Page 431 - Civil Engineering Formulas

HYDRAULICS AND WATERWORKS FORMULAS 357

The coefficient of permeability may be determined in the laboratory by tests
on distributed or remolded samples of the soil.

Quantity of Seepage. The quantity of seepage can be computed directly
from a flow net. Or, in certain instances, from charts or equations without the
construction of a flow net. The formula for computing the quantity of seepage
from a flow net is
n f
Q k h t L (12.211)
n d
where Q quantity of seepage in length of dam under consideration
k effective coefficient of permeability which is k max k min
n number of flow channels of net
f
n number of equipotential drops of net
d
h head difference between headwater and tail water
t
L length of dam to which the flow net applies
The ratio n / n is termed the shape factor of the flow net.
d
f
Approximate solutions for the quantity of seepage underneath an impervi-
ous embankment founded on a pervious foundation have been developed by
Terzaghi. The definitions of terms for the equations are as given above or as
defined in Fig. 12.32:
Case I. When I > 2U,
0.88
hk
Q I L (12.212)
U

Pervious
Line of seepage
Impervious

Pervious
Impervious
Pervious foundation with cutoff

B Pervious
shells
Variable Blanket
Impervious
I
b U
Variable Pervious underground
FIGURE 12.32 Blanket diagram. (Davis—Handbook of Hydraulics, McGraw-Hill.)