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.

[saveasteading]

15 hours ago, Ed_ said:

seems to go away.

That's very promising then. I was imagining it seeping out of a contour on your slope.

 

Get your spade going for the tests. Even if you plan to get someone else to be involved, it's worth doing these yourself and having a think first.

 

[Ed_]

So i dug a hole, it drained away quite nicely - 40cm in <4 hours. All good I thought.

Just been back to check and record it more officially and there was 20cm of water in the hole. Scooped a bit of soil out in case it was clay settling at the bottom but made no difference. I suspect I have perched water, as it hasn't rained much recently and it drained away in November, plus it is higher than surounding ground so unlikley to be the water table.

If this is the perched water table, at a depth of 60cm, a soakaway seems unlikely to pass, as the minimum depth crate I can find is 40cm which if placed at 60cm depth only gives 20cm of cover which is probably a bit low. Does anyone know?

 

My next plan is to cast around a bit to see if there is a better spot, where either the soil drains better or the perched water is lower.

Would another option be to make the soakaway larger so it has enough volume even with 15cm of water in the bottom from the perched water table?

Or has anyone done a rain garden with no overflow?

[saveasteading]

1 hour ago, Ed_ said:

has anyone done a rain garden with no overflow?

Yes.

Depending on your topography you could have a lagoon and rain garden . this then works as a soakaway but also loses water through evaporation and from transpiration of any plants.

In the winter it doesn't work so well though.

so it has to be big, and you must be prepared for it to dry out in summer. any rain will disappear before reaching it (see 'French drains' later) .

I discussed this with a Naturalist, and he agreed with me that nature finds a balance and it isn't for us to save every creature from the drying or flooding cycling.

 

BUT I had the feed drains built as French drains so that only overflows from them reached the lagoon. also rainwater harvester inline.

It worked well for 1000m2 of roof.

The planners and the LA 'nature' advisors resisted at first because it didn't fit with their hierarchy of solutions*, but I convinced them.

So yes, but don't count on approval.

* they have lists and like to see 'things'. so they wanted a green roof and crates. I explained how this prevented the water from reaching the harvester, and added tonnes of steel (so carbon) to support it.

 

You do still need the water to soak  away.

 

Is it perched water?  unlikely I think. Otherwise you would have seen it running from the sides of the hole even as you dug it.

Was the ground you dug pretty well saturated? It may simply have been oozing from there.

 

Your percolation test is decent and that is what matters. 

 

A crate will fill with water and so provide a head of water which will head downwards.

You appear to have space, otherwise a rain garden wouldn't be  a consideration, so you can have a bigger area of 200mm deep crates. OR simply do the drainage field principle as in the building regs.

 

Caution: if your rain garden or lagoon overfills, where  does it go next?

[Ed_]

I dug the hole 8 weeks ago, so it wasn't oozing then, it was dry. Now it has 20cm of water which seems to be an equilibrium. Whilst i'm generally unconcerned about it not working perfectly, as overflow will run off safely down the gentle slope, I dont want to create problems I don't understand. SI report found perched water at around this depth, but the boreholes were quite far away.

I think you are suggesting 200mm crate with 400mm of cover? This makes sense to me - thanks.

 

 

[saveasteading]

and French drains leading to them. Once the amount of storage is assessed you will know whether a controlled overflow is a good plan. I guess you could reverse this, with water to pond/rain garden and overflow to soakaway.

[Ed_]

On 29/12/2025 at 17:33, saveasteading said:

Once the amount of storage is assessed

With the prodigious amount of rain in January the water level in my test hole ended up about where the top of the soakaway would be, for about a month. Not promising. 

I was resigned to attenuation and discharge to the sewer, but the costs of doing the tank and the connection to the sewer are at least £30k, so thinking again about options.

 

Has anyone designed a swale or rain garden? I have a garden 12.5 x 25m, gentle slope (0.5m) away from house. Wondering about running a french drain towards the end of the garden and then making a depression there to hold whatever doesn't soak in by the time it gets there. I'm not really concerned if I get a lake in winter.

 

Where I'm a bit stuck is how to design it. I've looked at the suds manual CURIA 793 but it's 800 pages and not really aimed at single houses. If I just size my depression to the 1 in 100 year rainfall with 40% for climate change then is that going to satisfy a BCO?

Edited 9 hours ago by Ed_

[kandgmitchell]

How about suggesting to them a normal pipe away from the dwelling for about 5m or so and then swapping for a perforated pipe in a shingled trench lined with permeable fabric running to your "detention pond". Sketch it out, throw some rough figures at it and present it to the BCO. That's the only way to tell. Despite some horror stories most BCO's are pragmatic. At the end of the day as long as it doesn't undermine your or neighbours foundations and doesn't create a flood elsewhere then it ought to be ok.

 

Given that next year is forecasted to be even dryer than this, then the neighbours will be wanting to dip their buckets in your pond!

[saveasteading]

22 minutes ago, kandgmitchell said:

Despite some horror stories most BCO's are pragmatic. 

And for some reason don't seem much interested in drainage. Perhaps they only want a proposal that they can file.

Suggestions.

Yes build a swale. it has the benefit of a large surface area of ground and of air, so percolation and evaporation as well as storage. The problem may be the depth available. 

Feed it by French drain all the way, perhaps with parallel branches to maximise what percolation there may be. The length of run increases the chance of finding a route for the water. An old tree for example.

 

How deep was the test hole? There is often a crust below topsoil.

When did the hole dry out?

 

For your own benefit add water butts which you want to be empty before the storms.

 

[saveasteading]

36 minutes ago, kandgmitchell said:

next year is forecasted to be even dryer than this, 

Next summer yes, but winters likely to be as recently. More big storms too.

 

Bring back drizzle.

[kandgmitchell]

1 hour ago, saveasteading said:

Bring back drizzle.

Good point, can't remember the last grey drizzly February Sunday afternoon, mind you I haven't been to Scotland for a while!

[saveasteading]

3 minutes ago, kandgmitchell said:

haven't been to Scotland for a while!

If you were  watching the commonwealth games you'd have seen a real downpour. Also flood damage in recent months to a bridge over the Spey.

We are in for continental weather patterns.