Explain the differences between Bligh’s Creep Theory and Lane’s Weighted Creep Theory in finding out pressures due to seepage of water beneath the Hydraulic structure.
Answers
Answer: Lane's in his theory proposed that horizontal creep is more effective than the vertical creep. Explanation: He proposed that horizontal creep has less head and is less effective in reducing uplift than vertical creep. A weightage factor of 1/3 is proposed for horizontal creep and 1.0 for vertical creep.
Bligh's creep theory • According to Bligh's Theory, the percolating water follows the outline of the base of the foundation of the hydraulic structure. In other words, water creeps along the bottom contour of the structure. The length of the path thus traversed by water is called the length of the creep, L.
Explanation:
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Answer-
Difference between Bligh's creep theory and Lane's weighted creep theory in determining the pressure due to seepage of water under a hydraulic structure are given below-
Explanation-
Bligh's Sneaking Theory:
Bligh hypothesized that seepage water follows the contour of the base of the structure in contact with the subsoil.
The length of the path travelled by the seeping water is called the creep length or creep length.
He further assumed that head loss per unit creep length (i.e., H/L), which is called as hydraulic gradient, is a constant throughout the percolation passage, i.e., the head loss is proportional to the creep length.
The inverse of the hydraulic gradient (i.e. L/H) is also known as the Bligh creep coefficient.
Where C lies between 5 and 15 and depends on the types of soil.
To ensure safety against pipeline failure,
Percolation coefficient:
It is defined as the ratio of the coefficient of permeability to the porosity of the soil.
Lane's weighted creep theory:
Lane concludes horizontal creep is less effective in reducing lift than vertical creep. So he suggests a factor of 1/3 for horizontal creep versus 1 for vertical creep.
Assumptions:
1. Slopes steeper than 450 are considered perpendicular (d)
2. Slopes less than 450 are considered horizontal (l)
Creep length
Safe creep length
L = C.H
C = Lane's coefficient
Khosla theory:
“Khosla’s Theory: states that the seepage water do not penetrate along bottom contour. It moves along the system of streamlines. To calculate the head pressure and the outlet gradient, the theory takes into account the flow pattern of impermeable base of the hydraulic structure.
Where,
a = b/d
∴ According to Khosla's theory, the outlet gradient is infinite in the absence of the discontinuity in the flow direction.
According to Khosla, water does not rise along the bottom of the floor as it was suggested by Bligh's theory of flow, but moves along a fixed set of flow lines.
The corrections adopted in Khosla's theory are:
a) Correction of modular pile interference
b) Correction for floor thickness
c) Slope correction
increase the pressure
U=γwHkN/m2
Note 1: Due to the negligible thickness of t bearing surface, the buoyancy pressure will remain same for both theories.
Note 2: For significant thicknesses, plate thickness correction should be considered. In such case the buoyancy pressure u/s will be higher when always calculated using Bligh's creep theory.
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