French Drain Depth

[Danny-r]

Hi Guys,

I'm in the process of sorting some french drains myself. I'm doing them the recommended way, trench lined with a geotextile fabric, perforated drain at the bottom and then filled with stones and wrapped into a burrito. On top of that we will be putting decorative stones.

The width of the trench is 30cm, and we had planned to go 30cm deep. I've begun digging and have found that it becomes significantly harder around 27cm. More so towards the back, thicker clay and rubble pretty shallow (typical new build). Had to pull a paving slab out that was mostly in my garden but buried next door too, luckily only slightly so I didn't cause their garden to collapse!)

I've been reading about it, and there's mixed views on it as always! The main concern when it comes to depth is if turf goes back over and someone was to try and stick a fork in it. Nobody is going to stick a fork in it as it's got stones on top and even if it had turf, it'd hit the stones around the drain first anyway. Also encountered one object at one point that is 10cm deep that I'm pretty sure I won't be able to get out, it looks like an old branch that is significantly running across garden borders into nextdoor. Looks very much like a pipe but I am thinking surely not at 10cm depth.

The drain itself is 110mm wide, so I can use the first 20cm for the drain then 7cm is more than sufficient for the decorative stones, if not a little overkill.

Am I missing anything?

[markc]

Depth depends on where it will run to and distance. A drain can be pretty much level but the water has to go somewhere

[Tosh]

What is it you're trying to solve? High water table/poor surface water drainage or both? Protecting foundations/walls? Will it be just pedestrian traffic or in a driveway? Where will it go to? I put ours in at 700/800mm deep, that was to deal with high water table and to help get water away from foundations (900 deep)around 2 x sides of our building, basically like an interceptor moat around the house. This is then connected into the main storm water that runs down to our pond.

[Danny-r]

It's just to reduce waterlogging in our garden. Typical new build / clay soil issue. Grass area is 10m front to back and probably 5m wide. I've attached an image of my plans that include the french drain. The full perimeter will have as well as wrapped around a patio, and the garden will be dissected in the middle vertically with a drain too.

 

It will all be running to the front of the garden, where it will drain off.

Edited April 22, 2022 by Danny-r

[saveasteading]

Drain off to where?

Typically a metre of rains falls on your whole garden in a year. 50m2 of garden becomes 50m3 of water, so it has to go somewhere.

Currently it is soaking slowly through the ground or eventually evaporating. Sending it all to the bottom of the garden could become interesting.

 

[Danny-r]

1 minute ago, saveasteading said:

Drain off to where?

Typically a metre of rains falls on your whole garden in a year. 50m2 of garden becomes 50m3 of water, so it has to go somewhere.

Currently it is soaking slowly through the ground or eventually evaporating. Sending it all to the bottom of the garden could become interesting.

 

There's a "wall" of sorts, more of a fence but it allows water through it. At the other side of this from the grass area is a pebbled area. It'll drain through to there and then eventually evaporate rather than sitting in puddles on top of the turf.

[saveasteading]

4 minutes ago, Danny-r said:

a pebbled area. It'll drain through to there

And this is your land too?

[Danny-r]

Yes it is. I've attached a previous picture before I started digging, it may help. Water visibly drains between the two pieces of concrete, gaps on the sides and also a significant gap at the base when you pull the stones back a little.

 

 

Edited April 22, 2022 by Danny-r

[saveasteading]

OK I understand now, and obviously you don't mind that gravel area being under water.

 

I would be more inclined to make a catchment up on the higher level, so a big pit, that would be a pond. Then it holds the water, allows half into the ground and half to evaporate with wind or plant respiration, and meanwhile be attractive.

Shrubs would help too, in drinking the water and breaking up the soil over time.

 

Failing that, build the french drains with additional pipes to create more void space, tto hold the water. 

Gravel is one two thirds or more stone, and doesn't hold much water.

 

[Danny-r]

There's going to be just 46.22m of drainage pipe. And although the garden is roughly 50m2, it's lost some of that to current work as I've installed a base for a hot tub and a patio is going down within the next few weeks. The patio itself is 12.48m2 and the hot tub base is 5.34m2 which leaves 28.4m2 although I get what you're saying, obviously it doesn't effect rainfall as there will be run off from these. Both the patio and hot tub base will have their perimeter lined with the drain also. We're working on roughly 1m of drain per 1m2 of garden which I think is sufficient.

 

My main concern, going back to the OP, is depth. The garden is typical of that of a new build. I've got to 27cm in parts with relative ease, it seems getting another 3cm out of it is harder than the first 3cm, so my main query was about it really mattering as I've read mixed views on it minimum depth. Had a few close calls already with hitting paving slabs that crossed boundaries but luckily only slightly so I could pull them out.

[saveasteading]

The top is probably compacted by builders vehicles and containing bricks and junk.

After about 300 the effect of wheels is usually much reduced.

on the other hand the original ground may be dense , but at least should not have stuff in it.

 

Whether the ground level is similar to the existing, or has been cut or filled, perhaps you know or can establish.

 

Soakaways and french drains work better with depth because there is an increase in surface area, and the 'head' of water  pushes water downwards a bit more.

 

That is all theory though. If you can dig 1m down you may see all the layers of new and original ground. you could also do one or more  percolation tests for your own interest .(see earlier discussions)