70 ft deep, 8 ft diameter manhole concrete and foundation design 1

[sengineering]

Hi,

I'm working on designing the walls, rebars, top and bottom slabs for a 70' deep, 8 ft diameter manhole considering H20 loading (meaning there is a minimum of 32 kips truck load on the top slab). I've never designed a structure like this and i'm wondering if there are major issues to look out for.

 

[nackra]

here are some references to get you started:

http://www.concretepipe.org/pipe-box-resources/design/design-data/

contact the precast concrete plant near your project to review your design conditions

 

[sengineering]

Thanks for the reply. I did, no one does manholes that deep, so we have to do a cast in place. At least for the bottom 30 ft of it & they don't deal with the foundation.

 

[JedClampett]

You need to design this as a pipe (just hoop stresses) and ignore the effect of the base slab on it. See thread507-429011 for a similar case. For the base slab, put the truck tire on the lid, carry its load down to the slab, add in the dead load of the shaft and design for that load divided by the area. It's such a short span that a 12 inch base is likely OK if (big if), flotation is not a concern. Then the thickness and diameter is probably controlled by providing enough weight to keep the thing from floating.

 

[BridgeEI]

I'm having to do something very similar but making sure it meets AASHTO. I'm assuming the bearing strength of the concrete governs the design. Does that sound right or is there another place in ACI that spells it out? Im having to use AASHTO LRFD as the design code and its very silent on situations like this...

 

[Sawbux]

Try this link:

http://www.concretepipe.org/wp-content/uploads/2014/09/DD20_CircularPrecastConcreteManholes.pdf

Good info on concrete manhole design.

With a 70 foot depth, how is it going to be installed? Pretty deep for digging out & shoring. Possible caisson construction?

Why so deep? Are the collection pipes that deep, or do you just need the volume?

With this depth, you probably won't need to worry about buoyancy as there should be plenty of skin friction from soil to resist.

 

[TehMightyEngineer]

I'm not sure why cast-in-place should work but precast would not. The only thing the depth will do is maybe require special detailing on the base slab and any pipe penetrations. The standard manhole riser sections should be sufficient up to absurd depths as it's all compressive hoop stress.

BridgeEI: I'm surprised that you're needing to look at AASHTO. I've never had to design a manhole or other buried structure to AASHTO that I can recall. Only ASTM C478 and ACI 318 (maybe ACI 530 on occasion). Only culverts and at-grade structures have ever needed AASHTO design.

Sawbux: Good link. I also agree; why so deep?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[sengineering]

Thank you for your replies & links.

The pipe has to go through a hill and thats why the manhole is so deep. I have checked precast calculations from manufacturers and their manholes don't work for depths below 50 ft.

I also don't know if caisson would work, I've look at a lot of resources and it doesn't seem like the best option. I couldn't find enough information on method of construction. But if I use cassion for construction the bottom slab can only be as big as the manhole and we have to rely on skin friction. Right now the slab comes off as 16'x16' and 24" thick; I'm probably over designing it a bit...

 

[TehMightyEngineer]

I assume the 50 ft limit is driven by the base section? Have them construct a custom base section that can take the soil pressure and get you the anti-floatation properties you need.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/