What is piping?

Piping is the process in which water flows beneath a flood defence or sheet pile wall through the sand layer, carrying sand particles with it, so that a hollow channel — a “pipe” — forms and works its way back from the exit side towards the inlet side. Once that channel reaches the water on the other side, resistance collapses, the ground is washed out within a short time and the dike or excavation wall can fail.

How it develops

During high water, the level outside a dike stands higher than inside. That head pushes water through the sand layer beneath the dike. On the inner side the seepage water surfaces again, often in a ditch or hollow. If the flow velocity there rises, the finest sand particles are carried along. The first visible sign is a sand boil: a small crater ringed with sand, in the land behind the dike.

Once a channel exists the process accelerates: resistance falls, velocity rises, more sand comes out. It is a self-reinforcing mechanism, and that is exactly what makes it dangerous.

Where it occurs

• River and sea dikes on sandy ground with a covering layer of clay or peat.
• Excavations with sheet pile walls, where the water level inside is much lower than outside.
• Quays, locks and culverts, where a structure cuts through the subsoil.

Seepage path and assessment

The decisive factor is the seepage path: the length of the route the water must travel beneath the structure, set against the head difference. The longer that path for the same head, the lower the risk of piping. Dutch flood defences are assessed using the Sellmeijer model, which accounts not only for seepage length and head but also for the grain size and permeability of the sand. Older rules of thumb, such as Bligh’s rule, were considerably cruder.

Countermeasures

• Lengthen the seepage path with a sheet pile wall, a clay cut-off or an extended toe on the outer side.
• A vertical sand-tight geotextile within the dike, which passes water but holds back sand.
• A filter layer or gravel blanket on the exit side: the water may leave, the sand may not.
• Apply counter-pressure with a berm or, in an emergency, sandbags ringed around a sand boil. Never plug it — the boil must keep discharging water, just without sand.
• In an excavation, limit the head by recharging groundwater or driving the sheet piles deeper.

Related terms

• Seepage
• Sheet pile wall
• Excavation
• Flood defence
• Dewatering

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