Ian takes us on a recap of earthing in difficult geologies, you can find the webinar replay here.
Ian: Hello and welcome.
How many times have we faced with the install team, or you are doing tests on site. You come back, and the readings are high, far higher than expected, and you are scratching your head wondering what is going on.
My name is Ian Griffiths, I am the Principal Consultant here at GreyMatters. In today’s webinar, we will take a step back, and we are going to revisit a webinar that was a piece done by Hugh a couple of years ago now, but it has come up is very topical. It is something that we get asked a lot of questions on. So that is the plan for today, let us dive in.
Hugh: Straight into earthing in difficult geologies. Earthing is a poorly understood topic in all areas. But I think how to deal with difficult geologies is something that is particularly poorly understood.
Firstly I am going to start by talking about the key factors that soil resistivity depends upon. Next I am going to talk about why difficult geologies come about. I am going to cover how to address those challenges and what specialist products you can use. And then I am going to talk through an example of how we approached a difficult geology scenario. Then at the end, I am going to summarize up and talk about what you can do next to take things forward.
I mean it depends on a vast range of factors, a lot of which are site specific. There is four key elements that are the most important factors. The first is the water content of the soil. The amount of water retained, particularly in the top layer of the soil. Alongside that is the electrolytes, the salts that allow the electrons to travel through the soil. Another key factor is the granularity. How big the pieces of the soil are and how that affects the connectivity between the electrode that you are burying and the greater mass of soil. And finally, temperature. If your soil starts to freeze, then it becomes less effective at transferring those electrons through the greater mass of soil.
What makes a difficult geology?
There are three major factors that lead to a difficult geology. The first one is soils with a large particle size. We have lots of rocks in and these lead to a situation where you have poor connectivity between the copper electrode you have buried and the greater mass of soil. The second category is soils that are free draining. You will spend a lot of money burying copper in the first meter or so of soil, but there is not much water retained in that meter of soil so the earth electrode will not perform very well. And then finally, there’s geologies that just extremely high resistivity. These are things like granite, and other types of rock where there is just nowhere for the electrons to go. What is important to note is that these three categories take different approaches.
What do we do?
The thing I really want to stress is there is not much you can do about geology. You must work around it. And I am going to introduce some of the methods you can use to work around geology. Unless you are God, you are not going to change the geology with which you are working.
How do we deal with large particles?
What we what we can do here is we can bring in a material of small grain size quite commonly. Consultants recommend that you use engineered backfills like Bentonite and Marconite. These are very good for filling in the gaps that are created by large particles but equally if you if you’re clearing a lot of topsoil off your site that’s just as good at filling in those gaps and it’s a whole lot cheaper than going to a supplier and buying a load of bags of Bentonite or Marconite and that’s ultimately good practice.
What about free draining soils?
Well, normally the free draining structures tend to be fairly high up in the topology of the soil. And you find that the further down, there is going to be a bedrock structure, which means that the water does not go down any further. You end up with a high resistivity top layer, and then a really low resistivity area underneath. What you can do there is you can drive deep electrodes to penetrate that low resistivity area and use that to lower your earth electrode resistance.
Now, for anything greater than ten meters of depth, and often more than five meters, it becomes extremely hard to drive the rods mechanically. We normally recommend that once you get to about ten meters, you start investigating boreholes. That is just another cost to add in when considering your design.
Dealing with high resistivity geologies
High resistivity geologies are the most difficult scenario because you cannot change the structure that you are installing your earth electrode in. There is not an off the shelf product that will change that. All Bentonites or Marconite is going to do is improve the connectivity from the copper you’ve buried into that surrounding material.
So here you have got to look at designing your electrode cleverly. Doing some more soil resistivity testing to identify small pockets of low resistivity around your site. Sometimes you can look at things like sea electrodes. Sometimes a product like ChemRod is appropriate, which I will introduce in a minute. But the key thing is that there is no good substitute for copper in the ground. And if your soil resistivity is high, you will just have to install more copper.
We can look at the commercial backfills. The most commonly used one, I think is a product called Bentonite, which is a natural clay. It is not only common in the earthing industry but also among the geotechnical investigations industry. And they quite commonly backfill their boreholes with Bentonite anyway, that comes in at about £25 per bag.
Another common product called Marconite, which improves the conductivity of your cement. What a lot of people do not realize is that cement is already quite a low resistivity structure. Having material in cement, having copper in cement is fairly effective as an earth electrode already. But if you add in a product like Marconite, then that improves the conductivity of that cement. And so, you get better connectivity from your conductors to the greater mass of soil.
There is also another product out there, which is called Conducrete. This is a little bit more expensive again, and that is another premier cement product with exceptionally low resistivity indeed.
There is also a product called ChemRod, which I suppose you would call it an active electrode. ChemRod is an electrode that is filled with an electrolyte powder and over time. The electrolyte powder disperses into the surrounding soil duping out and lowering its resistivity. This can be useful in certain rocky structures where the rock is quite fractured to reduce the resistivity of the rock surrounding the electrode and improve the performance that way. But I will say it is quite an expensive solution. You also have got to drive it into a borehole for it to be most effective.
Could Chem-Rods be the solution to Nordic Earthing challenges? Find out here.
Example – Wind Turbine
Let us have a look at an example for this. Here is a wind turbine project we looked at in North Wales a few years ago. This is on a quite high soil resistivity. You can see in this slide, the top 15 or so meters of soil is quite high resistivity, and it drops off a little bit under that, but it is still quite high.
This came about for a number of reasons, there’s parts of the world are just not particularly good geology. And the other factor is that the particles in the topsoil layer were quite large. We also had an issue in that there were some domestic suppliers taken from the transformer at the base of the turbine. We had to manage the issues associated with hot site classification as well because is the wind turbine had a couple of neighbours.
This soil model represents the horizontal layer realisation of the soil in which the earthing is going to be installed. The resistivity column represents how resistive the soil in that area is and the height column represents how deep that goes in the soil. This is the key site input for our earthing calculations because this is what the outputs of earth resistance and step and touch voltages depend upon.
What does the electrode we designed for the site look like?
On the right-hand side of the large plot we can see we have the wind turbine foundation. We then have a ring of borehole earth rod which goes and 50 meters and then an additional outer ring of bare conductors. Then we have a long 11 KV cable trench, which goes to the DNO point of connection. Which is a second substation.
What did we do?
We took advantage of what the geology gave us and what the other disciplines on site were doing to give us the optimal electrode for the conditions.
We drove deep boreholes to take advantage of the low resistivity layer from about 15 meters down. But we still had to build quite a big Earth electrode. And even then, we still had a hot site so we had fit and isolation transformer on domestic suppliers. But we took advantage of the excavations that were already planned. We also took advantage of the slab reinforcement because that is a readymade earth electrode.
Managing step voltages
Because of the high earth potential rise associated with this electrode, we had to take some clever measures to manage the step and touch voltages on the approach to the substation. We use what I call a top hat arrangement. Instead of just having a single ring electrode around the base of the substation. We had three rings at successive depths. So that the surface voltages on the soil decay away slowly, instead of just dropping off a cliff which they would do in this high resistivity soil.
With that, you can see in this plot how the earth potential rise. So this is voltage at the soil surface drops off quite slowly in the area near the substation, and then drops off a bit quicker as we get further away from the substation.
Hot Site Contours
I mentioned hot sites earlier, let us have a look at the hot qualifying contours for this site. Let’s zoom right in on this, you can see that the second green line is the 1150-volt contour. Even though the electrode’s large, that 1150-volt contour is kept quite close to the electrodes. We are not causing a problem for the neighbours. so that is this second green line. Then obviously the 430-volt contour, which is this yellow line is still quite large, but because we are in a fairly rural area, we did not affect any other people in this area. Luckily all the neighbours were round here. It was just a question of making sure that this 430-volt contour did not extend further to the bottom of the drawing.
What is the second touch voltages look like?
Well, as you can see, the touch voltages around the substation plinth are acceptable. And all the way around this top hat arrangement the step voltages are again within acceptable limits. We have done everything we need to do to keep people safe. You can see the step voltages just plotted and the step voltages are going to be highest at this corner of grid here.
What was the earth resistance?
So, for our 597-amp fault current with we have an EPR of 3.5 kV, which is by any means of measurement extremely high in earthing, but it was the best we could do on this site. And so even with this fairly extensive electro we have only achieved an earth resistance of about six ohms.
What I wanted to summarize with this is that even in difficult circumstances regarding geology, you can still achieve a standard compliant design without going for commercial products like ChemRod and Bentonite, just by applying clever design. It is really important to take advantage of any and all excavations were possible. If anyone is digging a hole for something else, there is often no reason you cannot just put an earthing conductor at the base of that trench. There’s just no easy way of getting around having metal in contact with the soil. That is ultimately what we do as earthing designers is to tell you where to put metal in contact with the soil.
In my presentation, I talked about how different geologies require different approaches. I talked about some of the different design approaches you can use to work around difficult geologies. But the one thing I want you to take from this session is that commercial products are not a magic bullet. It is often cheaper to install more unconventional earthen components in a bigger area around your site, often that achieves better results for less money. As I said earlier, there is no good substitute for copper in contact with soil. And with that, let us open up to some question.
What are the requirements for temporary systems?
Ian: Okay. If you’re looking on site, and there’s a requirement to energize the site before completion, then there is a duty of care if you like. From the design team, as well as the client, that people coming into the site to carry out the installation of whatever it happens to be on a construction site, they still need to be safe, basically. Even though a site might have a temporary situation, there still needs to be a due diligence piece done to make sure that the people on site doing that going about their work are not exposed to any hazards during that period.
So, I am hoping that answers your question, Tony on that one. Tends to be a fairly light touch approach. You do not have for temporary systems, the hazard could be equally as hazardous as when the construction is completed. So, if that is the case, then it would be a good idea to do an earthing study pre completion. Just to make sure that things are okay. After all the electrode that is managing the hazard could be a partial system which has an associated higher risk to it.
Can you describe the top hat idea in a little bit more detail?
Yes, sure. So, what is happening there is, if we took a look at Hugh’s piece, where you have a square kind of substation. It sees a very high EPR. Without any measures, if you went into that substation, you would see a dangerous touch and step voltages.
The mitigation for that is to obviously put a grid in so you can equalize the potential. But sometimes when you have got a small substation like that, and without the top hat, the top hat is that gradient, so you’ve got a ring around the outside of the base, and then maybe a meter further out, you’d have another ring, but sort of half a meter deeper, or even a meter deeper. Then there may be a third ring, which goes deeper again.
The reason for that is that you are looking to provide almost like a ramp. So that when you are stepping into the hot area, or the where the EPR is at its peak, that it’s not a big jump. There is a significant difference between what your first foot and your trailing foot. That is the idea of having that graduated kind of approach that is commonly known as the top hat. Is that it provides that ramp so that the differences in potential between your back foot and your front foot are suppressed, or they are moderated.
What about if you already have an earthing system installed, but it is not working as it should do?
How can you detect what to do about the soil resistivity?
You have an existing site, it is not performing. You have got extremely high readings, very high EPR. As a result, what are you going to do about it? And when can you know something about the geology. So, if it is if it is a site that is new to you, you obviously need to unpick what is going on with the geology that it is sitting in.
That may mean actually going to do a soil resistivity survey around the site. You can start to get insights into what is happening under the ground. In the geology itself, if you are not able to do that, then a useful alternative is to look at some geological records that might already be in circulation. So, you have geophysical reports from when the site was first constructed. In the UK, we are blessed with British survey, geological survey maps, which are a legacy record that dates back sort of 100 years, where you have locations where boreholes have been cut and drilled. Also the data from scientific research is fed back into this. So, one, you cannot beat direct measurement, if you can do a soil resistivity test, do it. If you cannot, though, then you might have to lean on the records, Legacy records that might be available.
What to do in a desert situation?
We had a question come through from somebody that was experiencing a desert situation. So, you can imagine, underneath the sand is also pretty poor geology in terms of earthing. And it was a real struggle to get any kind of connection to the earth, you know, sand is not very conductive at the best of times. When you take out the moisture element, then the sites in the Middle East can be a challenge.
So, in that case, if your moisture is being extracted, especially on the surface level, then there might be an opportunity that might be some body of moisture deeper down. Use the boring techniques to get down to the moister conditions. If it is a site that is on near the coastline, then use the same similar kind of strategy, then because you get permeation of the surrounding sea, at certain levels. And especially if it is Tidal, because you get that kind of pumping action. But if those are not options to you, so if there is a complete absence of moisture, then you have got little option.
But to try and work away of either doping, or combination of doping and introducing moisture, by means of like an atomizer. There’s interventions that we can make to give ourselves a little bit of leeway, and headroom. And it is not perfect by any stretch of the imagination. What we are trying to do is to try and control something that we have little control over. But we can make certain influences and improvements.
Have we had experience of using Rocksalt?
Yes, we have. First thing is to say, consider the environmental side of things. Once that has been done, and you’ve risk assessed that there is going to have minimum impact, then rocksalt can be used as a doping agent. It is and it is often used as part of the recipe for some of the engineered backfills. So yes, it certainly can have a positive impact.
The downside is if you are in moderately temperate climates that have rainfall. The doping effect will lessen will degrade with time because the rainfall will kind of just wash it away. So, there is a maintenance thing that must be considered. So yes rocksalt but there are environmental things that you must look at, you are not impacting adversely and to the maintenance aspect. It is not a fit and forget kind of deal you must top up every now and again.
You have installed a copper mesh and the top layer/the backfill that the copper mesh sits in is not the native geology that you’ve previously tested.
We will take that scenario, first, so it has been like a manmade backfill. Do we exclude that first layer?
No. Even if it is an educated guess as to what that layer might be because the electrode is sitting in it. That is effectively the electrical connection that you have got with the wider geology. So, it is important to understand that connective layer as much as the underlying layers that go beneath it.
Thanks for your engagement today.
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