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

Chapter 5 BEARING CAPACITY THEORY
5.1.1.2 General Equation For Bearing Capacity
Various equations have been established for calculating the bearing of shallow foundation. A
comprehensive one which takes into consideration the shape of the foundation, inclination of
loading, the base of the foundation and ground surface is as follows
(GEO, 1993):
q u = Q u
Bf'Lf'
c'Nc ζ cs
ζ ci
ζ ct
ζ cg
+ 0.5 Bf' γs' Nγ ζ γs
ζ γi
ζ γt
ζ γg
+ q Nq ζ qs
ζ qi
ζ qt
ζ qg
(5.1)
Where:
Nc, Nγ, Nq = general bearing capacity factors which determine the capacity of a long strip
footing acting on the surface of a soil in a homogenous half space
Q u = ultimate resistance against bearing capacity failure
q u = ultimate bearing capacity of foundation
q
= overburden pressure at the level of foundation base
c’ = effective cohesion of soil
γs’ = effective unit weight of the soil
Bf = least dimension of footing
Lf
= longer dimension of footing
Bf’ = Bf – 2e B
Lf’ = Lf – 2e L
e L = eccentricity of load along L direction
e B = eccentricity of load along B direction
ζ cs
, ζ γs
, ζ qs
= influence factors for shape of shallow foundation
ζ ci
, ζ γi
, ζ qi
= influence factors for inclination road
ζ cg
, ζ γg
, ζ qg
= influence factors for ground surface
ζ ct
, ζ γt
, ζ qt
= influence factors for tilting of foundation base
Figure 5.1 shows the generalized loading and geometric parameters for the design of a shallow
foundation. The bearing capacity factors are given in Table 5.1. Equation 5.1 is applicable for the
general shear type of failure of a shallow foundation, which is founded at a depth less than the
foundation width. This failure mode is applicable to soils that are not highly compressible and have a
certain shear strength, e.g. in dense sand. If the soils are highly compressible, e.g: in loose sands,
punching failure may occur. Vesic (1975) recommended using a rigidity index of soil to define
whether punching failure is likely to occur. In such case, the ultimate bearing capacity of the
foundation can be evaluated based on Equation 5.1 with an additional set of influence factors for soil
compressibility (Vesic,1975).
5-2 March 2009