Seepage
The voids of a soil and of most rocks are connected together to form
continuous passageways for the movement of water. Water moving
through these passageways is driven by a difference in water head
and is called “seepage.”
Engineers develop models of seepage called “flow nets.” Such mod-
eling helps in the design of de-watering and drainage systems and
can also be used to help evaluate the effectiveness of dams and dikes
that are already in place.
In the diagram above, the head of water on the left side of the barrier
is greater than that on the right side. Water flows from the left to the
right, following the flow lines shown.
To calculate the rate of flow, the engineer first draws equi-potential
lines across the flow lines. The head of water is constant along each
equi-potential line. The result is a system of “squares.” The volume
of water flowing through each square is the same. Total seepage can
be calculated by adding up the squares.
The idealized flow net shown above is correct only for a homoge-
neous soil or rock mass. In practice, seepage is much more compli-
cated to analyze, and data from piezometers installed at selected
depths is required to define flow patterns.
Basic Soil Mechanics
2-13
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