Moisture cut-off in embankment sections: When the subgrade is about 0.6–1.0 m above the water table (embankment), what thickness of a coarse sand cut-off layer is typically provided to prevent upward moisture movement?

Introduction / Context:Moisture ingress from a nearby water table can weaken subgrades, reduce bearing capacity, and accelerate pavement deterioration. In embankment situations where the subgrade is moderately above the water table, a thin capillary cut-off of coarse sand is commonly provided to interrupt capillary rise.

Given Data / Assumptions:

• Water table lies roughly 0.6–1.0 m below subgrade level (embankment condition).
• Capillary rise through fine soils can reach the pavement unless interrupted.
• Coarse sand has low capillarity; a thin layer can act as a capillary break.

Concept / Approach:

The objective is to create a drainage/cut-off interface that prevents moisture migration to the upper layers. For moderate separations from the water table, a 15 cm coarse-sand blanket is usually adequate as a practical and economical capillary break.

Step-by-Step Solution:

Verification / Alternative check:

For more severe conditions (water table near subgrade or highly fine soils), thicker blankets or additional drainage layers may be specified, but 15 cm is a common baseline for the stated condition.

Why Other Options Are Wrong:

• 20–45 cm: used in more severe capillarity or perched water problems; not the typical minimum for the given range.
• “None of these” contradicts standard practice.

Common Pitfalls:

• Relying on coarse sand without providing outlets for intercepted moisture.

Final Answer:

15 cm.