Factor of Safety and Allowable Pressure
Allowable bearing pressure, from shear strength consideration, is only a fraction of the ultimate bearing capacity and is defined by the equation:
qallowable = (qultimate / Fs ) + gDf
- The factor of safety against shear failure is given by Fs
- gDf is the effective overburden pressure at the foundation depth Df
Important Features Affecting Factor of Safety
The required factor of safety depends upon:
- Type of structure permanent or temporary
- Sensitivity of structure
- Extent of soil exploration
- Nature of loading considered and assumption made in the design
- Extent of quality control during construction.
It is recommended that the factor of safety should be between 2 and 4. The following table may be used as a guide for permanent structures in reasonably homogeneous soil conditions.
Factor of Safety for Shallow Foundations
Below mentioned is a table giving minimum values of factor of safety for the design of shallow foundations. This is as per Vesic,1970.
Table.1:Minimum value of safety factor for design of shallow foundations (as per Vesic, 1970)
|Typical Structure||Characteristics of the category||Soil Exploration|
|Railway bridge, Warehouses, blast furnaces, silos, hydraulic retaining walls||Maximum design load likely to occur often, consequence of failure disastrous||3.0||4.0|
|Highway bridge, light industrial and public buildings||Maximum design load may occur occasionally, consequence of failure serious||2.5||3.5|
|Apartments and office buildings||Maximum design load unlikely to occur||2.0||3.0|
- Only after properly assessing the reliability of all parameters governing the structure, the Factor of safety can be decided. These parameters will include the loads, strength and the deformation properties of the soil mass.
- The above mentioned values have to be reduced to 75% with value not less than 2, for temporary structures.
- In the case of chimneys and towers, or in common tall buildings where there is chances for progressive collapse, it is recommended to increase the value by 20 to 50%.
- Adequate consideration must be given to take into consideration scour and excavation overburden.
Factor of Safety and Allowable Capacity of Pile Foundation
The factor of safety for both downward and upward loading in pile foundations are explained below:
1. For downward loading
Fs (Factor of safety) = 2.5 , when both end bearing and shaft resistance are considered. This value should be reduced upto 2.0 if sufficient number of pile load tests are conducted to ensure that Fs will never fall below 2.
Fs = 1.5 for shaft resistance and Fs =3 for end bearing. However, Fs based on both end bearing and shaft resistance should be greater than 2.
First approach should be given greater weightage.When allowable load is estimated based on first approach, it is ensurd that it is not less than estimated by second. It is essential to have higher factor of safety in end bearing than in shaft resistance. With negative stress friction acting, factor of safety as above should be obtained. With negative friction, the factor of safety should not be less than 2.
2. For uplift conditions
Fs = 2.5
Fs could be reduced to 2, if the capacity of pile could be established by a pull-out test. Fs could be further reduced to 1.5 if the weight of the pile itself is 0.75 times the uplift force or greater.
Factor of Safety For Embankments & Earth dams
The factor of safety for earth dams, embankments, dams will be less compared to the factor of safety of other structures. Providing high factor of safety will result in uneconomical sections of earthwork structures. It is satisfactory to provide lower values of safety. Table-2 below shows the factor of safety for different earthwork structures.
Table.2: Factor of Safety for Different Earthwork Structures
|Description of earthwork||Safety Factors|
|Embankments, end of construction||1.0 to 1.2|
|*cuts, end of construction||1.2 and over|
|Embankments, long term stability||1.2 to 1.4|
|*cuts, long term stability||1.2 to 1.4|
|Earthdams||1.5 and above|
|Earthdams – extreme conditions of loading (i.e. severe flood followed by sudden drawdown)||1.1 to 1.25|
Note: For cuts, safety factor has usually higher values at end of construction rather than at some future time.