We’ve got a very wet garden. Clay soil, and we’re in a dip. Water table at the moment is only a few inches below the surface. (I dug a couple of holes)

I’ve got a quote for someone to install 100m of French drains. 23cm wide, depth depending on position, 100mm perforated coil pipe, drainage gravel wrapped in geotextile. couple of lorry loads of topsoil on top to smooth and

It’s a lot of money, it’d be a good start towards getting the windows done. It would mean we’d be able to actually use the garden for more than 3 months of the year.

My concern is how long it will continue being effective. Will the geotextile silt up in a couple of years and put us back to square one?

When we bought our house, which was a new build the garden just didn’t drain at all, again we have clay soil.

Builders put drainage in to the same spec as your quote with the water running into the main drain. It made a difference but it was still pretty soggy.

The thing that made the most difference was putting borders round with bushes and trees.

If it’s a new house go back to the builders and ask them to sot it.

We live at the foot of a sloping field and our garden was a sopping marsh, the lawn was moss and you couldn’t walk on it. Got a substantial breeze block wall built along the edge with drainage behind taking water into the brook that runs under the house and the garden is now dry and grass growing in place of the moss. I expect that to be pretty much permanent.

Can’t predict your situation though.

Not new. So no great difference from the drains then Gary? 😕

Hmmm.

So the wall and drainage is catching the water before it gets to your garden, globalti?

I wouldn’t say no great difference, it did make a difference but not sure I’d have been happy with how much of a difference if I’d been paying for it.

Sounds like they have quoted to do the job properly, so no reason why it won’t do the job for many years to come. What can happen, is the soil can form a barrier to the drainage, so it is important to maintain a route to the system.

Ideally you should create core holes of min 25mm dia through the soil and back fill with sand. The number will depend on your situation.

If you really have ground water at that level, you’re on a hiding to nothing.
Options are to drain to surface water system, raise garden level with free draining material and reduce water inputs into your garden, or a combination of all three.

Depends on your location and surroundings as to what will work.

I put a French drain in a couple of years ago, the previous one had failed – but it had been put in 1910. So you’ll probably be fine 🙂

It sounds as though the clayey topsoil overlies a clay subsoil. The clay subsoil will act as an impermeable barrier for water and the water table that you describe is more like a perched water table.

The French drains that you have described should work fine as long as it drains freely away from your property, and make a noticeable difference.

Also, as you say that your garden is in a dip, then look for ways to prevent surface water runoff from entering your property.

I put a French drain in a couple of years ago, the previous one had failed

Maybe it was just on strike 😆

What is the French drain draining into? If you’re on a heavy clay and in a dip I’m not sure it’s going to help that much as any water you do drain away is just going to be replaced by water from the surrounding higher land.

Maybe it was just on strike

More likely surrendered 🙂

there’s a road uphill from us, but the road drainage works well, so surface runoff isn’t an issue, just groundwater from surrounding higher ground. There’s a stream at the bottom to drain into, so that bit’s easy.

We’re getting in quite a few tonnes of topsoil – hopefully easy draining, rather than heavy/clay! – so that’ll raise the level at the lowest points and allow the rain to get to the drains.

Main concerns are
– coil rather than smooth PVC (he says doesn’t matter – textile will block slit from getting to the pipe)
– fine particles blocking textile, making the whole drain useless

And I’d be interested to know how steep the gradient is of the water table in the soil – say it’s 40cms below the surface in the drain itself, bit higher in the soil next to the drain, how far away will it be pretty much unaffected by the drain?

Our garden was the same with having clay around a foot underneath the soil. We dug a trench, around 60cm wide x 1.2 deep, the length of the garden at the lowest point. Filled it with 2/3 lose rubble, weed barrier, pea shingle and then soil. Garden has no drainage problems now. New neighbours did similar but used those steel mesh cages with the rocks in.

I am not an expert but I tend to wonder what the geotextile is there for – as you say in the OP, it will just get clogged up.

If you continue to be stuck and need an answer I could try asking my father-in-law (a retired construction engineer who has designed a great many geotextile solutions for all sorts of uses).

john, the geotextile is to stop the fine particles from entering the pipe and clogging it up.

I’d be really interested to hear what your FiL has to say, if you don’t mind asking? My chap said he struggles to get decent answers on this from his suppliers.

The geotextile will prevent fines getting washed into the drainage but would not normally be placed direct against the clay but using a blinding layer. Something ‘cleverer’ and more costly than a simple filter geotextile could be used such as Macdrain. But sounds like they have specced a fairly standard general arrangement used all away roads and railways.

I would suggest some thought given to making your surface more resistant as well though. Maybe a celluar geogrid to limit erosion on paths and other heavy traffic areas etc across the lawn. Also look at installing herring bone style french drains to drain your land (or as posted above vertical drains)… you can get pre-fabricated ones but probably overkill..esp. to install them….) tied in with again a french style cut-off drain.

You haven’t got an inefficient soakaway drainage system or leaking buried pipe have you? Sure?

cheers mugsysm8. I’ll look up blinding layer.

Not sure about soakaways. There’s only one downpipe on that side of the house, and I’ve fed 13m of jetting cable into it. Pretty sure that drains to the sewer, because the stream’s less than 10m away, and nothing popped out.

No idea how I’d find out about leaking pipes. Water main is in the street. Can’t think where other misc pipes under the garden would lead.

As a counter proposal, embrace the damp. Stick in a pond, grow bog garden plants, pave an area for year round use. Remember nature always wins in the end.

The geotextile can become clogged with silt over time, however, there are loads of different geotextiles to choose from and a geotextile to suit your soil conditions (clay) should be used. Plenty of information on the interweb if you do some research.

Plant a few willow trees. They can grow well in damp conditions, and take up quite a lot water.

Got a couple of willows already and a few other bog-loving plants, they do take up a lot of water (they grow a lot!) but most rain comes in the winter, when they’re not growing, so it’s an incomplete solution.

Toddboy – you’re right, there are lots of different types, and there is a lot of information. Trouble is, you either want the fines to stay on one side of the fabric or you don’t.

To minimize the risk of clogging, follow this criteria:
• Use the largest opening size (O95) that satisfies the retention criteria.
• For nonwoven geotextiles, use the largest porosity available, never less
than 30%.
NOTE: For critical soils and applications, laboratory testing is recommended to determine geotextile clogging resistance.

You mean you don’t have a soil testing lab in your workshop? 🙄

I thought everybody on STW had one 😆

Don’t get pipe with holes in it – coil or otherwise. Get standard brown waste pipe and slot it with an anglegrinder (with a decent width blade, couple of mm or so shoudl be fine). This won’t silt up 😉

Not sure I would entirely bother with the geotextile myself, just pop it in a gravel filled trench (you could even do away with the pipe and just have the trench I suppose….). Bung the whole lot in the stream at the end.

Of course, if you were feeling even more extravagant you could run your drain into a storeage tank, pop a pump in it and use the water for other purposes around the place 😉 Rainwater harvesting!

(My drains are usually getting rid of “stuff” into the ground rather than getting rid of clean water, but the principle is basically the same!)

Always a good starting point…

we had a stream running in our garden whenever we had heavy rain ,run off from the school field behind our house. i put 100m coil of 3″ perforated drain in wrapped in geotextile in gravel, went a bit extravagant and bought a 1000ltr tank. Me and junior spent the day digging a large hole in the garden installed it with a pump feeding tap at the top of the garden pipe work layed in the same trench as drain. tank is never empty don’t know where all the water used to go,probably got no foundations left. I had to put an 35mm overflow pipe in it was that bad. Best move we ever did garden is fine all year round and we have our own water tank for the garden.

The perforated pipe is cheaper than the gravel isn’t it? So if a big trench, use lots of pipe!

Dig a well? Sell bottled water or start making “Artisan Gin” from your mineral water.
10% share please.