Building on wet clay

[88v8]

Soggy bottom... sounds like a location or a Steve King novel.

Unfortunately it's the bottom of the trenches for our garage. Standing water 2" deep, and where we've dug 4ft down for RSJs, 24" deep. Clay that would delight a pond builder. Or a hippo.

Our builder, a diligent and gung-ho chap in his mid-20s who has no incentive to bodge as he's on dayworks, is of the opinion that we can pour the footings (36x 18") as 'the water will float to the top'. Well, yes, but shrinkage.... settlement...

Options seem to be compacting perhaps a 4" layer of inch stone into the bottom, or treading a concrete mix into the bottom and leaving it to set, or ???
His uncle, long in the business, has mentioned putting 'polystyrene' in the bottom to counteract heave. Cordek have a product but it's not meant for footings and in any case needs a level, sound, base.
http://www.cordek.com/construction-products/ground-heave/cellcore-hx/benefits/

The garage is below the size that needs Building Regs, so we can do our own thing. Which is all very well if one knows the thing to do :?

There is also a raft to do but that can wait, even until the roof is on, as we're coming up off the footings.

Other than waiting perhaps weeks for the ground to dry out, or something exotic like bored piles, are there any good options? Not getting any younger here, I want it done.

Ivor

 

[Feltwell]

I suspect you'll be OK pouring the footings as you are. It only needs to be a relatively weak mix, what you're putting in will almost certainly be stronger than is needed, and it will push the water to the top as your builder says. It will weaken the mix a bit but it's unlikely to matter.

Harking back to my civil engineering days - rather bigger structures such as bridges - the first bit of concrete to go in as known an "blinding", and is just a few inches of a weak mix to give a level dry surface to build the reinforcement steel cage on for the foundations proper. The soil the blinding went on had to reasonably dry, if we couldn't scrape back to dry surface we would sometimes chuck a few bags of cement down and mix it up bit, which would stiffen the surface mud, or if it was really wet we'd dig a small sump to one side, get some perforated pipe, drop a submersible pump inside to get rid of the water and then pour the concrete, either leaving the pipe in place, cutting it off and filling it full of concrete, or pulling it out as the concrete went in - often they would stick fast though.

If you're worried then you could do a version of this. If you didn't want to buy pipe then dig a corner outside of your footing and outside of the structure altogether, slightly deeper than the footings, make up some shuttering out of timber to keep the concrete out but still let the water in, and hire a submersible pump for the day. Once you've poured your concrete you just backfill the hole, and if it's outside of your structure and settles a bit more for being disturbed then it doesn't really matter.

If the bottom of your footings is still too soggy for comfort then putting some cement powder or stone down won't do any harm.

As to the dangers of heave - not something I've any experience of, but I would suggest it is differential settlement or heave that would be your biggest problem, i.e. if one corner was much wetter and on more dense clay than the rest. If the whole of the footings are on similar soil then I can't see too much of an issue for relatively small structure such as a garage.

 

[Zebra]

What expertise we have on here!

 

[Feltwell]

If you spot any do let me know :wink:

For me the reason the forum works is the spread of experience we've got between us across such a wide range of topics - from leadwork to groundworks, upholstery and decorating to electrics, and everything in between!

 

[stuart45]

Footings are normally pumped out first and then creted on site. You can't see the pegs when full of water. Clayboards are used on the sides, rather than the bottom of trenches as shown here.