Soil mechanics: black cotton soil is generally unsuitable for shallow foundations primarily because of which property?

Introduction / Context:
Foundation performance depends on soil behavior. Black cotton soils (expansive clays) are notorious for distressing lightly loaded structures due to swell–shrink cycles driven by moisture changes.

Given Data / Assumptions:

• Soil type: expansive clay (black cotton soil).
• Shallow foundations without special ground improvement.
• Focus on the most critical adverse property.

Concept / Approach:
Expansive clays contain montmorillonite minerals that absorb water, increasing volume (swell), and lose water, decreasing volume (shrink). This cyclical volumetric change causes differential heave/settlement, cracking walls and floors, and rotating foundation elements—more severe than issues from mere low bearing capacity.

Step-by-Step Solution:
Identify hallmark of black cotton soil: high swell potential and shrinkage upon drying.Relate to foundations: volumetric changes produce seasonal heave and settlement, undermining serviceability.Select the option highlighting volumetric change due to moisture variations.

Verification / Alternative check:
Design remedies (under-reamed piles, moisture barriers, soil replacement/stabilization) specifically target swell–shrink, confirming that volumetric instability is the dominant concern.

Why Other Options Are Wrong:

• Low bearing capacity: may occur but is not universal and is secondary to swell–shrink risks.
• Uncertain permeability: not the core issue for foundation unsuitability.
• Cohesion: clays are cohesive, but cohesion itself does not cause the damage mechanism; volume change does.

Common Pitfalls:

• Designing shallow strip footings without accounting for expansive behavior, leading to cracking and tilting.

Final Answer:
property to undergo a volumetric change due to variation of moisture content