Expansive Soils – Behaviour of black cotton soil (high-plasticity clay) Which statements describe the characteristic behaviour of black cotton soil?

Introduction / Context:
Black cotton soils are expansive clays prevalent in many regions. Their volume changes dramatically with moisture content, posing challenges for foundations, pavements, and lightly loaded structures.

Given Data / Assumptions:

• Soil rich in expansive clay minerals (e.g., montmorillonite).
• Seasonal wetting–drying cycles occur.
• Foundations are shallow unless specifically treated.

Concept / Approach:
Expansive clays adsorb water into their lattice and diffuse double layer, increasing interparticle spacing and causing macroscopic swell. During drying, loss of water collapses the structure and causes shrinkage and cracking. Both behaviours lead to differential movements affecting structures.

Step-by-Step Solution:
Identify mineralogy: high plasticity clays with swelling potential.Relate wet season to heave (swell) under foundations.Relate dry season to shrinkage and surface cracking.Conclude that large volumetric changes are inherent in such soils.

Verification / Alternative check:
Laboratory tests (free swell, swell pressure) and field observations (cracks in summer, sticking doors post-monsoon) validate the described behaviour.

Why Other Options Are Wrong:

• Each of a–d accurately describes expansive clay; thus the comprehensive answer is “All of the above”.

Common Pitfalls:
Designing shallow strip footings without moisture control; better approaches include moisture barriers, soil replacement, under-reamed piles, or flexible detailing to accommodate movement.

Final Answer:
All of the above