Civil Engineering Reference
In-Depth Information
Fig. 2.29
Example
2.11.
Beneath the fine silt layer there is a stratum of water-bearing gravel with a water
pressure of 155 kPa. The surface of the sand is flooded with water to a depth of 1 m.
Determine the quantity of flow per unit area in mm
3
/s, and the excess hydrostatic
heads at the sand/coarse silt and the coarse silt/fine silt interfaces.
Solution:
12
k
z
=
=
3 75 10
.
×
−
5
mm s
/
4
2 0 10
4
4 0 10
4
2 0 10
+
+
.
×
−
4
.
×
−
5
.
×
−
5
Taking the top of the gravel as datum:
Head of water due to artesian pressure
=
15.5 m
Head of water due to groundwater
=
3
×
4
+
1
=
13 m
Therefore excess head causing flow = 15.5 − 13 = 2.5
=
15.5
−
13
=
2.5 m.
2 5
12
.
Flow q Aki
= =
=
3 75
.
×
×
10
−
5
=
7 8 10
.
×
−
6
mm s
3
/
This quantity of flow is the same through each layer.
Excess head loss through fine silt:
h
Flow
=
7 8 10
.
×
−
6
=
2 0 10
.
×
−
5
×
4
Therefore
31 2 10
2 10
.
×
×
−
6
h
=
=
1 56
.
m
−
5
Excess head loss through coarse silt:
−
6
7 8 10
.
×
×
4
h
=
=
0 78
.
m
4 10
×
−
5
