Foundation engineering — typical safe bearing capacity (SBC) range for black cotton soil In routine building design practice, the safe bearing capacity of expansive black cotton soil is generally taken to lie within which of the following ranges (in t/m²), assuming conservative values for shallow foundations?

Introduction / Context:
Black cotton soil is an expansive clay that undergoes large swelling and shrinkage with moisture changes. For shallow foundations, designers adopt conservative safe bearing capacity (SBC) values to limit settlement and mitigate seasonal movement risk. Knowing the usual SBC range helps in early sizing and feasibility decisions.

Given Data / Assumptions:

• Near-surface, natural condition black cotton soil without extensive ground improvement.
• Shallow foundation context (footings/rafts), typical serviceability criteria.
• Conventional safety factors and empirical ranges used in preliminary design.

Concept / Approach:

Expansive clays generally have lower allowable pressures than dense sands or stiff clays because of low shear strength in the wet season and high compressibility. For black cotton soil, many handbooks indicate a cautious SBC window around 5 to 7.5 t/m² for preliminary design unless improved. Lower values like 2–3 t/m² can occur in very soft conditions; higher values (8–12 t/m²) are more typical of competent clays/sands or improved ground.

Step-by-Step Solution:

Verification / Alternative check:

Settlement calculations using undrained shear parameters and compressibility often corroborate that higher contact pressures are risky without improvement or deep foundations in expansive clays.

Why Other Options Are Wrong:

• 2–3 t/m² is overly low for typical undisturbed sites unless soils are very soft.
• 8–10 and 10–12 t/m² are optimistic for expansive clay unless treated or on stiff crusts.

Common Pitfalls:

• Ignoring seasonal moisture variation; SBC must ensure performance in the wettest condition.
• Using high values from point tests without consolidation and swell considerations.

Final Answer:

5 to 7.5 t/m².