Civil Engineering Reference
In-Depth Information
The total contact stress
q
between the soil and the base of the pier (i.e. the bearing
pressure) arises from the weight of the concrete and the applied load and is
10
3
10
F
A
=
1
×
q
=
γ
c
H
c
+
(4
×
20)
+
=
180 kPa
(a) At low tide:
σ
=
γ
z
+
q
=
(20
×
2)
+
180
=
220 kPa
z
u
=
γ
w
h
w
=
2
×
10
=
20 kPa
σ
z
=
σ
−
u
=
220
−
20
=
200 kPa
z
(b) At high tide (note that
q
is reduced by uplift from the water pressure below the
foundation):
σ
=
γ
z
+
γ
w
z
w
+
(
q
−
γ
w
z
w
)
z
=
(20
×
2)
+
(10
×
3)
+
(180
−
(10
×
3))
=
220 kPa
u
=
γ
w
h
w
=
10
×
5
=
50 kPa
σ
z
=
σ
−
u
=
220
−
50
=
170 kPa
z
Notice that in this case the increase of the water depth has reduced the effective
stress in the ground; this is because of a reduction of the bearing pressure due to
uplift.
Example 6.4: Calculation of stress below an embankment
The soil profile in Fig. 6.15
consists of 4m clay over 2m sand over rock: the unit weights of all the natural materials
are 20 kN/m
3
and the steady state water table is at ground level. A wide embankment
4 m high is constructed from fill with a unit weight of 15 kN/m
3
. The total and effec-
tive vertical stresses at the centre of the clay and at the centre of the sand (a) before
Figure 6.15
Stress below an embankment - Example 6.4.
