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garden drainage plans - any drawbacks to this?

Hi all.

Currently landscaping the garden (10m x 8m) and want to deal with drainage issues.


Garden is boggy in rain and drainage is generally poor. 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 (we could level the grass to the highest point, but we'd lose fence height to the right and will be overlooking neighbours). 


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

  • Approx 1ft of topsoil;
  • approx 1ft compacted soil (rock hard when dry but when saturated is quite soft;
  • below this is a "sandy clay loam". Have done a jar test and it's approx 78% sand, 22% clay. Compacted, but easy to dig up. Nice and porous (have done a percolation test and it drained nicely).

We plan to have a gradual slope from the top of the garden towards the patio which we will turf.


However this will create further drainage challenges (which I think are solvable) as I'll have less water retaining topsoil above the compacted layer due to the levelling process.

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.


Proposed soakaway & french drain solution:

As there is this free draining layer beneath the compacted layer, I plan to put a soakaway about 2 ft in front of the patio, with a piped (80mm perforated?) french drain system leading to it, placed around the perimeter and middle of the garden.

This will help carry the water down addressing the sodden grass and then deal with the patio rainwater. It'll be a 1m3 soakaway crate kit, requiring digging a hole 1200x1200, 1650 deep.

The french drain depth will also break through the compacted layer to the free draining layer below for additional drainage along the channels.


Apart from the graft and cost, are there any drawbacks to this plan?


I’d be left with 3-4 inches of topsoil on the compacted layer in some areas - do I need to work some compacted layer so I have say 6 inches of top soil all around?


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

Cheers

Posts

  • punkdocpunkdoc Sheffield, Derbyshire border.Posts: 13,027
    If you want to grow anything you will need more than 4 inches of soil.
    There are ashtrays of emulsion,
    for the fag ends of the aristocracy.
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