Concrete base for a hot tub on wet clay soil

Firstly, I'm loving this forum and very glad I found it.

Secondly, after using the search function and reading as many similar posts as possible, I'm still worrying about a problem we've got in our garden.

We've been planning on putting a hot tub in our garden and we're in the process of enlarging the patio area to do it. When digging out the extra area and digging down a little, the clay soil underneath is wet. I mean water comes out the ground and fills the hole that's maybe a foot below the level of the current patio, at most.

This is the area we're planning on having a 2.5m reinforced concrete base put on. The base will be 150mm thick, or thicker if needs be. I should point out that the patio around the new area we've dug out has been there for years and is perfectly level.

The builders who dug the new area out haven't yet returned to lay the base. My worry is whether it's actually suitable to lay a base for something that heavy, on what is clearly wet clay. If I pump the pool of water out, it will just refill.

I should add that there's a lot of clay around our area and we live almost at the bottom of a hill. We're used to the clay soil being wet (as it usually is) but I wasn't expecting there to be a constant pool of water when we dug down, prior to laying the base. I've had the drains checked and there are no leaks. Soak-away works OK. Yet water fills a hole even after no rain. I have previously put a 50ft fence in along that side of the garden and when digging the holes out for the concrete posts, they would fill with water even in late Spring, but those holes are reasonably deep. This fills with water and it's less than a foot down.

Is what we're doing a very bad idea and if so is there anything that can be done? To be perfectly honest, I'm not getting good vibes off the builders about it which is why I'm worried. They may be able to lay a base on it but by the time it comes to put a hot tub on and fill it with water, that will weigh about 2,000 kgs. Will the base work OK on soil like that? The builders will be long gone by then and the tub will be a huge job to move.

Many thanks in advance for your help. I'm beginning to wish we hadn't started with this so I hope you can set me mind at ease. I fear it could be a very costly mistake.

Lots of clay in the top soil isn't necessarily an indication whether the subsoil is load bearing or not.

Neither is the hole filling with water...that could be just an indication of your geography and the time of year. Once ground gets saturated there it has no capacity to absorb more.

Clay is non porous so a hole in the ground won't soak away.


The bottom of the area dug out should be reasonably good load bearing soil...hard blue grey clay or hard subsoil. If it's yellow clay it's not so ideal. They should be going down deep enough to get to sub soil and putting on a layer of type 1 hardcore well compacted with a whacker plate.

If they make a reinforced base at least 150mm thick, it shouldnt crack, it may just move a little...because it's floating rather than on deep foundations on super hard ground.

A 2 ton load isn't much, think about the weight of an extension sitting on foundations...now that's quite a load.

I would only be concerned if you had lots of trees nearby that might cause shrinkage, if it's shrinkable clay.

Excavating was a mistake. The water will soften the surface meaning that more of it needs to be removed when it comes to laying the concrete.

Ongoing drainage will always be a problem as water is the main cause of ground movement. So that needs to be considered.

Thanks for the replies.

The clay is the dense grey/blue variety and there are no large trees within 15m of it. It's not the top soil but it doesn't take much digging down to get to it. The area we live in (Burgess Hill, Mid Sussex) used to have a lot of potteries because of the clay. The whole area around here is like it and it's a bit of a pain.

Within 1m of the hot tub base is our conservatory which has brick walls around the lower half. Not exactly the weight of an extension but it's been there long enough and is fine. We're putting channel drains in along one edge of the hot tub base where it will border next door. It's just that where the trench has been dug for this, it's partly full of water, much like the area the hot tub base is going on.

I was reluctant to put a french drain in, in case it dried the clay out too much and being close to the house, that could have caused big problems. Our house has been here for 50 years and doesn't seem to suffer from subsidence.

Would a membrane of sorts be needed underneath the concrete base, to stop water soaking into it?

I’ve got a non reinforced concrete base on similar sounding clay and it’s been fine with the tub on it for a year. You could probably find it on the search, try ‘sex pond’

I think I put a plastic sheet under the concrete and put 20mm insulation on the top.

https://www.diynot.com/diy/threads/hot-tub-area-ideas-needed.537716/

That’s it, it looks like soil but it was yellow/orange clay before I walked all over it.

Hello. How did it work out with the celotex insulation board under the hot tub ? I’m thinking of doing the same. be grateful if any advice
Thanks

Jane Carter 123 said:

Hello. How did it work out with the celotex insulation board under the hot tub ? I’m thinking of doing the same. be grateful if any advice
Thanks

Click to expand...

Don’t use Celotex, you need the EPS stuff which won’t crush.

Thanks for your reply. Shame as I have some celotex left over from an attic conversion! Has EPS made a big improvement to preventing heat loss? Is it worth the investment? thanks

I’ve never had the tub on anything else so I couldn’t say if it’s been worth it or not sorry.

Update on the concrete base. The area was dug down and a type 1 sub base was laid and compacted. This was still damp so a waterproof membrane was laid down before 150mm of concrete with 2 x wire mesh laid in, one at a 45 degree angle to the other.

It seems OK but my only concern now is how to leave it. It's a bit chilly at the moment so it's covered with a fabric dust sheet and then plastic sheeting over the top to stop it getting too cold or dry. Well I say plastic sheeting, it's the left over waterproof membrane which is black.

I want it to dry out slowly and not be exposed to any frost. How long should it remain covered for though?

The builders told me it was a pretty stiff mix they made, considering the weight of what's going on it. The hot tub doesn't get delivered until May so I'd have thought that should give it plenty of time to dry properly and harden?

Mine took 4 days to fully dry at a similar time of year and I thought that was ages.

Just checked on the concrete and it's still quite damp, but I'm leaving it covered for now.

More of a concern is that if I put a 6ft level over it, (total size of the slab being 2.4m x 2.2m) it's pretty flat against the level, apart from 30/40cm in on from one edge. Here it starts to curve downward towards that edge and there's a drop of roughly 15/20mm. I did ask the builders if it was level as to my eyes it didn't look right. He assured me it was, turns out that coming in from that edge it isn't.

As the concrete isn't fully set, can I mix up some extra to lay along that edge to make sure the slab is flat? Smooth it out with a trowel and sort of blend it in? This will all be covered so you won't see it. I just worry about one edge of the hot tub that's going there, being slightly off the ground. That surely won't do it any good.

Cheers

You might be able to do it with grit sand and cement. Did they build a wooden frame around before they poured?

They didn't use a wooden frame as it's dug down into an existing patio with slabs, so they sort of form the edging for it. The fella levelling it out also used a 6ft level, which isn't as wide as the base, which is why I think the problem has occurred. I hope that makes sense but If they'd used a straight edge as wide as the base, then I can't see how it would have happened. I took a photo of him doing it as I was worried he didn't have the right tool for the job.