Help with water under floorboards on clay soil

[andytheflyer]

Hi James,

My comment about removing the inside water was because I'd misread your post.  I thought you'd filled the outside hole first and run out of bentonite, hence see what happens.  I'd expect that if the bentonite does what it should and you'd put it in the outside hole first, drying out the inside hole should mean it will stay dry, if the outside hole is the pathway.

It may be anyway that there's stored water sitting somewhere in the ducting though so the picture will not be as clear cut as maybe I'm seeming to make it, which would mean that you could dry the inside hole, with bentonite in the outside hole, but that the inside hole refilled.  That would make it look as though the bentonite wasn't working, but in actuality the water was coming from a stored resource inside the house, probably in the ducting.  Sounds complicated, but it isn't really. You are dealing with a micro-site (in my terms!) so you may get some unexpected flows, but the bentonite will do its job given a few days.  As others in this thread have said, bentonite is widely used for sealing up groundwater flows, which is what you have here - just on a very small scale compared to what we normally use bentonite for.

Runny mixture is what you should aim for, and yes, fill that duct as it's a pathway.  As thick as you can make it but will still flow - the thicker the better but it's got to flow to where you need it.  If it's too thin then the bentonite may be fully hydrated and can't take in any more water, so can't swell in-situ.  Too thick and it won't fully permeate the duct.  A tricky balance but single cream should be about right. IME, our drillers were not too picky about the mix and it always seemed to work.  We could test our groundwater instruments and they worked, regardless of the bentonite mix used.  It wasn't rocket science - just as well as most of our drillers struggled to add 2 and 2 and get 4.  Which was fairly fundamental when I needed to know how deep my samples were coming up from!

If you've still got free water and slurry on the inside, maybe give it a stir to mix the two together. 

As I said though, if you've got the inside hole full of slurry, the density of that slurry is now greater than 1, aka the density of water.  So, provided you've not got a huge head difference between outside and inside, there's very little chance of water from the outside forcing its way inside through the slurry.  Laws of physics apply. Does that make sense? 

By filling the inside hole first you've gone straight for the solution, from your point of view, but obscured the validation of the solution - if you get the idea!  Not a problem, I'd have started outside first but apologies, maybe I should have given you a method statement.  Anyway, you are on the right lines.

[rider1972]

Makes perfect sense. Let's see what happens now, hoping my small issue can come to a conclusion, and hopefully the extra ventilation and air flow will help too.

I'll update once I've filled the outside hole and duct.

Thanks Andy

James

[rider1972]

Ok - update time, and not so great news...

I've been checking the inside hole every day since I filled it and today, upon checking, I could see there was actually a layer of clear water on top of the bentonite mixture I poured in last week. As in, around 1cm of water pooling on top of the entire hole surface area. It's not soaked through (probably because the bentonite is acting as a seal etc) but now I am flummoxed as to how/why there is water on top!

Could it be water that's naturally come up through the bentonite mix I poured? Was that to be expected? I was expecting the bentonite to stop the water rising?

Really, really odd...

Thanks,

James

[andytheflyer]

Hi James,

Assuming the bentonite plug has a putty-like consistency, then there's virtually no chance that the water is coming up through the plug.  The coefficient of permeability of the hydrated bentonite will be around 10^-13m/s, compared to a typical clay around maybe 10^-10. Sand would be around 10^-3 to -5m/s. Of that order.  So the bentonite is typically 1000 times less permeable than a natural clay, give or take.  We aimed for less than 10^-9m/s for a landfill liner.

So, water in the soils under your house is moving around the bentonite plug.  If it's sitting on top, that's because the standing level is higher then the plug top.  It's finding its own level. 

If you've filled the outside hole and the pipe duct, then that's not the only pathway.  The water is permeating through the soils under your house.  The duct flow was part of it, but not the whole part.  I'd suspect that the water is actually permeating from outside to inside on a much wider front.  Without seeing how your house sits in the landscape it's difficult to advise further.  Water flow occurs naturally in the ground, from high areas to low areas - obviously.  The depth to the groundwater surface varies greatly, from many metres, to almost at the surface, which is what you'd have in low lying areas, such as flood plains.  That why we sometimes see housing being flooded from below as the g/w rises near rivers in in wet weather.  The river surface may well be the local g/w surface too. 

If you have a hole outside the standing water level in that hole (may take a few days to equilibrate) should be similar to that above the bentonite plug inside.  Bear in mind that the flow direction may not be the simple route from the outside hole to the inside hole.  It may be the other way, or a variation of it, with the water actually coming from the other side of the house, through the external wall.  Does that make sense?

You could try a drop of drain tracing dye in the water in the outside hole and see if appears in the inside hole.  It may not if the flow is actually the other way. 

Whatever, if you've plugged up the pipe ducts, the flow is from somewhere else.

IME, it's unusual to have g/w as high as you are seeing.  You'd have well-vegetated garden and lawn because there's abundant water, so I'm wondering if you have a leak in your mains supply (or next door's?) somewhere that's feeding the underfloor area.  I recognise that your supplier said it wasn't mains water but not sure how they'd know that by the time it had passed through several metres of soil, filtering out and adding substances as it goes.

I'd suggest some simple holes around the outside of the house where you can to observe the water levels across the floor plan. If you go that route, you need to prevent rainwater runoff filling them up - it's the water level in the soil you need to find.

More investigation required I'm afraid.  Ultimately, if it's not a mains leak, then you may need a cutoff drain around the outside of the house to intercept g/w flow and take it away from the house.  Typically, a trench say 1m deep with a perforated pipe in it and a gravel backfill. 

Are you near a river or watercourse?

[hatler]

Am I alone in finding this thread fascinating ?  (Other than the OP and atf obviously.)

[Kim]

Nope.

[hatler]

Nope.

Phew.

[rider1972]

Hi Andy

Yes, its a putty like consistency. If I push my finger in to it a little, it makes a hole, but stays a hole shape, its not wet enough to just move around.

I only filled the outside hole yesterday, that's when I noticed the inside water as I was finishing off. So I might need to wait now to see if that has actually helped. I didn't have enough to fill the duct on the inside, but its thoroughly covered in bentonite on the outside so theoretically water shouldn't be able to enter it - let's see. I have also tried using the dye stuff in all drains on my property and it never shows up in the hole.

Thing is though, the inner of the duct on the inside is never wet for as far as I can reach down it, so I don't believe its rising to the top and then filling the hole from above. Weird.

Surrounding the hole both in and out is undisturbed clay so I'd not expect water to easily permeate that high enough to then get on top of the bentonite on the inside hole.

I am not near a river but there is a canal not a million miles away but can't see that being anything related. Just spoke to the neighbours either side and none of them have ever seen an issue with this under their floor. The directly attached house (semi) totally refurbished his a year ago and had all the floors up and there was nothing).

This is not my precise location but I live on this street:

https://goo.gl/maps/rywVrUFBQTLHG99d8

Is there any way / tricks of how I can monitor how / where the water is getting through from the sides of the hole? As the inside hole is not completely filled to the level of the ground around it (ran out), if I was just to fill it more, I suspect no water could get in, but if its sitting on top of it now, I suspect its not coming through, but around/in from the sides?

The plot thickens...

James

[Ham]

Nope.

Phew.

...and I even sit on gravel

[andytheflyer]

Hi James,

Seems we have an audience.........

You've got the bentonite mix spot on - putty-like is exactly what you want. It went in as a slurry, hydrated and probably picked up some more water from the surroundings, and it's set off. It'll go harder if it dries out, and may even crack, but it'll heal itself and swell up when it gets wet again.

Canal is not an issue.  I know roughly where you are.  I'll dig out the geological maps and get back to you on those.

The clay under your house will not be a particularly good seal.  It'll have lots of disturbance as a result of your house being built, and there will be brick ends etc incorporated, so it won't be particularly 'waterproof'.  There may even have been some previous use of the site - you can check this on the Old Ordnance Survey websites (for free).  You can go back to the 1st Ed of the OS, usually around 1850.  You may have had a bleach works for example since you're here up north, or some industrial facility on your property before.  That'll have disturbed the ground too so what you might think is a clay and should be a seal, in fact is riddled with discontinuities which can transmit water flow.

FWIW, when I was building landfills, even if we were sitting on metres of clay, the top metre or so came up and was replaced in layers, rolled and compacted.  There's a lot of QC associated with that process.  That's because there's micro-fissuring etc in the clay as a result of freeze-thaw at the end of the glaciations, and desiccation as a result of weather, and there are always inclusions of granular material in the North - so a natural clay may not be a particularly good seal.  You might get away with it in the Oxford, Kimmeridge and London Clays, but no chance in the northern clays.  The northern 'clays', other than the Coal Measures clays, tend not to be very good at shrink and swell too, and their clay mineral content can be low, being replaced with rock 'flour' from glacial action.  Given your location, I'd not expect the local clays to be a particularly good seal, and my earlier point about water flow under the house is even more valid. 

Monitoring:  not easy, particularly given the 'micro' nature of 'the site'.  Is there anywhere else you can get a look at the soil under the floor?  If it's more of a widespread phenomenon, then that tells me it's a wider flow under the house.  If other places are dry, then it's probably a confined pathway associated with that water pipe, as you expected. Don't forget, that it's laid in a trench in the clay and that pathway will have a very, very large effective permeability when compared to the surrounding clay.

Can you investigate the pipe trench further under the house, i.e away from the outside wall?  I wonder if the water's coming that way rather than from the outside, i.e. the water flow direction is the reverse of what's been assumed so far.  That was my pont about dye tracing, although the natural filtration effects of water passing through soil may take the dye out, so it's not a foolproof test.

This is not an easy issue to resolve - given the small size of the site in geological terms, and the limited access.

Finally, are you sure you don't have a water pipe leak?  That would fill the pipe trench to the top so it overflows across the soil surface under the house - it would then pool on top of the bentonite. Got a water meter?

[andytheflyer]

Nope.

Phew.

...and I even sit on gravel

Ah, you may get wet feet occasionally then, but it drains quickly!

[Ham]

Nope.

Phew.

...and I even sit on gravel

Ah, you may get wet feet occasionally then, but it drains quickly!

Indeed, gravel (a Hackney Gravel Member, apparently) on London clay. Oh, with an underground river nearby. I have dug a sump with a pump in the cellar for when it starts rising but otherwise the three-brick-deep victorian " foundations (ok, maybe slightly more) haven't shifted.

[rider1972]

We do indeed have a water meter but there is no movement on the dial at all. Can't hear it moving at all - like we can when water is actually being used.

Just going to wait now and see if the outside hole being filled solves it, then we know that it was coming that way.

There  is no wattled under any other part of the house...

[hatler]

The leak could be upstream of the meter of course.

[rider1972]

Checked a few times all along the water pipe from the hole to the meter and nothing

[Defblade]

Am I alone in finding this thread fascinating ?  (Other than the OP and atf obviously.)

Another avid reader here.
Big hand to Andy for the trouble and effort he's going to to help with this, btw... it's the expert knowledge that makes such good reading



Our house is from the early 1700s and just sitting on top of about 2m of soil then lime or sand stone bedrock (it varies around here). Has the odd bit of damp, but it's been up for the best part of 300 years so we try not to worry about it too much

[aidan.f]

Just a tip on the water leakage front. Fit a pressure guage inside the house*. Then turn house incoming stop cock off and note rate of pressure drop (should have a half life of hours) . Then house on / street water inlet off. I diagnosed an underfloor leak in this manner. Does rely on stop cocks not leaking so pressure not being 'topped up' but you can check this before the test. (overnight dribble check into a pan in sink with the cock off). IME this will be far more sensitive than eyeballing a water meter which won't resolve slow leaks due to internal mechanical sticktion.
* Think track pump style

[rider1972]

Ok, I am back... not good news I'm afraid.

Water has filled the hole to the top, and it has made the bentonite mix gooey and sludgy and now it's just a bit of a mess.

I could visibly see a tiny stream of water entering the hole from one side of the hole, just above the level which I filled the bentonite up to. So either I need to go higher and right to the top of the hole, or come up with another idea.

When I emptied it, it filled back up.

This is during heavy rain today. It's definitely not a leak or the water pipe, it's definitely when it's raining and wet outside.

Again once the hole is filled, it never fills any more, and not higher than the tip of the hole.

Thanks for all the advice so far, another step closer but also away

Thanks

James

[andytheflyer]

Hi James,

The bentonite will go soft at the top.  As it continues to adsorb water molecules into the interlayer of the clay mineral 'layer cake', the molecular bonds across the layer that hold the clay together become more and more satisfied, and that reduces the total bond attraction across the layers.  The 'sponge' layers are therefore more free to move as they are less strongly bonded to the jam layer. Conversely, when the bentonite dries out, fewer of the bonds are satisfied, so the bonding becomes stronger and the clay becomes stiffer.  The sponge tries to grab the jam more strongly as there are unsatisfied bonds.

That's why 'mud' becomes more slippery when it's wet.  The strength of the clay is inversely proportional to the moisture content.

Normally, we'd cover the top of the bentonite with a layer of something to mechanically protect it, sand for example.  The original intention of the bentonite was to seal up the duct and the pathway around the pipe, so the top of the plug didn't matter.  I'd suggest that the bentonite has done that.

The bentonite looks like it's shown that the water flow is not via the pipe backfill, it looks as though it's coming across the top of the soil surface under the floor.  Probably in the most disturbed layers at the top of the soil profile.  This was then accumulating in the holes you dug down to the pipe and it looked like the flow as associated with the pipe - and it may have been, but that wasn't the full story.

You now have to ask why?  What's changed, or has it always been like this but you never noticed?  I suggest you try to work out the flow direction so that you can intercept the flow upstream and drain it away.  Has anyone put any paving down nearby, next door maybe?  Block paved a driveway?  Broken or blocked surface water drain?  If the water is a new phenomenon, then something's changed to cause it.

I still need to look up the geology but been a bit busy the past few days, wife in hospital.

[Canardly]

Combined or separate drainage system? Blocked soakaway/weeping gulley?

[rider1972]

Hi Andy

Hope your wife is better. Did you get chance to look at the geology for this area?

With regards to any changes, we've been here for 10 years and nothing has changed, no changes to the outside at all, or next door. I've used plenty of dye down all drain etc

Was awaiting for more bentonite to arrive so I've now made a thicker mix and ensured the inside hole is more than filled and also around all the sides to try and stop any flow over the top and in to the hole. Whilst this isn't "solving" the problem I want to see if the hole gets any more water in it now it's surrounded by the bentonite slurry which tells me that water was flowing across the soil and the hole was the lowest point so just settled in there.





I'll let you know over the coming days/weeks!

Thanks

James

[rider1972]

Combined or separate drainage system? Blocked soakaway/weeping gulley?

Canardly, not sure it combined or not. The soil pipe from the bathroom has its own larger downpipe externally, and all the other guttering downpipes go in to their own drain but under the ground it might all join? How to tell?

Thanks

James

[Kim]

Combined or separate drainage system? Blocked soakaway/weeping gulley?

Canardly, not sure it combined or not. The soil pipe from the bathroom has its own larger downpipe externally, and all the other guttering downpipes go in to their own drain but under the ground it might all join? How to tell?

Lift a likely-looking manhole cover and see which direction the pipes are going in?

[rider1972]

That would mean there being a manhole cover in sight, never found one on this property.

[andytheflyer]

James,

PM sent.