Seasonal Analysis of Soil Models for Accurate Earthing System Design
Soil electrical resistivity is not a fixed property. It varies throughout the year with air temperature, rainfall, snowfall and freezing conditions – and these variations can dramatically change the performance of an earthing system. A soil model obtained from field measurements on a warm, wet summer day may look very different on a cold, dry winter day, and this difference can lead to unsafe touch and step voltages if left uncorrected.
This tutorial explains how seasonal temperature and moisture variations affect soil resistivity, how to derive a worst-case corrected soil model from a single set of measurements, and how to apply these corrections inside SafeGrid Earthing Software. Worked examples are provided for both hot-moderate and cold climates, followed by a real-world case study of a 132 kV substation earthing design in Sydney, Australia.
Soil Electrical Resistivity
Soil electrical resistivity (Ω.m) has a major impact on earthing system performance, and field testing is vital before designing any earth grid. The Wenner 4-point method is the most commonly used test technique.
Several factors affect soil resistivity:
- Temperature – resistivity increases as temperature decreases.
- Moisture content – resistivity decreases as moisture increases.
- Soil type, compactness and mineral content.
Ideally, measurements should be taken during adverse weather conditions (cold periods after an extended dry spell, or when the soil is frozen) to obtain conservative results. In practice this rarely happens – so a seasonal correction must be applied to measurements taken at a favourable time of year.
Multilayer Soil Modelling
Interpreting apparent resistivity measurements is usually a difficult task. The objective is to derive a soil model that is a good approximation of the actual soil – a perfect match is generally not achievable. The vast majority of real soils are multilayered, so multilayered models give the most accurate earthing calculations. As a rule of thumb, a soil model with an RMSE below 15 % is considered a good fit to the field data.
Equivalent two-layer soil models can still be derived manually, but multilayer interpretation is best performed using dedicated software.
Soil Resistivity Effects on Touch Voltages
Touch voltages near an earth grid depend primarily on the resistivity of the topsoil, while the earth grid resistance depends mainly on the deeper soil layers. This split sensitivity is why shallow-layer seasonal variation is so important for safety.
| Soil Model | Grid Resistance | Grid Potential Rise | Max. Touch Voltage |
|---|---|---|---|
| 100 / 1000 Ω.m | 12.10 Ω | 12104 V | 1042.4 V |
| 500 / 1000 Ω.m | 19.19 Ω | 19189 V | 3572.9 V (+243 %) |
| 1000 / 100 Ω.m | 3.04 Ω | 3035 V | 1387.1 V |
| 2000 / 100 Ω.m | 3.16 Ω | 3159 V | 1513.1 V (+9 %) |
Soil Temperature versus Electrical Resistivity
Electricity moves through soil primarily by electrolytic processes. Lower temperatures reduce electrolytic activity and reduce soil conductivity. When soil freezes, conductivity drops sharply and the soil begins to behave more like ice.
As a result the relationship between soil resistivity and temperature is divided into two distinct segments – above and below freezing:
- At +20 °C, soil resistivity decreases by approximately 50 % compared with values just above freezing.
- At -10 °C, soil resistivity increases by 400–1000 % compared with values just above freezing.
This step change at the freezing point is the main reason cold-climate earthing systems are so sensitive to seasonal effects.
How Weather Affects Soil Temperature
Weather conditions directly affect ground soil temperature. The ground surface has a complex and balanced heat transfer system consisting of conduction, convection, thermal radiation and moisture evaporation. Soil temperature at any depth is closely linked to the history of ambient air temperature, smoothed and time-delayed the deeper you go.
Soil temperature on any given day can be predicted using the soil thermal diffusivity, the annual average air temperature, the yearly air temperature fluctuation, and the date of the first day of Spring (the day on which the average daily temperature equals the annual average).
Soil Moisture versus Electrical Resistivity
Soil resistivity also depends strongly on moisture content. Water content has a strong influence on resistivity up to about **20 %** saturation – beyond that, the effect flattens out.
Soil moisture varies with rainfall, diffusion, evapotranspiration, surface runoff and deep percolation, and is also a function of depth. The effect of rainfall on soil moisture reduces with depth and effectively ceases after a critical depth of about **1 metre**. This means seasonal moisture correction mainly changes the resistivity of the top one to two metres of a soil model – exactly the layers that control touch voltages.
How to Consider Seasonal Variation
The general calculation procedure is:
- Calculate the multilayer soil model(s) from the field measurements.
- Process the temperature and rainfall / snowfall data (optional).
- Calculate the effects of soil temperature and moisture content on soil resistivity for every day of the year.
- Identify the worst day – the day with the maximum average soil resistivity.
- Correct the calculated soil model(s) for the worst day.
Data required:
- Multiple sets of soil resistivity measurements.
- Weather data – annual average air temperature, yearly air temperature fluctuation, annual monthly rainfall and snowfall.
- Soil type data or a geotechnical report.
Key assumption: one soil type is assumed to apply for all shallow layers, since temperature and moisture variations only affect shallow depths.
Example Calculations
All examples use a simple 20 × 20 m square earth grid. Four scenarios cover both hot-moderate and cold climates, with and without rainfall / snowfall effects:
| # | Climate | Correction Applied |
|---|---|---|
| 1 | Hot-moderate | Air temperature only |
| 2 | Hot-moderate | Air temperature + rainfall |
| 3 | Cold | Air temperature only |
| 4 | Cold | Air temperature + rainfall + snowfall |
Example 1 – Hot Climate, Temperature Effect
Clay soil, southern hemisphere, average annual ground temperature 19.45 °C, yearly air temperature fluctuation 3.16 °C, measurement day 5 February (summer), worst day 8 August (winter). The corrected model shows higher resistivity in the first three layers – enough to cause a significant increase in touch voltage.
Example 2 – Hot Climate, Temperature and Rainfall
When rainfall is included, the worst day shifts (to 31 July in this case) because shallow soil moisture on the measurement day was much higher than on the worst day. Predicted resistivity of the shallow layers rises considerably – layer 1 jumps from 23.34 Ω.m to 82.58 Ω.m.
Example 3 – Cold Climate, Temperature Effect
Clay soil, northern hemisphere, average annual ground temperature 3.14 °C, yearly air temperature fluctuation 17.81 °C, worst day 23 January. Shallow layer resistivity is dramatically higher than the original measurement – the top layer rises from 23.34 Ω.m to 2214 Ω.m because of sub-zero soil temperatures.
Example 4 – Cold Climate, Temperature and Rainfall / Snowfall
Adding precipitation data to the cold-climate case further increases shallow-layer resistivity. The top layer reaches 5036 Ω.m on the worst day, compared with 23.34 Ω.m on the measurement day.
Touch Voltage Effects for All Examples
Comparing the uncorrected baseline (280 V) with each corrected case:
- Example 1 (hot, temp only): 313 V – +11 %.
- Example 2 (hot, temp + rain): 1158 V – +313 %.
- Example 3 (cold, temp only): 373 V – +33 %.
- Example 4 (cold, temp + rain + snow): 1171 V – +317 %.
Ignoring seasonal effects can more than quadruple the real maximum touch voltage.
Real-World Earthing Design Example – 132 kV Substation
A 132 kV substation is being installed in Sydney, Australia. Soil resistivity measurements were taken on 15 March 2024, and seasonal variation (temperature and rainfall) is applied using SafeGrid Earthing Software to determine the worst-case soil model and maximum touch voltages.
Weather inputs:
- Annual average air temperature: 22.7 °C → average soil temperature 23.7 °C.
- Yearly air temperature fluctuation: 23.6 °C.
- Monthly mean rainfall taken from Australian Bureau of Meteorology data.
- First day of Spring: 1 October.
Soil type: from the geotechnical report the layers are 0.3 m silt, 1.9 m clay, then clayey sand. Silt is selected for the seasonal correction because it is in the topsoil and has a higher thermal diffusivity than clay, so temperature changes propagate deeper.
Worst-day results:
- Soil temperature variation only – worst day 5 July.
- Soil temperature and moisture combined – worst day 30 July. Combined temperature + moisture effects produce the largest change in the soil model.
Touch voltage calculations (20 × 20 m grid):
- Seasonal variation not considered: 429.5 V.
- Soil temperature variation only: 730.2 V.
- Soil temperature + rainfall: 1150.7 V – a 168 % increase in maximum touch voltage.
Touch voltage calculations (larger 300 × 300 m grid):
- Seasonal variation not considered: 85.2 V.
- Soil temperature variation only: 157.6 V.
- Soil temperature + rainfall: 365.3 V – a 328 % increase in maximum touch voltage.
The percentage impact of seasonal correction is actually greater for larger grids, because their touch voltages are more strongly dominated by shallow-layer resistivity.
Conclusions and Recommendations
- Annual soil temperature and moisture variations have a significant impact on soil electrical resistivity, especially for depths up to about 10 metres.
- It is very unlikely that field measurements were taken at the worst time of year – this must be checked. The resistivity of the shallow layers could be much higher than the measurement indicates.
- Touch voltages are greatly affected by shallow-layer resistivity, and can be much higher than expected if seasonal correction is not applied.
Recommendations:
- Always apply a seasonal correction to soil electrical resistivity measurements.
- Apply it in both dry-climate installations and cold-climate installations where the soil may freeze.
Related Tutorials
- Multilayer Soil Modelling in SafeGrid Earthing Software
- Touch and Step Voltage Calculations
- Finite Element Method for Earthing Analysis
- Earth Grid Resistance and Grid Potential Rise
- Frequency-Domain and Time-Domain Earthing Calculations
