Pond outlet culvert-HW & anti-seep collar question 1

[kedpe]

I want to use a 12" pipe to outlet a detention pond. It is only 30' length going thru the embankment. The profile will slope up 4:1 to provide 2' of cover over 10' of the pipe, then slope back down 4:1. I planned on using a HW at the inlet and outlet end. When is anti-seep collar needed?

 

[BelspringKen]

Check with the state agency for the site in question on this. Depending on where you are working, there is a good chance you will need the anti-seep collars.

I don't know where you are working - in some states you need a minimum of a 24" pipe (3' area) to outlet a pond.

Again - check the state or local regs.

 

[LHA]

Pp 59-61 here have an easy to follow method:

http://164.156.71.80/VWRQ.asp?docid...aspx?fs=2087d8407c0e00008000047b0000047b&ft=1

Engineering is the practice of the art of science - Steve

 

[fattdad]

Just for an opposing viewpoint (circa at least 20 years ago): don't use anti-seep collars! There have been dam failures directly related to the use of anti-seep collars, in spite of how they seem to help. The fact is that compaction surrounding the anti-seep collar is never as uniform as you want it to be. As a consequence, the hydraulic conductivity, the shear strength, the bouyant unit weight, etc. is less than favorable for the seepage forces and you don't quite get the "weighted creep ratio" that you are hoping for. Rather, design your penetration using a concrete cradle for the upper 2/3rds of the run and a gravel blanket for the lower 1/3rd of the run. For the concrete cradle, overexcavate the pipe bedding chair up the pipe and pour the cradle to the spring line. Now you have a good surface for placing the compacted backfill (i.e., don't have to worry about compaction below the haunches). For the gravel bedding, overexcavate the pipe bedding, place gravel (yes you can use filter fabric first if you must) below the pipe, beside the pipe and over the pipe. Then place compacted backfill.

Any body in their right mind will ask, "heck, the water will just pour through that gravel, eh fatt-dad"? Well, I guess. What happens actually is whatever water does make it along the upper 2/3rds will issue in to the gravel bedding but the confining stress will be too great to exceed the critical gradient (i.e., and therefore begin piping failure). I'd look at this before I was bullyed by a local jurisdiction into using seepage collars.

f-d

 

[cvg]

ditto to fattdad.

Collars are not allowed around here for outlet pipes through dam embankments. A filter diaphram is required, regardless whether any of the pipe is placed in a cradle. Make sure that your granular filter material is filter matched to the structural fill. this might require sand instead of gravel or a two layer granular filter.

 

[GeoPaveTraffic]

fattdad is right on.

I have also seen the pipe encased in concrete for the upper portion of the run instead of just placing concrete up to the spring line.

Remember that the concrete cannot be bank poured. It is important to COMPACT the soil backfill/fill up against the concrete to reduce the seepage.

With all of that said, if I'm reading the original post correctly, there is less than 10 feet of head on the pipe and the side slopes are 4(H):1(V). With that low of seepage head and the length of the seepage path, it is unlikely to matter how the pipe is installed unless the soils are very errodable.

 

[fattdad]

Quote: With that low of seepage head and the length of the seepage path, it is unlikely to matter how the pipe is installed unless the soils are very errodable.

I would agree other than to say, I worked on a job with very similar conditions and during a storm (i.e., water just pouring across the outlet structure and high gradients throughout), there was a catistrophic failure along the pipe. I wish I could post a photo, but the upshot of the whole thing related to unsuitable compaction along below the spring line of the pipe. At least a concrete cradle to the springline confirms the presence of something in that little area you can't see and allows for a proper surface to place the compacted soil trench backfill.

f-d

 

[rconner]

I noticed some interesting pictures at

http://www.mde.state.md.us/assets/d...nduit Guidelines for April 2003 workshop.pdf.