Introduction
Measuring soil resistivity by direct electrical measurement is fundamental to designing a safe and cost-effective earthing system. It is the foundation on which good design is built on. If the soil resistivity input data is wrong, then the entire earthing design will usually be wrong and this can lead to costly delays and rework.
Soil Resistivity testing is an apparently relatively simple process, but contains a lot of hidden subtleties and it is easy for inexperienced personnel to get things wrong.
At the most basic level, understanding the soil resistivity lets you identify what type of earthing system will be needed and how large it will be. Some soil is so rocky, that rods won’t work – or are not practical.
Soil resistivity plays a crucial role in determining the site earth potential rise (EPR) and how far it extends away from your substation/site. Lower resistivity, the EPR will drop away much quicker. Conversely high soil resistivity soils and the EPR will extend further away and may impress onto other nearby infrastructure. The complexity of this increases when considering different layers and thickness, that can be picked up through direct electrical measurement.
Geology, Soil and Typical Values
This may seem obvious, but the results obtained will vary depending on the site location. Typical values of soil resistivity (from BS7430) are shown below, and general trends within the UK can be seen from the British Geological Society (BGS) map. Whilst these general trends are important, there can be a lot of local variations in an area, which can be due to underwater springs, local hills and outcrops and areas of reclaimed land.
Check the site geology before you attend is always a worthwhile process, as it helps validate the results. Generally an experienced tester should know what to expect as the result come in.
| Soil Type | Typical Resistivity (Ωm) |
| Loams, Graden Soils etc. | 5-50 |
| Clays | 10-100 |
| Chalk | 30-100 |
| Clay, sand and gravel mixture | 40-250 |
| Marsh, peat | 150-300 |
| Sand | 250-500 |
| Slates and Slatey Shales | 300 – 3,000 |
| Rock | 1,000-10,000 |
The Wenner Test
The Wenner method for soil resistivity measurement, is usually taken as the best way to assess the soil resistivity as a function of depth. This insight not only allows us to design the optimum and most efficient electrode arrangement but also guides decision on whether deep earth rods are necessary.
A number of traverses are taken across the site to confirm trends and soil resistivity. It is recommended to carry out at least 3 traverses, as this helps identify any problems with the test or stray readings. The standard spacings used are given in BS EN 50522 as The electrode spacing are be 1m, 1.5m, 2m, 3m, 4.5m, 6m, 9m, 13.5m, 18m, 27m, 36m and 54m per traverse.
For larger sites, additional readings should be taken at 80m and 100m. It is important to ensure that the wide spacings are carried out for big sites, as it provides valuable information on deeper soil layers – many subcontractors prefer to avoid these wide spacing as they are time consuming to carry out.
Choosing a test location is important. Testing should always be done on flat land, as hills and slopes confuse the test meter. It is also important to test away from any sources of potential interference or low resistance paths. Most test kits inject only a small amount of mA, so nearby buried cables or overhead lines can distort the readings making them look higher or lower than expected.
One issue that often surprises people is that the value of the test result, usually (not always) falls with the wider spacings. The reason for this is actually simple, remember that the Wenner test is measuring a volume of soil in the ground, and from basic theory we know that ρ = 2πaR. So if ρ is constant – as the spacings get wider, then R has to fall. This isn’t always the case though – in rocky areas, such as Cornwall, Wales and Scotland there can be an increase, as the tests identify the deeper rock layers.
Other methods such as the Schlumberger method are possible, but based on our practical field experience, although they are quick they are generally less reliable and lead to more chance of error in the system design.
Real World Problems
In the real world, problems are often encountered, and a test engineer needs to know how to work around them. Construction on a site may have started, farmers may not allow access, sites may be on built-up land, or in the middle of a city. So how do we tackle measurements, in areas where it isn’t feasible to conduct measurements on the site or at the site of interest location?
Some will say estimations using uniform soil from standards or the BGS maps is acceptable, but this can give very misleading results and lead to a very under-estimated (unsafe) or over-estimated (expensive) design. The best way round this is to utilise the nearest open land around the site ideally N, S, E and W, to get a geographical idea of the ground conditions around site. If a land area if flat, and doesn’t have any unusual features, then geology doesn’t tend to alter much over a few hundred meters, so testing around an area and building up an average picture is a good practical approach.
Below is an example, where we might want to design an earthing system for a new hospital, but don’t have easy access on the site. Prior to attending the site a desktop survey is carried out to identify areas nearby that may be accessible, and could be used to build up a good understanding of the surrounding area.
Interpreting the Results
Before leaving site, it is always best to check the results. Do they look reasonable and are the three traverses showing similar results? If there are differences why? Do the results align with the expected soil geology? When you are on site, running an additional traverse is can be irritating, especially if there is a long drive to do, but it is a lot less painful than having to revisit the sites.
Summary
This crucial soil resistivity information is then utilised to develop a tailored electrical earthing system design for your site. Getting the input data correct is essential for striking the right balance between efficient design, cost and safety, and then avoiding unpleasant surprises in the post installation FOP test.
Get in touch with Aurora’s experienced earthing consultants for site specific soil resistivity measurements utilising the Wenner method for your earthing system’s needs.