Submerged Tank 6

[dik]

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

 

[Ussuri]

When I look at submerged tanks for flotation I usually consider two cases.

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.

 

[theonlynamenottaken]

I'd consider the soil but I'd apply a .6 factor to the dead load of the soil and the concrete as in ASCE 7 service load level (ASD) load combos #7 & #8. So the concrete base would have a density of [(.6*150)-62.4] and the soil (100-28)*[(.6*gammasoil)-62.4]*20.

 

[SlideRuleEra]

I would include the weight of both the concrete and the soil, as you have suggested. Also would include the dead weight of the empty tank. Be very conservative on the assumption of the high water table (here, we assume ground level - which is often true).

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.

http://www.SlideRuleEra.net

 

[Ussuri]

Can I ask why you are using the submerged weight of the concrete? (I think thats what the units are telling me, the density of water is 62.4 pounds per cubic foot?)

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.

 

[SlideRuleEra]

Ussuri - In the area where I work, goundwater is semi-infinite. It permeates all of the (sandy) soil. That is what is different from your assumptions. If this foundation were constructed here (coastal South Carolina) everything would be completely saturated - the soil under, around, and above both the foundation and the 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).

http://www.SlideRuleEra.net

 

[dik]

ussuri:
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

 

[dik]

theonlynamenottaken:
Sorry, the 0.6 isn't extreme considering you're likely using service loads...

Dik

 

[JStephen]

It does seem to be overkill to use the 0.6 factor. If you're designing for wind uplift or something of that nature, there is considerable variation possible in the actual overturning load that makes it worthwhile to be conservative. But with simple hydrostatic loading like that, you should know the loads involved fairly precisely, and a lower safety margin should be acceptable.

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.

 

[Ussuri]

Slide

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.

 

[dik]

It's a lift station, dry and completely submerged in the soil. The top extends approximately 1' above grade. It's not a wind design issue, only floatation. The discussion has centered around my using the submerged concrete weight.

thanks, Dik

 

[theonlynamenottaken]

JStephen

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.

 

[BSE05]

I have always used the buoyant weight of concrete for the base..... or another way of thinking of it is that the buoyant uplift force includes the weight of water displaced by the concrete pad.

Think of designing a concrete boat. (or submarine?)

 

[miecz]

JStephen

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.

 

[SlideRuleEra]

Here is a Canadian Water Control Structures Guideline (Table 8-2) that gives a range of flotation safety factors of 1.1 to 1.5, depending on the use.

http://www.infratrans.gov.ab.ca/INFTRA_Content/doctype125/Production/S8DsgnGdln.pdf

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.

http://www.SlideRuleEra.net

 

[Ussuri]

The British Standard BS8007 states that a factor safety against flotation of 1.1 should be used when the water levels are accurately known. If not, it is up to the designer to determine a suitable one to take into account the unknown factors.

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.

 

[SlideRuleEra]

And here is the US Army Corps of Engineers document that the Canadian Guidelines (safety factor of 1.1 to 1.5) is based on, see page number 2

http://www.usace.army.mil/usace-docs/eng-tech-ltrs/etl1110-2-307/entire.pdf

http://www.SlideRuleEra.net

 

[kslee2006]

In the practical design of dams and submerged structures, it is usually first to compute the hydrostatic pressure, measured from the free water surface to any surfaces exposed to the uplift due to water (p = 62.4*h). Then compute the uplift force (U = Sum*A). The last is to compute the downward forces (W) using "Dry Weight" of the components. The factor of safety SF = W/U >= 1.5, usually 2 or greater for critical facilities under normal operating conditions. 1.5 is satisfactory for the the case - the free water surface is at the maximum expected elevation, backfill completed to the level of the free water surface, the tank is empty, and the soil on top of the tank is removed (construction condition).

 

[kslee2006]

The downward force include the total weight of the soil (= submerged soil weight + weight of water) on top of the concrete mass for the case that the foot-print of the tank is smaller than the concrete base.

 

[JStephen]

If the design becomes an issue, I would suggest checking into the UL or NFPA codes governing underground tank construction. Underground tanks are very common, but don't seem to be included at all in IBC, so I would be reluctant to assume that they ought to fall under it. From a liability standpoint, it would be a whole lot simpler to explain why you used an underground tank standard for an underground tank than to explain why you used a building code. You'd have the same issue with underground pipelines as well.

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:

http://www.fema.gov/pdf/fima/pbuffd_chapter_3-2.pdf

which defines: "FS is a factor of safety to be applied to the computation, typically 1.3 for tanks."