SMALL DAMS PETITS BARRAGES

to laying a completely impervious layer horizontally for only xr in the upstream reservoir. And
the quantity of seepage in the foundation ground qf is obtained by equation (2)
qf =
k・
h・
d
x
r+ xd
………………………………….………………………..…….(2)
Where:
qf:seepage in the foundation ground (m 3 /s)
h:difference between the reservoir water level and downstream water level(m)
xr:effective seepage route length(m)
xd:width of the dam body base (m)
5.7.1.3 Artificial Blanket
h
t
d
(Head loss caused by the blanket)
(Δhb )
xr
Fig. 5.8 – Natural Blanket (Impervious)
xd
(Hydraulic gradient of
pervious layer)
The necessary length of an artificial blanket x is calculated by equation (3)
xr =
Where:
e
2ax
-1
2ax
a ( e + 1)
…………………………………………………........…………(3)
k1
a =
t・
k・
d
x:required length of the blanket(m)
xr:effective seepage route length(m)
The value of q f is determined based on the allowed leakage of the reservoir, xr for this is
obtained by equation (1), and the required length of the blanket x is obtained by substituting xr
in equation (3). The standard for the thickness is 1/10 of the water pressure. It is often from 1.0
to 3.0 m and the closer to the dam body, it is thicker, and the further upstream, it is thinner.
But care is necessary, because in ground with a large coefficient of permeability in the
horizontal direction, only executing a blanket may not necessarily provide adequate resistance
to piping.
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