Soil Resistivity | Why & How to Perform Soil Resistivity Surveys

Published 19 Oct 2020

Soil Resistivity Surveys

Special thanks to Andy Chaluda Business Development Manager - Technical Services at AN Wallis

The soil resistivity survey by AN Wallis is the first step in ensuring the correct design of an earth electrode system. It is essential that accurate measurements are taken at this stage as this data is used to determine what conductors are required in the finished earthing system to give a safe and suitable design. Corrupt data taken with inadequate test equipment could lead to a vastly over- or under-engineered solution.

Why We Carry Out Soil Resistivity Surveys

To complete an earth electrode system analysis, we need to know what the electrical properties of the ground we intend to install the earthing into.To do this, we would need to complete a soil resistivity survey of the site, or if not directly at the site, a location near to the site on a similar elevation.

How To Perform A Soil Resistivity Survey

Soil Resistivity

Four probes are inserted into the ground to give a good connection. The four probes are equidistantly spaced apart from one another to the given separation distances, we tend to use the separation distances given in BS EN 50522 as a set value. The distance the probes are spaced apart, dictates the depth into the ground you are testing.

The largest separation distance would normally depend on the size of the proposed substation site, for instance a 5x5m substation may only need to have, for example a 13.5m separation distance, where as a larger substation will need larger separation distances, this would be determined by our Earthing Design professionals when attending site. We must remember that ultimately the surveys will be limited by the land available to us and can therefore take this into account should we not have sufficient space.

It may be possible to compare the derived soil model to the local geology if that is known as shown below:

Type Of Soil	Soil Resistivity
Marshy Soil	5 to 40
Loam, Clay, Humus	20 to 200
Sand	200 to 2,500
Gravel	2,000 to 3,000
Weathered Rock	Mostly below 1,000
Sandstone	2,000 to 3,000
Granite	Up to 50,000

For instance, in London, we would expect a soil model to be around 20Ω-m, as we know London is in an area of clay. If the values are dissimilar to those shown in the table extracted from BS EN 50522, we may have a problem with our surveys or the land is not natural, such as a landfill site or former quarry.

Once the first survey is complete, further surveys will be completed at different parts of the site until the data gained collates and a ‘clean’ data set is derived.

We are vigilant not to survey near buried metallic objects such as pipelines or parallel to overhead lines, as these may influence the readings taken.

Once the raw data is gained, this is then processed using the CDEGS RESAP software to gain an indicative soil model of the site and therefore completing the Soil Resistivity Survey.

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