huangyhg

sea water retaining structures
hi,
i have one project in which i have to provide gates for sea water collection during tide in an artificial pond. the gates are having width 1.5m. velocity of tide is 6 knot. please help me to decide whether i have to go for rcc pillars or stone masonry pillars to support gates. we have to provide 50 consecutive gates.
thanks
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any retention of earth work will sustain some bending action. stone masonry is dear. hence the logical choice seems reinforced concrete.
thanks ishvaaag
but which type of foundation is proper against overturning of structure during tide when gates are closed.
piles are good in sea water ?
virtualvision,
i assume the gates will have to withstand a maximum difference of the hydrostatic water pressure between high tide elevation on one side of the gate and the low tide elevation on the other side. the pillars will have to withstand the reactions of the gates.
the design of the pillars will depend on several factors:
1) the maximum range between high and low tide
2) the depth of the water at the gate
3) how are the gates connected to the pillars. do the gates pivot at the top or at the bottom?
4) how fast do the gates close
5) the effect of the tide velocity (6knots) on the gates and pillars should be minor, but if there is the possibility of waves breaking against the gates, their effect should be considered.
i agree with ishvaaag that reinforced concrete pillars is the logical material to use for the pillars. i would provide a concrete cover over the reinforcing steel of at least 3 inches.
the type of foundation will depend on the existing subsoil and the depth of water.
steel or concrete piles could be used in seawater. prestressed concrete piles are preferible. if steel piles are used, their upper portion should be protected against corrosion. about 5 feets below low water, corrosion decreases because the lack of oxygen.

aef
thanks dlew,
the points are
1. the water level at tide will be 6 feet.
2. difference between two side water levels will be 3 feet
3. sub soil is soft.
4. frames of gates will be fixed with pillars.
5. gates will be slided upward in frame with screw.
6. pillars will be 6 feet long and just like 50 channels will be made of length 6 feet.
7. wooden piles will do or not ?
prestressed piles -and more in poor soil should be best, maybe even in foundation to substitute stabilization by sheer mass: in this case please ensure redundancy, for relying on a single pile might otherwise mean the failure of the gates. however, for any part exposed to hammering by waves i would undoubtely would rely in mass to keep the thing in place, this irrespectively of the actual structure that keeps the things tight.
you may link both by projections of the structure unto rock or wall wave breakers. it is implied that if wave hammering action may happen, the exerted forces will be higher than hydrostatic pressure.
use concrete of very low permeability (normally associated to big quantities of cementitious materials) to help durability. ensure through consulting or study that the components are adequate for the saline coast and submerged environment.
don't forget to check the general failure of the containment by surfaces of failure -as it is done for slope stability analysis- this might happen on soft soils and any susceptible of liquefaction, and has happened many times.
wooden piles may work, but if therer are exposed parts their durability should be lower than a good pc pile, but the differences are not big; there are wood piles standing tens of years without problem.
hi ishvaag!
don't you think that prestressed concrete piles will be a poor choice in a severe environment like this: alternate wetting and drying with wave action in sea water is the worst environment (even worse if there's possibility of frost action). so i'd go in for massive ordinary concrete piers (unless there's considerable bending, in which case rcc is inescapable). the concrete would also have to be made with slag cement/ fly ash cement / supersulphated cement, and any reinforcement will be only for thermal/ shrinkage cracking.
abhijeet oundhakar
design engineer
stup consultants ltd.
bombay, india
hi, virtualvision.
if you are thinking seriously about timber piles, just make sure that you do not have a risk of attack by teredo (ship-worm) or any other marine beasties. they can make a meal (literally) of exposed timber quite quickly.
find a marine engineer.... seriously, there are many other factors such as wave action, wind action, and tidal forces that will actually govern your design. wind and wave will literally rip your structure apart if you do not design for them properly.
imagineer
thanks friends
i am going for cast in situ concrete piles what are the requirements for that ?
virtualvision,
disadvantages of wood piles when compared against prestressed concrete piles are:
1) their expected service life is shorter. could be attacked by teredos or other borers.
2) they are more flexible under load, and their deformation could introduce additional stresses on the gates frames.
the only advantage i can think of is initial cost. in the usa the cost of treated wood pile is in about $5 to $10 per linear foot cheaper than prestressed concrete piles.
the piles have to be embedded into good soil to resist the lateral forces imposed by the differential water pressure and the energy of waves pounding on the gates (if this is an exposed location). there are formulas to determine the length of embedment for piles subject to lateral loads. a publication from the federal highway administration ( i do not have this reference handy) has a very straightforward formula.
i would recommend the book "port engineering" by per bruun as a very good reference for your project. it has chapters on wave action and on the stability of tidal inlets.
about cast-in-place concrete piles:
is the existing location under water now?
are you going to tremie the concrete underwater?
the disadvantages of poured-in-place concrete under sea water are many and serious.
chlorides in the sea water will contaminate the fresh concrete, shortening considerably its service life.
if you tremie the concrete, i think you will need to drive first a form (a steel pipe?) before placing the concrete. in soft soils "driller mud" (bentonite) may not work.
i do not know in what part of the world is the project located, but if prestressed concrete piles are not available there, i would use regular precast concrete piles.
their expected service life would be shorter than the prestressed concrete, but still longer than wood piles.
aef

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