Tank Wall Hydrostatic Pressure 1

[PittEng88]

Hi Everyone,

I'm looking at designing a fully buried wet well for a pump station, inside dimensions are 30' Long x 8' Wide x 27' Deep. The geotechnical report indicates that bedrock is pretty shallow in the area so the well will need to be keyed in. The problem is, that we are also next to a creek so the water table is pretty high. The geotech is recommending that we backfill with #57's and design for a saturated condition using 110 psf/ft. At 27' that is almost 3,000 psf of hydrostatic pressure at the base and the walls are becoming unreasonably thick. As an alternate I am looking at backfilling with flowfill, in lifts to help alleviate that hydrostatic buildup. Has anyone ever done this before? If anyone has any other recommendations as well, it would be greatly appreciated.

As a side note, I am using a combination of the PCA tables and RISA-3D to determine the forces. Additionally, I spoke with the process guys and pressure relief valves are not an option since they will overload the tank capacity during flood events.

Thank you!

 

[LittleInch]

What's unreasonably thick?

And why so deep? That's a vast hole to drop a concrete box in.

Can't you make it circular?

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

 

[PEinc]

Even if you backfill with flowable fill, won't the high hydrostatic pressure still push on the back of the flowable fill which then pushes on the walls?

http://www.PeirceEngineering.com

 

[Rabbit12]

I'm guessing Pitt is planning to drop the design pressure to 62.4 pcf. Maybe that's OK.

What is unreasonably thick? With that depth your wall is likely 30" which isn't out of the ordinary for that wet well. You could add pilasters to you can get some 2-way action on the 30 foot side.

 

[PittEng88]

Thanks for the replies. I'm coming up with 36" walls right now based on the hydrostatic pressure alone, but there is a surcharge load from the pump station building which is pushing the walls to 42".

@LittleInch: The well is this deep based on pipe inverts, site constraints, and required volume. I'd love to do circular but the site constraints won't allow it.

@Rabbit12: you are correct, the flowfill will take out some but not all of the pressure. I just want to get away from the 110psf/ft.

 

[SlideRuleEra]

PittEng88 - Suggest you follow the Geotech's recommendation using #57 stone because:

1) This cast-in-place structure will likely be constructed inside a temporary cofferdam that is 27+ feet deep. Keep The Design Simple. Stone can be placed quickly... don't force the Contractor to "fiddle" around placing flowable fill lifts inside the cofferdam (wasting time working in dangerous conditions).

2) Assumed pressure reduction by using flowable fill is questionable at best. Don't take "shortcuts" when working with hydrostatic pressure... the forces from it are real (not statistical probabilities like most loads) and they never "take a day off".

3) The permanent structure will have significant buoyancy. Nice to have "thick" walls instead of having to have a super-thick base slab to get sufficient concrete weight to resist uplift.

My advice, if "thick" walls solve the design problem, make it so. You, the Contractor, and the Owner will thank you in the long run. This type work (below the water table) is inherently expensive and potentially dangerous. Don't Cut Corners... You (and probably others) WILL Loose.

 

[Rabbit12]

SRE, good point about buoyancy. Don't ignore that. It's not uncommon to see that be a headache to deal with.

 

[nackra]

buoyancy reference:

https://precast.org/wp-content/uploads/2022/08/NPCA-Buoyancy-White-Paper-2022.pdf

Concrete canoes float

 

[HTURKAK]

IMO , 110 psf/ft. pretty high which makes at 27' ht almost 3,000 psf ..


If the bedrock is pretty shallow, can the casting of the RC walls against rock or filling the annular space with lean concrete be an option ? In this case , the trust of granular fill may be eliminated .. Another option, sharpen your pencil and calculate the active thrust of fill

Can you post the soil profile with proposed GWL ?








I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[JoelTXCive]

I've done structures that deep; and 36" walls at the base are not unreasonable. I've seen 48" thick ones.

Are you open cutting this; or will it be a caisson structure that is sunk?

If open cut, then you can hopefully sidestep the buoyancy issues by having the foundation extend a few feet wider than the tank. That lets you pick up a significant amount of earth load.

For the earth pressures, can you add weep holes & drains so that the fully hydro component can be avoided? Or, do your water table elevations prevent this?

Other ideas.. Can you put a top slab on the wet well, or add a series of beams across the 8ft wide portion so that load can be shared from one side to the other?

 

[JedClampett]

I want to echo what everyone else has been saying. But I'd add that ACI 350 used to have what they called a durability Load Factor of 1.3 that they would put on top of the 1.7 LL for a 2.21 factor (on bending only, not shear). I'd use that too. You don't want to cut any corners on these designs. Water pressure is unrelenting and high.
You definitely don't want me there in the future, on expenses, tut tutting on a failed wall.

 

[bones206]

You can also consider shear reinforcement in the lower portions the walls with the highest shear demand.

 

[PEinc]

The geotech is recommending that we backfill with #57's and design for a saturated condition using 110 psf/ft.

To me, the geotech's recommended 110 psf/ft sounds like the total "equivalent fluid pressure" (buoyant #57 stone plus water). Then the question is if the geotech recommended using an active or at rest earth pressure coefficient. There isn't much sense in us throwing around total lateral pressure values without clarifying the unit weight of the #57 stone, its internal friction angle, and whether the geotech recommended using an active or at rest earth pressure coefficient. Given the thickness and stiffness of the concrete wet well, the geotech probably recommended an at rest coefficient. In any case, as HTURKAK mentioned, 110 psf/ft sounds heavy.

http://www.PeirceEngineering.com

 

[SlideRuleEra]

Agree that 110 psf/ft equivalent fluid pressure seems high, but most of that value is water pressure (62.4 psf/ft).

Submerged stone EFP accounts for only 47.6 psf/ft (110 psf/ft - 62.4 psf/ft = 47.6 psf/ft).

Even if submerged stone EFP is, say, 20% lower (38.1 psf/ft), total EFP is still 100.5 psf/ft.

 

[HTURKAK]

Mr. PITT,
I do not want to discuss basic concepts here but the proposed lateral pressure at 27' p= 3,000 psf is wrong .

If we assume the water depth 27' , γ w=62 lb/ft3 , γs=110 lb/ft3 and assume friction angle Φ=30 degr.,

Ka =(1-SINΦ)/((1+SINΦ))=0.33 and at rest pressure coeff. Ko=(1-SINΦ)=0.50

The lateral pressure at the bottom ;

for active pressure Ph= 62*27+ (110-62)*27*0.33=2100 psf

for at rest case Ph= 62*27+ (110-62)*27*0.50=2300 psf

I would like to you remind that , the subject active pressure which may develop will be less due to silo effect. and suggest you ask to geotech how did they calculate 3,000 psf ..If the 3,000 psf factored load for strength design, this is another discussion..













I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[Smoulder]

@htukak That bouyant weight is too low, will be about the same as water density because buoyancy only acts on the solids.