This set of Irrigation Engineering MCQs focuses on “Rock Fill Dams – Seepage Control Through Embankments”.
1. A rock toe and a horizontal filter is provided on the downstream base of an earthen dam in order to _________________
a) prevent piping action in the dam body
b) prevent piping action in the dam foundation
c) reduce the seepage quantity by blocking its flow
d) collect and drain out the seepage flow
Explanation: Such provision reduces the pore pressure in the downstream portion of the dam and thus increases the stability of the dam permitting steep slopes. The seepage is collected through these drains and moved to a point where it can be safely discharged.
2. What is provided in the form of a transition zone between the homogenous embankments-fill and rock toe?
a) Toe filter
b) Horizontal blanket
c) Chimney drain
Explanation: The toe filter consists of three layers of fine sand, coarse sand, and gravel with the stone size or rock chips varying from 15 to 20 mm. It acts as a transition zone and prevents choking of the toe and height of the rock toe is kept between 1/3 to 1/4 of the dam height to achieve good results.
3. The length of horizontal blanket should be equal to _________________
a) 3 times the height of the dam
b) 4 times the height of the dam
c) 5 times the height of the dam
d) 6 times the height of the dam
Explanation: Blanket is a layer of impervious material laid at a natural ground level extending from the d/s end of the toe. A sufficient length equal to 3 times the height of the dam is provided as per the filter criteria to reduce the seepage pressure.
4. ____________________ is provided when large scale stratification occurs.
a) Chimney drain
c) Relief wells
Explanation: A vertical filter (or inclined u/s or d/s) is placed along with the horizontal as to intercept the seeping water effectively in case of large stratification. This arrangement is called a chimney drain.
5. The only provision among the following which can help control the seepage through the body of an earthen dam and thus to keep the phreatic line well within the dam width is _________________
a) upstream impervious cut-off
b) drain trench along the downstream toe
c) relief wells
d) chimney drain
Explanation: In case of large scale stratification a vertical filter is also provided along with the horizontal filter so as to effectively intercept the seeping water. This arrangement is nothing but is termed as chimney drain. It helps in keeping the phreatic line well within the dam body and also helps in rapid consolidation.
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