Help with water under floorboards on clay soil

[rider1972]

I'm wondering if anyone could offer any advice.

We moved in to a 1930's semi in Manchester 10 years ago and I noticed, a few years ago, some water under the floorboards at the front corner of the house. Never really thought too much about it at the time and just thought it must be due to rain etc, but having found rotten joist ends a few months back I thought I'd better take a closer look at why it's there and what's going on.

The water is actually in hole that had been dug in to the clay when a new water mains pipe was fitted at some point in the past (before we moved in), and they did not bother filling the hole back in with the clay they removed under the floor, and it's just piled up alongside the hole.

There is no water anywhere else under the floor in any parts of the house.

I've been monitoring the hole for a few months and it never moves. As in, it never soaks away, but never gets any higher; staying around around 3-4cm lower than the rest of the surrounding ground level. However, if I empty the hole of water (which is around half a washing up bowl worth), it slowly fills back up over a few days, or quicker if it rains. I can't see any route in from above/side and there are no leaks or pipes close to it, apart form the main water pipe. I have had the water board out to check the water in the hole and no chlorine was present and they couldn't find any evidence it's related to the water pipe, just a coincidence that it comes in where the water is standing.

I'm going to replace the floor joists but naturally before doing so I need to find the source of this water, and whether it's an external fault or just ground water table/level. There is no water above the normal clay surface, only in this hole.

I searched this forum and found this topic which is similar but of course each situation will be unique.

Any thoughts on what I should do? It's not a massive amount of water but enough to make the joists degrade and rot due to the constant stagnant water source.

Images (ignore the rubble, i'll get that cleared out too) below:











N.B. I've been messing with it today so the water is dirty, but normally its crystal clear, and no smell.

Thank you in advance,

J

[PaulF]

I'd suspect a leak in the water pipe; that was the cause when I had something similar.

[rider1972]

Hi PaulF,

This was my obvious first thought, but there are no joins and its not wet to touch (the blue plastic pipe) and no water is in that grey duct. The water is just a little lower than the duct itself...

The person from the water board said it definitely couldn't be the water supply as it would have shown up on the sample he did on-site 

The fact it also appears to fill up quicker when its raining outside suggests something else, but not sure what at this stage, hence the investigation 

It's so strange that it always fills back up to just below ground level and then never higher. One would have thought a leak or something else would just keep going...

[LittleWheelsandBig]

Natural groundwater level? What happens if you dig a small hole nearby?

[rider1972]

I will try tonight and see, but I did put a stick into the clay by about 30cm, 10cm to the side of the hole, and there was no water, just clay substance.

I have a feeling that if the hole would have never been dug, there wouldn't be anything coming through, as all around it this is the case.

If I was to concrete the hole in, to just above the level of the ground around the hole, it might be enough to stop it pooling? After all, it's the stagnant pool of water that's causing the same to be worse. what do you think?

[LittleWheelsandBig]

Groundwater moves slowly through clay. It takes days for water to refill this hole. A nearby hole should take days too. If it is groundwater, filling the hole should solve the problem.

[aidan.f]

Water apart, to prevent rot you need to pay attention to good underfloor ventilation probably hard to achieve in this circumstance.

[rafletcher]

Natural groundwater level? What happens if you dig a small hole nearby?

Yep, high water table was my thought too.

[Canardly]

Ditto.

[matthew]

My guess is the pipe outside is in a trench that was probably backfilled with gravel around the pipe, potentially without a sealing layer of clay on top. Therefore the rain water is ponding in this trench as external ground water and seeping through the bricks or in the annulus around the pipe penetration. I would consider waiting for a dry period (you do get those in Manchester occasionally), and hand dig outside against the wall to get down to where the pipe enters. If there is a backfill of either gravel or sand then look to ensure there is a layer of clay (or a water proof product) against the house foundations to seal out any water that may otherwise collect in the trench material.

[andytheflyer]

My guess is the pipe outside is in a trench that was probably backfilled with gravel around the pipe, potentially without a sealing layer of clay on top. Therefore the rain water is ponding in this trench as external ground water and seeping through the bricks or in the annulus around the pipe penetration.

As a formerly-Chartered Engineering Geologist, now retd (thankfully), I've seen a lot of clay and groundwater in my 40 professional years.  Unless you are in a particularly low-lying spot, with wet ground all around you, then I reckon Matthew wins the prize.  I suspect it's water sitting on top of the clay, constrained by the pipe trench.

[rider1972]

Great help guys - thanks.

Here's a pic of the outside hole where I dug down to find the pipe entrance. This was a while ago, not today, when I previously tried to figure it out and failed



Ignore the plastic tupperware boxes, I was using those to remove some water. There is no sand or gravel, just dug out soil/clay/earth to get to that

I think it is definitely related to the trench, because when I removed water externally in this outside hole, water from the inside hole drained to the outside hole so there is definitely a route/path. To confirm, there is no other water coming up anywhere under the floorboards - I've checked all the house. If I poke a stick down in to the clay, 10 cm from the inside hole, I do not hit water, so it must just be finding a way in to that hole somehow.

The question now is what should I do on the outside/inside to prevent it.

If I concrete the hole on the inside where the clay had previously been removed, to just above the rest of the ground soil, will it prevent the water from coming up from the outside hole? Should I externally as well?

I really want rid of the water pooling on the inside

Thanks

James

[andytheflyer]

Hi James,

In my working career I specified many many groundwater monitoring instruments (and installed more than a few myself). These are usually sealed into a borehole with a runny clay mix; sodium bentonite.  (Not the calcium derivative - although that could work)

I didn't think you'd have any chance of buying any less than a 25 kg bag of the stuff, but I am amazed to find you can buy Na-bentonite on eBay, as a beauty product!  I wouldn't have it anywhere near my skin as it brings me out with eczema!

However, that's what you need.  You don't need much.  You mix it into a thickish, but pourable, slurry and simply pour it into the hole.  I'd do both holes.  Na-bentonite is a clay mineral with a huge affinity for water.  It's adsorbed into the mineral structure, causing the clay lattice to swell, and it's widely used in the geotechnical business for sealing in the ground.  In dry weather it could dry out a bit, but when it rains again the bentonite re-hydrates.

I would have thought a couple of kg of the stuff should be enough.  Add the powder to some water in a bucket - start with a little water and add more water and powder as you need.

It does not set as such, but thickens up with time but stays plastic, so in the external hole you'd always be able to poke a finger into it - so you'll need to arrange a cover of some sort.  You can mix bentonite powder into cement and make a weak, plastic mortar - so that might be an option.  For the inside hole you could just fill up to the ground level below your floor joists.

Bentonite is your friend!

(It's actually a variety of clay (and there are many) called montmorillonite.  The bentonite variety originally came from Fort Benton, in Wyoming IIRC.  The montmorillonites have the huge capacity to take in and release water - not all clays do that).

[rider1972]

Hi Andy

Thanks again for you experienced point of view.

I managed to track the stuff down on eBay - does this look reasonable?

https://www.ebay.co.uk/itm/232728244166

On the outside, do I need to dig any more out or is the amount in my picture enough? And if I fill it up  with the sodium bentonite mix, to just below the surface then put some mot/sand back down to re-lay the paving flag is that fine?

On the inside do I just fill it with the same mix to just above the ground level of the surroundings of the hole? What about inside the plastic pipe collar which protects the actual water pipe? Safe to fill that too as it will be touching the blue pipe...

I'm finally looking forward to getting this sorted, as it's been on my mind for several years now and I didn't know where to turn.

Thanks

James

[andytheflyer]

Hi James,

That's the stuff.  Inside the house, I'd just pour it in the hole and let it go where it wants to.  Fill the hole to the ground level below your floor.

Outside, I'd fill to ground level and let it go off for a few days, then dig back a bit to lay sand or whatever for your paving.  Maybe put a piece of geotextile fabric (like the stuff you may use in a garden to supress weeds on flower beds) to support the sand layer and act as a separator.  If you haven't got any of that, a bit of a synthetic fabric, say nylon or Terylene or similar would do.  You want a synthetic so it does not rot.

Bentonite is inert so absolutely no issues letting it fill the duct that carries your water pipe - indeed that duct may also be conducting water under the house.  I used thousands of tonnes of sand mixed with bentonite over the years as a liner for new landfills, and that layer (300mm thick) is then covered with a 2 or 2.5mm high density polyethylene liner.  If there were any possibility of the bentonite attacking the membrane we'd not have used it as the combined lining system has to last for decades. We used to mix the dry bentonite powder with sand to achieve a specified low-permeability.  Effectively, we were making our own clay when a suitable clay was not available near the site.

Water pipes are also polyethylene (medium density IIRC). The bentonite will have no effect on your pipes.

I'm amazed from the Fleabay listing that people actually drink the stuff! But the point in that listing about not using metal utensils if you are drinking the stuff is appropriate.  The point about the montmorillonites is that the clay mineral (think of it as a Victoria sandwich layer cake) has a very large capacity to take in cations into the middle layer - these are adsorbed and become ionically bonded, so the clay sucks in the metals and they stay bonded.  It will also adsorb water molecules, but these are more weakly bonded, and can be desorbed in dry conditions - hence the ability of the clay to take in and give out water.  A reversible sponge if you will.  Other clays (e.g. the illites) have a much lower capacity to adsorb into the interlayer. 

In your situation, effectively you are aiming for a pourable plug (maybe like a single cream consistency), that stays plastic as it's always in contact with water trying to get in from the outside as the clay mineral is trying to suck up water.  You might want to experiment a bit to work out a suitable mix.  We used to do it by the mixer load so in small quantities a bit of experimentation would be useful to get the mix right.

[rider1972]

Brilliant, I'll go down that route. Do I need to wait until the ground has been drier for a few days outside or can it be poured in any condition?

Is this stuff safe to handle or do I need any specific PPE?

Finally, and I think I already know the answer, but is this better than just filling both holes with concrete because of its properties?

Thanks again,

James

[andytheflyer]

The holes don't need to be dry, but if you can get most of the free water out that would be best otherwise you'll have more dilution of the slurry than you might want.

You don't need any specific PPE - that eBay listing shows a young lady being covered with it - but it gives me eczema so up to you.  I think a pair of nitrile gloves would be sensible, and try not to breathe in the dust  - it gets everywhere!

Concrete (or mortar) would slow down the percolation from outside to inside along the pathway you've identified, but it'll never seal.  The bentonite will form an always-plastic and swelling seal if there's water about.  It'll shrink and crack when there's no water in a hot summer maybe, but once water gets in there, it swells up again and will push against the sides of the holes.  Concrete won't do that, and as the clay soil naturally moves with the seasons, you'll get gaps between the clay and the concrete. The bentonite plug will respond to the seasonally-changing moisture in the ground and seal up once it re-hydrates, pushing against the sides of the holes.

I can't remember the swelling pressure that a montmorillonite can exert, but it's substantial.  One of the first jobs I did as a young professional engineering geologist was to be sent to Mauritius (well, someone had to...) to find out why the segmental vertical concrete walls to an in-construction water service reservoir (about 5m deep IIRC) were waving about relative to each other.  I took some block samples of the clay they were founded on and took these to the local uni soils lab.  Set them up in a special apparatus, flooded the samples and measured the pressures when the clay swelled.  It was way more than the pressure of the concrete down on the foundation - the clay was lifting about 5m of concrete when it got wet.  As it does in a tropical island that gets cyclones.

Mauritius is a volcanic island, and the lavas weather down to montmorillonite-rich clays.  So they shrink and swell.

There's a bit of science to it!  The bentonite plug is the best solution you are going to get, other than somehow keeping the water out altogether.  We also used to use bentonite plugs to seal pipe trenches when we were cleaning up contaminated sites such as old gas works.  Pipe trenches act as conduits for water-borne contaminants and a metre or 2 of bentonite-rich pipe trench backfill around the pipe would stop that transmission, or at least severely retard it.  Not so good at stopping hydrocarbon contaminants though - the bentonite needs the water molecule to make it swell.

Good luck!

[Canardly]

I knew about betonite slurry being used on some sites for dewatering but all the above is really interesting stuff and most useful to know.

[andytheflyer]

I knew about bentonite slurry being used on some sites for dewatering but all the above is really interesting stuff and most useful to know.

Yup, useful stuff bentonite.  Used it too for slurry cutoff walls, fill a trench with the slurry, often with a bit of cement or PFA (Pulverised Fuel Ash - comes from the chimneys on coal power stations (or it used to!)) and it forms a permanent, plastic underground wall to keep out groundwater - handy for digging big underground basements, tube stations, underground car parks etc.

[Little Jim]

I knew about bentonite slurry being used on some sites for dewatering but all the above is really interesting stuff and most useful to know.

Yup, useful stuff bentonite.  Used it too for slurry cutoff walls, fill a trench with the slurry, often with a bit of cement or PFA (Pulverised Fuel Ash - comes from the chimneys on coal power stations (or it used to!)) and it forms a permanent, plastic underground wall to keep out groundwater - handy for digging big underground basements, tube stations, underground car parks etc.

...and also for lubricating giant caissons so they sink slowly into the ground.  It might well be very useful but it's horrible stuff- you get one small speck of it on your hand and then 20 minutes later you are covered in it.  It seems to multiply and move around completely on its own.

[andytheflyer]

I knew about bentonite slurry being used on some sites for dewatering but all the above is really interesting stuff and most useful to know.

Yup, useful stuff bentonite.  Used it too for slurry cutoff walls, fill a trench with the slurry, often with a bit of cement or PFA (Pulverised Fuel Ash - comes from the chimneys on coal power stations (or it used to!)) and it forms a permanent, plastic underground wall to keep out groundwater - handy for digging big underground basements, tube stations, underground car parks etc.

...and also for lubricating giant caissons so they sink slowly into the ground.  It might well be very useful but it's horrible stuff- you get one small speck of it on your hand and then 20 minutes later you are covered in it.  It seems to multiply and move around completely on its own.

Yup.  Been there, done that.  Mind you, I worked with a geophysicist for a few years.  He always looked smart and well turned out.  Spent a few weeks with him working on mudflats.  At the end of the day he looked like he'd just arrived on site.  I looked like the monster from the black lagoon.  Every day.  I just seem to attract the stuff.

[matthew]

I knew about bentonite slurry being used on some sites for dewatering but all the above is really interesting stuff and most useful to know.

Yup, useful stuff bentonite.  Used it too for slurry cutoff walls, fill a trench with the slurry, often with a bit of cement or PFA (Pulverised Fuel Ash - comes from the chimneys on coal power stations (or it used to!)) and it forms a permanent, plastic underground wall to keep out groundwater - handy for digging big underground basements, tube stations, underground car parks etc.

Also useful to prevent a plume of ground water contamination continuing to migrate. E.g a fuel oil spill in proximity to a field of greensands abstraction boreholes.

[rider1972]

Update...

Managed to dig the outside hole again today and remove any bits of gravel and wet ground to just underneath the incoming pipe.

Also cleaned up under the floor in the inside and mixed/poured the sodium bentonite in to the hole. Unfortunately 3KG of the stuff mixed into a single cream consistency didn't quite cover the inside hole, and not touched the outside one yet, so probably need another 3KG of the stuff off fleabay...

Pics here - https://photos.app.goo.gl/zjW2XYfKwhmkbgZ98

Silly question, but one that just came to me, is that is it actually better having that clay mix in the inside hole over stagnant water, seeing is most of it is water mixed in anyway! I'm sure there is... but will it still cause dampness, enough to affect the timbers.

I've also added a double airbrick from the outside to vent the area (it was previously an old single airbrick)

Finally, how do I actually know if it's worked? The hole is filled with this runny clay, so there still might be water getting in and affecting the mix?

Thanks!

James

[andytheflyer]

Hi James.

Those latest photos show a hole a good bit bigger than I thought it was - so you'd need a bit more bentonite.  Starting with the outside hole is a good idea as if water is flowing from out to in as you thought then that's the hydraulic gradient and the bentonite slurry will go that way too. 

The slurry will thicken up but it never sets hard, and you don't want it to - it needs to be plastic.  Give it a few days. 

We used to pour the slurry into water-filled boreholes, and it can be pumped in under water to form a seal.  The slurry is heavier than water so should sink to the bottom of the hole and seal from there - which is what you want.  As the clay soaks up water (i.e. it hydrates) the permeability of the mass reduces and seals up the hole.

Ideally, you'll see traces of the clay appearing in the inside hole, which is good, because that means the clay is permeating through the pathway and will eventually seal.  Don't think of it like a silicone mastic, it's more of an insidious permeator that gets everywhere and will swell to seal.

That last photo in the set looks perfect - that looks just like I'd expect to see it.  I'm sure you are on the right lines, just give it a bit of time to do its stuff.  Maybe remove water from the inside hole if you can and see what happens?  Always difficult to advise remotely when I can't see it with my own eyes, but this is is what bentonite does for a living.

I'd not get any more bentonite for the inside hole yet.  Dry the inside hole out as best you can and see what happens over the next few days. 

[rider1972]

Hi Andy

As the pictures show, I've actually filled the inside hole first, just because I didn't want it filling back up quickly if it rains and it's a pain to empty manually at that depth. Hope that's okay. At the moment it's still quite liquidy, like you said, single cream consistency so I'll leave it a few days and see if it thickens up any.

I can't fill the outside hole until more bentonite arrives, probably Tuesday/Wednesday this week!  I am also going to fill the duct that the pipe coming in through too... same runny mixture okay?

Any thoughts about if it will potentially cause issues on the inside as it's stil a pool of water (albeit thicker and mixed with clay.

What did you mean by Maybe remove water from the inside hole if you can and see what happens...

Thanks

James