small pond outlet culvert - anti seep collar?
small pond outlet culvert - anti seep collar?
(OP)
Hello,
I am working on the design of a culvert that serves as the primary outlet of a stream-fed pond. I have sized the structure as an 83" x 57" pipe arch. 100-yr storm flow is 163 cfs, and the pond rises 3.25' during the storm. The embankment of the pond is about 10' tall at its highest point, and is about 60' wide.
I have questions regarding anti-seep collar design:
1. The literature I've read so far has rejected antiseep collars for large structures in favor of filter diaphragms. Is my smaller structure OK to use ASC's, or should I be thinking about a filter diaphragm design instead? If ASC's are still fine for my structure, then:
2. The soils onsite are clayey, and consideration for a clay collar was asked for. Is there a spec for the type/thickness of clay used in an antiseep collar?
Thanks in advance,
I am working on the design of a culvert that serves as the primary outlet of a stream-fed pond. I have sized the structure as an 83" x 57" pipe arch. 100-yr storm flow is 163 cfs, and the pond rises 3.25' during the storm. The embankment of the pond is about 10' tall at its highest point, and is about 60' wide.
I have questions regarding anti-seep collar design:
1. The literature I've read so far has rejected antiseep collars for large structures in favor of filter diaphragms. Is my smaller structure OK to use ASC's, or should I be thinking about a filter diaphragm design instead? If ASC's are still fine for my structure, then:
2. The soils onsite are clayey, and consideration for a clay collar was asked for. Is there a spec for the type/thickness of clay used in an antiseep collar?
Thanks in advance,





RE: small pond outlet culvert - anti seep collar?
RE: small pond outlet culvert - anti seep collar?
Here's an example of what I'd do: If you have a 10-ft high embankment and there's an 8 ft crest width with 3H:1V sideslopes the run of pipe would be about 70 ft. I'd place the upper 2/3rds of the pipe in a concrete cradle to the springline and backfill above the springline with compacted structural fill (i.e., just like you'd use for the embankment). For the lower 1/3rd, I'd surround the pipe with at least 12 in of open-graded aggregate. This may warrant the use of a separation geotextile. Then I'd backfill over the pipe drain with normal embankment fill. At some point where the pipe daylights by the toe of slope, I'd include a perforated pipe in the open-graded aggregate to allow the free flow of water from the aggregate.
This will likely be all that you need.
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?
RE: small pond outlet culvert - anti seep collar?
The structure is pretty small. The dam impounds about 6 ac-ft, which puts us well under our state's jurisdictional threshold (12 ac-ft).
The pond is an existing farm pond that failed in the past (right now the location of the embankment is a chewed-out part of the stream); our proposal is to repair the breach, provide new drainage, and build a driveway/rural road on top of it. The 10' depth is at the breach; the length (perpendicular to flow) of the embankment is about 30', ending where it meets existing grade.
The permanent pool of water is set at the invert of the pipe, the only soil saturation around the culvert will occur during (big) storm events.
Does any of this change your thinking?
RE: small pond outlet culvert - anti seep collar?
RE: small pond outlet culvert - anti seep collar?
During periods of high flow (i.e., when the water is above the normal pool elevation and the pipe is running full), you will have transient conditions where flow will want to travel along the outside length of the pipe. This flow will result in seepage pressures. You or your contractor will have diffuculties compacting the backfill of the pipe at the haunches (i.e., below the springline) and these seepage pressures will worry away at that poorly compacted backfill. The best way to correct this is by using a concrete cradle (upper 2/3rds) and a pipe drain (lower 1/3rd). This is an appropriate design for a regulated or unregulated dam.
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?
Last I knew, the state of Florida required seepage collars on some dams. When I worked there (>20 years ago), we just left a space so they could have the concrete bedding envelop the lower part of the collar. Then, we required small equipment (Wackers) around the upper part. If constructed well with proper compaction around them, I'm not convinced that collars are a major detriment (as opposed to being approximately worthless), but proper compaction around them is often neglected. It's less trouble to bed the pipe in lean concrete and wrap a filter diaphragm around it.
Regards,
DRG
RE: small pond outlet culvert - anti seep collar?
In Virginia our Stormwater Management Handbook also references the use of anti-seep collars; however there is an out in that you can actually get an engineer to design something else. In many states, it remains possible for a barber to design a small embankment but not for an engineer to cut hair. Go figure. . .
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?
RE: small pond outlet culvert - anti seep collar?
On the matter of tolerable risk, bear in mind that the OP is a chemical engineer and may not have any perception of risk with respect to using anti-seep collars or poorly compacted pipe backfill.
I've seen the results of failure and it's not pretty, even when it's not a matter of human risk. Sometimes it's just big amounts of sediment that end up in the adjacent wetlands. Something about environmental regulators and sediment in wetlands that creates a bad day, eh?
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?
I've seen the effects of a dambreak with just sediment inundating downstream farm fields. Three lawsuits later and everybody has been sued and the lawyers are now dreaming about the fourth one - the cleanup would have been cheaper than the attorneys fees.
RE: small pond outlet culvert - anti seep collar?
Penetrations through dams are a different matter (in my mind). If you have a dam that does not need internal drainage for the typical embankment condition, then the only precaution is to control drainage along the penetrations. For this case, that's when I'd just consider the use of a pipe drain for the lower 1/3rd of the penetration.
Hopefully, we have not confused the OP at this point. . .
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?
RE: small pond outlet culvert - anti seep collar?
My thoughts at this point are that since I am only dealing with limited periods of innundation (greater than 1-year storms), I will be OK with an anti-seep collar.
All that being said, I am now looking into a precast concrete box culvert as an alternative. Will anything change if we decide to go this route as opposed to a metal pipe arch? My preliminary thoughts are that with gravel bedding, I will be providing a preferental pathway for any seepage that would occur and due to the greater mass, would provide a more stable structure to build my road on top of.
RE: small pond outlet culvert - anti seep collar?
Irrespective of whether you use a pipe or a culvert, I'd do the same thing. I'd envelop the lower third of the culvert in open-graded aggregate and I'd make sure that the upper two-thirds of the box culvert was in intimate contact with the subgrade soils and backfilled with embankment fill. I would not "bed" the culvert in open-graded aggregate for the full run as that would certainly create a preferential flow path.
It never hurts to ask local engineers what they do either. . .
f-d
¡papá gordo ain't no madre flaca!
RE: small pond outlet culvert - anti seep collar?