In Plaxis 3D, the surface prescribed displacement for determining the ultimate bearing capacity is typically set to a value large enough to capture the full load-settlement behavior but not excessively large to cause numerical instabilities. A reasonable range is between 10% and 20% of the foundation width (B), depending on soil conditions and failure mechanisms.

To distinguish between shear failure and settlement failure, examine the load-settlement curve and shear strain distribution:

• Shear failure occurs when there is a clear peak in the load-settlement curve, beyond which the load starts to drop or stabilizes with excessive deformations. Significant plastic strains and failure zones will be visible in the soil.

• Settlement failure happens when there is no clear peak, but the displacement continues to increase without a corresponding increase in load. This is typically governed by serviceability limits rather than ultimate strength.

If failure occurs due to excessive settlement, the surface prescribed displacement should align with the allowable settlement criteria for the structure, commonly 2-5% of B for serviceability checks.