duct bank question

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transman2

Senior Member
Location
Brooklet,Ga.
Are duct bank spacers designed for concrete only applications or can they be installed without? An EE told me that since they were not adding concrete to the duct bank that spacers could not be used because the Civil Eng said they needed to meet certain "what ever" to patch the 3' wide asphalt where the conduit lay. Can chairs cause them from being able to pack the dirt down to a certain hardness and cause a bad patch. I was wanting chairs as the specs said but the EE came back with this answer. Has anyone heard of this before? Throwing 30 conduits in a hole on top of each other seems crazy with no spacing. 480V feeders.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Concrete is fluid enough to flow around the upper ducts and fill in the entire cross section without major (or even minor) voids. In some cases this might be improved by using vibration as well as simple tamping of the poured concrete.
Dirt, on the other hand, does not flow as easily and might well leave voids which would put the entire weight of the fill and traffic above it on some of the ducts.

If you were to fill to the top layer of the duct bank with clean sand, I do not see the engineer having grounds for objection, but simply dumping dirt back into the trench is not a great idea, IMHO.

Note that the problems of ordinary dirt not filling the cross section completely will also apply when the ducts are resting on top of one another, the voids will just be smaller, so I think the engineer may be missing the point if he is recommending that.
 

luckylerado

Senior Member
Are duct bank spacers designed for concrete only applications or can they be installed without? An EE told me that since they were not adding concrete to the duct bank that spacers could not be used because the Civil Eng said they needed to meet certain "what ever" to patch the 3' wide asphalt where the conduit lay. Can chairs cause them from being able to pack the dirt down to a certain hardness and cause a bad patch. I was wanting chairs as the specs said but the EE came back with this answer. Has anyone heard of this before? Throwing 30 conduits in a hole on top of each other seems crazy with no spacing. 480V feeders.

I am a little surprised that a 30 raceway ductbank under a paved area does not require concrete encasement or at least a flowable fill.

I can see how a multi-tiered set of chairs would cause excessive voids and would change the bearing capacity of the material above supporting the paved areas. Even sand would struggle to firm everything without vibrating it in.

Are you taking a design responsibility on this? Such that you might be on the hook if the road settles or cracks in the near future?
 

luckylerado

Senior Member
What does your product data sheet say?

Carlon says:
"...Carlon Snap-Loc duct spacers provide stability,
consistent separation and relieve direct stress for
duct materials encased in concrete, and direct
bury applications...."

BUT goes on to say:

"IMPORTANT:
1. The use of duct spacers for direct burial may result
in excessive point deflections unless proper design
engineering is applied, such as the proper compaction
of the appropriate backfill material.
2. Carlon is not responsible for Snap-Loc Spacers used
in direct bury applications... design engineers and
contractors are responsible for the design of the
installation."

Allied product data says:
"...Used in both EB and DB installations..."

IPEX:
"...IPEX’s interlocking spacers provide uniform separation between the rows and columns of conduits and when assembled with conduit create a stable structure for the concrete...."
 

Smart $

Esteemed Member
Location
Ohio
Are chairs or supports designed for concrete, slurry, or flowable fill only.
IMO no... as long as the product data sheet doesn't say otherwise. Most of us are not privy to a lot of design parameters though. We are usually only concerned with listing requirement and compliance. You'll have to refer to product data sheet for specifics of permitted or not uses.

That said, a soil backfill job with stacked conduit bank should be done in layers equal in height to each layer of conduit and compacted to spec'. By the time you do backfill layering, there really isn't much purpose in using chairs.... but I don't see why you can't.
 
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