Civil Engineering Reference
In-Depth Information
Fig. 9.6
Terzaghi's bearing capacity coefficients.
The increase in the value of N
c
from 5.14 to 5.7 is due to the fact that Terzaghi allowed for frictional
effects between the foundation and its supporting soil.
The coefficient N
q
allows for the surcharge effects due to the soil above the foundation level, and N
γ
allows for the size of the footing, B. The effect of N
γ
is of little consequence with clays, where the angle
of shearing resistance is usually assumed to be the undrained value,
φ
u
, and assumed equal to 0°, but it
can become significant with wide foundations supported on cohesionless soil.
Terzaghi's solution for a circular footing is:
q
=
1 3
.
cN
+
γ
zN
+
0 3
.
γ
BN
(
where B diameter
=
)
(7)
u
c
q
γ
For a square footing:
q
=
1 3
.
cN
+
γ
zN
+
0 4
.
γ
BN
γ
(8)
u
c
q
and for a rectangular footing:
B
L
+
B
L
q
=
cN
1 0 3
+
.
γ
zN
+
0 5
.
γ
BN
1 0 2
−
.
(9)
u
c
q
γ
Skempton (
1951)
showed that for a cohesive soil (
φ
=
0°) the value of the coefficient N
c
increases with
the value of the foundation depth, z. His suggested values for N
c
, applicable to circular, square and strip
footings, are given in Fig.
9.7.
In the case of a rectangular footing on a cohesive soil a value for N
c
can
either be estimated from Fig.
9.7
or obtained from the formula:
