How do I improve drainage vertically through clayey soil?

Question

I live on a hill where the soil has lots of clay. My back garden has been terraced with small (1m) retaining walls. The net result is that we have several lawns / gardens with the following properties:

1. Surrounded on all sides by concrete retaining walls.
2. The top 20-30cm is good soil; the remainder is clay.
3. At the bottom of some of the retaining walls (i.e. a bit less than 1m below the surface), there are periodic drainage holes.

So the drainage is quite terrible -- the water can't get out the sides, and it can only sink slowly through the clay. To improve this, my idea was to dig a trench ~30cm wide along the side of the retaining walls (where the drainage holes are), fill the bottom 50cm with sand, and the rest with dirt, and then run drainage pipes from the rest of the lawn to this trench.

On the other hand, I have also read: "Don't mix sand and clay; that's how you get brick". Is this a good idea? Will it work? Are there better options?

Edit: Photos below (excuse the laundry -- we can't let a sunny day go to waste :-) )

The corner with the manuka (centre first image, bottom-right second image) is evidently a little lower than the rest of that lawn, and thus in heavy rain it can be under 5+ cm of water.

Answer 1

Below is what I would consider a "Rolls-Royce" job, all of it might not need to be done if water doesn't puddle on the surface of the lawn...

French drains on the back face/side of the upper and lower retaining walls:

• Dig 300 to 450mm (12 to 18inches) wide trenches.

  • The trenches should run the length of the retaining walls.

  • The trenches should bottom out 100mm (4inch) below the existing drainage holes in the retaining walls.

  • Keep the "good" top soil for back-fill material.

• Clean out the existing drainage holes in the retaining walls.

• It's a little difficult to tell from the photos, but if you feel there are not enough drainage holes in the retaining walls or they are spaced greater than 2000mm (78inches) centers, I would recommend drilling some additional drainage holes through the retaining walls.

  • Layout any new drainage holes at the same level as the existing ones and position them along the retaining walls as needed -- no greater than 2000mm (79inches) between them.

  • First drill pilot holes through the retaining walls using a 12.5mm (½inch) masonry drill bit, then use a 50mm (2inch) concrete core cutter to drill the "final size" drainage holes.

• Get some outdoor screen mesh fabric (small off-cuts) and place the pieces on the back face/side of the retaining walls over the drainage holes -- this will help keep them clear of debris and functional.

• Get enough "permeable" landscape fabric to line the trenches.

  • The landscape fabric should go 200mm (8inch) up the back face/side of the retaining walls, lay across the bottom of the trenches, then up the face of the trenches (opposite the retaining walls) to above ground level.

• Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

• Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

  • The drainage pipes should run the length of the trenches (retaining walls).

• Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

• Fold over the landscape fabric so it covers the top of the pea-gravel.

• Finish back-filling the trenches with the saved top soil back-fill material.

• Make good the surface to match in with the existing.

French drains on the outer face/side of the upper retaining walls:

• Dig 300 to 450mm (12 to 18inches) wide trenches.

  • The trenches should run the length of the upper retaining walls (minus the path/walk-way in the middle).

  • The trenches should bottom out at the same level as the trenches dug behind the lower retaining walls.

  • Keep the "good" top soil for back-fill material.

• Get enough "permeable" landscape fabric to line the trenches.

  • The landscape fabric should go 200mm (8inch) up the outer face/side of the upper retaining walls, lay across the bottom of the trenches, then up the face of the trenches (opposite the retaining walls) to above ground level.

• Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

• Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

  • The drainage pipes should run the length of the trenches (upper retaining walls, minus the path/walk-way in the middle).

• Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

• Fold over the landscape fabric so it covers the top of the pea-gravel.

• Finish back-filling the trenches with the saved top soil back-fill material.

French drains on the lower level lawn areas, running perpendicular between the upper and lower retaining walls:

• Dig 300 to 450mm (12 to 18inches) wide trenches.

  • 3 number per lower level lawn area.

  • 1 on each of the inside faces of the "side/wing" retaining walls. Position 1000mm (39inches) away from the "side/wing" retaining walls.

  • And the 3^rd positioned in the middle of the 2 (near the "side/wing" retaining walls).

  • The trenches should run between the trenches on the outer face/side of the upper retaining walls and the trenches on the back face/side of the lower retaining walls.

  • The trenches should bottom out at the same level as those trenches.

  • Keep the "good" top soil for back-fill material.

• Get enough "permeable" landscape fabric to line the trenches.

  • The landscape fabric should go up one face of the trenches to above ground level, lay across the bottom of the trenches, then up the opposite face of the trenches to above ground level.

• Fill the bottom 100mm (4inch) of the trenches with pea-gravel.

• Lay 100mm (4inches) diameter "perforated" underground plastic drainage pipes (enclosed in a polymer liner/sock) on top of the 100mm (4inch) thick layer of pea-gravel. Position the pipes "horizontally" in the middle of the trench.

  • The drainage pipes should "T" into the drainage pipes that are running along the "face" of the upper and lower retaining walls

• Fill the trenches with pea-gravel up to 300 to 450mm (12 to 18inches) below ground level.

• Fold over the landscape fabric so it covers the top of the pea-gravel.

• Finish back-filling the trenches with the saved top soil back-fill material.

---

If water doesn't puddle on the surface of the lawn I would considering not doing the work under heading "French drains on the lower level lawn areas, running perpendicular between the upper and lower retaining walls."

---

Good luck! and I hope the above makes some kind of sense (is somewhat helpful/useful). If you have any questions please ask, I will then do my best to address them...

Answer 2

Plant daikon radishes in the area affected these radishes are 3 feet deep/long. After each plant makes these "holes" and rot. You will have better drainage. The rotting root/radishes will become humus which will also make the soil more drain-able. effect Seeds are cheap and the leaves and root are also edible.

Answer 3

I agree with the very helpful advice above, but depending on the thickness of the clay, you may find a soakaway trench doesn't work (the trench would just fill, like a pond). You may need to drain the water into the the drainage system, fairly easy if the slopes work, If not build a sump and pump the water into the drainage system.

Hope this helps you can find information for garden drainage and sump pumps here. http://www.flowerpotman.com/lawnsandgardendrainage.html

Answer 4

Depending on the level of work you are willing to put into it...

You could use something like Haydite or another expanded shale product. It's cheap and they make it all over the country so it's not too difficult to come by.

Then you dig up your terraces, mix the Haydite to the right consistency (you can ask the source for instructions, especially if you bring a sample of what you're working with) and re-seed your lawn.

It's fairly cheap and straight forward. Just some excavation which is not so small.