Civil Engineering Reference
In-Depth Information
B
L
z
B
N
c
=
5 1 0 2
+
.
1 0 2
+
.
with a limiting value for N
c
of N
c
=
7.5(1
+
0.2B/L), which corresponds to a z/B ratio greater than 2.5
(Skempton,
1951)
.
9.3.4 Summary of bearing capacity formula
It can be seen that Rankine's theory does not give satisfactory results and that, for variable subsoil condi-
tions, the slip surface analysis of Fellenius provides the best solution. For normal soil conditions, Equations
(6)-(9)
can generally be used and may be applied to foundations at any depth in c-
φ
soils and to shallow
foundations in cohesive soils. For deep footings in cohesive soil the values of N
c
suggested by Skempton
may be used in place of the Terzaghi values.
9.3.5 Choice of soil parameters
As with earth pressure equations, bearing capacity equations can be used with either the undrained or
the drained soil parameters. As granular soils operate in the drained state at all stages during and after
construction, the relevant soil strength parameter is
φ
′
.
Saturated cohesive soils operate in the undrained state during and immediately after construction and
the relevant parameter is c
u
. If required, the long-term stability can be checked with the assumption that
the soil will be drained and the relevant parameters are c
′
and
φ
′
(with c
′
generally taken as equal to zero).
Example 9.1:
Ultimate bearing capacity (Terzaghi) in short-
and long-term
A rectangular foundation, 2 m
×
4 m, is to be founded at a depth of 1 m below the
surface of a deep stratum of soft saturated clay (unit weight
=
20 kN/m
3
).
Undrained and consolidated undrained triaxial tests established the following soil
parameters: c
u
=
24 kPa,
φ
′
=
25°, c
′
=
0.
Determine the ultimate bearing capacity of the foundation, (i) immediately after con-
struction and, (ii) some years after construction.
