This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Design Criteria of Earth Dams”.
1. What is the U.S.B.R recommended value for freeboard when the height of the dam is more than 60 m?
a) 2 m to 3 m
b) 2.5 m above the top of gates
c) 3 m above the top of gates
d) More than 3 m
Explanation: For controlled spillway, if the height of the dam is less than 60 m, the minimum recommended freeboard value is 2.5 m above the top of gates. If the height of the dam is more than 60 m, the minimum freeboard value is 3 m.
2. What is the recommended formula for top width of a very low dam?
a) H + 3
b) 0.2H + 3
c) 0.2 H
d) H + 5
Explanation: The top width (A) is generally governed by minimum roadway width requirements in case of small dams. The top width of the earth dam for very low dams is given by –
A = H/5 + 3 where H is the height of the dam.
3. What is the Terzaghi’s recommended value of U/s side slope for earth dam of height less than 15m of homogenous silty clay?
a) 2: 1
b) 2.5: 1
c) 3: 1
d) 3.5: 1
Explanation: For earth dams of homogenous silty clay of height less than 15 m, the recommended u/s slope is 2.5: 1 and d/s slope is 2: 1. For height more than 15 m, u/s slope value is 3: 1 and d/s slope is 2.5: 1.
4. If the height of the dam is 10 m, then the value of top width (A) according to Strange’s recommendations is _________________
a) 1.85 m
b) 2.5 m
c) 3.0 m
d) 4.0 m
Explanation: If the height of the dam is up to 7.5 m then the top width value according to Strange’s recommendation is 1.85 and if the height is in between 7.5 to 15 m, then the recommended value is 2.5. If the height is in between 15 to 22.5 m then the top width is 3.0 m.
5. When the height of the dam is in between 7.5 to 15, the Strange’s recommended value for maximum freeboard of low earth dams is ______________
a) 1.2 to 1.5
b) 1.5 to 1.8
Explanation: For the height of the dam up to 4.5 m, the maximum freeboard is 1.2 to 1.5 m and for the height range 4.5 to 7.5 m, the maximum freeboard is 1.5 to 1.8 m. If the height of the dam is in between 7.5 to 15 m, the maximum value is 1.85 m.
6. Calculate the top width (A) of the earth dam of height 50 m.
a) 5.0 m
b) 4.75 m
c) 6.10 m
d) 3 m
Explanation: For dams higher than 30 m, the top width (A) is given by the following formula-
A = 1.65 (H + 1.5)1/3 where H is the height of the dam.
A = 1.65 (50 + 1.5)1/3 = 6.13 m.
7. A phreatic line in seepage analysis is defined as the line on which pressure is _______________
a) equal to the atmosphere
b) greater than atmosphere
c) lower than atmosphere
Explanation: The line which joins the points in a dam section at which pressure is equal to the atmospheric pressure is called phreatic line. It is also called as line of seepage or saturation line. There is capillary fringe i.e zone of capillary which has negative hydrostatic pressure above this line.
8. Provision of horizontal berms at suitable vertical intervals may be provided in the downstream face of an earthen dam in order to _________________
a) allow the movement of cattle
b) allow the inspection of vehicles to move
c) reduce the erosion caused by the flowing rainwater
d) increase the erosion
Explanation: The provision of berms serves the following purposes –
- It behaves like a good lining for reducing losses and leakage.
- They provide protection against erosion and breaches due to wave action.
- They help the channel to attain regime conditions as they help in providing a wider waterway.
- It can be used as borrow pits for excavating soil to be used for filling.
9. During seepage through an earthen mass, the direction of seepage is ________________ to the equipotential lines.
c) not defined
Explanation: The seepage through a pervious soil material for 2-D flow is given by Laplacian equation and the graphical solution suggests that the flow through the soil can be represented by flow-net. It consists of 2 sets of curves equipotential line and streamlines which is mutually perpendicular to each other.
10. The upstream face of the earth dam is considered as _____________________
a) equipotential line
c) streak line
d) path line
Explanation: Equipotential lines are the lines of equal energy. Every point on the upstream face of an earth dam will be under equal total energy, this line acts as an equipotential line. Similarly, the upstream floor of a weir acts as an equipotential line.
10. The effects of capillary fringe are on the slightly safer side and are neglected.
Explanation: When there is appreciable flow through the dam body below the phreatic line, it reduces the effective weight of the soil and also the shear strength of the soil due to pore pressure. The capillary tension in water leads to increased intergranular pressure as the insignificant flow through the fringe leads to greater shear strength. Hence, the effects are neglected.
11. Which of the following line acts as a dividing line between dry (or moist) and submerged line?
a) Equipotential line
b) Path line
c) Seepage line
d) Streak line
Explanation: The soil above the phreatic line or seepage line will be taken as dry and the soil below it shall be taken as submerged for computation of shear strength of the soil. This helps in drawing the flow net.
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