Earth dam seepage

Water always seeps through and under an embankment dam. The aim is not to stop seepage entirely but to control it: keep the quantity acceptable, keep the line of saturation away from the downstream slope, and make sure that where seepage emerges it cannot carry soil particles with it. Uncontrolled seepage causing internal erosion, or piping, is one of the main causes of embankment dam failures.

The top flow line through the dam is the phreatic line. Above it the soil is partly saturated; below it, pore pressures act. For a homogeneous dam on an impervious foundation with a horizontal drain at the toe, Casagrande showed that the phreatic line is close to a parabola, Kozeny's basic parabola, with its focus at the upstream end of the drain, corrected near the upstream face. The seepage quantity per metre run follows directly from the parabola.

Zoned dams control seepage better. A low-permeability clay core cuts the flow; filters of graded sand and gravel downstream of the core stop core particles moving while letting water pass; chimney and blanket drains collect the water and keep the downstream shell dry, which improves slope stability. Under the dam, cut-off trenches, grout curtains, upstream blankets and relief wells control foundation seepage.

Flow nets give pore pressures and exit gradients for analysis. In operation, seepage is measured with weirs and piezometers; rising flow, muddy water or wet patches on the downstream slope demand immediate investigation.