Explain the differences between Bligh’s Creep Theory, Lane’s Weighted Creep Theory and Khosla Theory in finding out pressures due seepage water beneath the Hydraulic structure
Answers
Explanation:
Bligh’s Creep Theory:
Bligh assumed that the percolating water follows the outline of the base of the structure which is in contact with the subsoil.
The length of the path traversed by the percolating water is called the length of creep or creep length.
He further assumed that the head loss per unit length of creep (i.e. H/L) which is called hydraulic gradient is constant throughout the percolating passage i.e. Loss of head is proportional to length of the creep.
The reciprocal of hydraulic gradient (i.e. L/H) is known as Bligh’s Coefficient of creep.
Where, C lies between 5 and 15 and depends on the type of soil.
To ensure safety against piping failure,
Percolation Coefficient:
It is defined as the ratio of coefficient of permeability to porosity of soil.
Lane’s Weighted Creep Theory:
Lane's concluded that horizontal creep is less effective in reducing uplift than vertical creep. Therefore, he suggested a factor of 1/3 for horizontal creep against 1 for vertical creep.
Assumptions:
1. Slopes steeper than 450 are taken as verticals(d)
2.Slopes less than 450 are taken as horizontals(l)
Creep length
Safe creep length
L = C.H
C = Lane's coefficient
Khosla Theory :
"Khosla’s Theory: It states that seepage of water does not creep along the bottom contour. It moves along a set of stream lines. In order to calculate the uplift pressure and exit gradient theory takes into account the flow pattern impermeable base of hydraulic structure.
In the Khosla’s theory complex profile of weir or barrage is broken down into a number of simple profiles and mathematical solution is obtained for such profiles.
Exit gradient (E) according to this theory based on mathematical solutions adopted by Khosla is given as:-
Where,
α = b/d
∴ As per khosla theory, the exit gradient in the absence of down stream cut-off is infinite.
According to khosla water doesn’t creep along the bottom of the floor as stated as per Bligh’s creep theory but it moves along a fixed set of stream lines. The seepage in the vertical plane for a homogenous soil can be expressed by the Laplace equation.
Corrections adopted in Khosla’s theory are:-
a) Correction for modular interference of Piles
b) Correction for floor thickness
c) Correction for slope
Since in both the cases, the seepage loss occurs as the water seeps. Also uplift pressure is proportional to the residual head which is maximum at the upstream of the hydraulic structure.
Uplift pressure,
U=γwHkN/m2
Note 1: For negligible thickness of the bed surface, uplift pressure will remain the same for both the theories.
Note 2: For considerable thickness, correction for the thickness of the slab should be considered. For such case the u/s uplift pressure will be more when calculated using Bligh’s creep theory always.