Civil Engineering Reference
In-Depth Information
Excess head loss through fine sand:
−
6
7 8 10
.
×
×
4
h
=
=
0 16
.
m
2 10
×
−
4
Excess head at interface between fine and coarse silt
=
2 5 1 56
.
−
.
=
0 94
.
m
Excess head at interface between fine sand and coarse silt
=
0 94 0 78
.
−
.
=
0 16
.
m
2.15.4 Seepage through soils of different permeability
When water seeps from a soil of permeability k
1
into a soil of permeability k
2
the principle of the square
flow net is no longer valid. If we consider a flow net in which the head drop across each figure,
Δ
h, is a
constant then, as has been shown, the flow through each figure is given by the expression:
q k h
b
l
∆
=
∆
If
Δ
q is to remain the same when k is varied, then b/l must also vary. As an illustration of this effect
consider the case of two soils with k
1
=
k
2
/3.
Then
h
b
l
1
1
∆
q k
=
1
∆
and
h
b
l
h
b
l
2
2
∆
q k
=
∆
=
3
k
∆
2
1
2
2
i.e.
b
l
b
l
1
2
=
3
1
2
If the portion of the flow net in the soil of permeability k
1
is square, then:
b
l
1
3
b
l
k
k
2
2
2
2
1
2
=
or
=
The effect on a flow net is illustrated in Fig.
2.30.
2.15.5 Refraction of low lines at interfaces
An interface is the surface or boundary between two soils. If the flow lines across an interface are
normal to it, then there will be no refraction and the flow net appears as shown in Fig.
2.30.
When the
flow lines meet the interface at some acute angle to the normal, then the lines are bent as they pass into
the second soil.
