Soil resistivity is the input that weighs most in the resistance of a grounding system. It depends mainly on moisture and salt content; temperature matters only near freezing. IEEE 80 gives orders of magnitude by soil type, from 10 Ω·m for wet organic soil to 10,000 Ω·m for bedrock, and requires measuring the actual value at the site.
Short answer. Soil resistivity, in ohm-meters (Ω·m), measures how much the ground opposes current flow. It is the input that weighs most in the resistance of a grounding system and in step and touch voltages. In soil, conduction is mainly electrolytic, so it depends on moisture and dissolved salts. IEEE Std 80-2013 gives orders of magnitude, but makes clear that the design value is measured at the site.
The soil as a grounding medium
Clause 12.1 explains that a buried electrode behaves as if the soil were both a conductor with resistance and a dielectric. Except for high-frequency waves with a very steep front in highly resistive soils, the charging current is negligible compared with the leakage current, and the soil can be represented as a pure resistance.
What affects resistivity
| Factor | Effect according to IEEE 80 |
|---|---|
| Voltage gradient | Does not affect it as long as it stays below a critical value of the order of several kilovolts per centimeter. The grid is designed with much stricter step and touch limits, so the gradient always stays below that |
| Magnitude of current | A high, prolonged current can dry the soil near the electrode and raise its resistivity. A prudent value cited by Armstrong is not to exceed 200 A/m² for 1 s |
| Moisture | The resistivity of most soils rises sharply when moisture falls below 15% by weight of the soil. It barely changes above roughly 22% |
| Temperature | Almost negligible above freezing; at 0 °C the water freezes and resistivity rises quickly |
| Salts, acids and alkalis | They can change it considerably; for example, common salt markedly reduces the resistivity of a soil with 30% moisture |
| Layered structure | Soil is rarely uniform in depth and in area; there are almost always several layers, each with a different resistivity |
The guide warns that its moisture, temperature and salt curves are meant to show the effect, not to calculate. Actual resistivity is determined on site with tests such as those in IEEE Std 81.
Typical ranges by soil type
Tables in the literature give ranges by type of soil and rock. IEEE 80 includes the one by Rüdenberg, which has the advantage of simplicity (clause 13.2):
| Soil type | Mean resistivity (Ω·m) |
|---|---|
| Wet organic soil | 10 |
| Moist soil | 100 |
| Dry soil | 1000 |
| Bedrock | 10,000 |
These are orders of magnitude. When the design depends on resistivity, the guide itself says that estimating by soil classification gives only a rough approximation and that actual measurements are imperative, taken at several places in the yard. It also recommends recording the soil temperature and moisture when measuring, and any known buried metallic objects in the area. How it is measured in the field is covered in resistivity measurement with the Wenner method.
Why the soil below matters
A site with uniform resistivity over its whole area and to great depth is uncommon. There are normally several layers with different resistivities, and sometimes lateral changes too, which are more gradual than the vertical ones. Soundings and geological studies help to understand the composition and resistivity range of the site. How it is measured and interpreted for design is covered in clauses 13.3 and 13.4 of the guide. When resistivity changes with depth, a two-layer soil model is used.
Soil treatment to lower resistivity
Sometimes the desired resistance cannot be reached by adding more conductor or more rods. Clause 14.5 proposes another route: increase the effective diameter of the electrode by modifying the soil around it. The layer of soil closest to the electrode contributes most of its resistance to remote earth, which is why treating it works. The guide describes four options:
- Salts (sodium chloride, magnesium and copper sulfates, calcium chloride) around the electrode. Authorities may not allow them because of the risk of the salt migrating to neighboring areas, and the treatment must be renewed periodically.
- Bentonite, a natural clay. It is non-corrosive and stable, with a resistivity of 2.5 Ω·m at 300% moisture. It does not migrate because it is part of the clay. With enough water it swells to as much as 13 times its dry volume and adheres to surfaces. It depends on soil moisture: in a very dry environment it can shrink and separate from the electrode, which increases its resistance. If left exposed to direct sun, it tends to seal over.
- Chemical electrodes: a copper tube filled with salt, with holes through which moisture enters and dissolves the salt, and the solution leaves into the soil. They are installed in a bored hole and usually backfilled with treatment material.
- Soil enhancement materials, some with resistivity below 0.12 Ω·m, placed in a bored hole or a trench, dry or premixed as a slurry. Some are permanent and do not release chemicals into the soil; others are mixed with the local soil and leach out over time, so their resistivity drops with time.
The gravel layer and moisture
The gravel surface layer, 0.08 m to 0.15 m thick, slows the evaporation of moisture and limits drying of the upper soil layers during prolonged dry periods. Its other effect, increasing the contact resistance of the feet, is covered in clause 7.4 of the guide.
We measure your grounding system
We measure the resistivity of your site with arrays at several spacings and at different depths, and interpret it for the grid design. Field measurements are done in Colombia, or elsewhere on request. Our free grounding resistance tool has a “Resistivity” tab where you can average your Wenner readings and fit a two-layer model. See our testing and commissioning service or write to us through contact. If you need the grid design with that data, see our engineering design and studies service.
Related guides
- IEEE 80: Substation Grounding Safety
- Soil Resistivity Measurement: Wenner Method
- Two-Layer Soil Model from Field Measurements
- Ground Resistance Test: Fall-of-Potential and IEEE 81
- Grounding of Distribution Substations: Design and Tests
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 12.1 to 12.4 (soil as a medium, effect of gradient, current, moisture, temperature and chemical content), 12.5 (surface layer), 13.1 and 13.2 (soil structure investigation and resistivity ranges; Table 8) and 14.5 (soil treatment).
- IEEE Std 81, IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System.
Frequently asked questions
Can a resistivity table be used to design the grounding grid?
No. IEEE 80 states that estimates based on soil classification give only a rough approximation and that actual resistivity tests are imperative, at several locations in the yard. Tables are useful only to get an order of magnitude.
How much does resistivity change with temperature?
Above freezing the effect is almost negligible. At 0 °C the water in the soil starts to freeze and resistivity rises rapidly.
Is treating the soil with salts or bentonite a permanent fix?
It depends on the product. Salts must be renewed periodically, and authorities may prohibit them because of the risk of leaching into neighboring areas. Bentonite is stable and non-corrosive, but it needs moisture: in a very dry environment it can shrink and separate from the electrode.