Modes of shear failure

There are three modes of soil shear failure, i.e., general shear failure, local shear failure, and punching shear failure primarily based on soil and footing depth with respect to its width (i.e. D / B ratio).

The bearing capacity failure of the foundation base can occur in three alternative ways based on the type of soil and the location of the foundation.

Here we will learn about general shear failure, local shear failure & punching shear failure.

Types of Shear Failure:

1. General Shear Failure:

In this modes of shear failure, a slight downward movement of the foot develops completely plastic areas, and sudden failure happens with an appreciable elevation of the bottom surface adjacent to the footing.

They are characterized by well-defined failure patterns including a wedge and slip surface and heaving of the soil adjacent to the footing.

Often in the shear failure of soil, there is a sudden collapse as well as a bending of the foot, this type of failure happens in case of dense sand or stiff cohesive soil support that supports a footing.

In this case, the failure load is well defined, the load-settlement diagram is similar to the stress-strain for dense sand or extra cohesive soils.

Failure occurs where the soil is placed, the brittle type of stress-strain behavior can be identified by:

• Sudden and catastrophic failure with tilt of the base.
• The ground surface extends adjacent to the foundation.
• Failure occurs at very small vertical stresses with large lateral stresses.
• The load settlement curve indicates an accent peak.

2. Local shear failure:

In this modes of soil failure, the failure pattern has a wedge and slip surface but is defined only under the foot, slight elevation of the soil surface occurs when bending of the feet is not expected.

In this mode, there is a large deformation prior to the development of the felting zone, i.e. a large vertical settlement before a slight bulge of the ground surface.

The final load is not well defined in local shear failure, it occurs in moderately compacted soil or loose sand, i.e. in high compressive soils.

The yields are close to the lower edges of the crop, many yield growth can occur with settlement in a series of shocks.

And the bearing pressure at which the primary yield happens is called the first-failure pressure or first-failure load.

Failure occurs where soils with elastic-plastic stress-strain behavior can be identified:

• There is no sign of visual collapse nor substantial bending of the foundation.
• Minor bulge of ground surface adjacent to foundation.
• Difficult to find the failure point (load) on the load-settlement curve.
• Failure occurs under any very small stress and large lateral stress.

3. Punching shear failure:

In this modes of shear failure, the pattern is not effectively defined in shear failure punching soil.

There is no elevation of the floor on the ground nor the tilting of the feet, the yield surface in the vertical plane adjoining to the foundation edges.

The floor of the ground will be dragged downward with no bulge of the surface and failure occurs instantly after footing and the surrounding soil stays relatively unaffected.

Perforated shear failure occurs in weak vertical soils with considerable vertical settlement i.e. in very excessive compressive soil and low compressibility if the foundation is positioned at substantial depth.

After the first yield, the load-settlement curve will be slightly stable but will remain fairly flat and if the foundation is located at a considerable depth, punching shear failure may also occur in low compressive soils.

The failure occurs in soils with plastic type stress-strain behavior and can be identified by:

• Sinking of footing into the soil.
• Poorly defined ship surface.
• It is very difficult to locate the failure point on the load settlement curve.

Difference between general, locak and punching shear failures:

Features	General shear failure	Local shear failure	Punching shear failure
Relative Settlement	Less	Large	Large
Bulging	Significant	Less	No
Tilting of Footing	Expected	Not expected	Not expected
Ultimate Load	Well defined	Not well defined	Not well defined
Failure Pattern	Wedge + Slip Surface + Bulging	Wedge + Slip Surface + Bulging(no or less)	Not well defined
Occurs in (Soil Type)	Dense	Less compressible	Highly Compressible

Also read: Types of Foundation, Soil Classification & Types of Footing

Conclusion:

Depending on the stiffness of foundation soil and depth of foundation, the following are the modes of shear failure experienced by the foundation soil.