Affect of Frozen Soil on Resistivity Measurements 1

[livewire9]

Anyone have knowledge of how frozen dirt may affect measurements of soil resistivity? We need work done in northern Colorado to design the ground grid for a new substation and it appears that some of our recent cold weather has frozen the soils down to about 30".

Christopher Renedo, P.E.
ESC Engineering
Fort Collins, CO

 

[rcw retired EE]

Resistivity of soil is affected by temperature. Fig 18 in IEEE 80 "IEEE GUIDE FOR SAFETY IN AC SUBSTATION GROUNDING" shows variation for a sandy loam soil, 15.2% moisture by weight. The effect is "negligible" above freezing. "At 0 degrees C the water in the soil starts to freeze and resistivity increases rapidly." The curve jumps from about 150 ohm-meter to around 350 right at 0 C and increases from 350 to 5000 ohm-meters between 0 and -15C.

You could pull samples and do lab tests on the soils' top layers.

My field experience is that you can get resistivity numbers for the frozen soil and the layers underneath it using the 4-point Wenner method. But it is very difficult to get a good test connection to the soil. I had to salt the test probes, drill holes, drive deep stakes and used other tricks to get enough test current for the readings to stabilize. The results were suspect, but the wider spacing tests gave reasonable results for the deeper layers.