Civil Engineering Reference
In-Depth Information
Fig. 2.24
Dam resting on an impermeable soil.
relationship between a and
Δ
a in terms of
α
, the angle of the downstream slope (Fig.
2.24b
). In Fig.
2.24
the point J can thus be established and the corrected flow line sketched in as shown.
2.15 Seepage through non-uniform soil deposits
2.15.1 Stratification in compacted soils
Most loosely tipped deposits are probably isotropic, i.e. the value of permeability in the horizontal direc-
tion is the same as in the vertical direction. Present practice is to construct most embankments, spoil
heaps and dams by spreading the soil in loose layers which are then compacted. This construction tech-
nique results in a greater value of permeability in the horizontal direction, k
x
, than that in the vertical
direction (the anisotropic condition). The value of k
z
is usually 1/5 to 1/10 the value of k
x
.
The general differential equation for flow was derived earlier in this chapter:
2
2
2
∂
∂
h
x
∂
∂
h
y
∂
∂
h
z
k
+
k
+
k
=
0
x
y
z
2
2
2
For the two dimensional, i.e. anisotropic, case the equation becomes:
∂
∂
h
x
2
∂
∂
2
h
z
k
+
k
=
0
x
z
2
2
