Bearing Capacity of Soil and its Factor of Safety

Fig: Effect of Bearing Capacity of Soil on Foundation



Bearing capacity of soils are reduced by a factor called factor of Safety to underestimate the bearing capacity of soils. This value of factor of safety depends on following factors:

1.  Type of soil

2.  Method of exploration

3.  Level of Uncertainty in Soil Strength

4.  Importance of structure and consequences of failure

5.  Likelihood of design load occurrence, etc.

Generally a factor of safety F = 3 is assumed for bearing capacity calculations,  unless otherwise specified for bearing capacity problems. Table-1 gives the details of factors of safety to be used under different circumstances.

Fig: Effect of Bearing Capacity of Soil on Foundation

Fig: Effect of Bearing Capacity of Soil on Foundation

Table-1: Typical factors of safety  for bearing capacity  calculation in different situations

Category Typicalstructures Characteristics of the category Design factor of safety
Thorough and completesoil exploration Limited soil exploration
A Railway bridges, warehouses, blast furnaces, hydraulic, retaining walls, silos Maximum design load likely to occur often,

consequences of failure – disastrous

3.0 4.0
B Highway bridges, light industrial and public buildings Maximum design loads may occur occasionally,

consequence of failure – serious

2.5 3.5
C Apartment and office buildings Maximum design load unlikely to occur 2.0 3.0