Thermal Resistivity and Underground Cable Sizing for a horizontal directional bore using bentonite

[mostaertt]

We have traditionally open trenched/backfilled or plowed our medium voltage runs from inverter to inverter (approximately 1000 LF). We are looking at utilizing a horizontal directional drilling for our triplexed MV cables (1/0-1250 KCMIL) + ground + fiber in duct. We would drill a 4" bore hole and the back ream while pulling the cables back thru the bore hole. We expect to use a bentonite mixture to keep the bore hole open and to remove the cuttings from the boring.

The question I have is how will the bentonite mixture affect the RHO value we assume for the soil surrounding the MV Cables ?? We typically do soil sampling of the existing soil to determine what RHO value to use in calculating our cable sizing/loading. However, I'd like to know more about how the bentonite mixture will affect the RHO values we should expect to experience surrounding our cables. What are the best practices for dealing with this situation ?

 

[jghrist]

I would expect a thin layer of bentonite to be insignificant compared to the thermal resistance of the duct itself. I would be more concerned about an air gap between the duct and soil.

 

[mostaertt]

The duct only goes around the fiber, not the MV Cables... So for this question - we can probably ignore the presence of the fiber in duct as we may need to run that fiber line separately anyhow.

 

[jghrist]

Does the bentonite remain after pulling the cable back through the bore hole? If not, you should rate the cable as in conduit, not as direct buries because there will be a thermal resistance of the air in the bore hole. If you are calculating ampacity by hand, you can just leave the duct wall out of the calculations. If you are using a computer program, you may have to enter a very small duct wall thickness.

If the bentonite remains, filling the bore hole, or if you assume that the bore hole collapses, you could account for the different thermal resistivity of bentonite (or loose soil) as a separate thermal resistance element. I don't know if cable ampacity software can accommodate this. You may have to check with the software company for support.