Design criteria of earthen embankment dam

An earthen embankment dam is a structure of compacted earth, and it is safe when it meets the following design criteria. The dam must be safe against all the failures: hydraulic, seepage and structural.

1. Safety against hydraulic failure

• Overtopping: The spillway and outlet should discharge the design flood without allowing water to go over the embankment. Freeboard (normal and minimum) is provided above the MWL, taking account of wave height, wave run-up, wind set-up, settlement and earthquake effects.
• Erosion of the u/s face by waves: Provide rip-rap or slope protection (stone pitching) on the u/s face, and sod or gravel on the d/s face against rainfall.
• Erosion at the toe: Provide a toe drain/protection.

2. Safety against seepage failure

• Piping: The exit gradient must be less than the critical gradient ($i_{exit}<i_c/FS$, $FS\approx3$-4). Use filters and cut-offs.
• Phreatic line: The seepage line should remain well inside the d/s slope. Provide a drainage filter (horizontal, toe or chimney drain) to keep the d/s face dry.
• Seepage quantity through the dam and the foundation should be small, and the foundation should have a cut-off (key trench, sheet piles or grout curtain) to reduce the flow.
• Filter criteria (Terzaghi): $D_{15}$(filter) $<4$-$5\,D_{85}$(base) and $D_{15}$(filter) $>4$-$5\,D_{15}$(base).

3. Structural safety (slope stability)

• Slopes are chosen (about 2.5 : 1 to 3 : 1 u/s, 2 : 1 to 2.5 : 1 d/s) so that the factors of safety (FS) are adequate:
  • Downstream slope — steady seepage: $FS\ge1.5$
  • Upstream slope — sudden drawdown: $FS\ge1.2$-$1.3$ (end of construction $FS\ge1.25$)
  • With earthquake: $FS\ge1.0$-1.2
• Checked by the method of slices (Swedish, Bishop) for circular slip surfaces and the wedge method for weak layers.
• Foundation: Bearing capacity and settlement must be within the limit; the foundation must be strong enough to carry the embankment; consolidation settlement is allowed for by camber (about 1-2%).

4. Other criteria

• Crest width: Sufficient for the road and construction (about $\tfrac{H}{5}+3$ m, or at least 5-6 m).
• Material: Compaction at the optimum moisture content; zoned section with an impervious core and a pervious shell wherever possible.
• Cracking: Avoid differential settlement and cracks in the core; provide filters to heal the cracks.
• Instrumentation: Piezometers, settlement gauges and inclinometers to monitor behaviour.
• Economy: Use local materials and keep the section minimum consistent with safety.