In order to allow the seepage water to escape without dislocating the soil particles a/an _____ is provided at the downstream end of the impervious floor of a weir.
- Alaunching apron
- Bgraded filter
- Cinverted filterCorrect
- Dtoe wall
Solution
Concept
An inverted filter at the downstream end of the weir floor lets seepage water escape without washing out soil particles, which prevents piping.
Step-by-step solution
1. Seepage water comes out at the downstream end of the impervious floor. 2. If it escapes freely, it can carry soil particles with it, causing piping. 3. To prevent this, an inverted filter is provided. It has layers of graded material, fine at the bottom and coarse at the top, loaded with concrete blocks. 4. Water passes through, but the soil particles stay in place. Answer is C.
Explanation
At the downstream end of a weir floor, seepage water exits with an upward force. An inverted filter has fine material at the bottom and coarse material at the top. It lets this water escape safely and holds the soil particles in place, so piping does not start.
Why the other options are wrong
A: The launching apron protects against scour by falling into the scour hole. It is not meant for seepage escape. B: Graded filter is a general term. For weirs, the standard name for this component is the inverted filter. D: A toe wall (cut-off) increases the creep length. It does not let water escape.
Shortcut
Seepage escape without soil loss at the weir end means inverted filter.
Common mistake
Choosing launching apron, because it is also placed at the downstream end of the weir.
Correct answer: C) inverted filter