Irrigation Engineering Questions and Answers – Gravity Method

This set of Irrigation Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Gravity Method”.

1. When the reservoir is empty, the single force acting on it is the self-weight of the dam which acts at a distance of ____________
a) B/2 from the heel
b) B/6 from the heel
c) B/3 from the heel
d) B/4 from the heel
View Answer

Answer: c
Explanation: The only single force on the dam when the reservoir is empty is the self-weight of the dam acting at a distance of B/3 from the heel. It provides maximum possible stabilizing moment about the toe without causing tension.

2. When the reservoir is empty, the maximum vertical stress equal to ________________
a) At heel = 2W/B and at toe = 0
b) At heel = 0 and at toe = 2W/B
c) At heel = toe = zero
d) At heel = toe = 2W/B
View Answer

Answer: a
Explanation: The vertical stress distribution at the base when the reservoir is empty is given as –
Pmax/min = V/B [1 + 6e/B] and V/B [1 – 6e/B] where e = B/6 and V = total vertical force = weight W
Pmax = 2W/B and Pmin = 0.
The maximum vertical stress at the heel is equal to 2W/B and at the toe is zero.

3. The two-dimensional stability analysis of gravity dams proves better for U-shaped valleys than for V-shaped valleys.
a) True
b) False
View Answer

Answer: a
Explanation: The transverse joints in the dam body are generally not grouted in U-shaped valleys but are keyed together in V-shaped valleys. In V-shaped valleys, the entire length of the dam acts monolithically as a single body. The assumption that the dam is considered to made up of a number of cantilevers of unit width each may involve errors here.
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4. Calculate the value of minimum base width for an elementary triangular concrete gravity dam supporting 72 m height of reservoir water and full uplift? (Take specific gravity of concrete as 2.4 and coefficient of friction as 0.7)
a) 36.28 m
b) 39.77 m
c) 51.5 m
d) 73.5 m
View Answer

Answer: d
Explanation: Using formula –
Case 1: B = H / (Sc – c)1/2 (For full uplift c = 1 and specific gravity of concrete = 2.4 )
= 72/ (2.4 – 1)1/2 = 60.85 m
Case 2: B = H/μ (S – 1) where μ = coefficient of friction taken as 0.7
B = 72 / 0.7 x 1.4 = 73.46 m
The highest among the two base width value is to be selected i.e. B = 73.46 m.

5. For usual values of permissible compressive stress and specific gravity of concrete, a high concrete gravity is the one whose height exceeds ______________
a) 48 m
b) 70 m
c) 88 m
d) 98 m
View Answer

Answer: c
Explanation: The limiting height is – Hmax = f / (Sc + 1) ϒw Permissible strength of concrete = 3000 KN/m2, Sc = specific gravity of concrete = 2.4
Hmax = 3000/[(2.4 + 1) x 9.81] = 89.9 m.
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6. For triangular dam section of height H for just no tension under the action of water pressure, self-weight and uplift pressure, the minimum base width required is _____________
a) H / (S-1)
b) H / S1/2
c) H / (S – 1)-1
d) H / (S-1)1/2
View Answer

Answer: d
Explanation: The minimum base width (B) of a gravity dam having an elementary profile –
B = H / (S – 1)-1 where S is specific gravity of concrete and H is the height of water.
If uplift is not considered – B = H/S1/2.

7. If the eccentricity of the resultant falls outside the middle third, the ultimate failure of the dam occurs by ______________
a) tension
b) crushing
c) sliding
d) overturning
View Answer

Answer: a
Explanation: When eccentricity is greater than B/6 (eccentricity falls outside the middle third), tension may develop. When tension prevails, cracks develop near the heel and uplift pressure distribution increases reducing the net salinizing force.
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8. What is the value of eccentricity for no tension condition in the dam?
a) e < B/6
b) e > B/6
c) e > B/3
d) e < B/3
View Answer

Answer: a
Explanation: The resultant of all the forces i.e hydrostatic water pressure, uplift pressure and self-weight of the dam should always lie within the middle third of the base for no tension. When e < B/6, the value of stress intensity at toe and heel are positive i.e compression on both sides.

9. What is the formula for limiting height of a gravity dam?
a) Hmax = f / (Sc + 1) γw
b) Hmax = f / (Sc – 1) γw
c) Hmax = f / (Sc + C) γw
d) Hmax = f / (Sc – 1) γw
View Answer

Answer: a
Explanation: The critical height or limiting height of a dam having elementary profile is –
Hmax = f / (Sc + 1) γw where f = allowable stress of the dam material, Sc = Specific gravity of concrete and γw = unit weight of water.
This limiting height draws a dividing line between a low gravity dam and a high gravity dam.
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10. Calculate the top width of the dam if the height of water stored is 84m.
a) 5 m
b) 2.5 m
c) 5.55 m
d) 7.75 m
View Answer

Answer: a
Explanation: Bligh has given an empirical formula for finding out the thickness of the dam at top.
A = 0.522 H1/2 = 0.522 x 841/2 = 5.05 m.
As per Creager, the economical top width has been found to be equal to 14% of the dam height without considering earthquake forces.

Sanfoundry Global Education & Learning Series – Irrigation Engineering.

To practice all areas of Irrigation Engineering, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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