Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 5 BEARING CAPACITY THEORY
5.1.2 Factors of Safety
The net allowable bearing pressure of a shallow foundation resting on soils is obtained by applying
a factor of safety to the net ultimate bearing capacity i.e.
q u = q ult
F
(5.2)
where
q ult = ultimate net bearing capacity
q u = allowable bearing capacity
F = Factor of safety
The net ultimate bearing capacity should be taken as (q u – γ D f ) where D f is the depth of soil
above the base of the foundation and γ is the bulk unit weight of the soil. The selection of the
appropriate factor of safety should consider factors such as:
(a) The frequency and likelihood of the applied loads (including different combination of dead
load and live loads) reaching the maximum design level.
(b) Soil variability, e.g. soil profiles and shear strength parameters. The ground investigation
helps increase the reliability of the site characterization.
(c) The importance of the structures and the consequences of their failures.
In general, the minimum required factor of safety against bearing failure of a shallow foundation is
in the range of 2.5 to 3.5. For most applications, a minimum factor of safety of 3.0 is adequate.
Although the factor of safety is applied to the bearing capacity at failure, it is frequently used to
limit the settlement of the foundation.
5.1.3 Effects of Groundwater
The ultimate bearing capacity depends on the effective unit weight of the soil. Where groundwater
is present, the effective stress and shear strength along failure plane will be smaller and the bearing
capacity will be reduced. The effect of groundwater is accounted for by adjusting the γ s ' in equation
5.1. and the three possible cases as shown in Figure 5.2 and describe below:
a) Case 1: D w < D
Use γ’ = γ b = γ - γ w
where γ b = weighted average buoyant unit weight
b) Case 2: D < D w < D + B
Use ′ w
1- Dw-D
B
c) Case 3: D + B < D w (no groundwater correction is necessary )
Use γ’ = γ
March 2009 5-5