Capillary cutoff with fine sand layer: To completely interrupt capillary rise beneath a pavement or track formation, the minimum practical thickness of a fine sand cutoff layer is taken as

Introduction / Context:
Capillary rise can bring water into upper subgrade layers, weakening pavement support and causing frost-susceptible conditions. A capillary cutoff layer using fine sand or properly graded non-capillary material is a common countermeasure in highway and railway formation design.

Given Data / Assumptions:

• We seek a practical rule-of-thumb thickness for a fine sand capillary cutoff.
• Objective is to break the capillary continuity between groundwater and subgrade.
• Design is for general guidance (exams), not a site-specific hydrogeological analysis.

Concept / Approach:
Capillarity depends on pore size distribution. A sufficiently thick layer of relatively uniform, non-capillary fine sand interrupts continuous small pores, greatly reducing upward movement of water. Many practice problems adopt about 0.6 m to 0.65 m as a reasonable minimum thickness to ensure an effective cutoff for typical soils.

Step-by-Step Solution:
Recognize target: break capillary continuity.Adopt the commonly cited practical minimum thickness for fine sand cutoffs, roughly 0.6–0.65 m.Choose the closest option representing this minimum: 64 cm.

Verification / Alternative check:
Design manuals vary, but exam keys and many field practices use ~0.6 m as the indicative thickness; 64 cm lies within this practical band, providing a safety margin for variability.

Why Other Options Are Wrong:

• 40 cm and 52 cm may be insufficient for reliable cutoff under variable site conditions.
• 76 cm or 80 cm provide more safety but are often beyond economical minima and not typically required by exam standards.

Common Pitfalls:

• Using poorly graded material that still sustains capillarity.
• Omitting drainage paths; cutoff is most effective with adequate sub-surface drainage.

Final Answer:
64 cm