Underground tanks subject to traffic loads

[RiverBeav]

Just looking for some background on how underground tanks resist traffic loads. This is still in regards to the problem that I have with a very large piece of machinery above some underground tanks (it is a thread on the 2nd page of the storage tank engineering forum), but I am looking for just some more general information.

From my research, I primarily see that the soil cover combined with a concrete slab above the tanks is generally how underground tank is protected. Also, it seems like the tank is backfilled very carefully so the bottom half of the tank is firmly supported by the soil. But curious how that works exactly . . . the physics behind the loading in the tank. Are they taking into account some arching action of the soil combined with the shape of the tank that disipates the soil vertical loads to the full height soil on the side of any tank? Or does the vertical pressure on the tank get resisted by some sort of axial force in the wall of the tank? Or is the protection of these tanks primarily based on rules of thumb and previous experience.

Thanks for any help.

 

[Snickster]

the following link to an ASCE paper on underground piping may help what you are trying to understand.

https://www.americanlifelinesalliance.com/pdf/Update061305.pdf

 

[LittleInch]

Do some searching on Iowa formula and Spangler formula.

These are the base of most of the codes or standards.

Also look up API RP 1102

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RiverBeav]

That is perfect! Thanks! I will run through this analysis. I have a way forward to avoid loading the tanks by installing an elevated slab on piles, but it is a lot of money to spend. It would be good to know for sure that this expense is required or if we can somehow avoid it.

Finally, the tank I have is a double wall tank with the outside wall being constructed of fiberglass. I worry about if the tank starts to become more oval in shape under load that it wouldn't crack the exterior coating, leading to decreased life and leaks of chemicals into the soil. How is that addressed?

 

[Snickster]

I have used protective concrete slabs over piping in the past for high traffic loads but without piles. The slab spreads out the load to acceptable values. I believe it was 6" to 8" thick.

 

[RiverBeav]

So my issue is the tanks are 10'-0" diameter, 1/4" wall with 3/8" x 5" stiffeners at some spacing (I am not sure of the spacing), and the machine that needs to work above the tanks weighs 266,000 lbs and is on tracks (like a track hoe), so instead of discrete point loads from tires, has more of a uniform pressure applied to the soils above the tank. Based on the bury depth, I suspect that at the worse loaded location, the pressure is about 3500 psf at top of tank elevation, or 25 psi.

I suspect that the stiffener spacing is determined by the loading from above . . . for the 3500 psf applied load, the tank would have needed stiffeners at about 10" on center just so the 1/4" wall could distribute the load to the stiffened strips of the tank. I suspect that for lighter design loads (the loads the tank was actually designed for), the stiffeners would have been spaced farther apart. I don't have any idea on how close the stiffeners are spaced, but I doubt that they put them 10" on center.

 

[CarlB]

Don't know how you came up with the required stiffener spacing. Agree that for 1/4" wall & large surface load, would need a lot of stiffening.
UL 58 does have a formula to determined required steel thickness and stiffener spacing. But it doesn't really account for external loading, other than soil backfill. I've seen some large diameter fuel and septic tanks, they typically are not intended to be buried with more than about 4 feet of cover. 3500 psf is equivalent to about 30 feet of cover, that's a large load for a tank to resist, if distributed over a large portion of the tank.
If you go with underground, may want to enclose tank in a concrete vault.

 

[r6155]

Who is thinking about inspection after installation and during service? Leak test?
How do you plan to access to make frequent inspections?.

Regards

 

[RiverBeav]

As an update, we have proceeded ahead with an elevated platform for the drill rig to operate from and avoid loading the tank. It was too big of a risk and too many unknowns. I reviewed the soil loads applied beneath the existing slab from HS20 truck loading, and the overburden pressure was around 500 psf. With the drill rig, the overburden pressure over the tanks was closer to 1800 psf (the slab distibuted the load out farther than I was calculating by hand in an earlier post, where I indicated 3500 psf overburden), which was nearly 4 times the 'baseline' design.