Subgrade moisture control (capillary cut-off): To prevent upward movement of moisture from a subgrade where the water table is at pavement level, what thickness of coarse sand layer is commonly provided as a capillary cut-off?

Introduction / Context:Capillary rise transports moisture from groundwater into the pavement structure, weakening the subgrade and base layers. A coarse sand cut-off layer is a practical means to break capillarity and protect the pavement.

Given Data / Assumptions:

• High water table reaching near pavement level.
• Provision of a free-draining, coarse-textured interlayer to interrupt capillary action.
• Typical highway construction practices.

Concept / Approach:

Capillary rise is driven by pore size: finer soils have smaller pores and higher capillary suction. A coarse sand layer presents large pores, dramatically reducing capillary continuity and allowing moisture to drain rather than rise. The layer must be thick enough to ensure a continuous break and provide a drainage path.

Step-by-Step Justification:

Verification / Alternative check:

Field experience and specifications often recommend around 30 cm of coarse sand (or a graded drainage layer) in high groundwater areas for reliable performance, alongside edge drains where needed.

Why Other Options Are Wrong:

• 15–20 cm: may be inadequate to maintain a robust capillary break under construction variability.
• 45 cm: offers more margin but is usually beyond standard practice unless conditions are severe.
• “None of these” is incorrect since 30 cm is a widely used value.

Common Pitfalls:

• Using fine sand that still supports capillary rise.
• Not providing drainage outlets, causing the layer to saturate.

Final Answer:

30 cm.