HyrdrostaticUplift force on buried pipes

[numerical1]

Hello everone.

I have a pipe buried at 3 ft below river bed at stream crossing. The water level is 10 m above river bed. When checking if the pipe is going to be uplifted under hydrostatic water pressure, I noticed that in literature when calculating upward hydrostatic force under pipe, only water height under river bed is considered. I meant that in the literature the water height above river bed is not counted to the upward hydrostatic force. Should not the uplift hydrostatic force be computed for the full water height (including the water height above river bed). (Gamma Water * height of water) ? Thanks for help

 

[LittleInch]

I really don'tunderstand your issue. If you mean buoyancy then height of water makes no difference.

A sketch would help.

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

 

[numerical1]

Thank you for reply. I attached a conceptual sketch showing the problem . The Idea is to get the uplift force at the base of the pipeline which is buried under 1 m of river bed and has 10 m height of water above it. I am trying to compute the uplift force under the pipe . Thank you for help.

 

[robyengIT]

force down : self weight of pipe + material inside the pipe + weight of prism of ground above the external diam of pipe (that is weight of ground immersed in water)

force up : weight of water volume calculated with the external diam of pipe (Archimede)

PS : with concrete pipe, generally speaking, to avoid buoyancy, it is enough one meter of ground above the pipe

 

[LittleInch]

The diagram is drawn badly and is not correct. Those arrows on the bottom of the pipe act all the way around the pipe. The arrows on the river bed are not forces and for this exercise I would delete them as the water pressure acts in all directions. And force down from the water is being resisted by an equal and opposite force vertically up from the soil in the river bed.

Roby eng has it nailed. The uplift force is the volume of the pipe times the density of the fluid, in this case water.

Down forces are the weight of pipe and coatings plus soil weight and soil shear strength. Initially soil strength is often ignored unless it becomes important.

Usually you aim for a minimum of 1.1 for forces down vs forces up.

This idea of hydrostatic forces under the pipe is not the way to think about this issue. Start thinking differently.

Depth of the river will affect the external pressure on the pipe, but that's something completely different.

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

 

[robyengIT]

furthermore I attache a drawing showing the pipe lay out in crossing a stream

 

[numerical1]

thanks for reply guys..the hydrostatic case is clear now. One more thing should not we account for hydrodynamic forces which become significant under flooding situations. Would anyone know sources for quantifying hydrodynamic forces affecting buried pipes ( say 1 m depth below river bed) at stream crossings.

 

[rconner]

I don't know about "hydrodynamic" effects specifically, but erosion is a very real force of nature, and in certain soil strata and current/wave conditions it can cut quite quickly. Originally buried pipeline crossings have been left suspended and/or bank approaches exposed in floods and storms. Liquefaction in seismic and artesian-type conditions may also occur. Strong pipe, sufficient bulk density, adequate and confirmed depth of cover, and even getting down in stable ground strata are in general good.

 

[LittleInch]

If your pipe remains buried then it matters not whether the stream is flowing at 0.1 m/sec or 10 m/sec.

If your pipe is exposed on the surface then yes there is a force from the flowing water.

If this stream can get to a high flood state then 3ft burial is looking rather low.

Scour and washout become your issue, especially if you've open cut this and then just dumped the soil/gravel back in.

All depends on your "stream".

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

 

[bimr]

Buoyancy is discussed in the steel handbook.

http://www.eng-tips.com/

Steel Handbook