Seepage under embankment dam on pervious foundation

When an embankment dam rests on a pervious foundation such as sand, gravel or fractured rock, a large part of the reservoir water flows under the dam. This is a serious problem for both safety and economy.

Consequences

1. Loss of stored water and reduced power generation, especially when the flow is large.
2. Uplift pressure under the d/s part of the dam reduces effective stress and the shear strength of the foundation, so the d/s slope and toe become unstable.
3. Piping and boiling: if the exit gradient at the toe reaches the critical value $$i_c=\frac{G-1}{1+e}\approx 1,$$ the soil is lifted (quick condition), and a pipe grows backwards beneath the dam, causing sudden failure.
4. Internal erosion of fine particles, settlement of the dam and cracking of the embankment.
5. Wet, swampy ground at the d/s side and damage to the toe.

How it is reduced

• Positive cut-off: compacted clay trench, concrete diaphragm wall or secant piles reaching the impervious layer.
• Partial cut-off: sheet piles or grout curtain when the pervious layer is deep.
• Upstream impervious blanket of clay, which lengthens the seepage path and reduces gradient.
• Relief wells and toe drain with filter to release uplift pressure safely.
• Downstream loaded filter / berm to resist heaving.
• Keep the exit gradient below the safe value, about $\frac{1}{4}$ to $\frac{1}{5}$ of $i_c$ for the design.