Causes of earthen dam failure; safe design

Causes of failure of an earthen dam

An earthen dam fails when water, or the soil itself, is not controlled. About 90% of failures come from three groups of causes.

1. Hydraulic failures
   • Overtopping because the spillway is too small, gates fail or free board is insufficient. Water flowing over the crest erodes the d/s face quickly.
   • Erosion of the u/s face by wave action, and of the d/s face by rain and surface runoff (gullies).
   • Scour of the toe by tail water or by spillway discharge.
2. Seepage failures
   • Piping: a thin channel forms along a weak path (conduit, cracks, animal burrows, a pervious foundation layer), carries soil particles and grows backwards towards the reservoir.
   • Sloughing of the d/s face when the phreatic line emerges on the slope, softening the soil.
   • Heaving or boiling at the toe when the exit gradient exceeds the critical gradient $i_c=\frac{G-1}{1+e}\approx 1$.
3. Structural failures
   • Slope slides: u/s slope during rapid drawdown, d/s slope under steady seepage, or both slopes at the end of construction (high pore pressure).
   • Foundation shear failure or excessive differential settlement causing cracks.
   • Earthquake: liquefaction or cracking of the embankment.

Criteria for a safe design

• No overtopping: spillway and outlet sized for the design flood; free board is the sum of wave height, wave run-up, wind set-up and a safety margin (normally 1.5-3 m).
• Seepage line inside the section: the phreatic line must stay well inside the d/s face. Use a core, a toe drain, a horizontal blanket or a chimney drain to bring it down.
• Exit gradient less than the safe value (about 1/4 to 1/6 of $i_c$); provide filters wherever seepage leaves a fine soil, and a cut-off for pervious foundations.
• Stable slopes under every condition, with the minimum factor of safety: about 1.5 for steady seepage, 1.3 for end of construction and rapid drawdown, 1.0-1.2 with earthquake load.
• Foundation: adequate bearing strength, small and uniform settlement, removal of organic or weak layers, and a key or cut-off trench.
• Slope protection: riprap or stone pitching on the u/s face; grass turf, berms and drains on the d/s face.
• Proper crest: width of at least $\frac{H}{5}+3$ m, with camber for settlement, and soil compacted in thin layers at optimum moisture content.