Percolation test on made ground - how deep?

[Ed_]

Doing a demolish and rebuild.

Existing rainwater went to a soakaway, which unfortunately had to be removed.

 

I have 1-2m of made ground over clay. Clay is, apparently, essentially impermeable. Don't bother with a percolation test they say. All well and good but I've been quoted £25-30k for connection to the surface water drain - 8m away across a quiet residential road! I'm sure I can find a cheaper quote, but got me back to thinking about soakaways.

 

Clearly the existing soakaway worked, I'm on a very slight slope and I guess the water percolates through the topsoil/made ground downhill, slowly adsorbing, evaporating and eventually ending in the river.

 

The advice on a percolation test seems to be to dig it at the level of the soakaway. If I choose a broad flat soakaway design, I'm sure I could bring this above the top of the clay layer and reasonably expect it to drain adequately.

 

Is there anything wrong with this logic? It seems percolation tests aren't exactly very well controlled. 

[Crofter]

I did perc tests for my build and the council were very hands off. I could probably have made up any old number if I'd wanted. But I wanted the system to actually work, so I did it all properly.

That was for foul water, where you have to ensure drainage in to the aerated upper level of the soil, whilst maintaining a minimum cover depth (can't remember the exact figure for that).

 

Your quote for a connection across the road sounds absolutely bonkers. What sort of size pipe are you talking about?

[Ed_]

100mm. Attenuated to 2l/s, I think.

[Gus Potter]

9 hours ago, Ed_ said:

Doing a demolish and rebuild.

Can I ask, did you buy this at auction?

[saveasteading]

The quote seems silly high, although it can cost thousands to get a road crossing licence and do traffic lights etc.... then the council will rightly want the highest highway standards for the reconstruction.

 

Percolation tests are described in the Building Regulations Document H. Please read then return with questions.

 

 

[Ed_]

5 hours ago, Gus Potter said:

Can I ask, did you buy this at auction?

No...

[saveasteading]

15 hours ago, Ed_ said:

Clearly the existing soakaway worked,

Meaning, the water went away. But on made ground it could cause settlement or erosion. On a slope especially, it could cause issues to you or elsewhere.

The solution is probably to spread the water over a large area, and distant from the house.

Doing the percolation test is a few hour's work with a spade, a bucket and a tape measure when at ground level, But your made ground may be an issue.

 

Do you have a design intention  for the foundations on that ground?

[Ed_]

26 minutes ago, saveasteading said:

The quote seems silly high, although it can cost thousands to get a road crossing licence and do traffic lights etc.... then the council will rightly want the highest highway standards for the reconstruction.

 

Percolation tests are described in the Building Regulations Document H. Please read then return with questions.

 

 

It says to a depth of 300mm below the invert pipe. That will be fairly shallow, within my made ground layer.

 

From part H the depth seems quite clear, but when I read about soakaways and percolation tests, a lot of the Information suggests I should conduct the test in the undisturbed layer, ie the clay below the made ground.

 

I feel reasonably content that doing the test in the made ground will give me a representative result, and if successful a working system like the existing one, but I am hardly an expert. 

[Ed_]

The foundations will be down to the clay, it's a basement so will be a sort of raft. Soakaway can be downhill and I could put it 15m-20m away I think.

 

You are right, best to dig a hole and see what happens first.

[saveasteading]

9 minutes ago, Ed_ said:

best to dig a hole and see what happens

That's the answer to so many queries!

 

I'm thinking your fill will change to clay somewhere on your slope so dig the clay there.

It will probably fail but that is a start. 

Then we can look at options.

 

At the bottom of your hill, what do we find? Fields or houses or what? 

[Ed_]

1 hour ago, saveasteading said:

At the bottom of your hill, what do we find? Fields or houses or what? 

Houses. This is an urban area.

 

All the houses up the hill, so far as I can tell, drain to back garden soakaways.

[saveasteading]

4 minutes ago, Ed_ said:

to back garden soakaways.

..and potentially down the slope,  along the clay surface to you. Any sign of that or does it seem to go away?

[Ed_]

It seems to go away. No signs of flooding or waterlogging.

 

The gradient is minor but still that would prevent any sort of flooding, the water would just run off. In the end there is a road at the lowest point, and if it gets that far it will enter the drains.