Bligh’s Creep Theory

Bligh’s creep theory states that, the percolating water creeps along the profile of the bottom of hydraulic structure, which is in contact with the subsoil. The creep length is the path traced by the percolating water. Hydraulic gradient is constant throughout its passage and it is the loss of head per cubic length. The creep length must be made as big as possible to prevent the piping action. By proving deep vertical cut-off or sheet it can be achieved.

According to Bligh’s creep theory, the total length for first drawing: L=B and for second drawing: L = B + 2 (d1 + d2 +d3).

If the total loss of head is H, then the loss of head per unit length of the creep shall be,

Bligh called the loss of head per unit length of creep as percolation coefficient. The reciprocal, (L/H) of the percolation coefficient is known as the coefficient of creep C.

Assumptions

• The hydraulic gradient is constant throughout the impervious length of the apron.
• The path traced by the percolating water is called creep length and it is the sum of horizontal and vertical creep.
• Stoppage of percolation by cut off possible only if it extends up to resistant soil strata.

Design criteria for Bligh’s creep theory

Safety against piping

The length of creep should be sufficient to offer safe hydraulic gradient according to the type of soil.

Safe creep length = L = CH, C = 1/c.

Limitations of Bligh’s theory

• This theory has not made any distinction between the vertical and horizontal creep.
• This theory did not explain any of the exit gradients. By considering a flat average gradient safety against undermining cannot simply be obtained, but by keeping this gradient will be low critical.
• Bligh’s creep theory has no distinction between the outer and inner faces of sheet piles or the intermediate sheet piles. It is very clear from the investigation, that the outer faces of the end sheet piles are much more effective than inner ones.
• A loss of head does not take place in the same proportions as the creep length. The uplift pressure distribution is not linear but follows a sine curve.
• In case of two piles, the width between the two piles should be greater than twice the head or the piles are not effective.

Questions:

• Explain Bligh's creep theory?
• Explain the limitations of Bligh's creep theory?