A bearing capacity failure is defined as a foundation failure that occurs when the shear stresses in the soil exceed the shear strength of the soil. Bearing capacity failures of foundations can be grouped into three categories, as follows:
1. General Shear Failure
As shown in Fig-1, a general shear failure involves total rupture of the underlying soil. There is a continuous shear failure of the soil (solid lines) from below the footing to the ground surface. When the load is plotted versus settlement of the footing, there is a distinct load at which the foundation fails (solid circle), and this is designated Qult. The value of Qult divided by the width B and length L of the footing is considered to be the ultimate bearing capacity (qult) of the footing. The ultimate bearing capacity has been defined as the bearing stress that causes a sudden catastrophic failure of the foundation.
As shown in Fig-1, a general shear failure ruptures and pushes up the soil on both sides of the footing. For actual failures in the field, the soil is often pushed up on only one side of the footing with subsequent tilting of the structure.
A general shear failure occurs for soils that are in a dense or hard state.
2. Local Shear Failure
As shown in Fig-2, local shear failure involves rupture of the soil only immediately below the footing. There is soil bulging on both sides of the footing, but the bulging is not as significant as in general shear.
Local shear failure can be considered as a transitional phase between general shear and punching shear. Because of the transitional nature of local shear failure, the bearing capacity could be defined as the first major nonlinearity in the load-settlement curve (open circle) or at the point where the settlement rapidly increases (solid circle).
A local shear failure occurs for soils that are in a medium dense or firm state.
3. Punching Shear Failure
As shown in Fig-3, a punching shear failure does not develop the distinct shear surfaces associated with a general shear failure. For punching shear, the soil outside the loaded area remains relatively uninvolved and there is minimal movement of soil on both sides of the footing.
The process of deformation of the footing involves compression of soil directly below the footing as well as the vertical shearing of soil around the footing perimeter. As shown in Fig-3, the load settlement curve does not have a dramatic break and for punching shear, the bearing capacity is often defined as the first major non linearity in the load-settlement curve (open circle).
A punching shear failure occurs for soils that are in a loose or soft state.
Table shown below presents a summary of the type of bearing capacity failure that would most likely to develop, based on soil type and soil properties.
Where (N1)60 is the corrected standard penetration test (SPT) value.