Outlet pipe sizes in retaining walls

[jplay2519]

I have a 14' tall granite rubble wall that I've designed and on site there is a 60" outlet pipe they want to run through the wall. I'm not sure how that would work. I think that at the location of the pipe there is no soil so there is some help there but that's a pretty big hole, almost half the height of the entire wall. I'm thinking about placing reinforcing around the hole and also providing a concrete collar around the pipe. Are there any guidelines on max sizes for holes in walls like this?

 

[JAE]

Did you mean 6' or 60'?

No guidelines that I'm aware of - just engineering principles.
Your idea of a concrete collar to protect the pipe from shifts in the rubble wall and crushing pressures sounds appropriate.

Design the concrete donut to take vertical crushing force from the weight of the rubble above it.



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[jplay2519]

I meant 60" so 5'. I totally did not proofread before posting that. Thanks for your response though!

 

[bridgebuster]

The FHWA has a manual on rockery walls, don't recall if it has any info on pipe openings. If you can't find it on line I'll post a link later when I get home tonight.

 

[XR250]

I can't add anything other than JAE needs glasses and jplay does not need to apologize as it say 60" :>

 

[LittleInch]

If you look carefully jplay edited his post. The original said 60 feet...

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[KootK]

Buster's reference: Link. Generally great but doesn't speak to this. How big and rectangular is your rubble? Is it even right to think of it as rockery? Any positive tieback into the retained soil?

I don't envy you this problem. Feels like you're heading for cowboy judgment territory. I'd recommend placing a call to somebody who manufactures related product like redi-rock. One would think that their engineering department has dealt with this before. When is picture



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[JAE]

XR250 - edited post. It was originally 6'

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[jplay2519]

It's definitely rockery, they are using a granite rubble wall so it won't be a segmental wall but granite blocks mortarted together. It's about 120" wide at the bottom and 18" wide at the top, front batter. We're not providing any geogrid, the original design was a basket wall with geogrid and it failed and left ripped away from the geogrid so they are requesting a gravity wall. I would have preferred a concrete cantilever wall but sometimes you have to provide what they ask for. The wall is about 120 feet long and we're at 40-50 feet mark along the length. I'm trying to get the elevation of the pipe to determine if it's near the top or bottom (yes this should be on the friggin plan but the plan stated 24" pipe so clearly it's not up to date haha)

JAE was definitely correct I said an impossible 60' in the original post

 

[oldestguy]

If this pipe will eventually carry water with much of the full area used, there should be a design with details needed for headwalls at outfalls, such as cutoff rings to prevent seepage erosion alongside the pipe, erosion protection outside of outlet, etc.

 

[LittleInch]

A diagram or sketch would help here a lot, especially X section, but I'm not sure what exactly your concern is?

The pipe (small tunnel) is hopefully man enough for the over burden pressure but in terms of the wall it just replaces some of the stones.. If it was a hole that needed support then that's a different matter, but I'm having a little difficulty understanding the wall and its cross section.

Is this a 90 degree triangle wall or an equilateral triangle wall?

Could you use gabions behind the front surface to give you more stability?

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[jplay2519]

I've down a 30" pipe but that's because this was the initial design, the pipe is for drainage of water from the site not a continuous full flow of water.

 

[bridgebuster]

Thanks for posting the link KootK

 

[cvg]

I would be very careful about settlement behind the rockery which could cause the pipe to leak with almost certain catastrophic result. Perhaps the concrete encasement should extend a lot farther.

I think you need to consider a cutoff wall to handle underseepage and I am wondering how you intend to keep the rock on top of the pipe from sliding off. Given all that, perhaps a 14 foot high headwall with a cutoff wall and apron would be a better idea.

 

[LittleInch]

OK, so this isn't really a rockery or a "rubble" wall, but a weak concrete wall with lots of big rocks in it - " granite blocks mortared together". Correct?

I can't see a pipe causing any really issues other than being a potential leak path and lack of friction for the rock mortar wall.

A few plates or bars welded on ( if it's metal) or rouging up the concrete it it's concrete to get a good key would seem like a good idea.

Can only assume this large pipe won't have any expansion or thrust / movement?? That could bring down your wall.

If it's a thin steel pipe then check the overload capacity, but if it's buried in soil coming into the wall it's almost certainly OK.

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