Foundation engineering – definition check: What is meant by the “ultimate bearing capacity” of a soil supporting a foundation?

Introduction / Context:
In geotechnical design, differentiating between ultimate, safe (allowable), and working bearing capacities prevents foundation failures. The ultimate value is a fundamental concept used to assess the strength limit of soil beneath footings before applying safety factors and serviceability checks.

Given Data / Assumptions:

• No numerical data are required; this is a definition and concept question.
• Failure refers to shear failure of soil or sudden intolerable settlement that indicates loss of support.
• Safe or allowable bearing capacity is lower than ultimate due to factors of safety and settlement criteria.

Concept / Approach:
Ultimate bearing capacity qult is the maximum contact pressure that soil can sustain under a foundation just at the onset of failure. Classical theories (Terzaghi, Meyerhof, Vesic) estimate qult using shear strength parameters and footing geometry. The safe bearing capacity qsafe is obtained by dividing qult by a factor of safety and often also checking settlement limits.

Step-by-Step Solution:

Verification / Alternative check:
Load–settlement curves from plate load tests show a peak or break point; this peak corresponds to ultimate capacity, beyond which settlements accelerate or load cannot be increased.

Why Other Options Are Wrong:

• Total load (Option A) ignores soil response; it is not a capacity.
• Safe load (Option B) is a design value, not the limit state.
• Consolidation alone (Option D) is a time-dependent volumetric change, not the ultimate failure condition.

Common Pitfalls:
Confusing allowable and ultimate values; equating gradual consolidation with failure; overlooking that both shear failure and sudden excessive settlement indicate reaching the ultimate limit state.

Final Answer:
Load intensity at which the supporting soil fails in shear or undergoes sudden excessive settlement