Insulating gravel vs coarse subgrade gravel
Insulating gravel vs coarse subgrade gravel
(OP)
Hi,
We are designing a grounding system for a pipeline terminal which is being supplied by a 25kV utility substation. The grading drawings for the plant suggest that the backfill material is ‘225mm pit run gravel and 75mm thick coarse gravel layers’ for the whole plant area plus substation fenced area. While running the soil and touch voltage calculations I have the following concern:
The 75mm thick coarse gravel layer is being qualified as an insulating gravel layer (as recommended in Canadian Electric Code, 36-304) by the civil guys. However, from electrical grounding experience, gravel layer consists of clean washed rock which is free of sand and fines (3000Ωm resistivity as recommended by CEC), is un-compacted and is very difficult for vehicles to drive on. From the info we have, it seems that the material being identified as insulating gravel is compacted for easy vehicular traffic which may introduce fines thus decreasing its resistivity.
My question is, can we go ahead with considering this material as a suitable insulating gravel layer or should recommendations be made otherwise?
We are designing a grounding system for a pipeline terminal which is being supplied by a 25kV utility substation. The grading drawings for the plant suggest that the backfill material is ‘225mm pit run gravel and 75mm thick coarse gravel layers’ for the whole plant area plus substation fenced area. While running the soil and touch voltage calculations I have the following concern:
The 75mm thick coarse gravel layer is being qualified as an insulating gravel layer (as recommended in Canadian Electric Code, 36-304) by the civil guys. However, from electrical grounding experience, gravel layer consists of clean washed rock which is free of sand and fines (3000Ωm resistivity as recommended by CEC), is un-compacted and is very difficult for vehicles to drive on. From the info we have, it seems that the material being identified as insulating gravel is compacted for easy vehicular traffic which may introduce fines thus decreasing its resistivity.
My question is, can we go ahead with considering this material as a suitable insulating gravel layer or should recommendations be made otherwise?






RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
Thanks for sharing your experience.
@stevenal
By qualified, I mean that the civil guys are persisting that this coarse gravel top layer is enough to be considered as an insulating gravel layer in our grounding study. But as a grounding engineer, it's my job to recommend if additional insulating gravel with the right properties is required in the substation area.
RE: Insulating gravel vs coarse subgrade gravel
IEEE 80 provides guidelines on how to calculate safe touch/step voltage levels for a given surface layer resistivity, upon which the 3000 ohm-meter crushed rock allowable touch/step potentials in the CEC are based on.
RE: Insulating gravel vs coarse subgrade gravel
The resistivity of a compacted gravel with fine could be estimated roughly 1000 ~1200 Ωm. To obtain the desirable insulating level the thickness could be increased to obtain comparable insulating layer if use clean washed rock which is free of sand and fines. Work with your civil team to excavate extra few inches if require meeting the approved grading and backfill plan.
Although you might face some challenges in obtaining the budget for extra test, here is couple suggestions few steps that could be implemented to mitigate the uncertainties meeting the safety requirement:
a) Perform a wet sample resistivity test on the prospective material to be used.
b) Perform a step and touch potential test on site to verify the calculation.
I hope this help
RE: Insulating gravel vs coarse subgrade gravel
Thanks for your suggestion. We did try completing a test for top soil layers. However early winter frozen soil + highly compact top layer effectively rendered those measurements useless with no budget for further testing. And we cant wait til summer conditions to redo the test as the substation has to be commissioned before that time.
@cuky2000
That really helps going forward. Also it confirms my suspicions that the current gravel top layer will act a pseudo-insulating layer (lower in resistivity than a standard utility-grade gravel of 3000Ωm).
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
And thanks very much for the IEEE paper. Will definitely go through it.
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
Fines reduce the resistivity substantially. Using the tested resistivity of the compacted gravel with fine, determine safe touch/step voltage levels.
I work for utility in Canada, and our standard practice is to use fractured rock with minimum wet resistivity above 3000 ohm-meters. If a tested gravel does not meet minimum resistivity requirements then new gravel source is considered.
RE: Insulating gravel vs coarse subgrade gravel
There are always some smalls and fines, the gradation test will confirm whether the test samples meet your specification.
Specify clear crushed stones not rock. Crushed stones have no rounds and they can set and pack while the rocks don't.
When testing the resistivity you have to use de-ionized water not tap water. No excess water csitting on the bottom of the test box and the way you damping the stones has to be consistent otherwise the results vary a lot.
RE: Insulating gravel vs coarse subgrade gravel
RE: Insulating gravel vs coarse subgrade gravel
How do you spell HUBRIS.
Point out to the civil group that they are not qualified to comment on or to change the grounding specs.
Bill
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"Why not the best?"
Jimmy Carter