Fig: Effect of Bearing Capacity of Soil on Foundation

Bearing Capacity of Soil and its Factor of Safety

FACTOR OF SAFETY FOR

BEARING CAPACITY OF SOILS

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