Soil Mechanics – Which ground has the highest ultimate bearing capacity? Considering common natural grounds, which category typically provides the maximum bearing capacity for shallow foundations?

Introduction / Context:
Bearing capacity is the maximum pressure that soil or rock can sustain from foundations without shear failure or excessive settlement. Selecting a suitable founding stratum is fundamental in geotechnical design and directly affects footing size and cost.

Given Data / Assumptions:

• Comparison among typical natural strata encountered in building works.
• Shallow foundation context.
• No special ground improvement assumed.

Concept / Approach:
Rocks, especially sound hard rock, have very high shear strength and stiffness, leading to the highest allowable pressures. Sands and clays have much lower capacities, with loose sands and soft clays being the weakest. Black cotton soils (highly plastic clays) undergo swell-shrink and have low safe capacities.

Step-by-Step Solution:
Rank strata qualitatively by strength and stiffness.Identify hard rock as the most competent bearing stratum.Select “Hard rock”.

Verification / Alternative check:
Typical safe bearing values: soft clay ~50–100 kPa, loose sand ~100–200 kPa, dry dense/coarse sand higher, and hard rock can exceed several MPa depending on quality and weathering—far greater than soils.

Why Other Options Are Wrong:

• Black cotton soil: expansive and weak in saturated states.
• Loose fine sand: low density and strength.
• Dry coarse sand: stronger than loose sand but below rock.
• Soft clay: very low capacity and high compressibility.

Common Pitfalls:
Equating “dry” condition with high capacity in sands; density and confinement matter. Always verify weathering and discontinuities in rock as they can reduce actual capacity.

Final Answer:
Hard rock