This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Failure of Earthen Dams”.
1. Stone pitching or riprap is generally provided to avoid _______________
b) erosion of U/s and d/s face
c) cracking due to frost action
d) gully formation
Explanation: The waves developed near the top surface due to the winds notch out the soil from the U/s face and the D/s toe of the dam may get eroded due to the cross currents and due to tailwater. Stone pitching or riprap should be provided to avoid such failures.
2. Provision of sufficient freeboard eliminates _______________
i. Over-topping failure ii. Erosion of U/s face iii. Erosion of D/s face iv. Cracking due to frost action v. Gully formation
Which of the following is/are correct?
a) i and v
b) ii and iii
c) iv and v
d) i and iv
Explanation: Over-topping of the dam generally occurs due to insufficient free-board, inadequate spillway capacity, and clogging of the spillway, etc. Frost causes heaving and cracking of the soil with seepage and consequent failure, an additional freeboard allowance of about 1.5 m should be provided as an additional safety measure.
3. The collection and removal of water before it acquires high downward velocities is ensured by provision of ____________________
c) burrow pits
d) spoil banks
Explanation: The rainwater that acquires high downward velocities causes consequent erosion which can be reduced by the provision of berms.
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.
4. In order to prevent the possibility of the cross-flow towards the earthen embankments, it is necessary to provide _____________________
a) counter berms
b) sides walls of sufficient height and length
c) spoil banks
d) sufficient freeboard
Explanation: The cross-currents that may come from the spillway buckets is one of the reasons for the d/s toe erosion. To prevent the possibility of cross-flow towards the earthen embankment, sidewalls of the spillway also called diaphragm walls of sufficient height and length is provided.
5. Sloughing is the process of _________________
a) progressive erosion
b) subsequent removal of soil grains from within the dam
c) uncontrolled seepage through the dam body
d) progressive removal of soil from the D/s face
Explanation: The progressive erosion and the subsequent removal of soil grains from within the dam or from the foundation is piping. The progressive removal of soil from the d/s face is sloughing.
6. In foundation slide failure___________________________
a) top of embankment gets cracked and lower slope moves outward forming large mud waves near the heel
b) top of embankment gets cracked and lower slope moves inward forming large mud waves near the heel
c) bottom of embankment gets cracked and lower slope moves outward forming large mud waves near the heel
d) bottom of embankment gets cracked and lower slope moves inward forming large mud waves near the heel
Explanation: The entire dam may slide or just one face slides over the foundation when the foundation of earth dams is made of soft soils. Then the foundation bulges out on the side of the movement. The top of the embankment gets cracked and lower slope moves outward forming large mud waves near the heel.
7. What is the most critical condition of slide of the U/s slope?
a) The sudden drawdown of the reservoir
b) Reservoir full condition
c) Steady seepage condition
d) Sudden drawdown and reservoir full condition
Explanation: When the upstream slope is steep and when the soil used in the construction of the dam is poor and compaction is not adequate, the sudden drawdown of water causes sliding of the upstream face. This failure seldom leads to catastrophic failures.
8. When the reservoir is full, the slope which is most likely to slide is _____________________
a) the upstream slope
b) the downstream slope
c) both upstream and downstream slope
d) no effect
Explanation: The D/s slope is most likely to slide when the reservoir is full. For downstream face also, when the slope is steep and soil used is poor with inadequate compaction, saturation and softening of soil due to rainfall and seepage causes sliding of the downstream face.
9. In order to keep the saturation line in a canal embankment well within the toe, it is necessary to provide _____________________
a) spoil bank
b) counter berm
Explanation: The saturation gradient line may cut the downstream end of the bank even after providing sufficient section for bank embankment. By the provision of counter-berms, the saturation line can be kept covered at least by 0.5 m.
10. Which of the following failures generally occur due to the development of unaccounted pore pressures?
a) Hydraulic failures
b) Piping through the dam body
c) Sliding in embankments
d) Sloughing of the d/s toe
Explanation: Excessive pore pressure developed due to consideration of clay embedded between sands or silts may reduce the shear strength of the soil. It becomes incapable of resisting the induced shear stresses leading to the failure of dam foundation without warning.
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