Sanitary pipe under detention pond

[SMIAH]

Reviewing plans from another engineer.
A detention pond (with a permanent pool of 2') is to be constructed over a 24'' sewer pipe.
There is less than 4' of cover over that pipe (bottom of the detention pond).

Water will freeze in winter (Canada). Do you think there should be some special considerations for this particular design?

 

[civilman72]

What is standard burial requirement for sewer in your district?

While it's possible that the frost line will not be as deep under the pond or that flows in a 24" pipe will be sufficient to prevent freezing, it should be properly documented. The easiest way to address the issue is to insulate the pipe.

 

[SMIAH]

Standard burial is 6' here.
This conduit will be 4' + 2' deep (2' water).

 

[jgailla]

That doesn't sound right to me. Water has high thermal conductivity. 2 feet of water is not the same as 2 feet of soil. This is coming from someone in Florida, where we don't have a frost depth.
I would be concerned about the sewer running under the pond. What happens if the sewer breaks and leaks into the surface water?
This would not be allowed under any jurisdictions I've worked in.

 

[SMIAH]

I have many concerns aswell. It's not usual design.

 

[cvg]

24 inch sewer main? that is a major interceptor. I really doubt that the sanitary district would approve of that. Is it an existing pipe? If so, then how did they get permission to build on top of it?

 

[Twinkie]

This is just a bad idea no matter how you look at it. 24" sanitary is a huge line, is there not an easement above it that prevents the basin? Is there enough fall in the pipe to route it around the footprint of the basin somehow?

 

[SMIAH]

There would be enough fall to route it outside the basin (e.g. under the access road).
Regarding freezing, the conduit has to be isolated (wasn't shown on the plans).

I don't like this design at all (looks like I'm not alone). Made my point to my superiors and waiting for more validation from the engineer signing the plans.

 

[beej67]

If I were forced by circumstance to show this on a plan, by not being able to route it any other way, I'd probably try to come up with a way to sleeve it inside a larger pipe. If there's enough fall in the pipe to route it around, I'd definitely do that instead.

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[SMIAH]

My superior is not too concerned about this.
Nor the engineer in charge of the plans.

They say we'll be able to divert the sanitary and repair it if necessary. And we rarely have to repair a 24'' sanitary pipe.

Routing it would be extra $$$ and that basin couldn't be placed somewhere else (i.e "perfect" spot).

 

[jgailla]

CYA. Good luck. I can't believe someone will approve this.

 

[SMIAH]

Indeed I will.

 

[cvg]

are they concerned about the sewer pipe floating to the surface of the pond? since sewers rarely flow full, bouyancy is a significant concern

 

[SMIAH]

It's a Bar Wrapped Concrete Cylinder Pipe.