Precast Plank Roof for Underground Tank

[pricklyPete]

Can anyone tell me if they have had experience with using precast plank for an underground tank. This particular tank is 70 ft square. The interior columns are going to be 17-6" apart. The tank has about 5 1/2 feet of soil over the top.

I find that if you are going to design the tank to be full of water with no backfill (as in a leak test), that precast plank presents some real concerns in regard to not pulling apart due to the tension forces that would occur in the top of the wall. The connection from the top of the wall to the top slab would also be an issue.

If this condition were to never occur I would feel pretty comfortable with plank because everything is in compression. I am told that no leak test will be performed for this tank, but even so I still don't think the ACI 350R code allows me to not design the top slab and the connections from the wall to the top slab for the tension forces. Does anyone have any thoughts. Is a poured slab pretty much the way that I have to go?

 

[SperlingPE]

Can the cast in place walls be designed to span horizontally?

 

[pricklyPete]

The walls cannot be designed to span horizontally or be cantilevered.

 

[SperlingPE]

What width (2', 4',8') of plank?
What are you considering for the connection at the precast? Welded? Reinforcing bars?
I have used precast plank below grade, but not for a water containment structure. I could see some problems for the connection (corrosion).