Soil Resistivity Measurement

[xabproject]

Client's specification calls for soil resistivity to be measured before and after backfilling if the thickness of back-fill exceeds 1m. Wonder why this should be done!?

Does this have any reference to IEEE 80 / 81? I am working is IEC area.

Thanks in advance!

If there is a separate forum, please advise re-direction.

Rgds

 

[Humble2000]

We install ground grid .6m below the grade.
What depth is the grid going to be installed?

 

[rcw retired EE]

The ground grid design may be based on soil resistivity tests done prior to excavation and fill. Testing after installation verifies the design.

One practical problem is that the presence of ground grid conductors, buried metal pipes, rebar and foundations can all affect the soil resistivity measurement. It becomes difficult after construction to measure the soil resistivity because of the other objects in the soil. A soil sample can be sent to a lab for a measurement, but I never got good correlation between field measurements and lab results.

 

[jghrist]

If the ground grid is in the backfill, the grounding design needs to consider the resistivity of the backfill. Resistivity measurements will include various depths, so there is no real need for a separate measurement before backfill, except to allow design to proceed before the backfill is completed.

 

[dpc]

It would be good to know the resistivity of the native soil since that is what the ground rods will mostly be in.

 

[EEJaime]

The respondants above assume that you are talking about soil resistance for a grounding system, which is probably the case. I just wanted to be sure you weren't talking about soil thermal resistivity for feeder sizing calculations, which would be an entirely different matter.

We just had a hell of a problem with a large feeder installation wherein the contractor instead of the concrete encasement specified, backfilled with sea sand, (native "soil"), and it turned out to have a thermal "Rho" of almost 400 Deg C-cm/watt, whereas the conductor ampacity tables in most code books are based on 90 Deg C-cm/watt. This resulted in excessive heating in the duct banks and premature feeder failure. And of course the inevitable, interminable and exasperating conversations with those of the legal ilk.

 

[advidana]

Why Don't You ask the person that wrote the specification. Some times they just copy another persons specification because it is faster that way to get the spec out on time.