29
Exercise B - Seepage Through an Earth Dam
Objective
A frequently encountered type of seepage in hydraulic engineering is seepage
through an earth dam. The term "earth dam" includes dams constructed from
materials ranging from rock-fill to silt and clay. The services earth dams render are
not limited to the impounding of water as in a reservoir. They also serve as dykes,
levees, banks of canals, banks of aqueducts (with water level above the surrounding
terrain), part or all of the perimeter of a catch basin. Even the earthworks for
highways or railways sometimes function as dams, especially when culverts and
bridge openings become clogged from heavy rains and water is impounded behind
the earth fill.
Apart from its practical importance, seepage through an earth dam is interesting from
a theoretical point of view. It typifies a group of engineering problems in which
boundary conditions are not completely specified by the geometry as they are in
problems similar to that of seepage under a sheet pile wall.
The objectives of the experiment are as follows:
1. To determine the position and shape of the flow line representing the
uppermost free water surface inside the body of the dam.
2. To visualise the flow lines system and to show each flow line starts
perpendicular to the upstream slope of the dam and the slope is a boundary
equipotential line.
3. To determine the rate of seepage through the dam.
Equipment Set Up
A segment of a dam of trapezoidal cross section is formed from sand in the tank so
that the base of the dam is about 150mm above the bottom of the tank. The
upstream slope can generally be steeper than the downstream one and its toe should
be approximately at the overflow outlet.
For current laboratory sands, a stable dam cross section will have an upstream slope
of approximately 1:3 to 2:3 and a downstream slope of about 1:5 to 1:4. The height of
the dam should be about 250mm, the crest width approximately 75mm. The
