Flexible pavement – recommended sub-base thickness for a poorly graded subgrade soil For a poorly graded (weak) subgrade soil under ordinary traffic, which thickness of sub-base is commonly provided?

Introduction / Context:
The sub-base layer distributes wheel loads and protects the underlying subgrade. When the subgrade is poorly graded or weak, designers increase the sub-base thickness to improve support and durability.

Given Data / Assumptions:

• Poorly graded subgrade indicating low bearing capacity.
• Ordinary rural/urban highway traffic (not expressway level).
• Standard granular sub-base material.

Concept / Approach:
Empirical catalogues and experience suggest thicker sub-bases for weaker subgrades to limit stress and rutting. A thickness around 25 cm is a common baseline for such conditions; lighter subgrades may require more, while strong subgrades may allow less.

Step-by-Step Solution:

Verification / Alternative check:
Checks using CBR-based design charts typically map low CBR (poor subgrade) to higher granular thicknesses near 0.25 m or more for conventional traffic categories.

Why Other Options Are Wrong:
10–20 cm are often inadequate on weak subgrades; 30 cm can be used for heavier traffic but is beyond a common baseline.

Common Pitfalls:
Ignoring drainage and frost; not compacting to target density; skipping a filter/separator geotextile where fines pumping is expected.

Final Answer:
25 cm