Environmental Engineering Reference
In-Depth Information
Figure 10.6.
Earth dam with internal drains of inadequate discharge capacity.
Permeability
K
2
L
1
w
h
2
Permeability
K
1
h
(a)
Vertical drain
(b)
Horizontal drain
Figure 10.7.
Design of drain dimensions for discharge capacity.
of the downstream slope and also potentially leading to piping failure in the down-
stream fill.
Such lack of drain capacity can also occur for dams on permeable rock foundations
where earthfill dams such as those shown on
Figures 10.5
and 10.6 are used. It can also
occur where dirty rockfill has been used as the downstream zone.
Cedergren (1972) gives a design method for estimating the discharge capacity of a hor-
izontal drain without pressurization based on:
2
kh
2L
1
q
(10.1)
1
where
k
1
permeability of the drain material - m/sec
h
vertical thickness of the drain - m
L
1
length of the drain - m as shown in Figure 10.7b.
discharge capacity per metre width of drain (width measured across river) -
m
3
/m/sec.
The capacity of the vertical drain is seldom a critical issue because the quantity of seep-
age through the earthfill is small and the vertical drain width is dictated by construction
factors. However its capacity should be checked by:
q
khw
L
22
q
(10.2)
2
2
where
k
2
permeability of vertical drain; h
2
, L
2
are as shown in Figure 10.7a.
width of drain.
If there is any doubt about the capacity, a two (2A, 2B) or 3(2A, 2B, 2A) layer filter drain
system should be used.
w
