Bligh's Creep Theory
Design of impervious floor for sub surface flow: Iit is directly depended on the possibilities of percolation in porous soil on which the floor (apron) is built. Water from upstream percolates and creeps (or travel) slowly through weir base and the subsoil below it. The head lostby the creeping water is proportional to the distance it travels (creep length)along the base of the weir profile. The creep length must be made as big as possible so as to prevent the piping action. This can be achieved by providing deep vertical cut-offs or sheet piles.
According to Bligh’s theory, the total creep length for first drawing: L = B and for second drawing: L = B + 2(d1 + d2 + d3)
If H is the total loss of head, 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.
The length of creep should be sufficient to provide safe hydraulic gradient according to the type of soil.
Safe creep length = L = CH, C = 1/c
Let h' = uplift pressure head at any point of apron (Hydraulic gradient line above the bottom of floor)
The uplift pressure = wh' where w = density of water. If t = thickness of floor at the point, l = specific gravity for floor material. Then, downward force per area (resisting force) = t.w.e or wh' = t.w.e
For portion of floor upstream of barrier only nominal thickness need to be provided since the weight of water will counterbalance the uplift pressure.
A certain minimum length of impervious floor is always necessary to the downstream of the barrier (thickness of downstream floor for worst condition)