Factors of safety for bearing capacity is a value which is established based on the type of soil, method of exploration, level of uncertainty in soil, importance of structure and consequences of failure, and likelihood of design load occurrence. So, it provides enough room to accommodate uncertainties and possible over loading during life span of the structure and its foundation through the reduction of ultimate bearing capacity of soil to allowable bearing capacity.
The allowable bearing capacity is computed by dividing ultimate bearing capacity by factor of safety. Generally, a factor of safety of (3) is assumed for bearing capacity calculations, unless otherwise specified for bearing capacity problems.
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.
Factor of Safety for Bearing Capacity of Soils
Table 1 provides typical factor of safety for bearing capacity calculation in various conditions. These factors of safeties are conservative and generally limit settlement to acceptable values, but economy may be sacrificed in some cases.
Factor of safety 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 factor of safety.
Table 1 Typical factors of safety for bearing capacity calculation in different situations
| Category | Typical structures | Characteristics of the category | Design factor of safety | |
| - | - | - | Thorough and complete soil 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 |
Also Read: Effect of Water Table on SBC of Soil