil subgrade rating by Group Index (GI) A soil subgrade has Group Index GI = 7 as per the group index method. Based on standard GI classifications for highway subgrades, how should this soil be rated?

Introduction:
The Group Index (GI) method provides a quick empirical rating of soil subgrades for pavement design. It supplements the soil classification by estimating support quality: lower GI indicates better performance under traffic, higher GI indicates poorer support and greater required pavement thickness.

Given Data / Assumptions:

• Group Index for the subgrade: GI = 7.
• Standard GI rating bands used in highway practice.
• Uniform subgrade conditions assumed.

Concept / Approach:

Common rating bands: GI 0–1 excellent/very good, 2–4 good, 5–9 fair, 10–20 poor to very poor. Therefore, a GI of 7 lies within 5–9 and the subgrade is rated “fair.” Designers use this rating to guide layer thickness and improvement measures.

Step-by-Step Solution:

Verification / Alternative check:

Numerous design manuals tabulate GI ranges and corresponding qualitative ratings that match the above assessment.

Why Other Options Are Wrong:

“Good” applies to GI 2–4; “poor/very poor” apply to GI ≥ 10; “excellent” corresponds to GI near zero.

Common Pitfalls:

Assuming GI alone dictates design without considering CBR, drainage, and frost action; misreading the band boundaries.

Final Answer:

fair