Seepage theory | Bligh's Creep Theory | Irrigation engineering

THEORIES OFSEEPAGE

* Whenever a hydraulic structure is founded on a pervious foundation, it is subjected to seepage of

water beneath the structure, which may cause the failure either by

1. piping

2. direct uplift

* According to Bligh’s Theory

(i) The percolating water follows the outline of the base of the foundation of the hydraulic structure.

(ii) Safety against piping can be ensured by providing sufficient creep length.

(iii) The water standing on upstream floor counterbalances the uplift caused by the same water, and

hence, only a nominal floor thickness is required on the upstream side.

* Lane’s weighted creep theory stipulated that the horizontal creep is less effective in reducing uplift

than the vertical creep.

He suggested a weightage factor of 1/3 for the horizontal creep as against 1 for vertical creep.

However, since it is only empirical formula without any rational basis, it is generally not adopted

in any rotational basis, it is generally not adopted in any design.

* According to Khosla:

1. Seeping water moves along a set of streamlines.

2. The first streamline follows the bottom contour of the structure.

3. The remaining streamlines follow smooth curves, transmitting slowly from outline of foundation

to a semi-ellipse

The exit gradient is said to be critical, when the upward distributing force on the grain is just equal

to the submerged weight of the grain at the exit.

* Owing to the simplicity, Bligh’s theory is still used for design of small works, but it should never be

used, as it may lead to expensive and unsafe designs.