Formation protection — moorum blanket is primarily used to prevent percolation into which problematic subgrade?

Introduction / Context:
Moorum (a lateritic gravelly material) is widely used as a blanket layer in railway formation to control infiltration and improve drainage. Its use is particularly important over expansive soils that undergo large swell–shrink cycles with moisture changes, threatening track stability.

Given Data / Assumptions:

• We seek the subgrade type where moorum blanket is most essential to prevent moisture ingress.
• Moorum provides a capillary break, better drainage, and reduced volumetric changes in the subgrade.

Concept / Approach:

Black cotton soil (expansive clay) exhibits high swell and shrinkage due to montmorillonite content. Percolation into such subgrades triggers heave in wet seasons and shrinkage in dry, causing differential settlements and track geometry defects. Moorum blanket reduces water entry, moderates moisture variation, and improves bearing performance.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Design manuals recommend subgrade improvement/blanketing over expansive soils specifically; case studies show reduced maintenance where moorum blankets are provided over black cotton formations.

Why Other Options Are Wrong:

Sandy soils drain readily and need less capillary control; generic clayey soils may need improvement but the classic critical case is black cotton soils. “All the above” is too broad for the primary use case asked.

Common Pitfalls (misconceptions, mistakes):

Treating all subgrades as equally problematic; underestimating swell–shrink risks of expansive clays.

Final Answer:

black cotton soil