What are the different types of shear failure in foundation?

Soil mostly fail in shear, thus when foundation load is transferred to the soil, the soil may fail in shear in 3 modes names as; (a) General Shear Failure (b) Local Shear failure (c) Punching shear failure;
The type of failure that will occur in the soil due to foundation load will depend upon the type of soil, its state of compactness or gradation in case of granular soil;

Failure Types in foundation

a)    General Shear Failure;

The most common type of failure in foundation is general shear failure; general shear failure mostly occurs in dense or very dense granular soil which might be sand or gravels or it may be stiff or hard cohesive soil which is well compacted or having low moisture content. In load deformation curve there is relatively small deformation with increasing load showing a clear peak represented by a sudden drop of load after peak.
In the failure of the soil mass the failure plane will extend to the surface of the soil and there is clear bulging extending out of the soil surface. In case of soil sample the failure will occur at peak load and the failure planes will move relative to each other.

(b) Local Shear Failure

 Local shear failure mostly occur in loose and medium dense granular soil which might be sand or gravel and soft and medium cohesive soil which might be medium compacted or high moisture content.
In load deformation curve; there is no peak and after a certain load there are sudden jerks, in the soil mass there is small broken bulging and the failure surface is not that clear. Same is the case will the soil sample failed in local shear failure while testing with triaxial or unconfined compression test.

(c) Punching Shear Failure:

Punching shear failure occurs in soil which is very loose granular or very soft cohesive soil. The settlement in foundation is considerable with increasing the load and load deformation curve is very steep. In soil mass the failure surface will not extent to the ground surface.
 The test sample may not be handled or may slump when taken out of the sampler.

Keywords for this post

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