Seepage and uplift Seepage and uplift pressurepressure

Seepage and uplift pressure

• What is it?• Methods• Bligh’s creep theory• Lane’s weighted creep theory• Application

• According to Bligh’s theory, water creeps along the bottom contour of the structure.

• The length of the path of water is called the length of creep • The loss of head is proportional to the length of creep.• The loss of head per unit of creep length is called the

hydraulic gradient. (HL / L)

Bligh’s Creep Theory

• The loss of head is proportional to the length of creep • Dissipation of head per unit length of creep is constant

throughout the seepage path.

• LBligh = CBligh . H

• CB = Bligh’s coefficient of percolation • Your views on the theory?• Bligh’s theory makes no discrimination between

horizontal and vertical creeps.

• Safety against piping or undermining• Sufficient creep length• H/L < 1/C

Bligh’s Creep Theory

• Safety against uplift pressure• If the uplift head at any point is H1 (metres of water),

then uplift head has to be counterbalanced by the weight of floor thickness.

• Weight of water upstream?

Bligh’s Creep Theory

Example

• Calculate hydraulic gradient and uplift pressure at point C • Specific gravity of concrete is 2.4

Lane’s weighted creep theory

• Horizontal creep is less effective in reducing uplift than vertical creep.

• Lane suggested a factor of 1/3 for horizontal creep against 1 for the vertical creep.

• Calculate creep length using Lane’s theory.

Definitions

• Percolation is the flow of water under the ground

surface due to an applied differential head

•  Percolation length (creep length) is the length to

dissipate the total hydraulic pressure on the structure

•  Undermining (Piping) is to carry away (wash) soil

particles with flowing water below the ground surface

causing collapse or failure of the above structure

A weir on solid rock (impervious foundation) does not need long apron (Floor), but needs sufficient width “b” to resist soil stresses.

A weir on pervious soil needs length “L” to:• Cover creep or percolation length,•Resist scour from falling water

L` = 2 t + L•If L` > LB   (Design is safe, no possibility of undermining)•If L` < LB   (Design is unsafe, undermining occurs, leads to failure)

L` = L + 2 t + 2 S1 + 2 S2 •L`  LB   (design is safe, no possibility of undermining)•L` < LB   (design is unsafe, undermining occurs, leads to failure)

• Distance between sheet piles a-a and b-b > d1 + d2

• Water percolation length takes the right path, hence is safe

• Distance between sheet piles a-a & b-b < d1 + d2• Water percolation length takes a short cut from a to

b• Actual percolation length is smaller than designed