Tank Slab

[BarrySanders]

I am currnetly designing a tank for a detention basin. The walls have been sized according to PCA rectangluar tanks and foundations sized to support the wall reactions similar to a retaining wall. My base slab will be used to as a diaphragm to support the bottom of the wall to assist in sliding resistance.

My question is in reagards to the detail between the foundation footing and the base slab. If the slab is considered to be "floating" but is poured in contact with the foundation, will it still act as diaphragm, or will I need to cast the slab in place with the footing? The contractor would actually prefer to pour the slab on top of the footing. I would usually just size the footing/retaining wall base to resist sliding alone, but this would result in a large footing. I am assuing this type of construction is typically used when designing a basement in homes, is this correct?

Also the tank bottom is slightly below the water table, appx 1 ft, so i will need to provide a waterstop at the joint between the floating slab and the footing. The slight uplift pressure will be resisted by the weight of the slab alone (empty tank), but Im not sure this is a good detail. Again should I consider the slab to be connected to the footing?

Suggestions are appreciated.

 

[BarrySanders]

I should have noted the tank is below grade with a will height of 12 Ft.

 

[37ed]

Barry

Sounds to me like pouring the slab and footing at the same time is the way to go here.I saw not to long ago this very situation in action.Water will float almost anything,and will seperate you footing and slab under the right conditions.Are you sure about the uplift?It would not be a bad idea to check you math on that issue.A gold mine of infomation on this subject can be found on the U.S. Army corps of Engineers web site.

 

[BarrySanders]

I have to agree with you here, 37ed. Im curious how basements floor slabs in houses/buildings handle this problem when there is a water table. I know they assume the slab supports the bottom of the basement wall, but do they use a waterstop at joints to keep the water out?

I also have some high surcharge loads adjacent to the tank due to assumed traffic loads in a parking lot. Im thinking I will jsut design the slab assuming it is connected to my foundations. Seems to be a better detail, I dont want any joints in my slab if it used to for sliding support.

 

[MotorCity]

BarrySanders,

I guess I'm a little confused about your design approach. What site conditions would create a sliding condition of the tank? Is it only partially below grade?

 

[BarrySanders]

My design approach is as such:

-It is a fully buried tank (appx 11 ft, w/ 300 psf surcharge load).
-Tank walls are designed per moment coefficients out of PCA - Conc. Rectangular Tanks (Could use Moody Charts of USBR too).
-Tank is assumed to be free at the top and fixed at bottom, similar to cantilever retaining wall design.
-In order to assume a fixed condition at the base, sliding must be resisted by my base slab as a diaphragm otherwise the base would be huge. Also moments will be resisted by my footing base similar to a cant retaining wall design.

Not sure exactly how I should detail the slab. I would think just assume a solid slab, but the contractor is looking to pour the base slab either on top of the footing (similar to a basement) or perhaps flush to the top of the footing. Since I need the base slab to provide wall support at the base, Im not so sure if there should be a joint between the slab and the footing, especially with uplift loading the slab however slight. I have not performed any basement designs so I'm iffy how this works.

Is it ok to have the slab separate from the footing is my question (i.e. poured against the footing)w/ a waterstop across the joint....will the diaphragm action occur if this detail is used. Im inclined to just assume a solid slab.

 

[MotorCity]

If the weight of the tank is large enough, sliding should be ok even if the backfill is not evenly placed around the tank. Depending on your backfill material, it would have to be a pretty small tank to slide.

Usually on tanks I have designed, I have used a thickened slab at the walls which is essentially a slab and footing cast together. On top of the slab I will detail a keyed joint with a pvc waterstop. The thickened slab allows less form work with fewer pours and also will prevent any sliding if the sides are not backfilled at the same time. If there is uplift on the tank, the slab will be pushed up into the wall. Also, uplift at the slab-wall connection is aided by placing L-dowels in the wall and slab. Good luck.

 

[BarrySanders]

Thanks for your comments MotorCity, you just described what I had in mind. I will size the thickened slab to resist my foundation/wall lateral loads and moment and then check the slab thickness base on uplift due to water...again it is slight.

What about settlement at the foundations? This could add loads to the slab too. I was planning on checking the slab thickness/steel in the thinner portion of the slab as well assuming a uniform pressure distribution on the total base area. This should cover any settlement issues (basically assumes a slab on grade).

I think you misunderstood my sliding question though...Im not concerned about sliding of the tank as a whole. I am considering a cross section of one of the walls. The wall reactions at the foundation. My wall design assumes a fixed base per the PCA Charts, I am trying to resist the local shear at the foundation due to lateral soil loads. I am designing the footing assuming it is a retaining structure base. Without a slab diaphragm, the assumed base will be HUGE. I was curious if just butting a "floating" slab at the foundation would serve this purpose.

I have seen other threads about the base slab for basements (buckling stiffness), but Im still not sure if the slab needs to be cast with or dowelled into the footing. it seems to me in most basements the base slab supporting the wall just butts against the wall, but I was not sure.

Too...LONG, sorry.

 

[BFPartners]

A tank in the ground will not slide if there is equal soil loads on all sides of the tank. However, there is the possibiliy that the tank could float out of the ground (upward)when then tank is empty and a hydraulic pressure is surrounding the structure. A basement wall of a house will develop cracks (yield line theory) based on the end restraint at the edges of the wall. If you decided to use deflection charts make sure you supply the edge restraint as dictated by the chart or the wall will not react as you might think.

Most contractors do not want to form the base slab continuous with the walls since this will complicate construction. Do not give into the contractor and then have cracking occur where you might not want it.