Submerged Tank
Submerged Tank
(OP)
I've got a 6' dia tank X 20' long tank submerged vertically in soil. The x-section of the tank is about 28 ft^2 and the conc base I'm proposing is about 10'X 10'. How do other engineers address this type of problem. I've got a bit of a discussion going on at the office. Assuming the entire system is submerged, is it reasonable to assume that the concrete base has a density of 150-62.4 psf and the included weight of soil is (100 - 28) * (gammasoil-62.4)*20' (assuming no angle and no friction) to resist floatation? or would you neglect the weight of the soil?
Dik
Dik






RE: Submerged Tank
The first with full backfill and ground water level at the maximum possible over the design life of the tank. In that situation I would take the weight of the soil into account. It is also possible to take skin friction into account to give you a bit of extra resistance.
The second case would be the just constructed case when there was no backfill. This is to cover the fact that it may be left unbackfilled and the ground water level rises, or it gets caught in a heavy rainstorm. For this I would consider a lower water level but no soil on the skirt.
As for the flotation part of it. You need to ensure the weight of the structure is greater than any buoyant force exerted on it, with a adequate factor of safety. I calculate the weight in air of the structure as a whole and then calculate the external volume of fluid displaced (archimedes principle).
I'm not really sure that answers you question though.
RE: Submerged Tank
RE: Submerged Tank
Also give consideration to how the tank is anchored to the concrete (how to keep the empty, submerged tank from "popping" loose from the foundation).
Apply a safety factor, such as the one suggested by theonlynamenottaken, to all the calculations.
If the resistance to flotation does not check out, usually the easiest solution is make the concrete foundation larger, thicker, or both.
www.SlideRuleEra.net
RE: Submerged Tank
To my mind you need to look at the structure as a whole. You would calculate the tank weight in air, from the concrete in the base, weight of the tank, weight of a roof (if you have one), weight of any plant or equipment inside the tank etc.
This will give the total weight as if it was sitting on the ground. The buoyant force is then calculated based on the volume of water displaced. The submerged weight of the structure is given by the weight in air minus the buoyant force. If that figure is negative your structure is buoyant.
As I see it the concrete base (is part of a larger structure and acting as a plate to keep water out) is not actually submerged as it is only in contact with the water on three sides (assuming the tank is empty). The net load on a submerged structure would be zero, this is not the case for a tank as the base will tend to hog as it is submerged due to an external pressure.
Let me try an illustrate how I am thinking, consider, if you please, that the base is actually formed with the same material as the walls (say thin sheet steel) and concrete placed inside the tank. In this instance the concrete would not be in contact with any water but the weight of the structure would remain the same. Would you use the submerged weight of concrete in this instance?
This is always how I have been taught to look at the these things. I would be interested to know if that is actually wrong.
RE: Submerged Tank
With this being the case, Archimedes Principle does apply, assuming the tank/foundation do not float, then they displace their VOLUME in water. The weight of the this VOLUME of displaced water is the flotation force. The only "thing" holding the structure down is the "excess" weight (in that these materials are heavier than water) of the concrete, the soil directly above the concrete/tank, and the tank itself. This assumes the worse case when the tank is empty.
Here is a sketch of my understanding of the conditions
This begs the question how do you build this structure. The above situation is steady-state. Ground water moves relatively slowly - often simple pumps (usually diaphragm pumps - Mud Hogs) can keep groundwater in an excavation depressed. Vacuum well points and deep-well pumps are more sophisticated, long term ways. Slurry walls, ground freezing techniques, and of course cofferdams can be used if there is a need (and a large budget).
www.SlideRuleEra.net
RE: Submerged Tank
Thanks for the inclusion of the construction time loading. I had taken care of this by including it in the notes, but now I'll check the load. The discussion in the office is related to using the submerged weight of concrete...
The walls of the tank and base are included by virtue of the OD of the tank included for the displaced water. Like SRE's neat sketch except that I've included soil sitting on the base.
theonlynamenottaken:
In Canada, the old NBCC required a factor of 0.85 and the new NBCC requires 0.9 for resistance and a factor of 1.5 for uplift; this had already been considered. The 0.6*150 seems a little extreme even for lightweight concrete<G>. Also using limit states...
SRE:
The dead weight of the tank has been included. Granular under the concrete base so uplift of the base has been included. Your sketch is almost the way I'd envisioned it except I'd considered soil... How do you create these sketches?
The tank is secured by means of an annular ring of concrete poured around the tank and anchored to the base.
BTW, I still have my Pickett, Hemi 10" and 6" slide rules as well as my small cylindrical 'hand crank' calculator... use my TI89 at the office and my HP48G at home...
Dik
RE: Submerged Tank
Sorry, the 0.6 isn't extreme considering you're likely using service loads...
Dik
RE: Submerged Tank
Don't know if anyone noticed or not, but once the water gets up to the top of the tank, the uplift doesn't increase whether it's at ground level or where.
Also, if not done, you need to design the tank itself external loads, which may or may not be an issue, depending on the construction.
RE: Submerged Tank
Nice sketch! From that I see we are talking about the same thing, but possibly from different angles.
You could use the submerged density if you calculate the buoyant force on the structure to the top of the foundation and then add the submerged weight of the found after.
RE: Submerged Tank
thanks, Dik
RE: Submerged Tank
From a technical standpoint I agree with you that a .6 factor seems like an abnormally high safety factor (1/.60 = 1.67) for this situation. That number comes from ASCE 7 "Minimum Design Loads...", a recognized and published value, so I wouldn't deviate from that (in the U.S.). I just won't use lower safety factors than those published after hearing an engineer (that I agreed with) try to justify his using lower than published values in court.
RE: Submerged Tank
Think of designing a concrete boat. (or submarine?)
RE: Submerged Tank
I agree that a 1.67 seems like an abnormaly high safety factor for this situation. It's certainly higher than the 1.15 safety factor traditionally used in our office. However, our traditional method ignored the resistance from the wedge of soil outside the footing. NAVFAC DM 7.2 puts this angle at 20 degrees for sandy soils. See thread 507-46927 for a discussion on this. In order to meet the safety factors required by ASCE7, I think it is appropriate to include the wedge of soil for resistance.
RE: Submerged Tank
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dik - The sketch is just pen on bond paper, scan it, size and compress the file, upload it to a website (mine in this case), and link to. A little tedious, but not difficult.
Glad to hear that you used a Pickett, the differences between them and the other brands reminds me of the old comparisons between Texas Instrument and Hewlett Packard (RPN) calculators.
www.SlideRuleEra.net
RE: Submerged Tank
dik, I think the upshot is that you can use either the in-air weight, or the submerged weight depending on how you approach the calculation.
RE: Submerged Tank
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www.SlideRuleEra.net
RE: Submerged Tank
RE: Submerged Tank
RE: Submerged Tank
In doing some searching, I find different sources that list 1.3 or 1.5 safety factors for uplift on underground fuel tanks (including soil loads). For example, this reference:
Principles and Practices for the Design and Construction of Flood Resistant Building Utility Systems November 1999, found at:
h
which defines: "FS is a factor of safety to be applied to the computation, typically 1.3 for tanks."
RE: Submerged Tank
RE: Submerged Tank