Factor safety in Determining Bearing Capacity
Bearing capacity is the ability of soil to safely carry the pressure placed on the soil from any engineered structure without undergoing a shear failure with accompanying large settlements. Applying a bearing pressure which is safe with respect to failure does not ensure that settlement of the foundation will be within acceptable limits. Therefore, settlement analysis should generally be performed since most structures are sensitive to excessive settlement. Unpredictivity of soil property introduce the term factor of safety which provide a degree of underestimation of bearing capacity dividing it by some factor.
Table below illustrates some factors of safety. These FS’s are conservative and will generally limit settlement to acceptable values, but economy may be sacrificed in some cases.
(a) FS selected for design depends on the extent of information available on subsoil characteristics and their variability. A thorough and extensive subsoil investigation may permit use of smaller FS.
(b) FS should generally be ≥ 2.5 and never less than 2.
(c) FS in Table for deep foundations are consistent with usual compression loads.
Typical Factors of Safety
|
SI
|
Structure
|
Factors
of Safety |
||
|
1
|
Retaining
Walls
|
3
|
||
|
2
|
Temporary
braced excavations |
>
2 |
||
|
3
|
Railway
|
4
|
||
|
Highway
|
3.5
|
|||
|
4
|
Buildings
|
Silos
|
2.5
|
|
|
Warehouses
|
2.5*
|
|||
|
Apartments,
offices |
3
|
|||
|
Light
industrial, public |
3.5
|
|||
|
5
|
Footings
|
3
|
||
|
6
|
>3
|
|||
|
7
|
Deep
Foundations |
With
load tests |
2
|
|
|
Driven
piles with wave equation analysis |
2.5
|
|||
|
calibrated
to results of dynamic pile tests Without
load tests |
3
|
|||
|
Without
load tests |
3
|
|||
|
Multilayer
soils |
4
|
|||
|
Groups
|
3
|
|||