Lift Station Floatation

[bimr]

Has anyone had the experience or has heard of a lift station floating out of the ground during a flood?

I am doing some lift station calculations on floatation. It looks like the sliding resistance to resist floatation is so significant that it would take extremely poor soil conditions or a shallow lift station for such an event to occur. The sliding resistance is the lateral pressure on the side of the lift station times the friction coefficient between the lift station and the soil.

 

[RWF7437]

"The sliding resistance is the lateral pressure on the side of the lift station times the friction coefficient between the lift station and the soil."

In my experience, this frictional resistance is often ignored for two reasons. One, the groundwater may rise during construction before the backfill is in place. Two, the friction coefficient of wet soil against the wall material of the station is difficult to determine but is almost certainly much lower than for dry soil or rock backfill.

It is obviously conservative to make this assumption and to also assume the station is "empty" and use the lowest likely weight for the station and contents. Counterweights are pretty cheap insurance.

 

[PELS]

Not exactly your scenario but similar.

We have designed sewer lift stations where we utilized an "off-the-shelf" packaged product that is composed of a fiberglass wet well. We have used this product in areas where there was high ground water, and with the light weight of the fiberglass wet well, it required the need to compensate for buoyancy.

This was accomplished by designing a concrete anchor that encased the base of the wet well. We computed the amount of concrete (in cubic feet) required to overcome buoyancy force caused by the groundwater and added a 10% safety fator.

We tried to simplify our reasoning:

Basically, displaced water is the buoyancy force (BF). The compensating downward force is the weight of the concrete anchor (150 PCF) combined with the weight of the lift station and its equipment. So we determined the volume of displaced water (the space taken up by the wet well) and multiplied it by the weight of water (we used 65 PCF) to come up with a BF.

Then we determined the weight of the lift station (lbs.) and subtracted it from the BF to determine the net weight. The net weight was then divided by 85 PCF (the difference in weight between concrete and water) to arrive at the volume of concrete required to anchor the station.

 

[semo]

I've never seen one float; but, we are in a location where the majority of our lift stations are in sandy soils with extremely high water tables (some less than 5 feet).

I always look at flotation when making the design. As the others mentioned, I don't take friction into account. The friction values can change depending on the site conditions. Typically I place enough concrete to offset the bouyancy.

In our area, some of this is critical during installation as we caisson the wetwells down to eliminate the need for dewatering. This provides a poor design condition in respect to the friction coefficient and the difference in water levels between the inside and outside of the wetwell.

 

[rconner]

fyi (maybe the information at e.g.

http://www.concrete-pipe.org/pdfs1/2008-05-DD_41-manhole-flotation.pdf

would be of help)

 

[rconner]

[For whatever it is worth, in the USA there are so many utilities and regulatory agencies that have specific calculation etc. requirements in their specifications for this that I suspect at one time or another in the past actual problems have probably occurred somewhere. An example is the Ten States Standards for Wastewater Facilities, e.g. accessible at

http://10statesstandards.com/wastewaterstandards.html#34

, that includes the passage:

“42.24 Buoyancy

Where high groundwater conditions are anticipated, buoyancy of the wastewater pumping station structures shall be considered and, if necessary, adequate provisions shall be made for protection.”]

 

[jartgo]

Where I'm at, the state requires that we submit buoyancy calculations with other pump station calculations as part of the permitted design. We do as PELS mentioned above. One thing often overlooked (and PELS correctly pointed out) is that the buoyant forces will also act on the concrete, so instead of using 150pcf for concrete as counterweight, you should use 85pcf or so.

 

[rcooper]

http://www.nmenv.state.nm.us/fod/LiquidWaste/septic.do.nots.html

The above link shows some examples of concrete tanks floating.

 

[SkinnyDip]

If you end up with a lot of concrete you can make it more efficient by extending it outside the manhole so it has resistance due to weight of soil.

 

[lilrascal]

Usually discount sliding friction. Not worth the time to calculate. The soils above the extended base will normally provide a large amount of resistance force and is worth calculating.