Correct option is D
Expansive Soils (e.g., Black Cotton Soil):
Expansive soils are prone to significant volume changes due to moisture variations, causing swelling when wet and shrinkage when dry.
This creates substantial stresses on foundations, leading to cracks and structural instability.
Under-Reamed Piles:
Under-reamed piles are specifically designed for expansive soils.
They have one or more bulb-shaped enlargements (under-reams) near their base, which anchor the pile firmly into stable soil layers below the expansive zone.
These bulbs resist uplift forces caused by soil swelling and provide excellent stability.
