Linear drain concept for lawn - Use a shingle bed rather than land drain

[Toby O]

I have a 15M by 15M garden now levelled close to the final levels. I need to remove approx 200mm more of the subsoil to then refill with topsoil and turf. The base level is 1.2M above the patio level, so I have to build a timber wall to finish the edge of the garden.

I have very heavy subsoil with sandstone chunks in it. The water seems to soak through it very very slowly. To dig out channels for land drains will be hard due to the sandstone. I am also wondering whether it will be effective as the water does not seem to move 2.5M through the clay.

I thought I could remove the subsoil to leave a 1:100/1:200 gradient. Then I could put back some aggregate/sand to act as a filter bed with terram on top of it and then top soil. The water should run to the edge and drop through the vertical height (via standard land drain build or fin drain?) behind the timber wall to be piped off to drain. I cannot let it drain off the other sides of the garden.

The main questions are;
Do I need Terram or some other fabric over the subsoil to stop the aggregate bedding into it?

What layers of aggregate and of what type would be suitable?Does it need to be compacted?

How much topsoil is suitable over the top? Too little and the aggregate may move as we walk on the lawn, too much and it may not drain perhaps?

Is a fin drain overkill for the vertical drop compared to 200mm of aggregate and a land drain?

[Pablo]

Blanket drainage systems like this are usually very successful over small lawn areasup to about 20mx20m.100mm of 20mm clean aggregate with a membrane on top is grand for the drainage course with a min of 150mm of UNSCREENED topsoil over that. I try for at least 250mm of soil. If the soil is quite heavy add plenty of grit and organic matter. If it's a light soil add organic matter. One problem with this system is that the soil compacts very quickly and easily because it drains so quickly then it gets too dry and a cap forms preventing deluge water from getting away quickly so you get puddles etc. If you get the soil stucture right and have a decentwater and annual maintenance schedule then you will be grand. Fin drains are overkill landrain with mebrane surround will be perfect for you. If you increased the aggregate depth to 150mm with a few 2 inch pipes to remove the water then there would be no need the cut any gradient. Forgive the spelling mistakes i can't turn off the overtype mode meaning everything needs retyped. Nightmare. Good luck and let us know how you get on.

[Tony McC]

An alternative to the layer of agg would be a horizontal drainage composite, of the type now commonly used with green roofs. These are only 25mm or so thick, and can be buried with Pablo's recommendation of 250mm of topsoil no problem at all.

Although the cost is higher than for agg and filter geo-textile, you will have less excavation to do, less cart away, and less trouble when it come to installation.