Vol. 13, Bund. F
2
INTRODUCTION
The ultimate bearing capacity of strip footings can be determined using the method
proposed by Terzaghi (1943):
1
2
ucq
q
cN
qN
BN
γ
=++
(1)
Where
q
u
= ultimate bearing capacity,
c
= soil cohesion,
q
= surcharge,
γ
= soil unit
weight,
B
= width of strip footing.
N
c
, N
q
and
N
γ
= bearing capacity factors which depend only
on friction angle of soil,
φ
.
Several methods have been proposed to calculate the bearing capacity factors of
N
c
, N
q
and
N
γ
. The exact values of
N
q
were obtained by Prandtl (1921) for strip footing on weightless
subsoil, from which
N
c
can be derived. However, exact values of
N
γ
have not been obtained
for quite a long time; numerical solutions have been obtained by many researchers including
recently by Martin (2005) using the method of characteristics.
There are several quite different methods for determination of ultimate bearing capacity,
i.e. limit equilibrium, slip line method, finite element method, finite difference method and
limit analysis. By using these methods, several results of bearing capacity factor
N
γ
have been
investigated. Sokolovskii (1965), Booker (1969), Hansen (1970), Bolton and Lau (1993) and
Kumar (2003) have obtained
N
γ
using slip line method. Griffiths (1982), Frydman and Burd
(1997) obtained
N
γ
using finite element method and finite difference method. Chen (1975),
Michalowski (1997), Soubra (1999) and Zhu (2000) obtained
N
γ
using rigid block upper
bound limit analysis. Zhu et al (2001) employed slice method of limit equilibrium to calculate
N
γ
. Recently, upper bound finite element analysis, combined with optimization procedure,
provided a rigorous numerical solution to ultimate bearing capacity problem (Sloan, 1989).