This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Gravity Dams – Galleries Construction”.
1. The provision of drainage gallery in a gravity dam helps in reducing ____________________
a) hydrostatic pressure
b) seepage pressure
c) silt pressure
d) both hydrostatic pressure and seepage pressure
Explanation: An opening or passage left in the dam which runs longitudinally is called the gallery. This is to provide space for drainage of water percolating through the upstream face of the dam or seeping through the foundation. It is also used for inspection purposes and for the mechanical equipment used in the operation of gates in spillways.
2. Which of the following attempts are made to reduce the uplift in order to economize on the provided section of a concrete gravity dam?
i. Providing drainage gallery to collect seepage water
ii. Constructing cut-off under upstream face
iii. Pressure grouting in dam foundation
iv. Provision of shear keys or keyways
a) i and ii
b) i, ii and iii
c) i, ii and iv
d) i, ii, iii and iv
Explanation: In a gravity dam, the grout curtain is provided near the toe to reduce the exit gradient. A drainage gallery with its drainage pipe system provided in gravity dam reduces the uplift pressure at all levels below the upstream water level. Vertical cut-off walls are the most effective to reduce seepage flow and uplift force.
3. Transverse joints in concrete gravity dams are the ___________________________
a) horizontal construction joints at each lift height
b) vertical construction joints of full height and width
c) diagonal construction joints for torsion
d) longitudinal construction joints of full width
Explanation: Transverse joints are vertical joints that run through the entire height and extend through the full width of the dam section. These joints are continuous from the upstream face to the downstream face.
4. Leakage through the transverse joints in a gravity dam is prevented by ____________
a) shear keys
c) water stops
Explanation: Water bars or water stops are provided in the transverse as well as horizontal joints in concrete adjacent to the upstream face of the dam. The openings of the joints are sealed properly with water stops to avoid passage of seepage of water through the body.
5. In order to reduce uplift on a gravity dam, the type of grouting done is ____________________
a) curtain grouting near the heel
b) consolidation grouting near the heel
c) curtain grouting near the toe
d) consolidation grouting near the toe
Explanation: Curtain grouting helps in forming a principal barrier against the seepage through the foundations and thus reduces the uplift pressure. This grouting can be accomplished from the foundation gallery or from other galleries within the dam.
6. The general value of lift for concrete is taken as __________________
a) 1.5 m
b) 2.5 m
c) 3 m
d) 4 m
Explanation: The concrete is poured from a certain height in the first attempt and this height is called a lift. Maximum height of single pour of concrete is usually about 1.5 m. If this is reduced, more horizontal joints will get developed.
7. The horizontal joints that extends through the entire width of the dam section and are developed at each lift height is called as ___________________
a) transverse joints
b) longitudinal joints
c) construction joints
d) contraction joints
Explanation: The longitudinal joint is developed at each lift height and extends through the entire width of the dam section. These joints shall run through the entire length of the dam but are staggered between transverse joints.
8. Which of the following joints justifies the two-dimensional analysis of gravity dams?
a) Longitudinal joints
b) Transverse joints
c) Construction joints
d) Contraction joints
Explanation: Transverse joints are vertical joints that divide the dam length into a number of vertical cantilevers each of which is independent of the other. It runs through the entire height and extends through the full width of the dam section. Hence, it justifies the two-dimensional analysis of gravity dams.
9. The foundation pressure used in the pressure grouting is equal to _________________
a) 2.0 D N/cm2
b) 2.5 D N/cm2
c) 5.0 D N/cm2
d) 3.0 D N/cm2
Explanation: The grouting pressure is kept as high as possible without lifting the foundation strata and the pressure used in this grouting is equal to 2.5D. It is carried out only after some portion of the dam section is laid and is generally done in stages of depth equal to 15 m or so.
Sanfoundry Global Education & Learning Series – Irrigation Engineering.
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