seepage theory
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Seepage theoryTRANSCRIPT
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Seepage and uplift Seepage and uplift pressurepressure
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Seepage and uplift pressure
• What is it?• Methods• Bligh’s creep theory• Lane’s weighted creep theory• Application
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• 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)
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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.
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• Safety against piping or undermining• Sufficient creep length• H/L < 1/C
Bligh’s Creep Theory
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• 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
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Example
• Calculate hydraulic gradient and uplift pressure at point C • Specific gravity of concrete is 2.4
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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.
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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
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A weir on solid rock (impervious foundation) does not need long apron (Floor), but needs sufficient width “b” to resist soil stresses.
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A weir on pervious soil needs length “L” to:• Cover creep or percolation length,•Resist scour from falling water
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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)
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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)
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• Distance between sheet piles a-a and b-b > d1 + d2
• Water percolation length takes the right path, hence is safe
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• 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