This document discusses geotechnical information and its application to electrical grounding resistivity calculations. It defines key terms like resistance, resistivity, and conductivity. It describes how soil resistivity affects grounding design and safety. Measurement techniques like Wenner and Schlumberger are presented. Factors influencing soil resistivity are outlined. Examples of soil resistivity curves and predictive models are provided. The importance of direct soil resistivity measurements for accurate grounding design is emphasized.

1. Geotechnical information and
application to electrical
grounding resistivity calculations
D. Nowicki 12/17

2. Agenda
• Basic reasons
• Definitions and concepts
• Resistivity of soils
• Measurement techniques
• Conical curves
• Examples
• Example calculations
• Quiz and review

3. Important point
• They’ll be times when we discuss detail – this is for future reference.
The important concepts are on the light blue slides.

4. References
[1] Parker, M. E.; Peattie E. G. (1995) Pipe Line Corrosion and Cathodic
Protection. Houston, TX: Gulf Publishing Company
[2] Wright, H.B. AIEE Vol 55 (1936) Formulas for Calculation of Resistance to
Ground Pages 1319 – 1328
[3] Boys, P. (2016) Resistivity Testing for Earthing Safety. IEEE
[4] Sen, P.K.; Mudarres, N.K. (1990) Corrosion and Steel Grounding.
Princeton, NJ; IEEE
[5] IEEE 80 Guide for Safety in AC substation Grounding
[6] IEEE 81
[7] Dr. John Louie, University of Reno, Department of Geophysics Lecture
Series
[8]NOAA National Weather Center Climate Predication Center
[9] Sen, P.K.; Nelson, John, (2002) Steel Grounding Design Guide and
Application Notes

5. Typical areas of where soil characteristics play
a role
• Electrical design and soils
• Resistance to ground – code compliance and personnel safety
• Step potential – personnel safety
• Touch Potential – personnel safety
• Thermal conductivity – duct bank design and heat derating
• Cathodic protection – pipe and underground corrosion
• Soil profiles – resistivity layers, moisture content, prediction

6. Soil resistivity – why does it matter
• Over designed /under designed - cost and safety
• Resistivity is tied to corrosiveness of the soil
• Needed for location of ground beds
• Accuracy of soil resistivity is wide ranging – worse case designs not based on direct
measurement.
• Single point averages, predication models based on generalized data is pretty much
useless
• Models fall apart very quickly given the range of published data
• Using measured values will produce results which are accurate to a design degree.
• Fairly consistent readings versus the models for a given system
• Code compliance
• Ground rods
• Substation, alternative energy and professional distinction.

7. Definitions
• Resistance Ω Ohms opposition to current flow
• Resistivity ρ (ohms per meter, ohms per centimeter ) intrinsic value of all
materials resistance per unit of material
• Apparent resistivity ρ (ohms per meter, ohms per centimeter ) resistivity
estimate of a material given a ½ space geometry. Spheres of radius r.
• Conductivity σ mhos/meter or Siemens equal to 1/ ρ degree of a material
to allow for current flow.

8. Definition
• E=IR or E=I(R+jX)

9. Grounding

10. Grounding

11. Resistance of any material per unit

12. Simple case - one rod

13. Ground rods resistances don’t exactly add as
if in parallel
•
1
𝑅𝑡𝑜𝑡𝑎𝑙
=
1
𝑅1
+
1
𝑅2
+ ⋯
• Reduction of the same resistance rod
• 2 rods 60 % reduction
• 3 rods 40 % reduction
• 4 rods 33 % reduction

14. Grounding
• Substations – 1 Ω or less IEEE 80
• Most others – 25 Ω or less NEC
• Step and touch potential – generally applies to substations, high
energy services pose a risk, however it’s not mandated except in a
OSHA general clause.
• Fence and other structures likely to become energized.

15. Ground rods and soil

17. Some important factors
• There is a minimum resistance to ground achievable based on the soil
resistivity where it is unproductive to add rods to reduce resistance.
(overlapping spears)
• Frozen soil has some of the highest resistivity and should be subtracted.
• If you get the ρ right, these formulas are fairly accurate.
• After a certain distance the connecting cable becomes the major
contributor to reducing the resistance to ground.
• The surface layer and the first 10 to 20 feet of soil - exception – cathodic
protection and anode beds when the soil resistivity trends downward with
depth.

18. Effectiveness of
additional rods

19. Simple case - one rod

20. The resistivity of soils, Archie's Law
• The effective resistivity of a soil is
ρ e =a s-n Φ-m ρw
• Φ fraction pore volume (porosity)
• s is fraction of pores containing water
• ρw resistivity of the water
• A,n, and m are somewhat of a constant (material structures)
• Notice the type of material resistivity is not in this equation. Quartz,
clay, limestone, granite, etc.

21. IEEE 80 – Guide for grounding in AC
substations
• Typical soil resistivities

22. IEEE 80 Tables

23. Apparent soil resistivity of 10 ohms per meter
to 10,000 ohms per meter (wet/dry)
Low High
Soil resistivity p 1000 1000000 Ω cm
Ground rod length driven L 297cm 10' = 297 cm (9.5ft), effective lengths used - subtracts frost depth
Ground rod radius a 0.793cm 5/8"=1.59 cm radius =0.793cm, 3/4" = 1.9 cm, rad =0.95
Frost Depth Fd 0cm 4' = 121 cm, 2' = 61 cm
Two rod separation s 0cm Needs to be greater than length
Wire size raduis a 0cm #4/0 = 1.34 cm radius =0.67cm, #2/0= 1.05 cm, rad of 0.525 cm
Length of buried wire L 0cm 1foot = 30.48 cm
Depth of Buried wire s 0cm Needs to be greater than Frost depth 2' 6" 76.2
Low High
Single R R
3.38 3384.14Ω

24. IEEE 80 Based on a 2 layer model, ½ sphere
geometry, uniform horizontal layer

25. Factors that effect soil resistivity - Top to
bottom
• Porosity (Fractures and pores)
• Pore saturation (% air % gas)
• Hydrocarbon fluid saturation
• Water Salinity
• Clay Content
• Metallic sulfides
• Fluid Temperature - Freezing
• Rock Matrix – clay, swelling clays, volcanic glass, solid granite, metallic
sulfides, vermiculate)

26. Models and predicting soil resistivity
• Measurement of ρ, depth of boundary (vertical and horizontal plane)
• Averages, tables, predication models based general classification of
soil types typically have values so wide as to render them useless

27. Measurement of soil resistivity methods

28. Measurement of soil resistivity methods
• Pole –pole
• Dipole – dipole
• Pole - dipole

29. Measurement of soil resistivity methods
• Wenner
• Schlumberger
• Variations (3 wire, 2 wire)

30. Determining ρ and apparent resistivity
• Wenner Method
• Ρa = 2 π 𝑎
𝑉
𝐼
• V/I is the resistance

31. Apparent resistivity 2 layer model
• Wenner
• As you go wider you go deeper

32. Typical curves (conical model A) increasing
soil resistivity 2 layer
• ρ1 = 10 Ω m
• ρ2 = 100 Ω m
• Z = 10 m (depth interface)

33. Typical curves decreasing soil resistivity
(conical model Q) 2 layer
• ρ1 = 100 Ω m
• ρ2 = 10 Ω m
• Z = 10 m (depth interface)

34. Typical soil curves resistivity (conical model H)
• ρ1 > ρ2 < ρ3

35. Typical soil curves resistivity (conical model K)
• ρ1 < ρ2 > ρ3

36. Typical soil curves resistivity Faults
• ρ1 > ρ2

37. Typical soil curves resistivity Faults
• ρ1 > ρ2

38. Typical soil curves resistivity Faults
• ρ1 > ρ2

39. Graphing, layers
• Put it on log – log paper
• Look for discontinuations, steep versus steady
• Look for high readings
• Resistivity goes up slow and comes down slightly faster.
• Transition points (depths) are near the beginning of curve
• Estimate layers, multilayer of large depths are somewhat rare, it’s not
the material it’s the porosity.
• Use software to verify.

40. Ground modeling software

41. Actual readings
• Approximately all sites within 100 miles of each other

42. 0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
0 5 10 15 20 25 30 35
1
10
100
1,000
10,000
0.1 1 10 100
Raw Values for
Apparent
Resistivity
Apparent
Resistivity in Log-
Log graph format

43. 0
500
1,000
1,500
2,000
2,500
0 5 10 15 20 25 30 35
1
10
100
1,000
10,000
0.1 1 10 100

44. 0
50
100
150
200
250
300
1 2 3 6 12 15 18 24 31
Series2 Series3
1
10
100
1000
0.1 1 10 100
Series2 Series3

45. Practices
• Wenner method is by far the most accurate and common place. Specify NS,
EW spacing distances and center GPS coordinate. Stick with the same units.
• Determine number of layers and apparent resistivity. Error check EW, NS,
and data. Identify faults, vertical transitions, dips. Check data points.
• Estimate single rod to ground resistance based on an assumed high and
low apparent soil resistivity.
• The longer it goes the less the ground rods matter.
• What I’ve done for spacing between rods- any one segment is less than 25
ohms or a recommended value.
• Larger diameter ground rods usually have a bigger effect on lowering the
resistance than driving them deeper depending on the soil layers and
moisture.

47. Predication of steel helical pier resistance
Soil resistivity p 8427 11491 Ω cm
Ground rod length driven L 152cm 10' = 297 cm (9.5ft), effective lengths used - subtracts frost depth
Ground rod radius a 7.62 cm 5/8"=1.59 cm radius =0.793cm, 3/4" = 1.9 cm, rad =0.95
Frost Depth Fd 0cm 4' = 121 cm, 2' = 61 cm
Two rod separation s 0cm Needs to be greater than length
Wire size raduis a 0cm #4/0 = 1.34 cm radius =0.67cm, #2/0= 1.05 cm, rad of 0.525 cm
Length of buried wire L 0cm 1foot = 30.48 cm
Depth of Buried wire s 0cm Needs to be greater than Frost depth 2' 6" 76.2
Low High
Single R R
29.83 40.68

48. Predication and actual measured results
• Grounding report came up with two values – based on a high and low
soil resistivity for both a pier and a ground rod.
• Values for apparent resistivity came from a soils report.

49. Predication and actual measured results
• For a triad (3 rods connected equidistance) model predicted 4.48 to
5.59 ohms
• Measured 4.9 ohms
• For a helical pier predicted 29.6 to 40.68 ohms
• Measured 36 to 40 ohms
• Based on published data values ranged from 2 ohms to 200 ohms.

50. Geotechnical reports
• Don’t get a single point resistivity number or an average.
• Asked for soil resistivity.
• Look at moisture, depth, compaction and porosity.
• Brief check of materials for special cases– swelling clay’s, metal
sulfides, bedrock.
• Determine a high and low value for soil resistivity.
• Check with software.
• Do a single rod calculation
• Check step and touch potential in ETAP.
• Spend most of the time estimating in the upper levels