Garden drainage - soakaway solution?

19 Apr 2022
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United Kingdom
Currently landscaping our garden (10m x 8m) and want to deal with drainage issues.

Garden is boggy in rain and drainage is generally poor. It also slopes down about 4 inches from left to right. We have a patio at the end of the garden going across most of the width. As you look at it, the left patio side is currently lower than the grass level, so we need to level off the grass. Raising the grass and stepping down in to the patio isn’t an option due to fence height on the right side of garden.

I have dug down 1 meter to see what is below grass level:

  • Approx 1ft of topsoil; approx 1ft hardpan (rock hard compacted clay soilI believe); below this is what appears so be sand / clay mix - orange/red in colour. Compacted, but easy to dig up. Nice and porous (have done a percolation test as per soakaway instructions and it drained nicely)

We plan to have a gradual slope from the top of the garden towards the patio (which we will turf) which will address the current lower left side of the patio.

This will however create further drainage challenges (which I think are solvable) as I’ll be removing topsoil which will effectively lower the surface closer to the hardpan. Water is also currently pooling around the edge of the patio and takes ages to drain, sometimes covering the patio when it rains a lot, so drainage needs addressing.

Proposed soakaway & french drain solution:

As there is a porous layer beneath the hardpan, I plan to put a soakaway about 2 ft in front of the patio, with a piped french drain system around the perimeter and middle of the garden. This will help carry the water down addressing the sodden grass (and the soakaway will deal with the patio rainwater).

The french drain depth will break through the hardpan to the porous layer below for additional drainage benefit.

Apart from the hard-work and cost, are there any drawbacks to this plan?

I’d be left with 3 inches of topsoil on hardpan in some areas - do I need to break and work some hardpan so I have say 6 inches of top soil all around to further aid drainage?

Any drawbacks to putting a soakaway so close to a patio? (will be 5m away from house, and 2.5m from boundary).

Sorry for long post!

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where are you planning to route the french drain water to?
FYI we installed soakaway crates (possibly these ) as the rainwater drainage for a log cabin in our garden, since there was nowhere to direct run-off. Essentially a digger excavated a 2-3 cubic metre hole and we put the crates in with our gutter downpipe feeding in, and a foot of soil on top. No French drain, it just drains into the ground. This adequately deals with the run-off from a 6x11m roof footprint (~70m2) apart from in very heavy prolonged rain (once in 3 years).

But why is the garden boggy if it's on a slope? What is stopping it draining - is this more an issue with the soil type?
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Soakaways have to be more than 5m from any property, is 2ft from the patio far enough from the house?

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