Soil Resistivity Testing in the Wet – Ninja Tricks to Help You Spot the Enemy

Soil Resistivity Testing in the wet particularly saturated ground. Has the potential for many problems.

Here are some ninja tricks to help you spot the enemy lurking in the damp…

Soil Resistivity Testing is the absolute starting point for pretty much everything to do with design for Earthing, grounding and lightning protection. (click link to flow chart) This is chiefly because all these type of systems are connected to earth and rely on the quality of this interface.  Therefore, understanding the soil resistivity and what the representative ‘Soil Model’ will look like is without doubt the single most important foundation to all subsequent safety calculations.

The metaphor of using a sound foundation crops up in many cultures – like the foolish man who built his house on sand, whereas, his more intelligent counterpart built his house on rock.  Needless to say, back in the day (2 millennia ago) they didn’t have the know-how of piling technologies in sand, so we all can guess who stood up best when the storms arrived.

It’s the same with Soil Resistivity – given the importance of a robust Soil Model for a technically secure Electrical Earthing design – it’s not good enough to accept old data without checking that the Soil Model is providing a solid rock-foundation for YOUR Electrical Earthing System Design!

Hence the relevance of Soil Resistivity Testing. And avoiding potential problems when doing Soil Resistivity testing in the wet ground.

There are a number of sources of potential error. Can dramatically influence the resulting Soil Model.  Some of the more common ones are dealt with in previous posts, particularly, What is Soil Resistivity Testing & Why is it Important to Safe Electrical Earthing Design and in the 7 Deadly Sins of Electrical Earthing series.

Soil Resistivity Testing in the Wet

But today, continuing with a stormy theme, we’re going to explore what can happen during the soil resistivity testing task during, or after a rain storm – when the ground under test is saturated.

WET WET WET

Aside from being great ‘80’s pop band, when the ground underfoot is wet-wet-wet with standing water, or the testing is being attempted during a rain storm with horizontal rain … stop, and think!

What is this doing to the electrical signal generated by the instrument and being sent down the probes which sits in the top surface layer of the ground as part of your Wenner soil resistivity test?

At best, if the ground has not been fully saturated, then the moisture will simply artificially lower the soil resistivity for a given depth.  If so, this can be accounted for as a seasonal variation.  However, if the ground has been subjected to a monsoon of rain, or sustained rainfall over many days/weeks, then pockets of surface-saturated ground can form or subterranean bodies of water can collect before draining away (hydrology) as shown in the image above.

To the uninitiated Earthing Ninja, these signs (when ignored) play havoc on the process of deriving a sensible Soil Model (the Earthing Design foundation).

Soils are composed of solids (particulates, granules, chunks, minerals, etc) and VOIDS.  These voids are really important to us as they can be occupied by air (non-conductive) or water (conductive), which can influence a volume of soil massively.  Mechanically speaking there are 3 soil phases to consider, ‘Partially’ Saturated, ‘Fully’ Saturated and ‘Dry’.

For the most part, soil resistivity testing should be carried out when the soil is in a partially saturated phase, but life isn’t always that accommodating.  So, it’s useful to be aware that when the soil enters a fully saturated phase, you may be introducing a large source of variation, at best… or at worst… error!   Variation can be accounted for, but error is error… Sin #5 of Earthing Design- error, the enemy of all safety critical earthing design.

What’s happening?

During a Soil Resistivity test using the Wenner Soil Resistivity testing method, the signal current is sent down the current probes (C1, C2 shown below).  I

f the probes sit in a saturated layer of soil, the returning electrons will prefer to travel along the saturated layer or finite volume of soil instead of penetrating the ‘soil’.

 

This can lead to massive underestimations in the soil’s resistivity, and can prevent the instrument’s signal current from penetrating the deeper layers.  Naturally, this loss in signal strength can go unnoticed, unless the data is further processed and optimised using software tools such as the full RESAP module of CDEGS by a competent engineer.

Subterranean hydrology can also have a similar effect on testing.  See the example 1-D Soil Resistivity Plot image which examples a sharp drop in resistivity at ~12m depth (Inter-electrode spacing), which could indicate either a conductive buried structure (unlikely at 12 m) or hydrology.

pH balance in the soil is also something that can dramatically change as a result of soil saturation.  Given the majority of earthing arrangements sit in the upper layer(s), a change in the hostility to the Earthing design should also be considered – this is a topic for a future post.

What can you do?

If you cannot at least wait for the ground to reach a partially saturated state, you should account for the risk that the test readings may be subject to significant error.  In the hands of an 6^th Dan Earthing Ninja… technology, equipment, experience and judgement can certainly help to mitigate the potential for error, but it is unlikely to eliminate it.  To eliminate error, the tests may need to be repeated when the ground is not suffering from full-saturation.

Contact us for more Ninja Earthing skills or to learn how to de-risk your system and solve your Earthing or Lightning problems.