What Is a Soil Resistivity Test and What Does It Measure?
Soil resistivity is a property of the soil itself: it tells how strongly the soil opposes the flow of electric current. In field work the unit is the ohm-meter (Ω·m); the lower the value, the better the soil conducts. The value is controlled by moisture content, soil type and grain size, dissolved salt content, temperature and density, all of which change with the seasons. Moist clay containing dissolved salts gives very low values, while dry coarse sand or hard rock can be a thousand times higher. Guessing the value from how the soil looks is therefore no substitute for actual measurement.
It is important to be clear that this test is not a ground (earth electrode) resistance measurement. Ground resistance testing measures the performance of a grounding system that has already been installed; its unit is the ohm and it is done at acceptance. A soil resistivity test is done before design, to show how favorable the soil at the site is for building a grounding system.
Design of grounding electrode systems and ground grids for power plants, substations, factories and data centers · Calculation of step and touch voltages for personnel safety · Determining the number, length and layout of ground rods · Assessing the corrosion risk of gas pipelines, metal pipes, buried tanks and steel piles · Design of cathodic protection systems · Design of lightning protection systems
Which Projects Need This Test?
The test method is the same for every project, but the reason for measuring and the way the values are read differ by project type. The three groups SPN encounters most often are power plants and solar farms, gas pipelines and buried metal pipes, and data center buildings.
Solar Farms and Power Plants
During a fault, power plants and substations discharge high fault currents into the ground. The grounding system must dissipate that current quickly and widely enough that no dangerous voltage develops on the ground where people stand. IEEE Std 80-2013 uses soil resistivity as a fundamental input for calculating step voltage and touch voltage. Enter this value wrongly and the whole design is wrong along with it.
Solar farms have IEEE Std 2778-2020 (Guide for Solar Power Plant Grounding for Personnel Protection) written specifically for them, because the grounding system is spread across hundreds of rai rather than concentrated as in a substation. The soil data must therefore cover the entire site and go deeper than typical substation work; large projects may require electrode spacings on the order of several hundred meters. Another aspect is a service life of 25 years or more, with most module mounting structures founded on driven steel piles or ground screws that stay buried for the whole project life. Low resistivity indicates a high level of dissolved ions in the soil, which accelerates steel corrosion, so the results must be read together with pH, chloride and sulfate test results.
Gas Pipelines and Buried Metal Pipes
Natural gas pipelines, oil pipelines and metallic water mains read this value the opposite way from grounding work. A low value is not good news, because soil that conducts electricity well more readily allows electrochemical corrosion cells to form on the pipe surface. The value therefore indicates the corrosivity of the soil along the route and is input data for designing cathodic protection systems in line with AMPP (NACE) SP0169, including the selection of anode type and location and the estimate of the system's current requirement.
In practice, pipeline work focuses mainly on the soil layer at pipe burial depth, so readings at short to medium spacings a are taken in detail and test points are distributed along the length of the route. A single pipeline may cross several soil types, with stretches of saline soil or shallow groundwater, so the corrosion risk is not uniform along its length.
Data Center
Data centers need a grounding system that is safe for people and stable enough for electronic equipment that is sensitive to potential differences between grounding points. Designers therefore need the soil resistivity first, to size the ground grid, set the number and depth of ground rods, and judge whether the target ground resistance can be achieved. In high-resistivity soil, adding ground rods alone is often not enough; the grid has to be spread wider or the soil around the rods improved, and both options affect construction cost and the land that must be reserved from the design stage.
How Does the Wenner Four-Electrode Method Work?
The Wenner method is the method ASTM G57-20 specifies for field work. Four electrodes are driven in a single straight line, with equal spacing between adjacent electrodes. This spacing is called a.
The two outermost electrodes (C1 and C2) inject current into the ground, and the two middle electrodes (P1 and P2) measure the resulting potential difference. Knowing both the injected current and the measured voltage, the soil resistivity can be back-calculated.
Many meters already read out resistance R = E/I, so the formula can be written ρa = k × R, where k = 2πa is called the geometric factor. For example, at a = 2.0 m, k ≈ 12.6, and at a = 10.0 m, k ≈ 62.8. The word "apparent" matters, because real ground is not uniform with depth. The measured value is a weighted average of all the soil layers the current passes through, with the depth of investigation roughly equal to spacing a. Extending spacing a is therefore like looking progressively deeper into the ground, layer by layer.
Field Requirements to Control
Electrode embedment depth must not exceed 5% of spacing a (the B < A/20 condition), because the formula assumes each electrode is a point contact on the ground surface; driving deeper makes results deviate from theory. At a = 0.5 m, electrodes can therefore be driven no deeper than about 2.5 cm, which calls for care against loose electrodes with poor contact. Another good practice is to measure two perpendicular lines at the same point, because real soil is rarely the same in every direction (anisotropy). Two lines show whether the values are stable enough for design or so variable that more test points are needed.
The 3D view below summarizes the whole job on one page: the Wenner electrode line laid out in front of the solar arrays, the second perpendicular line, and where the measured values end up—the transformer grounding system and buried metal pipes, which read the same value with opposite meanings.
What Equipment Is Used and How Is the Fieldwork Done?
The main equipment used in the field is as follows:
- Earth resistivity meter: injects current and reads the resulting potential difference.
- Power supply: a 12 V battery connected to an inverter adjustable over 0–200 V, to deliver enough current when electrode spacings are long or soil resistivity is high.
- Four electrodes: the potential electrodes are usually copper rods and the current electrodes stainless steel rods.
- Cable reels: their length must cover the maximum planned electrode spacing. Tests with long spacings need large reels of around 300 m.
- Measuring tape and positioning equipment: to make the electrode spacings truly equal and record test-point coordinates for reference.
The field sequence starts by setting out test points according to the plan agreed with the designer, stretching a straight line with a measuring tape, driving the four electrodes at the first spacing a, connecting the leads to the meter, injecting current and taking the reading, then moving the electrodes out to the next spacing a until every spacing is done. It finishes by rotating the line 90 degrees and repeating the measurements on the perpendicular line.
Water makes soil conduct better, so values measured in wet soil are significantly lower than in dry soil. Testing for grounding design should be done with the soil in its naturally dry condition, because that is the critical design case. If measurements must be taken after rain or in the rainy season, the moisture condition and timing must always be recorded in the report.
What Is the Typical Resistivity Range for Each Soil Type?
The table below gives reference ranges for checking whether measured values are reasonable for the soil conditions observed. It is for preliminary checking only and must not replace actual test results in design.
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| Material / condition | Typical range (Ω·m) |
|---|---|
| Seawater | 0.18–0.24 |
| Clay, wet to moist | 1.5–3.0 |
| Silty clay / silt, wet to moist | 3.0–15 |
| Fresh water | 20–60 |
| Silt and sand, moist to dry | 15–150 |
| Heavily fractured bedrock with moist soil filling the cracks | 150–300 |
| Silty sand and gravel | About 300 |
| Sand and gravel with silt layers | 300–2,400 |
| Slightly fractured bedrock with dry soil filling the cracks | 300–2,400 |
| Coarse sand and gravel, dry | More than 2,400 |
| Solid, hard bedrock | More than 2,400 |
What this table shows at once is how wide the range is, from below 1 to several thousand ohm-meters. Putting a mid-range value into a design without measuring can make results off by a factor of tens. In coastal areas or saline soil, very low values are good news for grounding but bad news for metal pipes and steel piles that must stay in that soil for the whole project life. (Note on units: ASTM G57 reports results in ohm-centimeters, while grounding design in Thailand commonly uses ohm-meters; 1 Ω·m = 100 Ω·cm.)
How to Read Real Test Results: An Example from a Power Plant Project
This data set comes from SPN testing at one test point on a solar power plant project, measured on two perpendicular lines at the same point with spacing a stepped from 0.5 to 10.0 m.
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| Spacing a (m) | k = 2πa | R, E–W line (Ω) | ρa, E–W line (Ω·m) | R, N–S line (Ω) | ρa, N–S line (Ω·m) |
|---|---|---|---|---|---|
| 0.5 | 3.1 | 46.88 | 147.3 | 41.67 | 131.0 |
| 1.0 | 6.3 | 24.12 | 151.6 | 21.92 | 137.8 |
| 2.0 | 12.6 | 5.43 | 68.2 | 7.39 | 92.9 |
| 3.0 | 18.9 | 2.30 | 43.3 | 2.90 | 54.7 |
| 5.0 | 31.4 | 1.03 | 32.4 | 0.95 | 29.9 |
| 10.0 | 62.8 | 0.58 | 36.7 | 0.40 | 25.1 |
This data set tells three things.
First, the upper soil is clearly more resistive than the lower soil. At short spacings of 0.5–1.0 m, values are around 131–152 Ω·m, within the reference range for silt and sand in moist to dry condition. Extended to 5–10 m, values drop to around 25–37 Ω·m. This pattern is common where the surface soil is dried by sun and wind while the deeper soil still retains moisture.
Second, the two lines agree. The E–W and N–S lines show the same trend with values acceptably close; the largest difference is at 2.0 m (68.2 vs. 92.9 Ω·m). This consistency indicates that the values represent the real soil conditions at that point, not the effect of a foreign object along one line.
Third, use the value that matches the working depth. If the grounding system uses ground rods about 2.4 m long—the minimum length set by the EIT Electrical Installation Standard for Thailand—the relevant values are those at a = 2–3 m, not the value at 10 m. Likewise, buried pipeline work must use the value at pipe burial depth. Picking the lowest value in the table because it looks better is a mistake that makes the design inconsistent with actual conditions.
This article explains the test method and how to read the values; it does not set pass/fail criteria for any project. Whether the measured values are acceptable must be decided by the electrical engineer designing that project's grounding system, considering fault current magnitude, site layout, fault clearing time and the standards the project references. For the resistance of an installed ground electrode, the EIT standard specifies no more than 5 ohms, relaxed to 25 ohms where 5 ohms genuinely cannot be achieved and the electricity authority has approved.
Which Standards Apply to This Test?
← Scroll to see the full table →
| Standard | Covers |
|---|---|
| ASTM G57-20 | Field measurement of soil resistivity using the Wenner four-electrode method; the main standard for this test (the older edition still cited in some documents is G57-95a, which has been superseded) |
| ASTM G187-23 | Laboratory measurement of soil resistivity using a two-electrode soil box, reporting results in ohm-centimeters; used on soil samples collected from the field |
| IEEE Std 80-2013 | Guide for safety in AC substation grounding; uses soil resistivity to calculate step and touch voltages |
| IEEE Std 81-2012 | Guide for measuring earth resistivity, ground impedance and earth surface potentials of a grounding system |
| IEEE Std 2778-2020 | Guide for solar power plant grounding for personnel protection, covering issues specific to large, widely spread project sites |
| AMPP (NACE) SP0169 | Guidance on external corrosion control of buried or submerged metallic piping systems; uses soil resistivity to help assess the need for and design of cathodic protection (the latest revision is SP0169-2024) |
| Electrical Installation Standard for Thailand (EIT) | Grounding system requirements enforced in Thailand, including ground electrode resistance and minimum ground rod length |
Which Mistakes Make Test Results Unusable?
Measuring one point and assuming it represents the whole project. A solar farm of hundreds of rai or a pipeline route several kilometers long can have very different soil conditions within one project, especially where fill thickness varies. Test points must be spread to cover every zone where the soil boring results show different soil layers.
Maximum spacing a does not cover the working depth. If the system uses a widely spread ground grid and deep ground rods but testing only goes to a few meters, there is no data on the soil layers that actually dissipate the current.
Buried metal along the test line. Steel pipes, underground cables, reinforced drainage channels or piles already driven make the current flow through the metal instead of the soil, giving abnormally low readings. This is especially important on gas pipeline work, where the test line is often in a right-of-way shared with other utilities; check underground utility plans and avoid those lines before driving electrodes.
Driving electrodes deeper than specified. In hard ground where electrodes are difficult to drive, they are often hammered deeper so they will stand, so the B < A/20 condition no longer holds and results deviate from theory.
Not recording conditions and test date. The same value means something different when measured in the middle of the dry season versus after heavy rain. A report that does not state moisture condition, temperature and date leaves the designer unable to judge how much margin to allow.
How Does It Work with Other Tests?
Soil investigation (soil boring) provides the soil profile, SPT-N values and bearing capacity for designing the foundations of module mounting structures, inverter buildings, data center buildings and transformer foundations. Boring results are also used to zone the site and plan how many soil resistivity test points are needed and where. For the number and depth of boreholes, see How many boreholes and how deep?
Soil thermal resistivity testing measures the thermal resistivity of soil (m·K/W), a different property from electrical resistivity, used to calculate the current rating of underground power cables per IEC 60287. Substation and underground transmission projects often need both soil resistivity and soil thermal resistivity tests in the same trip, together with 24-hour soil temperature measurement (ambient temperature test) at cable laying depth.
Pile testing for solar farm projects covers uplift (pull-out) and lateral load tests, because solar module piles mainly resist wind uplift and lateral push. A plate bearing test is used to check the bearing capacity of surface soil or fill under internal project roads and shallow foundations.
For corrosion assessment, soil resistivity alone is not enough. Soil and groundwater samples should also be tested for pH, chloride and sulfate, because the three together form the picture of corrosion severity that designers need. See the scope of laboratory work at Soil investigation and testing.
What Are the Costs and Timeline?
Costs are estimated per project. The variables with the most influence are the number of test points, the maximum spacing a required, the number of lines per point, travel distance and site access. In terms of time, fieldwork at one point does not take long, and several points can be done per day on open sites. Combining this work with soil boring in the same trip noticeably reduces travel costs. Anyone wanting a price estimate should prepare project site information, a layout of the required test locations and the spacings a specified by the design engineer. Prices do not include travel and equipment mobilization, which are charged extra according to the distance to site — Get a free quotation within 24 hours.