5 story basement, 30' below water table

[SimpleStructures]

We're designing a building with a 5 story basement, and the lowest level will be roughly 30 feet below the water table. There is a permanent dewatering system for the project including an underground drainage system.

The footprint of this building is huge (about 320'x320'), and the levels below grade are only parking levels. I was thinking it would be prudent to specify "pop-up valves" or hydrostatic relief valves in case the dewatering system failed to let it flood the basement rather than fail the slab on grade that is not designed to take the pressure. I know these valves are installed in pools, but I cannot find any information on these valves for "building application". An alternative would be to provide breakaway sections of the slabs, but that would likely be more costly.

Does anyone have any experience with this or a resource?

Thanks!

 

[JedClampett]

We design water treatment plants and this is a common occurrence. We try to stay away from mechanical devices such as the ones you describe if we possibly can. They're not really reliable enough to trust. Plus I would worry that there would be constant seepage into the building, even when the underdrains are working. Check Neenah Foundry (

http://www.nfco.com/c/pdf/valvesgates.pdf)

or Zurn for some designs.
Can you anchor the building on piles or piers to resist uplift?

 

[SimpleStructures]

We do have drilled piers but it is not required by Code to design the slab on grade (or the structure) to resist hydrostatic pressure if you have a dewatering system installed (not to mention it would be impossible to convince the owner to spend another million dollars for a dual system).

We're looking for something relatively cheap as a prudent precaution. The building is square in plan and nothing inhibits water intrusion (in the event that the dewatering system fails) around the perimeter since there is a gap between the earth retention system and the edge of slab on grade - thus we are more concerned with the interior portions of the floor plate.

Maybe we don't even need the secondary system to relieve water pressure...

 

[JedClampett]

Not a bad idea to have a belt and suspenders design. I just wouldn't want to be the design engineer answering the phone when the owner calls saying the cars are floating in the bottom level. Expecially when the answer is, "it's supposed to work that way."
You might want to install sump pumps and alarms in the basement.

 

[flamby]

The yield of water from this much of area can be huge. I don't think it is going to be practical to dewater the area and ignore hydrostatic pressure. You may need to run the pump 24x7!

I think steel plate cladded design will keep the seepage out. But you should design it for whole uplift.

Ciao.

 

[LPPE]

30 feet of water pressure on your bottom slab is going to make the slab ridiculous (30x62.4=1872psf), unless you put your drilled piers at really close spacings.

I've heard of jobs using the relief valves, but have not heard of any results if they actually worked. They were all installed as "just in case", and apparently havent been needed yet.

 

[wiktor]

Instead of using valves or other mechanical devices use just simple pipes (or RC concrete boxes) inserted vertically in the bottom slab. Calculate maximum hydrostatic pressure the slab could possibly take without sustaining damage, and set the height of the “relief devices” slightly below. It would be prudent to install also some floating warning switches, setting some kind of alarm system.

Simple is beautiful...

 

[SimpleStructures]

It is a permanently dewatered site, with pumps running continuously I would assume (tho as structural engineer we did not design the dewatering system). The hole is 60 feet deep and 320'x320', so you can imagine the lateral earth pressures on the permanent shoring wall if you were to consider hydrostatic pressure - it would be far too impractical to construct. Similar with the slab on grade - the uplift pressures would be huge if you don't use a permanent dewatering system.

Thanks to JedClampett above, we're considering the Trumbal relief valves - these things open at about 9 inches head of water, and retract when the water retreats. They're only $57 apiece (4" diameter and 9" long) and they extend into the gravel drainage layer below the slab. We're going to install them at around 40' O.C. in both directions. It's a cheap safeguard that is hopefully never needed.

Thanks to all for your input.

 

[BigH]

Can I query about what kind of geological material you are in? Rock? Sand and gravel? clays? . . . and the stratigraphy of each? Thanks.

 

[dicksewerrat]

Are you putting alarms and elec. cutouts on the elevators?I wouldn't want to have the elevator dorr open to 5 feet of water.

 

[VAD]

With your dewatering system being permanent you should be wary about possible settlement issues related to the proposed and adjoining buildings. I presume that your geotech has looked into this matter.

In addition to BigH's question on soil stratigraphy and thickness, I would be curious if the water level reported was determined from single standpipes slotted all the way or from standpipes or piezometers placed at various levels.

Very often standpipe information can be misleading depending on the installation technique used. I am not saying that this is the case on your project as I presume that your geotech consultant would be very experienced for a project of this nature.

If there are any buildings with basements around what was their experience.

 

[SimpleStructures]

BigH: Clayey soils near the surface, underlain by sandy silts and silty sands that transition to PWR and then down to screaming hard bedrock. Bedrock elevation varies roughly 60' across the site - some blasting is now being performed in one corner of the site where rock is about 10' above the lowest parking level.

dicksewerrat: The dewatering system has an alarm in case it fails. Not sure about the electrical cutouts on the elevators.

VAD: Water table drawdown effects on existing adjacent structures has been studied in depth.

Thanks all for your comments....huge help.