Armfield Instruction Manual
22
water. The soil through which the water is pushed by the pressure head
resists its movement in much the same manner, as a rough surface resists, or
brakes, the movement of a sliding body.
The soil resistance to moving water is called viscous friction since it causes a
gradual dissipation of the kinetic energy in the moving water. Within
limitations it can be treated as a negative head.
Figure ii
Let us imagine a situation where water penetrates under a sheet pile wall
from basin I to basin II as shown above and let the pressure head between
the two basins be h. Water will enter the soil along the whole bottom of basin
I, and according to the location of its point of entry, each elemental volume
will follow a different path on its way to basin II. However, all the elemental
volumes, whatever paths they follow, will have the same potential at the
points of entry and, similarly, they will have the same potential when
eventually reaching the bottom of basin II. This follows from the fact that the
water tables in both basins I and II are horizontal as well as their bottoms. So
the pressure is constant along the whole bottom of each basin.
Now we pick two of the unlimited number of possible flow lines and denote
them by A and B as shown in Figure ii above.
The lines connecting points with equal potentials on different flow lines are
called equipotential lines. Thus we see that the contours of the two bottoms
represent two equipotential lines since they connect, respectively, points a
1
and b
1
having equal potentials, and points a
n
and b
n
also having equal
potentials.
Naturally, pairs of points having equal potentials must also exist between the
two pairs located at the beginning and the end of the flow lines. Examples of
such pairs are represented by points a
s
, b
s
and a
t
, b
t
. The connecting lines
a
s
b
s
and a
t
b
t
represent the equipotential lines between the pairs.
