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Types, causes of failure and functioning
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WEIR
What is a Weir?
WEIR
WEIR TYPES
Types of Weir
Gravity Weir
Vertical
Drop weir
Sloping
Weir
Masonry or Concrete Sloping Weir
Dry Stone Slope Weir
Parabolic Weir
Non Gravity Weir
Gravity weirs
The uplift pressure - seepage of water below the floor - resisted by the weight of the floor.
1.Verical Drop Weir• Vertical drop wall or crest wall - with or without
crest gates.• Cut off piles are provided at the U/S and D/S
ends.• To prevent scouring - aprons - at the U/S and D/S• Inverted filter - D/S end - relieve the uplift
pressure.• Are suitable for any type of foundation
Vertical Drop weir
Drop Weir
Masonry or concrete sloping weir• Recent origin.• Suitable for sandy foundations.• They are used when the difference in the crest
level and the D/S floor level is less than 3 meters.
• The main advantage - hydraulic jump is formed on the d/s glacis - dissipation of energy.
• Sloping weir - suitable for large drops.
Concrete sloping weir
Dry stone slope weirs• In addition to the main weir wall - number of
core walls. • The space between the core walls - fragments of
rock (called rockfill). • Requires a lot of rock fragments – economical -
rockfill is easily available• Suitable for fine sand foundation. • Obsolete these days.
Stone Weir
Parabolic weir
• Similar to spillway section of a weir.
• Body wall – similar to low dam
• Cistern is provided at the D/S to dissipate energy
• The U/S and D/S - similar to vertical dropweir
NON GRAVITY WEIR
• The floor thickness is kept less• Uplift pressure – resisted by bending action of
reinforced concrete floor.
CAUSES OF FAILURE OF WEIR
CAUSES OF FAILURE OF WEIR
PIPINGRUPTURE OF FLOOR DUE TO UPLIFT
RUPTURE OF FLOOR DUE TO
SUCTION OF STANDING WAVE
SCOUR ON U/S AND D/S
SIDE
PIPING• Occurs in permeable soils• Caused by groundwater seeping out of the bank face.• Grains – detached - transported away if there is sufficient
volume of flow. • The exit gradient of water - @ D/S base of the weir -
exceed a certain critical value of soil.• Result - the surface soil starts boiling & washed – by
percolating water. • The progressive backwash at the upstream results in the
formation of channel (pipe) underneath the floor of weir.
• Difference in head between U/S and D/S – constant flow of water
• Erosion continues – pipe formed – failure of D/S end
Remedies:• Decrease Hydraulic gradient - increase path of
percolation - sufficient length of impervious floor
• Providing curtains or piles at both upstream and downstream
PIPING OF SOIL
RUPTURE OF FLOOR DUE TO UPLIFT• Weight of floor < uplift pressure – floor bursts• Bursting – reduces – effective length of
impervious floor – failure of the floor
Remedies:• Impervious floor – sufficient length• Impervious floor – sufficient thickness• Providing pile @ U/S side
RUPTURE OF FLOOR DUE TO SUCTION OF STANDING WAVE
Standing wave – U/S of weir – suction
Floor thickness – insufficient – failure by rupture
Remedies:• Providing additional thickness of the floor• Constructing wall with concrete instead of
masonry
SCOUR ON U/S AND D/S OF THE WEIR• Natural waterway – contracted – water scours
– bed – U/S and D/S ends.• Scour holes – failure of the structure
Remedies:• Piles at greater depth than scour level • Launching aprons
BLIGH’S CREEP THEORY• The length of the seepage path transversed by
the water is known as the length of creep (percolation length).
• Bligh - the dissipation of head per unit length of creep - constant throughout the seepage path.
• Percolation length is the path length from (a) to (b)
L` = 2 t + L
L` = L + 2 t + 2 S1 + 2 S2
LANE’S WEIGHTED CREEP THEORY
• Lane suggested that a weight of one third should be given to vertical creep and a weight of one to horizontal creep.
• Lw = Cw H
• Lane percolation length L` = 1/3 L (horizontal) + L (vertical)
L` = 1/3 L + t + t + 2 S1 + 2 S2