Geotechnical Engineering

DHARM

SEEPAGE AND FLOW NETS 169

shown has been drawn within these boundaries. With the aid of flow net, we can compute the
seepage under the wall, the pore pressure at any point and the hydraulic gradient at any
point. A water pressure plot, such as that shown in Fig. 6.2 is useful in the structural design of
the wall.

Silty soil Flow

line

Equipotential

line

Impervious

a

l

m b j n

p q

e

a

ee

f f

g

j

g

j b c d e 0

(a) Flow net (b) Water pressure on the wall

l

Sheet pile wall

Fig. 6.2 Sheet pile wall

6.3.2 Flow under Concrete Dam

Figures 6.3 to 6.7 show a concrete dam resting on an isotropic soil. The sections shown are
actually those of the spillway portion. The upstream and tail water elevations are shown. The
first one is with no cut-off walls, the second with cut-off wall at the heel as well as at the toe,
the third with cut off-wall at the heel only, the fourth with cut-off wall at the toe only and the
fifth is with upstream impervious blanket. The boundary flow lines and equipotentials are
known in each case and the flow nets are drawn as shown within these boundaries. The effect
of the cut off walls is to reduce the under seepage, the uplift pressure on the underside of the
dam and also the hydraulic gradient at the exit, called the ‘exit gradient’. A flow net can be
understood to be a very powerful tool in developing a design and evaluating various schemes.

Dam H

Impervious

Fig. 6.3 Concrete dam with no cut-off walls