Civil Engineering Reference
In-Depth Information
3.5 Stresses induced by applied loads
3.5.1 Stresses induced by uniform surface surcharge
In the case of a uniform surcharge spread over a large area it can be assumed that the increase in vertical
stress resulting from the surcharge is constant throughout the soil. Here, the vertical total stress at depth
z, is given by
σ
= +
γ
z
q
z
where q is the magnitude of the surcharge (kPa).
Example 3.3:
Effective stress with surface loading
Details of the subsoil conditions at a site are shown in Fig.
3.8
together with details of
the soil properties. The ground surface is subjected to a uniform loading of 60 kPa and
the groundwater level is 1.2 m below the upper surface of the silt. It can be assumed
that the gravel has a degree of saturation of 50% and that the silt layer is fully saturated.
NB
γ
w
=
9.81 kN/m
3
.
Determine the vertical effective stress acting at a point 1 m above the silt/rock interface.
Solution:
G eS
e
+
+
9 81
2 65
.
+
0 65 0 5
.
×
.
s
r
Bulk unit weight of gravel
=
.
γ
=
w
1
1
0 65
.
+
=
17 7
.
kN/m
3
G e
e
+
+
9 81
2 58 0 76
1 0 76
.
+
.
s
Saturated weight of silt
=
γ
=
.
w
1
+
.
=
18
.6
3
kN/m
Effective vertical stress at 1 m above silt/rock interface
Uniform pressure
applied at ground
surface
Total pressure
due to weight
of soils
−
[
Water pressur
e
]
=
+
= +
60
( .
1 8 17 7
×
.
+
4 2 18 6
.
×
.
)
− ×
3 9 81
.
=
140 6
.
kPa
G
s
=
2.65
e
=
0.65
1.8 m
Gravel
1.2 m
G
s
=
2.58
e
=
0.76
Silt
4 m
Rock
Fig. 3.8
Example
3.3.
