Porous sub base for gravel

[nwmac]

Hi all,

I have a minor drainage problem in my back garden, close to the house. The house is brand new and soil is heavy clay type. Garden is approx 4m wide x 7m long. It has a slight downwards slope towards the house though it flattens out towards the rear. There's no obvious end point for a french drain.

Adjoining the back of the house are two rows of 60cm paving slabs, followed by 60-120cm of bare soil, then a circular lawn.

When it rains heavily the soil area between the lawn and slabs puddles and if exceptionally wet these puddles spread onto the paving slabs but don't reach the house. The majority of the puddles soak away within a few hours but at it's worst the small area looks pretty flooded. The lawn doesn't suffer these problems.

I'm planning on putting gravel down in the area between the circular lawn and the row of paving, where the pooling happens to be worst. I'm looking at digging down the depth of a kerb and filling it with gravel with potentially some kind of sub base.

The plan is the gravel sits nicely around the bottom end of the curved lawn and the trench it sits in acts a bit like a shallow soakaway, holding any puddles beneath the surface while it soaks into the soil.

Couple of questions:

A: is this likely to work. I don't think the drainage problem too bad and not having any grass in that area seems to highlght the issue.

B: What's a good porous subbase for beneath the gravel? Or should I just use loads of gravel? It's not a big area to fill but will be walked on a fair bit.

Sorry, this was a bit long-winded. Thanks for any info.

[haggistini]

A: if heavy clay no
b: any clean stone will drain if the soil allows it, I would dig out the messy soil to a reasonable depth lay teram or similar geotextile with an edging of some kind flush with the paving and grass to retain it and not cause a trip hazard fill it with clean stone compact and top up with a decorative chippings or gravel, this might make it cleaner for people to cross but still might not solve the problem as you say it is clay.

[haggistini]

and the water egres from the soil might be a bit cleaner too:)

[nwmac]

Thanks haggistini

To ease my mind about the drainage I read about the peculation test and had a pretty rough go at it. I dug the 30cm cubed hole in the worst area and filled it with water. Then did it again.

It's taking around 24 hours to drain. Maybe a little longer.

The interesting find was that about 25 cm down I hit what I presume was a the sub base of a previous building (the house is on the site of old hospital buildings).

It was made up of large bricks and stones, heavily compacted and difficult to dig up. I basically had to dig around the large bricks with a trowel (Time Team style) before levering them up - there's a drain cover 6 foot away and I was cautious about hacking away too hard in case I hit a pipe.

First question: How deep are these pipes buried? The cover is about 30cm squared and black. I presume it is access to a foul water pipe running between houses.

Anyway It looks as though the real drainage problem is not so much the top soil but this pretty impermeable sub base.

I suppose it does drain, though slowly, but unlike a clay soil problem it seems unlikely to further compact to the point at which it will no longer drain at all.

Do I:

1: Leave it. Dig the 15-20cm of topsoil from the area (as planned) and add the gravel, leaving the problem to drain under the surface at it's own rate.

2: Buy a pick axe and try to break through it to see how deep it is and puncture this solid layer (this hole is about 1.5 metres from the house and there's the aforementioned pipe in the area.)

3: Do another perculation test elsewhere in the (small) garden and if there's no tough sub base here, look at a french drain to channel the water to a different part of the garden maybe 15 foot away and which drains quicker. The garden slopes gently towards the house.

4: Do something else

Thanks for any suggestions

[seanandruby]

photos' please.

[nwmac]

photos' please.

This was taken a few months ago following very heavy rain. It was after I laid the lawn but prior to installing a moving strip, hence the bricks and trench. It's been this bad twice in 6 months, including the time from this photo. After normal rainfall it's not anything like this bad. It just pools in the soil closest to the square paving stones.

I dug the hole in the soil for the perc test in the area nearest to where the photos were taken from.

It's a new build house. Is it something I could ask the developer to sort out. I'd rather not have to have the lawn and paving stones ripped up, though these don't seem to suffer from flooding.

I can get a photo of the soil type in the hole I dug tomorrow, if that would be useful.

1: Stood at my back door and shows the full size of the garden.


2: From another angle, showing the drain I was talking about