# Thread: How much weight on top of underground conduit?

1. Originally Posted by kwired
Someone correct me if I am wrong, but wouldn't 40 inches deep water in the pool exert 40 inches of water column equally across the entire bottom of the pool?
If I am remembering correctly from the instrumentation class I took, each square inch column of water exerts it's force straight down and does not exert force to the sides. Of course, there are other columns of water on each side of the one we are discussing that are also exerting their force straight down. It doesn't matter the size of the column, the force is the same at any point on the bottom (as long as the depth is the same).

2. OK, so I'm supposed to stay five feet away, however the pool has a built-in motor with a wiring compartment so I have to run the conduit into that. Do I have to approach it at a right angle in order to stay five feet away as much as possible, or can I approach the compartment at a more obtuse angle meaning the conduit will be within five feet for a longer distance?

3. Originally Posted by Coppersmith
If I am remembering correctly from the instrumentation class I took, each square inch column of water exerts its force straight down ...
Not so. Fluids exert a force in every direction. The keel of a submarine's pressure hull encounters (essentially, less ρΔz) the same pressure as its deck and sides.

And to a reasonable approximation -- good enough for small-scale engineering & construction purposes -- soil is a fluid.

4. Originally Posted by Coppersmith
If I am remembering correctly from the instrumentation class I took, each square inch column of water exerts it's force straight down and does not exert force to the sides.
That is what I understand.

Originally Posted by drcampbell
Not so. Fluids exert a force in every direction. The keel of a submarine's pressure hull encounters (essentially, less ρΔz) the same pressure as its deck and sides.

And to a reasonable approximation -- good enough for small-scale engineering & construction purposes -- soil is a fluid.
With the submarine the pressure should be less on the upper portion of the hull then it is on the bottom larger the vessel the more noticeable this may be.

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dirt actually usually weighs more than water so weight is not the issue.

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Originally Posted by kwired
That is what I understand.

With the submarine the pressure should be less on the upper portion of the hull then it is on the bottom larger the vessel the more noticeable this may be.
Pressure in salt water increases by 1ATM every ~33' of depth, but here's a thought experiment: If a submarine is 33' from deck to keel and you run a pipe through it from top to bottom open to the sea, will water continually flow through it due to the pressure differential? I think not. Or maybe I have discovered a source of free energy.

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would only work if your sub was three times that fat.. roughly 100 feet... to get enough water movement to actually create scrw motion, if I remember the experiments... but once they figured out how to create power from nuclear energy the technology was not used anymore in experiments. a version of one of the methods they tried became the basis for some of the tidal energy systems that they have been trying to make, using a flapper to create energy, attached basically to a cork or bobber. In fact, they even were trying to create a method of allowing the seal teams to transition in and out of such a hole from either top or bottom of subs, including underwater sled parking, so that the subs did not have to surface to release the seal teams off of a coastline such as Korea... but in early eighties they still had not come up with a great version of that, even with the James Bond movies for inspiration. Lots of interesting things in first year of sub training in Dago...lol..wonder if that is gone like the rest of the base was last time I went there on a visit.

8. Originally Posted by kwired
... With the submarine the pressure should be less on the upper portion of the hull [than] it is on the bottom ...
Right you are. The pressure on the keel is greater than the pressure on the deck. This pressure difference can be calculated as the density (ρ) of water multiplied by the difference in depth (Δz).

If the pressure hull is ~10 meters high, this difference is about one atmosphere, which is essentially nothing when compared to the ambient pressure at maximum dive depth.

It's even more irrelevant in the case of underground conduit.

Originally Posted by ggunn
... If ... you run a pipe through it from top to bottom open to the sea, will water continually flow through it due to the pressure differential? ...
A submarine is not needed for this experiment. All that's required is a submerged vertical pipe, open at both ends.
There will be no flow. Although there is a Δp between top & bottom, there is exactly the same Δp in the ambient water. This is why a whale doesn't have a blood-pressure crisis when making a vertical dive, even though the blood pressure in its brain is 2000 mmHg greater than in its tail.

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Originally Posted by drcampbell

A submarine is not needed for this experiment. All that's required is a submerged vertical pipe, open at both ends.
There will be no flow. Although there is a Δp between top & bottom, there is exactly the same Δp in the ambient water. This is why a whale doesn't have a blood-pressure crisis when making a vertical dive, even though the blood pressure in its brain is 2000 mmHg greater than in its tail.
It was a joke.

10. Originally Posted by ggunn
It was a joke.
Sorry. Between Poe's Law and the vast number of Americans without a clue about elementary science, that's not always evident online.

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