This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Seepage Theories – Failure of Hydraulic Structure”.
1. What are the two main causes of failure of hydraulic structure on the pervious foundation?
a) Seepage and Over-turning
b) Undermining and Uplift
c) Over-turning and Piping
d) Overturning and Uplift
Explanation: Whenever a hydraulic structure is founded on a pervious foundation, it is subjected to seepage of water beneath the structure in addition to all other forces. The water seeping below endangers the stability of the structure and may cause its failure either by Piping or Undermining and Direct uplift.
2. The uplift pressure is not important for the upstream apron during considerations of failure due to uplift.
Explanation: When the apron is not sufficiently strong to counteract the excessive upward pressure, it may fail by rupture. The uplift pressure is not important for an upstream apron because the downward weight of stored water on the upstream apron is sufficient to neutralize the effect.
3. The structure finally subsides in the hollow formed in the foundation in which failure?
Explanation: Undermining takes place due to excessive percolation of water under pressure below the foundation. The seepage water when retains a sufficient residual force at the emerging downstream end, it may dislodge and lift up the soil particles leading to the increased porosity by progressive removal of the soil. This leads to the failure of the structure as it ultimately subsides in the hollow so formed.
4. Heave piping may occur on the _______________
a) downstream of sheet pile or cut off wall
b) upstream of sheet pile or cut off wall
c) body of earth structure
d) below the foundation
Explanation: As per Terzaghi, heave piping may occur within a distance of D/2 on the downstream of sheet pile where D is the embedment depth of sheet pile. The entire soil mass in the zone gets heaves up and is blown out by the percolating water when the seepage pressure at any level is greater than the submerged weight of soil above that level.
5. Backward erosion piping takes place when the phreatic line cuts the downstream face of the dam and seepage pressure is high.
Explanation: Backward erosion piping is an important failure mechanism for water-retaining structure and local disruption of the downstream top layer leads to concentrated seepage flow. This results in erosion that continues in the upward direction. It is ensured that the phreatic lines never touch the downstream side of the earth dam to avoid failure.
6. The safety of a hydraulic structure founded on a pervious foundation can be ensured by:
i. Provision sufficient length of its concrete floor.
ii. Providing sufficient depth of its concrete floor.
iii. Provision a downstream cutoff of some reasonable depth.
a) i and ii
b) i and iii
c) i, ii and iii
d) ii and iii
Explanation: The safety against piping can be ensured by providing sufficient creep length as per Bligh’s theory. The thickness of the concrete floor is generally increased by 33% to allow a suitable factor of safety for the worst conditions. A reasonably deep vertical cutoff is also provided at the downstream end of the floor to prevent undermining.
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