A bearing capacity failure is defined as a foundation failure that occurs when the shear in the soil exceeds the shear strength of the soil. The bearing capacity failures of the foundation can be divided into three categories as follows:
1. normal shear failure
As shown in Figure 1, a typical shear failure involves complete breakdown of the underlying soil. Failure is the continuous shearing of soil (solid lines) from the bottom of the base to the surface of the ground. When loads are plotted versus footings settled, there is a separate load at which the foundation fails (the concrete circle), and is designated WhyCorrect, value of WhyCorrect divided by width b and length Took considered to be final bearing capacity ,WhyCorrect, Notch. The ultimate bearing capacity is defined as the bearing stress which causes a sudden catastrophic failure of the foundation.
As shown in Figure-1, a typical shear failure erupts and pushes the soil up on either side of the base. For real failures in the field, soil is often pushed to only one side of the base with subsequent tilting of the structure.
A common shear failure occurs for soil that is dense or in a hard state.
2. local shear failure
As shown in Figure 2, local shear failure involves breaking of the soil just below the footing. The soil is raised on both sides of the footing, but the bulge is not as significant as in normal shear.
Local shear failure can be considered as a transitional phase between normal shear and punching shear. Due to the transitional nature of local shear failure, bearing capacity can be defined as the first major non-linearity in the load-disposal curve (open circles) or at the point where the settlement accelerates (solid circles).
A local shear failure occurs for soils in a moderately dense or firm state.
3. punching shear failure
As shown in Figure-3, a perforating shear failure does not develop the typical shear surfaces associated with general shear failure. For perforation shear, the soil outside the loaded zone is relatively unaffected and there is minimal soil movement on either side of the bed.
The process of footing deformation involves the compression of the soil just below the footing as well as the vertical shearing of the soil around the footing circumference. As shown in Figure 3A, there is no dramatic break in the load-disposal curve and for shear punching, bearing capacity is often defined as the first major non-linearity in the load-disposal curve (open circle).
A percolation shear failure occurs for soils in a loose or soft state.
The table below summarizes the types of bearing capacity failure that is most likely to develop, based on soil type and soil properties.
Where (n1,60 Correct is the Standard Entrance Test (SPT) value.
Read also: How to calculate the bearing capacity of a Shallow Foundation