Question

A filter is provided at the d/s toe of an earth dam
1. to prevent the piping action in the dam section
2. to reduce the seepage through the dam
3. to reduce the uplift pressure on the dam
4. to collect and drain out the seepage water

Answer 1

Correct Answer - Option 4 : to collect and drain out the seepage water

Explanation:

rock toe/toe filter

•  A toe filter (graded in layers) is provided as a transition zone, between the homogeneous embankment fill and rock toe. Toe filter generally consists of three layers of fine sand, coarse sand, and gravel; as per the filter criteria requirements.
• The principal function of a rock-toe is to facilitate drainage of water and protect the lower part of the downstream slope from tail water erosion.

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stone apron/falling apron

• A falling apron is a ridge of armor stone dumped at the toe of a hydraulic structure.
• It is also known as a launching apron as it is launched as a consequence of scouring.
• The apron is designed to fall into the scour hole when the scour develops in front of the toe and its main aim is to provide a protective layer on the slope of the scour hole.

Inverted filter

• The inverted filter consists of one or more layers of graded coarse-grained, free-draining material over less pervious soil.
• It is used to prevent dams from failing by water piping through the foundation.

toe wall

• A low wall constructed at the bottom of an embankment to prevent slippage or spreading of the soil.