Seepage around pipe piles in embankments

[Geotechnical76]

How do you consider the effect of seepage on the stability of the embankment if you have pipe piles peneterating
an embankment? We have an ebmankment nearby a river with piles penetrating the embankment and socketed in rock.
We have two cases, rapid drawdon and flood. I understand that when seepage occurs around the pipe piles, the
pile skin friction will be reduced.

I am wondering how to model the seepage around the piles in programs such as SEEP/W or hand calculation? Do we consider the pile width as the full blanket width? Do we assume that the pile will act as a vertical drainage blanket?

Any references, books, or articles dealing with seepage around pipe piles in embankments?

Thanks.

 

[cvg]

I am trying to understand how the piles will have any appreciable effect on the phreatic surface. Are these vertical piles? I am assuming that the piles were installed through the completed embankment. There may be some disturbance of the embankment soils that was caused by installing the piles. This would depend to a great degree on how they were installed. If they have loosened the embankment soils, perhaps it allows increased flow through that area. If they were driven piles, perhaps they have not done that. Not sure how the pile would act like a chimney drain unless you are assuming that either they are perforated pipes or there is unconsolidated material or granular material around the pipe which allows water to flow. A sketch might be useful...

 

[Geotechnical76]

These are vertical piles. The idea is that penetrating the permeable layer by many piles will create a seepage path that will affect the integrity of the levee by introducing seepage problems.

 

[dgillette]

In a levee or dam, penetrations are always to be avoided whenever possible, because they cause discontinuities in the embankment where internal erosion (piping) is more likely to initiate. That's your concern, right? Not slope instability.

The seepage analysis around the piles is 3D, not really possible with Seep/w. Probably much easier to design remedial measures, such as a weighted filter on the land side of the levee, if that's where the piles are. Post a sketch?

Seep/w and flow nets are continuum models, whereas internal erosion is more likely to initiate at discontinuities. Keep that in mind while analyzing.

Bon chance!

DRG

 

[Geotechnical76]

dgillete,
Thanks for the great advice. Any suggestion of a computer program that can do the 3D analysis? Do you consider a seepage path just along the pile or do you model a zone (say 0.5 ft) around the pile while considering the pile as a rigid body?
Thanks

 

[dgillette]

I don't believe I would do any 3D analysis unless I had a very clearly identified need for it, in addition to enough good soil info to make it meaningful. (Remember also the continuum/discontinuity issue I mentioned.) For a levee job, I would be a little surprised if either of those conditions are met. The analysis is bound to be expensive if you need to get more info to support it, and it isn't worth the paper to print out the results if the stratigraphy and material properties aren't well established, or at least well bracketed.

Therefore, I would start by looking at how the piles would create a failure, by identifying the chain(s) of events beginning with storage against the levee, initial particle movement, path of transported particles, etc., ending with the breach. If there is a plausible failure mode, look at what you can do to mitigate it. You might find the fix to be much cheaper than the analysis - maybe just a few truckloads of sand and gravel around the piles.