Soil resistivity data

My Multifuntion has a soil resistivity function which I guess might be helpfull to anyone wanting an idea how the local soil is prior to a supply being anywhere near or any earth rod in place. I`ve never needed to use it myself as I very rarely TT anyway and there`s always a supply nearby or I can do a 2 or 3 or 4 wire earth electrode test with the meter anyway

I was just wondering how much help such maps would be? - I guess they're based on something like 1km squares which are in turn based on possibly even wider spaced samples of rock types and so on - whereas actual conditions can vary enormously from metre to metre depending on what's happened to that particular patch of soil over the last few thousand years. From my own garage experience, I get consistently good results from a single rod at the back of the garage (into basically undisturbed garden soil) but a few metres away at the front of the garage the ground is completely different - having been an unadopted road for the past century, built up by perhaps a metre or more before that to level it, and before that being covered by several old privies and other outbuildings (according to old maps I've seen). Even in open countryside depth & makeup of soil can vary tremendously if even a small stream had happened to meander that way in the distant past.


For sure a survey might give you early warning if you were considering an area of widespread poor dry shallow soils, but I can't see it comparing with a decent site survey.


   - Andy.

Be aware that the maps are only a general guide - within that there are often  quite large zones that are erratic - especially in places that have seen glaciers in the past, it is not uncommon to find clay, sand and gravel soil types within a km of each other, or even the odd chalky outcrop, while the national map just says 'clay' and in terms of no nasty surprises nothing beats a local survey.

Actually you may have seen on archeology TV  programmes like "Time Team" and similar that a fine detail map of ground resistance, often done with towed spiked wheels or previously in a simpler form with a low paid student stabbing in electrodes on a preset frame at a regular spacing, can be used to find the course of old ditches or buried foundations of buildings and so on.

wikipedia article

Jam


If you have a 4 Wire earth resistance tester, or you hire one, you could do you own site survey.


I think I have a PDF on file that tells you how to do it. It's not a 5 minute job if you have a large site.

Thanks all.

Agreed we could "just" measure it, and in due course that will need to be done ... and indeed that's what's currently done (with Wenner method, generally coinciding with some other surveys). But we have to wait (and pay) for that, and I just wondered if you could get a sneak peak at an indication of what the site's like from the desktop to help with preliminary studies, and it might possibly also help with planning the survey and later interpreting the results and selecting possible electrode locations when read in conjunction with the actual site measurements.


As an aside, the user notes do say results are given on a 50m grid, which given that the minimum order is 1km and I do get to see some pretty large sites is fine with me.


Would love to know if anyone's used it though.

maybe this image is what became a poster I'd still treat it with a large grain of salt - they have not measured anywhere near me at a 50m grid. Resitances are not measured, but are based on knowledge of local rock types. I'd also be wary of data in an unreadable format -  it is a long way from being a google maps overlay ?

Whilst you will eventually need to measure it, you can do a lot of design based on an assumption of resistivity and a design solution that is "flexible enough" to make minor modifications later on.


As an example, I did  a 60MVA substation (x2) for a data centre client recently based on an assumption of 50ohm/metre resistivity. Using a number of modified equations for the foundation and interconnection to a typical hollow square and grid array we had a workable design based on 3.6m driven rods and tape - on measurement, the more global nature of the earthing arrangement allowed us to shorten to 2.4m rods - most of the design remained unchanged.


I appreciate a degree of conservatism does cost money - but you can often spend more money than you save if you look for fine granularity at the wrong time (ie too early).


I'd use the map to get a RoM value and go from there for the initial design  - you can refine once you get closer to site


Regards


OMS

Thanks again all for your thoughts.

Quick update for those interested: BGS have finally replied to say the data isn’t available as a poster map as they provide several data points for each coordinate (best case worst case etc). However it is sold via third parties so hopefully more cost-effectively for smaller orders...