Wet Well Bouyancy 1

[Kuhuh]

Ok, am I missing something here? I'm told that this thing will not float and is with in a reasonable S.F. but my calcs say the S.F. starts to reduce after the water table hits 1517.28. The wet well is flush top = el. 1531 ; concrete= 145 #/ft3 outer diameter is 112" (96"+8"+8"). There is a hatch that is 4'x8' but I neglected the weight of the hatch from my calcs due to just rough calcs. What am I missing?

http://files.engineering.com/getfil...dba-4a01-a4ed-6aed7939d977&file=DOC081312.pdf

 

[JedClampett]

Who told you it won't float? Unless they're taking into account the soil over the toe of the foundation, plus a lot more, it's floating.

 

[Kuhuh]

I forgot to show the existing grade to be at 1531 but I don't think that makes a difference.

 

[bimr]

You are missing a lot. Just using the weights of materials is a very simplistic analysis that is incorrect because it neglects the sliding resistance of the soil. The analysis method that you are using is correct if you are analyzing for floatation in a swimming pool, but is incorrect for something buried in the ground.

Because of the sliding resistance of the soil, it is nearly impossible to float a lift station. Compare the force necessary to pull a fence post out of a swimming pool to the force required to pull a fence post out of the ground.

Use the manhole floatation analysis procedures prepared by the Concrete Pipe Association to check floatation:

http://www.concrete-pipe.org/pdfdd/DD_41.pdf

If you still have a concern, add a l2-Inch lip around the base.

 

[BenJohnson]

That is my suggestion too... if you have the slightest thought that this wet well (and pump station?) might possibly reach the point of buoyancy, remember an extra 3 feet of concrete is concrete is very cheap insurance. Even the slightest shifting of the unit will break pipes and cause other damages. Make sure you put in a very generous factor of safety so the thing stays put! You don't want to spend a year involved with a malpractice law suit because you cut the design too tight.

 

[Maury]

I don't use the sliding resistance of the soil for two reasons:

1. The worst case may occur during construction, before there is any backfill.

2. I practice in an area with clay soils that in dry weather may shrink and pull away fron the walls, reducing the friction.

Some additional concrete is a relatively small price for security.

 

[JedClampett]

Yeah, we never use any kind of skin friction either.
Concrete is cheap, failures are expensive.

 

[Kuhuh]

So, it's better to design for a hefty S.F. without the soil friction? I agree that you can't always guarantee that the entire perimeter will be in full contact with the soil at all points along the height of the wet well. It looks like that equations only calculate the friction at 2/3s as a unit pressure I am guessing. There is soil pressure at that point, therefore I would assume that the soil will "pressed" against the side of the wet well. No? just better to be safe than sorry?

 

[chicopee]

I would pay attention to the first item that Maury is pointing out. In the event the excacation is left opened with the dry well in place during a weekend and it begans to rain, you may have that thing floating.

 

[jgailla]

I also do not use skin friction. I only use the weight of the soil above the lip, if any, the concrete weight, groundwater at ground surface level, and empty wetwell. I use a F.S. of 1.0, as all the above criteria have a small variability with conservative assumptions. It is far more important to review the geotechnical report for possible adverse soil conditions.

 

[bimr]

You should not be using any safety factor with the analysis method that you have used. There is no reason to use a safety factor greater than 1, if the weight of the structure is the primary force resisting flotation. There is little variability in the weight of the structure.

A safety factor is only appropriate if friction or cohesion or earth are the primary forces resisting flotation. The safety factor is used to account for the variability of the soil properties.

It is also not economical to add weight for floatation during the construction process. If the Contractor has concerns about floation, he can address that by adding wieght on his own dime. Construction (of the lift station) is considered to be "means and methods". The Engineer is not responsible for construction, the Contractor is.

In your example, the lift station is buried 30 feet in the ground. It would extremely unlikely to have 30 feet of clay soil. Note that you should have a soil boring for a project of this scope.

Another item missing in your analysis is the weight of soil on the bottom lip. The additional effective weight of the backfill in the cylinder above the lip can be added as an anchoring force. A conservative soil weight in the cylinder above the lip is 65,000 lbs.

 

[beej67]

Whenever I've done this, neglecting soil skin friction *is* my factor of safety.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[bimr]

One would think that some of the posters are neglecting to obtain competent geotechnical advice *based* on the comments. Maybe better opinions are available in the geotechnical forum.

Here is a link to a geotechnical report for a similar lift station project where the geotechnical engineer is using skin friction to resist uplift. See page 8.

http://www.houstonlwsforum.org/designCompetition/Suburban Res Challenge/TerraconGeotechReport.pdf

 

[fel3]

Two more points, one minor, one not so minor: Your sketch shows an 8'-diameter lid, but [1] there is no need to convert it to inches, then divide by 12 as part of your volume calculation (e.g. you didn't do it for the base) and [2] 8' converts to 96", not 112".

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Kuhuh]

8' lid with 8" walls = 96"+16"=112". this gives a 4" radius further for the base or 4" for the soil load to act upon. If they call for a 8' diameter wetwell lid the thickness of the wet well has to be greater than the cover.

 

[fel3]

My apologies, my eye caught the Section arrows and melded them with the interior dimension.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[fattdad]

There are two approaches to this problem. One is to use the soil shear that would act along the vertical plane that extends from the lip of the 10-ft diameter footing. That would essentially be the horizontal earth pressure times the friction angle. You could do this at various incremental depths to show how the vertical reaction varies with depth if you wanted to. The other way to do this is to consider the weight of the soil that's acting atop the lip of the footing. For this alternate method, you'd extend a line up from the lip of the 10-ft diameter footing that struck a 1/2:1 slope (horizontal:vertical) toward the surface. The volume and soil density of this hollow cone would provide supplemental resistance to uplift.

As long as the contractor owns the site, dewatering and bouyant conditions are his bother. Don't make them yours!

f-d

¡papá gordo ain’t no madre flaca!

 

[jgailla]

There is nothing wrong with using skin friction, an appropriate "cone" of soil based on phi, or a lowered groundwater table based on the geotechnical report.
However, these factors are subject to more variation than the simpler factors discussed above, and therefore an appropriate safety factor should be used.
I find it easier to simplify and use no F.S. On the few occasions I have checked, there is a minimal difference between the methods with a F.S. of 1.0 applied to the simplified method and an F.S. of 1.5 applied to the more detailed method. Concrete is cheap.
In my area, the groundwater level is typically very high, so the groundwater at ground level assumption is OK. In areas with low groundwater tables, this assumption could be excessively conservative.

 

[patswfc]

Agree with fattdad, we would usually consider the weight of a wedge of soil that will help increase the dead load.
The the UK the codes state that the min. F.o.S. should be 1.1