Types of shear failure of soil.
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Types of shear failure of soil.

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Types of shear failure of soil.

In the previous article (Shear Strength of Soil) we explained what meaning of shear strength of the soil and how we calculated it.

In this article we will explain what types of shear failure of soil, soil safety factor in designing foundations and determining the bearing capacity of the soil.

- Types of shear failure of soil.

We have two types of shear failure of soil:

- General shear failure.

In the general shear failure as the load increase, the amount of settlement associated with it increase. But soon the value of the settlement increase significantly. 
Noting that there is no increase associated with the load at small settlement values. This kind of failure occurs in condition of dense, hard, and highly compacted soils.

- Local shear failure.

In local shear failure the load increase with an increase in the amount of settlement associated with it, but without a maximum load and that the amount of settlement in this case is greater than its counterpart in the case of general failure. 
This type of shear failure often occurs in condition of soft, loose, and compressible soils.

The below figure show the relationship between load and settlement in both general and load shear failure.




It is often not assumed that the failure of the soil is by local shear when estimating the value of the maximum bearing capacity of the soil due to the need for the soil to be compacted at the site and thus it reaches a sufficient degree that its associate failure of the type of general shear failure.

- Soil safety factor in designing foundation and determining the bearing capacity of the soil.

To estimate and determine the bearing capacity of the soil without the occurrence of a collapse of the soil under the foundation as a result of the shear force, this is done by choosing an adequate safety factor and accordingly, to obtain the bearing capacity of the maximum soil allowed in the design it is equivalent to:
The bearing capacity of the maximum soil / factor of safety (F.S)

This safety factor value depends on and takes into account the following factors:

  • The expected differences in the shear strength of the soil.
  • High imprecision in theoretical and experimental treatments for determining soil bearing capacity when using these parameters.
  • Potential change in soil properties during foundation and structure implementations.
  • The total cost of implementing the foundations.
  • The importance of the structure and number of floors and nature of its structural elements.
Usually the value of the safety factor for the soil ranges between 2 and 4 is often used 3 in normal cases when affecting dead and live load only. And 2.5 if the earthquake and wind loads are taken into account when designing the foundations.

The reason for using a safety factor of a large value does not reduce the value of the used factors, but it is usually used as insurance against any unseen factor and because any future modifications or repair of foundations is usually expensive and difficult to implement due to the impact of the rest of the elements of the structure by what happens in the foundations. In opposite, the structure if there is any failure in one of its elements or parts is limited or repaired easier and less expensive and the increase in the safety factor is usually accompanied by an increase in the costs of the foundations by a small percentage if compared to the total costs of the structure and safety of its use during the period of its operation.


- Maximum bearing capacity of shallow foundations on multiple soil layers.

There are multiple methods for estimating and calculating the maximum bearing capacity of soil under foundation is based on multiple layers of soil with different properties. Among these methods is the following approximate method: 

Referring to the below figure 





Where the foundation rests on several layers of soil where in this case, each layer is treated separately, considering the width of the foundation (B) at each layer is the width corresponding to the stress distribution line in the vertical assumed inclination ( 2 vertical : 1 horizontal) in this case the maximum bearing capacity is calculated using the first layer properties and its thickness (h1) and using known equations.

Then the maximum bearing capacity of the second layer is calculated using the properties of this layer, assuming the presence of an imaginary foundation resting on its upper surface with a width of B1 =B+h1 (considering the distribution 1:2 of stresses)

The pervious step is repeated for the rest of the layers.

The maximum bearing capacity of the foundation is determined on the bass of the smallest value of bearing capacity of the different layers below the foundation calculated from the previous steps.














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