PVC lined metal ducts under floor slab

[BayPointArchitect]

Does anyone know where I can find a standard detail that illustrates how these are installed?

I have a particular concern that when the trench is filled with concrete to provide minimum 2" cover all around...

the pvc lined metal duct with become buoyant and pop up out of the concrete with no hope for setting it back into place. So how do these underground ducts get anchored down?

Thanks

ICC Certified Plan Examiner

Registered Architect

 

[Mark K]

Circular sheet metal ducts below the slab were used on several projects I engineered. These ducts did not have the concrete cover but I believe the issues would be the same since the concrete cover would only serve to protect the sheet metal.

I do not believe that it is feasible to place the concrete in one pour. I believe that you would have to place the concrete at the bottom first and then later try to place the concrete on the sides and top possibly in several pours. Given that you do not want to build forms it may make sense to use shotcrete.

With or without concrete cover these systems carry load by arching action in the soil. This arching action requires good compaction of the soil around the underground duct.

The weakness of the system is that it is sensitive to heavy point loads on top of the duct such as for example a heavy truck driving over the duct during construction. Two strategies to deal with this include increasing the depth of the soil on top or thickening and reinforcing the concrete slab to carry the point loads. I definitely would get input from the geotechnical engineer on the compaction of the soil.

 

[BayPointArchitect]

These ducts will be located inside and below insulated foundation and heated living spaces.

Never-the-less, to minimize the effects on joints related to thermal expansion and contraction, I am calling for approximately R8 or 2 inches of insulation all around heat supply duct. My guess is that if I don't specify insulation around the underground ducts, I should not expect insulation.