I need to install a new lawn on poor draining ground made up of heavy clay. They want the lawn to be well drained. I plan to do the following:
1. Rotovate the soil with a heavy duty rotovator whilst mixing in a fair bit of sharp sand.
2. Dig trenches (1.5m apart) to put in collector drains (1.2 metres deep at lowest point, with the whole trench filled with filter material, with perforated land drain at the bottom, and the whole thing wrapped in filter material).
3. Cover the whole area with 6 inches of good top soil.
4. Turf it.
Does this sound ok? The owner wants us to go over the top to ensure it drains well. All the perforated land drains will be draining onto low land, so there will be no problem with soakaways etc. What size filter material should I use to fill the trenches (gravel or larger)? Also, will I need to compact the trenches to ensure they don’t settle later and cause dips in the lawn? If so, do I compact over the trenches, but won’t this crush the land drain at the bottom of the trench? What else can I do to ensure the lawn doesn't 'settle' later and become uneven?
The other thing I thought about is to dig the land over with a mini-digger (whilst mixing in sharp sand) to improve the drainage of the lower layers of soil as opposed to just the surface layers using the rotovator. Do you think this is worth doing or will a rotovator be sufficient?
What you've suggested sounds fine. You might it useful to surround the gravel with a low cost membrane to stop the trenches from silting too quickly, but that might be overkill. 10mm shingle is ideal, because it settles well and 'locks' together thus reducing future settling.
With that much groundwork, you'll be incredibly lucky if it doesn't settle a little unevenly. I usually tell customers this when doing a job of this type so they don't expect it still to be bowling green flat in two years' time.
Don't put a membrane down to seperate the soils. This will attenuate the surface drainage which is the last thing you want. It will also prevent worms from doing their vital work.
Consider using a vibrating roller after the final cultivation, then blinding over the nicely compacted surface with rootzone (60%+ LB sand to dry topsoil) to about 20mm depth. This will help the turf take quickly and allow you to get in that last bit of levelling.
