Earthen embankment over existing ground with trenched in watermain

[jjgerrit]

Hello all,
I am an intern at a municipality and I am having trouble finding a solution to a problem my boss posed to me. The WISDOT is placing a 20 foot fill over our DR-18 S-900 water main pipe that is trenched in 6.5 feet below existing ground. All in all there will be 26.5 feet of cover over the pipe. We are trying to find out if this will cause too much stress on the pipe and if it should therefore be replaced with cased pipe. Thank you for any help.

 

[cvg]

replace it with ductile iron or install a steel casing

 

[coloeng]

I would be surprised if your boss just wants the answer. Since you are an intern, he(she) probably wants you to actually find out what the load on the pipe will be from the additional embankment and if the pipe is capable of supporting that load. To find the dead load of the soil you can probably use the Marston Formula for flexible pipes. You will still need to determine any live loads on the pipe from traffic, equipment, etc.

 

[francesca]

In my experience, anything going under 20-odd foot of highway needs to be in a steel casing. Much easier to repair/maintain if you don't have to dig up 26 ft of earth first. In fact, in IN the DOT requires anything under their state routes to be in casing.

 

[jgailla]

If it's DR18, it will probably pass a stress analysis under 26.5 feet of fill, depending on the soil type. You can find the formulas in the Unibell Handbook of PVC Pipe. However, I would agree with cvg and francesca that a steel casing would be preferable. At the very least, you should install mechanical restraints at all the joints.
What is the length of pipe that will be under the fill?

 

[oldestguy]

Ok here is something WisDOT did at EauClaire WWTP for an outfall pipe under use that cound not be shut down. They used what an old Iowa State Professor Spangler called "the imperfect trench". Might have been an idea of an earlier prof. Marston.

You should look it up, possibly on Google. Or see if you can find the test book from Spangler (who is long dead). There are several papers on the subject that I came u with with a Google search, including some design guidelines.

What it basically involved is excavating down some with a trench over the pipe. Fill it with compressible stuff, like corn stalks.

Then, since your fill goes above that, build a reinforced concrete slab over the Imperfect trench and for some distance on the sides, say 10 ft. each way.

then place your fill.

I suggest seeing if Interstate I-94 original plans show the work done at EauClaire. Done about 1960. After all it is still WisDOT.

A similar thing was done at Madison for an active MMSD WWForce main next to the Yahara River, under pressure, that had to be built over by a bank with footings bearing over it. They covered the trench area with a reinforced slab and went on with the work, but did not do the Imperfect Trench.

You make the ground alongside the trench carry the load.

I betcha none of the above commentators heard of it before. Once in a while it comes in handy.

 

[cvg]

regardless if the imperfect trench works for loading, I would not advise constructing a pressure waterline under such a tall embankment. The risk from leakage is way to high. There is no easy way to repair it and leakage could cause embankment failure. Put it in a casing.