Mechanism of piping

Piping is the progressive removal of soil particles by seepage water, forming a pipe-like channel beneath a hydraulic structure.

Mechanism:

1. Water seeps under the structure (dam, weir, sheet pile) from the upstream to the downstream side. The flow is concentrated near the downstream toe, where the flow lines converge and the exit gradient is highest.
2. When the exit gradient $i_e$ reaches the critical gradient $i_c = \dfrac{G-1}{1+e} \approx 1$, the upward seepage force cancels the submerged weight of the soil. Effective stress becomes zero and soil grains are lifted and boil out ("boiling").
3. The loss of soil creates a channel at the downstream end that grows backward (towards the upstream) because the shorter flow path increases the gradient. This channel eventually reaches the reservoir, and the structure fails by undermining.

Safety is checked by $F = i_c/i_e \ge 4$ to 5 (or the creep length methods of Bligh and Lane). It is prevented by longer cut-offs, sheet piles, downstream filters and blankets.