maximum burial depth

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ericsherman37

ericsherman37

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Oregon Coast
The minimum burial depth thread got me thinking... is there a maximum burial depth? I mean, is there some point where the weight of the covering dirt would crush something like PVC pipe? Or can you just throw it into the abyss and bury it?

Either way, I imagine that you can bury that stuff pretty deep.
 
Most of the force that might crush the conduit comes from "live" loads on the surface (like the concrete truck that broke my water line). Weight of the backfill is minor in comparison. As the burial depth gets deeper the weight/force of anything on the surface like a truck tire gets spread out reducing the force on the conduit. That is sometimes why you?ll see 2-3 feet of compacted earth brought onto a jobsite as a temporary roadbed for a heavy lift or transport.

The deeper the burial, the less likely there will be damage unless you drop heavy backfill on top of it from the top of the excavation.

A deeper conduit may be below the frost line and have less chance of damage from frost heave. We have had a few duct banks get broken open when they got too shallow in cold country.

Deeper conduits require more ampacity derating on power circuits (6% per foot) if you are using good engineering practice or following the suggestions in Anex B. This creates a practical maximum depth.
 
Thermal resistivity vs depth

Thermal resistivity vs depth

A general equation is:
T = p/2/Pi*ln(4*D/d)
where p = soil thermal resistivity
D = cable Depth
d = cable diameter

But you have to consider that as the depth increases, the ambient temperature decreases and the moisture content of the soil increases, improving the soil resistivity.
 
Yes and No

Yes and No

Greater depth means more thermal resistance to getting heat out of the wire and up to the surface. Yes, greater depth MAY mean more moisture so better thermal performance. But, in clay soils, you can drive the water away from the conduit and get thermal runaway. Is this a serious issue, probably not. But, engineers have to address the possibility.

Good example is the directional bore under a river through rock. Even though you are below the water table, you are in rock. With enough heat from the conductors, you can have problems.
 
beanland said:
Greater depth means more thermal resistance to getting heat out of the wire and up to the surface.
Click to expand...
Generally speaking, that is. Nothing says the heat has to go to the surface. It is trying to go anywhere that is cooler, which may be down (not to the magma).

The only way to know if the resistivity gets better or worse as you go deeper is to measure it. It can go up and down depending on the different soil layers and depths. But, we can make some general assumptions about the soils in certain areas and use conservative rules of thumb.
 
Cold Fusion said:
According to the arctic engineering class I had to take for my PE, the ambient temp increases with depth.

cf
Click to expand...
You have to read the fine print.

Here, let me increase the font size so you can read it:...as the depth increases, the maximum ambient temperature decreases...:D

On a large scale, we have liquid rock in the earth's core so of course we generally get hotter as we move toward the core. I think miners use about a 5 degree increase per 1000 feet (?). But near the surface where we play with cable, the soil can be seasonally cooler or hotter than the surface ambient. Sometimes the heat goes up, sometimes it goes down.
 
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