Civil Engineering Reference
In-Depth Information
From Fig.
3.15b
:
0 81 600
.
×
τ
=
=
155
kPa
π
For a square footing:
2
Area
= × =
5 5
25
m
Diameter of circle of same area:
25 4
×
=
5 64
. m
π
Hence the shear stress under a 5 m square foundation can be obtained from the bulb
of pressure of shear stress for a circular foundation of diameter 5.64 m.
z
B
=
5
5 64
=
0 89
.
.
From Fig.
3.15a
:
τ
=
0 2 600
.
×
=
120
kPa
These values can be combined if we proportion them to the respective areas (or
lengths):
15
15 5
τ
=
120
+
(
155 120
−
)
=
146
kPa
+
The method is approximate but it does give an indication of the shear stress values.
3.5.7 Contact pressure
Contact pressure is the actual pressure transmitted from the foundation to the soil. Throughout Section
3.5
it has been assumed that this contact pressure value, p, is uniform over the whole base of the founda-
tion, but a uniformly loaded foundation will not necessarily transmit a uniform contact pressure to the soil.
This is only possible if the foundation is perfectly flexible. The contact pressure distribution of a rigid
foundation depends upon the type of soil beneath it. Figures
3.16a and 3.16
b show the form of contact
pressure distribution induced in a cohesive soil (a) and in a cohesionless soil (b) by a rigid, uniformly loaded,
foundation.
On the assumption that the vertical settlement of the foundation is uniform, it is found from the elastic
theory that the stress intensity at the edges of a foundation on cohesive soils is infinite. Obviously local
yielding of the soil will occur until the resultant distribution approximates to Fig.
3.16a
.
p
p
(a) Cohesive soil
(b) Cohesionless soil
Fig. 3.16
Contact pressure distribution under a rigid foundation loaded with a uniform pressure, p.
