Math Question for Plumbing Nerds

[Caduceus]

I have a slab of concrete 100 ft x 20 ft that builds up storm water underneath and tends to lift the slab at its weakest points. So a 6" diameter pipe is inserted vertically through the concrete into the soil below.
The 6" vertical pipe rises 15 feet before going horizontal to gravity drain.
How many PSI needs to build up below the concrete to force the water 15' vertically in this 6" pipe?
Will the concrete heave/lift before that pressure is met?

 

[frodo]

1.47 gallons per foot x 15 = 22.5 gallons

22.5 gallons x 8.34 weight per gallon = 183.86 pounds

of water to push up. aint gonna happen,,,GOT PUMP?


water will equal itself out, it wont rise without the hole thing rising


i am not a math nerd, so my answer is most likely wrong..

 

[phishfood]

6 1/2 PSI.

Ask a structural engineer.

 

[phishfood]

Now that I have been sarcastic....

I don't know what concrete weighs per cubic inch. But with the standard 4 or 6 inch thick slab, I doubt that it weighs 6.5 lbs per square inch, so I think you need a pump.

 

[Chris]

There is 45,646 sq inches in a yard of concrete. Assuming the yard weighs roughly 3,000 pounds a cubic inch of concrete weighs .064 pounds

 

[Chris]

36.86 pounds per square foot at 4" thick.

 

[Matt30]

My head hurts

 

[frodo]

I got my shoes off and my pants down, anything over 23, i am lost

 

[frodo]

49.4 lbs at 12x12 x4'' according to google

2o x 100 = 2000 sg ft x 49.4= approx 100,0000 pounds

gonna need a jack hammer, bust it up, dig new footers, install a french drain

then pour it back, INMO you will not lift it with water,

i

 

[Mr_David]

I'm not an engineer but I have a good head for how things work mechanically.
There maybe other cause but here are two I can think of that is raising the slab.
The expansion of the soil as it becomes saturated. If it was sand it probably would not expand but clay is a whole nother thing.

Then there is also dynamics of hydraulics that allow a 5,000 Ton ship to float on water.
Concrete does not float unless it is hollow inside and water tight like a boat.
If your basement and foundation have no cracks and is water tight like a boat, then the hydraulics of water can lift VERY heavy objects.
Like an empty swimming pool can actually pop up out of the ground if left empty and ground became water saturated .

Putting a tube through the bottom of that boat is not going to relieve the pressure exerted on the hull.
If the tube is higher than the surrounding water level. It has no effect on the hydraulic pressure on the hull
.

If you did install a pipe to relieve the pressure, the water in the pipe is only going to rise as high as the water level saturation in the ground around the foundation. The hydraulic force on the slab will remain the same.

The pipe would only be relieving the pressure exerted at the location of the pipe.


If you had an empty water tight container with a straw that extended down through the bottom. Push that container into a larger one full of water. the water in the straw will only rise to the level of the water in the second outer container.


Does that make since?

 

[Mr_David]

The type of lift I am describing is caused by the displacement ratio of the boat in water. If the boat is heavier than the volume of water it is displacing it will sink.

Another scenario of your problem could be:
There are other types of underground water pressure like an underground flow of water that originates from a much higher level and the outlet is now restricted by a slab which has no other way to relieve it self, therefore building up pressure below the slab.

I have seen a ruptured water main raise a small 10' x 10' panel of concrete about 6 "

 

[Caduceus]

Thanks for the input guys. I have tried many times in the past to explain to the engineer that designed this relief system why it hasn't worked and can't make any headway with him. This was seen as a more cost effective method to solve the problem as opposed to the French drain. I was hoping to get new language from the forum so that I can explain it again in different words the next time the conversation comes up with him. I believe you all have given me the tools for a new approach as to how to create a visual image for him to understand. Sometimes engineers have delicate egos and their judgement is clouded by that.

 

[Chris]

I can run it by my engineer if you want.

 

[Chris]

What about a sump pump?

 

[frodo]

I had an enginer draw a 3'' floor sink with trap. smack dab in the middle of a 36'' x 24'' duct.

i sent him an RFI asking him to

please explain the method to install floor sink and associated piping in the same space that is occupied by a 36'' x 24'' hvac metal duct

the answer i received

physically with great prejudice manipulate the top of the duct to fit the piping.

he was telling me to beat the **** out of it, make it work. LOL

this was in the ceiling of a university kitchen, that was in operation.

I stole a stainless steel mixing bowl, that was on the shelf,, cut a hole in the duct, set the bowl in it, and screwed it down

then set the trap in the bowl

 

[Chris]

From my engineer.

It takes 6.5 psi to go 15'. The size of pipe doesn't impact the pressure it only impacts the volume. A concrete slab has 1042 psi per 1000 sq ft, so it should be OK. The best solution is to redirect the storm water away from the slab

 

[phishfood]

From my engineer.

It takes 6.5 psi to go 15'. The size of pipe doesn't impact the pressure it only impacts the volume. A concrete slab has 1042 psi per 1000 sq ft, so it should be OK. The best solution is to redirect the storm water away from the slab

Could you elaborate on that? PSI is per square inch, obviously, so the per 1000 square feet is throwing me for a loop.

 

[Chris]

I will ask him. This is above my simple mind.

 

[Chris]

From my engineer

The specific gravity of concrete is 150 pounds per cubic foot. There is a conversion to change it to psi based on a certain square footage. Some concrete can have slightly different specific gravities based on the mix design.

 

[Mr_David]

I have a slab of concrete 100 ft x 20 ft

Is that a typo error? If not, That is a lot of concrete.
is that concrete one continuous slab or does it have expansion joints?
Is this a basement slab below grade?