A clay cut-off is a thick but not very wide layer of clay placed in or under a dike to make it watertight. The Dutch name is kleikist — literally “clay chest” — and it describes it well: a vertical body of clay dug in to cut off the water’s path. Clay is virtually impermeable and so forms a seal within an otherwise permeable subsoil. In dam engineering the same idea appears as a puddle clay core or cut-off trench.
The clay body is typically some 50 to 100 cm wide, and wider where conditions demand it.
What it guards against
- Seepage — water finding its way under or through the dike to the landward side
- Piping — the most dangerous form of it: the seepage water carries sand particles with it and scours a channel under the dike that keeps growing until the dike fails
- Erosion and bank slumping — wash-out of the dike body or the bank
Where it is used
- Dike strengthening — a clay cut-off in or beside the dike body lengthens the path the water has to travel, which reduces the pressure gradient
- Sealing trenches — a trench cut through a dike is backfilled with clay to stop leakage
- Closing off old structures — at a disused sluice or culvert, clay is placed between the walls to block flow for good
Points to watch
- Compact in layers — clay that is not placed and compacted in thin layers retains voids and weak spots
- Do not let it dry out — dry clay cracks, and those cracks are exactly the leakage paths you were trying to prevent; a clay cut-off is therefore covered with soil or turf
- Quality of the clay — not every clay is suitable; the clay fraction and sand content matter
- Work on flood defences needs consent — dikes and embankments are managed by the water authority; digging or building there is not allowed without permission
Related terms
- Clay
- Flood defence
- Seepage
- Piping
- Dike watch
- Bentonite
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