Stabilising boggy ground

[Tony K]

Evening all.

My SB is built into the corners of my small plot, and has required me to excavate the footprint down about 800mm. Inside this area a host of steel screw piles have been inserted, and we are now beginning to create the slab. It's clay soil. 

 

The recipe for the slab is 50mm of concrete blinding laid on the soil, then 225mm of cordex (collapsible void former) on top, then the rebar gets attached to the pile heads, before the whole area is filled with concrete. 

 

Despite draining the surface several times, and the recent hot weather, the surface remains very boggy. We believe that though the clay is largely impermeable, some small amount of water is getting through - a white line is visible towards the base of the excavation walls which I presume is salt or something. Above the line the clay walls are bone dry, whilst below they are damp. This makes it hard to dry out the surface, and when we walk in it we sink in up to our ankles in places. 

 

This makes it hard to progress. We need the blinding to be solid enough to walk about on for a month or so while we build the rest of the slab, but as it stands the blinding will be pushed down into the bog when we walk on it. 

 

Possible solutions range from digging a ditch along the side and trying to drain from it, to extra deep blinding, to boarding the area prior to blinding it. One friend has suggested use of some kind of coir matting with crushed concrete over the top, though neither of us has any experience of this. 

 

Anyone faced and solved this problem before? 

 

Any ideas gratefully received as usual! 

 

Thanks 

[Temp]

My builder used loads of limestone hardcore to stabilise our clay site. He effectively built a road along one side so vehicles wouldn't churn up the place.

[Tony K]

8 hours ago, Temp said:

My builder used loads of limestone hardcore to stabilise our clay site. He effectively built a road along one side so vehicles wouldn't churn up the place.

 

Thanks @Temp

 

We had considered this option. Not cheap, but might be worth it. A geotextile firm I spoke to this morning didn't have any obvious solution they could offer. 

[Jilly]

I'm on clay too and it veers from what you describe, knee deep mud, to setting like concrete.  Is there any chance there could be an underground spring keeping the land wet at the moment as it's been pretty dry and we are in a concrete phase?  I would second placing the aggregate over a geo textile (coir will rot eventually) and I'm led to believe its better to do this job when the ground is drier as it seems to work better, as when it's wet, the clay can seem to swallow it (without the geo textile).  It might be worth speaking to a local drainage engineer. 

 

 

Edited June 10, 2021 by Jilly

[Ralph]

We had to put down tons of hardcore and dig a drain to get the water away. Ended up putting in a pipe and filling in that ditch with hardcore to keep the water away. There was no way the machinery would have coped with the level of clay mud.

 

[George]

With clay sites you have to put stone down - think about where patios/gardens will be so you can just put it down and leave it. Dig down 150mm and stone up. Any type 1 stone would work as well as limestone. A 6" layer of stone won't sink unless something really heavy is on top. 

 

I'd also consider encircling the house with a French drain, or at least sloping the ground away, but that depends more on the surrounding elevation/geography whether that's really necessary.

[joe90]

5 minutes ago, George said:

I'd also consider encircling the house with a French drain,

+1, my builder did this, we are on heavy yellow clay and the water table can almost reach the ground level in winter. We encircled the house and the garage with French drains and piped it into a nearby ditch and have had no problems since. 

[Tony K]

Thanks all. I'll try leveling the surface with the digger and creating a moat around its edges, just inside the excavation walls. I will pump any water away from that moat over the weekend and see if the main surface dries out properly. Then I'll consider extra material and possibly a matting underneath. 

[IanR]

6 hours ago, George said:

I'd also consider encircling the house with a French drain, 

+2

 

We installed a perimeter drain all the way around our build, going to below the foundation level. There were numerous seams in the clay with water constantly running through them.

 

[Tony K]

I doubt I could have a French drain around the house. It's built into the corners of an urban plot, so I've no space outside for it, plus the design has the slab backfilled around the edges in what I think they call a fit and forget arrangement. 

 

I will set up a temporary moat drain inside the footprint and try to save the cost of extra hardcore etc. 

[scottishjohn]

from your description you now have a clay lined 800mm deep pond?

 if that is correct then you will have to put a drain ,if only temporary  in there to drain it out --so that means it has to run downhill from the base of your clay pond to somewhere 

[Tony K]

2 hours ago, scottishjohn said:

from your description you now have a clay lined 800mm deep pond?

 if that is correct then you will have to put a drain ,if only temporary  in there to drain it out --so that means it has to run downhill from the base of your clay pond to somewhere 

 

It was a pond in winter! I've nowhere to gravity drain it off too but it doesn't matter as I've got the sewer run next to the site and a small pump is more than sufficient. 

 

It's just the base of the area. It dries hard enough in some places, but stays mushy in others, suggesting rising moisture. In fact when we literally hover one or two areas dry in the morning they have tiny puddles again by the end of even the hottest day. 

 

 

[Dave Jones]

talk to the people you use for muck aways they often sell cheaper recycled stone. Dig down 250 if you can and base of 150mm of 40-70mm clean, roll or wacker than top it off with 100mm of type 3 (recycled is good).  

[Jilly]

Have you got a SuDS planning condition? 

[Tony K]

2 minutes ago, Jilly said:

Have you got a SuDS planning condition? 

 

Nope 

 

[Jilly]

That's lucky, its a bit of a palaver

[Tony K]

It should be as simple as showing them your soakaway design shouldn't it? 

[scottishjohn]

If it is as boggy +clay as you say ,then you could have problems making a soak away work.