French Drain Installation Concerns

[kevsw]

I have recently had a French drain installed to side and rear of garage where surface water was collecting. The perforated drainage pipe was connected into the rodding pipe and wrapped in a fabric (geo-textile?) and the structure was backfilled with the original soil and turf- see photos. Not much stones was used in my view and I read elsewhere the fabric should be wrapped around the stones and not just the pipe itself so has this been installed effectively or is it fine?

 

[Hugh Jaleak]

The membrane should stop silt getting into the pipe and blocking the system up, but I would have expected a bit more gravel around the pipe for bedding, protection and to give the water a better chance of getting away.

 

[^woody^]

Yes, the membrane goes around the stones to stop soil grains clogging the gaps up.

Also, it looks like they have used stone chippings, when it should really be gravel which are round and have move voids for dainage.

 

[theprinceofdarkness]

As I see it French drains come is two parts, the collection bit (where the water is) and the dumping bit (where you want the water to end up). So for the collection bit, you need open gravel to the surface so the water can get into it, even a couple of inches of top soil can stop water draining away. So this means a membrane around the gravel to stop the soil being washed into it. Then for the dumping bit, remembering that the water is going to flow out of it, needs just a bit of plastic over the top, it is unlikely that the water flow is ever going to fill the pipe. But it needs to sit on a wide bed of gravel to eases the soaking away of the water within the pipe. It is paramount that the whole system is laid on a slope, else you have built a soak away, which might cure the problem, but only accidently.
The classic French drain is just an open ditch filled with large pebbles which is a container for the water, which the pebbles help to evaporate.
Frank

 

[r896neo]

As other have said burying a land drain pipe does not make a land drain. The water is collected by the large channel of clean stone which interrupts the flow of water which then trickles easily through the gravel and is then collected and removed by the pipe. Whilst the pipe is important the stones are almost more important, a land-drain made solely of stones would function better than what you have.

When doing this in grass i line the trench base with an inch of clean stone then lay in the membrane then another 2 inches of stone, then pipe, then fill with clean stone to about 3'' from the surface, fold the membrane over and then use grit sand over it for levelling and replace the sod. The membrane should wrap your whole pipe and gravel like an envelope. It's also importanty to use the right membrane, terram 1000 or similar rather than a generic landscaping geotextile. The speed of water passage through different types is significant.

If the trench is less than a foot wide the grass directly on top gets enough water from each side to remain in good health.