Gravel Force

[meca]

I have a 14" OD Pipe that is buried under 18" of gravel. Along this 14" pipe there are several branch connection, each branch consisting of a 3" Pipe sticking up through the gravel approx. 5'. The 14" pipe will be exposed to a temperature causing thermal expansion.

My question is how to calculate the drag force that the gravel will exert on the 3" branch pipes, due to the thermal expansion of the 14" line. Does anybody have any ideas?

 

[Flareman]

I just finished a similar study. The piping system that I analyzed was shaped something like a tunning fork. A main line came into a tee, at the tee the line ran to the right and left out to 90 degree elbows and then they ran out as long sraight runs to end caps. I had the same question about the gravel fill resistance and posted a similar question on the structural forum with very little response. What I concluded was that the pipes are going to expand and the fill is not going to stop it. The less sliding resistance the fill produces, the better.
To estimate the gravel fill sliding resistance I assumed a volume of fill above the pipe and the volume of fill at an angle of 30 degrees from the vertical. The weight of this fill(100 to 120 pcf) plus the weight of the pipe was mulitplied by a sliding friction factor of something like .30 or .35 (I'm doing this from memory).
For the pipe ends at the elbows and pipe caps I assumed a pasasive soil resistance equal to the volume of fill at the ends that could be moved by the pipe as it expanded. I again assumed a 30 degree soil shear angle.
I assume the gravel fill is used as an insulator to keep the pipes from getting too hot. Keep in mind that over time the gravel may settle and lose some of it's insulating qualities and the thinking is to add more gravel to cover the pipes. This will make the fill more resistant to movement and cause more stresses in the piping system. An alternative would be to add a cover that can be placed over the pipe that holds the fill but lets the pipe expand. Hope this helps.