Basic Concepts of Bligh’s Theory

Q. While designing a hydraulic structure, the piezometric head at bottom of the floor is computed as 10 m. The datum is 3 m below floor bottom. The assumed standing water depth above the floor is 2 m. The specific gravity of the floor material is 2.5. The floor thickness should be

1. 2.00 m
2. 3.33 m
3. 4.40 m
4. 6.00 m

Q. At a certain point in the floor of the weir, the uplift pressure head due to seepage is 4.8 m. If the relative density of concrete is 3.4, then the minimum thickness of floor required at this point to counteract the uplift pressure is ____ m.

1. 2

Q. The figure shows a section of a hydraulic structure with three sheet piles founded on previous soil.



If the specific gravity of the floor material is 2.24, as Lane's weighted creep theory, the minimum thickness required for the impervious floor at the point ‘A’ is

1. 3.65 m
2. 2.74 m
3. 2.42 m
4. None

Q. Consider the following statements about Bligh's Creep theory for seepage flow:
1. The loss of head is proportional to the length of creep.
2. For all types of soil, Bligh's coefficient is constant..
3. The hydraulic gradient must be kept under a safe limit in order to ensure safety against piping.
4. Generally, the thickness of floor is increased by 33%, so as to allow a suitable factor of safety.
Which of the above statements are CORRECT?

1. 1, 2 and 3
2. 2, 3 and 4
3. 1, 2 and 4
4. 1, 3 and 4

Q. Consider the following hydraulic structure being constructed on sand:

The uplift pressure at point A is __________ kN/m${}^2$.
[Use Bligh's creep theory and take ${\gamma }_w=10kN/m^3$]

1. 24
2. 20
3. 12
4. 10