CUT-OFF WALLS

• Cut-off wall is a device use to reduce or prevent the seepage of water through the pervious foundation.
• It is always provid below the upper surface of the foundation.
• If the cut-off wall extends through the foundation to impervious material below, it is called complete cut-off wall.
• If it penetrates only a part of the pervious foundation, it is know partial cut-off.
• Partial cut-off walls are not very effective in reducing the seepage through a pervious foundation.

Following are the advantages of cut-off walls.

1. Danger of seepage along the contact surface of the foundation and the dam embankment is reduced.
2. It reduces the seepage through horizontal cracks, fissures and pervious seams in the foundation.

• Almost all the soils deposited by water are stratified or have layers of materials of various sizes.
• A vertical cut-off will penetrate a number of these layers or strata and thus force the seepage to pass through different strata.

The cut-off walls may be of any form from the following:

(a) Cut-off trenches back filled with impervious material.

(b) Masonry or concrete walls.

(c) Sheet piling—mostly made of concrete or steel.

(d) Grout curtain—A barrier is develop by grouting cement deep below the foundation.

• Before choosing the type of cut-off to be use it would be appropriate if foundation conditions are fully investigat.
• If seepage is moderate, sheet pile treatment may be adequate.
• If seepage is very heavy and foundation is pervious for large depth, masonry or concrete cut-off.
• Grout curtain is a sort of preventive measure.