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

(12.211)

whereQquantity of seepage in length of dam under consideration
keffective coefficient of permeability which is
nfnumber of flow channels of net
ndnumber of equipotential drops of net
hthead difference between headwater and tail water
Llength of dam to which the flow net applies

The ratio nf/ndis termed the shape factor of the flow net.
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:
CaseI. When I> 2U,

Q (12.212)



hk

0.88

I

U

L



kmaxkmin

Qk

nf

nd

htL

Pervious

Line of seepage

Impervious

Impervious

B

Pervious foundation with cutoff

Blanket

Variable

Variable

Impervious

Pervious

shells

Pervious underground

b U

I

Pervious

FIGURE 12.32 Blanket diagram. (Davis—Handbook of Hydraulics, McGraw-Hill.)