Factor of safety for soil bearing capacity

Factors of safety for bearing capacity refers to a value that is based on the basis of the type of soil, method of exploration, level of uncertainty in soil, significance of structure and outcomes of failure, and probability of design load proceeding.

So, it creates adequate space to adapt uncertainties and potential over loading throughout the duration of the structure and its foundation through the cutback of ultimate bearing capacity of soil to permissible bearing capacity.

The permissible bearing capacity is measured by dividing ultimate bearing capacity with factor of safety. Normally, a factor of safety of (3) is presumed for bearing capacity calculations, if not mentioned for bearing capacity problems.

Bearing capacity means the strength of soil to bear the pressure securely that is provided on the soil from any engineered structure devoid of experience a shear failure with accompanying large settlements. Applying a bearing pressure that is secured with regards to failure does not guarantee that settlement of the foundation will remain inside tolerable ranges.

So, settlement analysis should normally be accomplished since most structures are sensitive to extreme settlement.

In the following table, you will get the standard factor of safety for bearing capacity calculation in different conditions. These factors of safeties are conservative and normally limit settlement to suitable values, but economy may be abandoned in some cases.

Factor of safety chosen for design is based on the extent of information obtainable on subsoil characteristics and their variability. A detailed and wide-ranging subsoil investigation may allow use of smaller factor of safety.

Category Typical structures Characteristics of the category Design factor of safety 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

Factor of safety for soil bearing capacity