Irrigation Engineering Multiple Choice Questions on “Gravity Dams – Stability”.
1. The factor of safety against overturning generally varies between ___________
a) 2 to 3
b) 1.5 to 2
c) 0.5 to 1.5
d) 1 to 2
Answer: a
Clarification: Factor of safety against overturning can be determined by the ratio of righting moments about the toe to the overturning moments about the toe. The value generally varies between 2 to 3.
2. What is the maximum permissible tensile stress for high concrete gravity dam under worst conditions?
a) 500 KN/m2
b) 500 kg/cm2
c) 5 kg/m2
d) 50 KN/m2
Answer: a
Clarification: The masonry and concrete gravity dams are usually designed in such a way that no tension is developed anywhere in the structure. The maximum permissible tensile stress for high gravity dams is taken as 500 KN/m2 under worst conditions. If subjected to such tensile stresses crack develops near the heel.
3. Which failure occurs when the net horizontal force above any plane in the dam or at the base of the dam exceeds the frictional resistance developed at that level?
a) Overturning
b) Crushing
c) Sliding
d) BY development of tension
Answer: c
Clarification: Sliding should always be fully resisted. At any horizontal section of the dam, the factor of safety against sliding is –
FOS = u Ph / Pv where u = coefficient of friction, Ph = Sum of horizontal forces causing sliding and Pv = Algebraic sum of vertical forces.
4. Which failure occurs when the minimum stress exceeds the allowable compressive stress of the dam material?
a) Overturning
b) Crushing
c) Sliding
d) By development of tension
Answer: b
Clarification: The compressive stress produced if exceeds the allowable stresses then the dam material may get crushed, a dam may fail by the failure of its own material. The allowable compressive stress of concrete is generally taken as 3000 KN/m2.
5. Tension cracks in the dam may sometimes lead to the failure of the structure by?
a) Sliding of the dam at the cracked section
b) Overturning about the toe
c) Crushing of concrete starting from the toe
d) Both overturning and crushing
Answer: c
Clarification: When tension prevails, cracks develop near the heel and uplift pressure increases, reducing the net salinizing force. This crack by itself does not fail the structure but it leads to failure of the structure by producing excessive compressive stresses.
6. The major principal stress at the toe of a gravity dam under full reservoir condition neglecting the tailwater effect is given by ____________________
a) Pv
b) Pv tanQ2
c) Pv secQ2
d) Pv sinQ2
Answer: c
Clarification: When there is no tailwater, the principal stress in such a case is Pv secQ2 where Pv is the intensity of vertical pressure. This value of principal stress should not be allowed to exceed the maximum allowable compressive stress of dam material.
7. Which of the following criteria has to be satisfied for no tension at any point on a gravity dam?
a) The resultant of all the forces must always pass through the mid-point of the base of the dam
b) The resultant force for all conditions of loading must pass through the middle third of the base
c) The resultant of all the forces must pass through the upstream extremity of the middle third of the base
d) The resultant of all the forces must pass through the downstream extremity of the middle third of the base