Author Topic: Help with water under floorboards on clay soil  (Read 2433 times)


  • Andytheex-flyer.....
Re: Help with water under floorboards on clay soil
« Reply #50 on: November 07, 2020, 10:39:24 am »
Hi James,

I've looked up your local geology.  You can do the same, the 1:50,000 and 1:63,360 sheets are all available to view online. The majority come in 2 versions, Solid, and Solid and Drift (or Superficials).  The former shows the rock strata under the ground, the S&D shows the soils and glacials etc that overlie the rocks.

You are on Sheet 85, Manchester.  You can get much larger scales, but you have to buy them, they are not online.

I've snipped out your local area and attach these.  You should be able to work out where you are from the M60 and the railway lines - assuming always that I've got it right in the first place.

James Superficials by AndyFtheflyer, on Flickr

James Solid by AndyFtheflyer, on Flickr

The top one shows that your area is all underlain by glacial till (aka Boulder Clay - the pale blue colour) and in your area this is normally a stiff sandy clay, probably grey or dark grey, but may be brown near the surface (where the iron oxidises) and will contain gravel and cobbles of mudstone and sandstone derived from the underlying Coal Measures.

The lower map shows the rock sequence.  This takes a bit of understanding but your house sits above the mudstones and coals right at the top of the Middle Coal Measures.  So, you'll be sitting on the heavy clay till, left behind by the last glaciation some 10,000 years ago (or so), and that sits on top of the mudstones of the Coal Measures.  The maps don't tell me how thick the till is.

In short, the ground in your area is not free draining, water will sit on top of the clay till and not go very far.  Equally, the groundwater surface, wherever it is, won't change much over the year.  The groundwater in your area will not be abstracted for public supply and annual fluctuations will be attenuated by the overall low permeability.  The water you see is almost certainly not the regional g/water surface - that will be deeper - but a perched water sitting on top of the till. 

If you have water under your house, my bet is still on a leak from somewhere, which may not be on your property.  Could be next door in the uphill direction.

That bentonite's looking good though - exactly what I hoped you'd get.

And just for info, nothing to do with the issue, you can see a line marked AGMB just N of your house.  There are a few of these. (The solid black lines are coal seams). They are Marine Bands, the AG refers to a certain fossil shell.  These are thin (maybe <300mm) bands stuffed with that fossil and can be traced many, many miles. They act as markers in the vertical sequence so we can work out where we are in that sequence - one coal seam looks much like any other until you know where you are - these fossil bands give us the vertical position.  Some of these can even be traced over to the West German coalfield (but don't ask me which ones - I've long forgotten), they are that extensive.  Given their thinness, that's some feat.

Re: Help with water under floorboards on clay soil
« Reply #51 on: November 07, 2020, 12:12:45 pm »
Thanks so much Andy.

So nothing strange showing on the maps and that it's just clay all around here... so likely that the natural groundwater level is below this and shouldn't be what my problem is related to.

I'll have a more detailed look later.

My neighbours house up from me is built around 3-4 bricks higher that my property so his drive/side/back is higher so suspect some natural run off from that but it never floods or visibly forms more surface water on my side.

Let's see what happens now...