Large Deep potable water service Reservoirs

[BRIS]

I am looking at options for large covered potable water storage reservoir. Apart from the standard rectangular RC and circular pre stressed options, I am considering the possibility of secant pile or diaphragme walls. I am searching for examples of where they may have been used before.

 

[JedClampett]

We like "hopper bottom" buried reservoirs in our parts. It makes the retaining walls a lot less heavy. The sloped slabs are tough to build, but a good contractor can do them.
If you're looking for other options, I'm afraid I can't help you. There are basically only a few types of materials (steel, concrete, prestressed concrete) and shapes (round, not round) so it's unlikely you're going to stumble on a concept that's much more efficient than the basics. There's a reason that they're most common.

 

[bimr]

Note that it is generally recommended that "At least 50 per cent of the water depth should be above grade." to prevent possible contamination of the stored water.

http://10statesstandards.com/waterstandards.html#7.0.3

 

[JedClampett]

We don't follow that section of the 10 States Standards in Arizona. We have lots of buried reservoirs. I've designed a bunch of them. And one in Texas.

 

[bimr]

That is the reason for the "general recommendation".

However, you are still required to contruct the tank bottom above the 100 year flood or protect it from the 100 year flood.

http://www.azdeq.gov/function/compliance/download/eb10.pdf

There are also good reasons to construct the tank above the ground water elevation as well.

 

[BRIS]

Thanks for the response - I am looking at very large reservoirs perhaps 100 million gallons. We are in a desert area, bottom will be above ground water level and we are above flood level. The client want to bury the reservoirs for aesthetic and security reasons. However I see little increased security with burying- the walls can't be breached if they are totally in excavation. A hopper or stepped floor will clearly have advantages in reducing retaining wall heights. My thoughts were that if the client really does want to spend his money burying the reservoirs then a secant piled retaining wall rather than the standard gravity retaining wall section might give the best solution. I guess I need to get on and do the work and make some cost comparisons !

 

[cvg]

excavate partially and place the excavated material around the sides of the reservoir. Saves on cost of hauling, screens the reservoir for aesthetics and safety and can keep the water cooler, a real problem in the desert. Also promotes storm runoff away from the reservoir. Check with city of Phoenix water services for some recommended designs. Note that a reservoir of this size will probably be considered a "dam" unless it is less than 6 feet tall on the down slope side. Keeping it below this height will be a primary consideration for safety and reducing permitting and regulatory requirements.

 

[JedClampett]

Do you want to bury the roof? This really increases the cost. But it's not unheard of.

 

[BRIS]

we are in the Middle East and logic does not always prevail. I guess we can argue that providing the walls are secure the storage function will remain (to some large extent) even if the roof collapses.

Maybe we bury the roof under 2 or 3 feet of fill (difficulty then is ensuring fill depths do not exceed design depths - i.e some future well meaning operations/maintenance personnel do not heap fill onto one or two panels).

Going back to my original question I see no difficulty in using a secant pile wall technique for the perimeter reservoir wall, a hopper shaped bottom or a stepped bottom and sticking some pumps in a caisson in the centre of the reservoir to get the water out.

 

[cvg]

the roof is likely the most expensive part of this type of reservoir and due to the high humidity which accelerates corrosion, generally will have a shorter service life than the rest of the structure. burying it 3 feet wil greatly add to the capital cost as well as the future maintenance cost