I'm getting to the point of buying the material for my French drain, which is suppose to go at the foundation of my house's crawlspace. Now the question, I couldn't find a definite answer to...the fabric that is used to wrap the PVC pipe.... I'm assuming it is the woven one because I want water to get to the pipe? I also plan on putting a sock on the pipe so that small particles don't clog it.
It depends on what your soil consists of. Actually, a nonwoven fabric is recommended for soils on the sandy side. However if your soil is a good part (>20%) clay, thats where things get tricky. The opening has to be smaller, about 70 US Sieve (.212mm), but there's a catch. The smaller opening will now be subject to clogging the fabric instead of the pipe and then the entire trench ends up rejecting the water. I'm currently reading about woven monofilament fabrics but the only viable option appears to be a Carthage Mills 20% Fabric. It's got a 70 US Sieve opening, 20% Open Area, 48 g/pm flow rate (may be an issue as it's half of what is common for nonwoven), and a permittivity of .68 sec which should be closer to 1.0 or greater.
It has been 2 years now with a lot of snow in the winter and plenty of rain during spring and fall...I'm happy about my drainage system...it is dry in the crawlspace and the house is not moving anymore!!! Nice side effect: No cold feet and I was able to lower the heating temperature in the house. About 67 degrees now compared to 72. The hard work paid off!! I used woven landscape fabric to keep the dirt out, laid the pipe with sock (because holes are bigger in pvc pipe to keep small pieces of rock out) directly on it with holes down then filled with rock around and on top.A clean out after every corner to be able to wash it out if I have to. I started at 1 foot down and ended up with 3 feet down after 200 feet of digging. I hope this helps everyone who wants to do the same. Don't underestimate the work especially if you have clay soil as I did .
I am working on a couple of lengthy French drains in Georgia clay... nasty stuff. If you’re working in clay, I recommend still using a non-woven fabric. The smaller holes of woven fabric will not help... they will clog and slow the water flow.
I’ve been studying my first drain (which is not quite finished – it’s 320 feet) during and after a few major southern storms. The clay will get through the fabric, but not a great deal. If it is allowed to pool on the top of the enclosed drain, it will slow but not stop the transfer of water. It does not seem to embed in the fabric... after it dries it can be rubbed and then mostly blown off with a leaf blower. Interestingly, our native topsoil pools the same way... I assume it has a fair amount of clay in it.
During a major downpour just today (with tornado warnings and hail even), there was a small amount of clay and topsoil pooling in areas of major runoff. Where the rain bounced off the bare soil or clay piles next to the unfinished (but covered) trench there were no pools... just a peppering of soil. This is while it was still raining, but not as hard.
Here’s the best part: at the spot in the trench I’ve worked up to, the fabric roll is still on the surface, beyond the end of the pipe (before I connect it to the the next length). It might be hard to visualize, but it creates a potential barrier to the water ... like a curtain being drawn across the path. There was a pool of clay-rich water at the base, but when I checked the flow at the end it was a steady, strong stream. In fact, I forgot that it was going through the fabric it was so strong, let alone through a pool of clay. And there was absolutely no pooling of water on the grassy surface anywhere... the owner had chronic problems of standing water and areas that were washed out. Not any more.
So here’s my opinion and approach:
You’re not going to keep clay or dirt out of your rock, and to choose a monofilament woven fabric is chasing an ideal that will result more quickly in a plugged or slow drain... not the pipe, but the fabric. The non-woven fabric, with it’s larger gaps, seems to “police” the flow of clay. It slows it down at the borders, but allows clay and water to find their paths. Not enough clay to choke the water lanes though. If you’ve ever seen a fight between clay and water, the water finds its way. That’s my opinion, here’s my approach:
Most water flow is runoff (this is why re-grading is the first line of defense). So most water and definitely the deluge of water during a major storm will come in through the top of your drain. I’ve chosen to keep my stone about 4” below the surface. I close the fabric in a double layer at the top (if possible). I tested this and it made no significant difference in drainage.
I then add 2” of organic topsoil (meaning composted with manure, etc... available @ $34/yard locally, same as 57stone). That’s got some of the largest particles and gaps when you lay it down, and I pack it down with a tamper to fight any invasion of the clay at the surface. Organic topsoil is very permeable at first, and allows the replaced sod to dig deeper roots. These roots allow paths for water, even after the organic matter is compressed. The layer also allows for sprinkler lines to cross the trench and the cable company not to screw things up (although we all know they will somehow).
So at the sides of my clay-bound trench I have a policed barrier for clay that water can still find a way through. At the all-important top, I have a blockade of topsoil and grass that keeps a clean fast pathway even after the soil compacts. I cut the sod at 2.5” and with the 2” of topsoil that amounts to 4.5” into a 4” slot, meaning a hump. This varies of course, but that’s the principle. I’m probably actually more at 1” but it’s a hump.
That hump might become a shallow recession after a year or so – if so overdress (rake in) any recession with more topsoil (keeping a pile of topsoil is always handy).
One last benefit of this approach is that you are building a thick layer of organic matter for your grass to survive in times of low rain. One negative about a modern French drain that has stone all the way to the top... it can starve the grass after a couple weeks of no rain. This will actually help with my approach because it will drive the roots deeper, creating more water paths. And the organic water should still have water and nutrients.
Hope that’s helpful, know it’s wordy, but I also know I worried about the clay/cloth problem and am happy to report my results. BTW I used SRW 4.5 ounce cloth... good stuff. I also have an access at the high point to flush the system if needed.
Finally, it’s been an incredibly difficult thing to do... don’t let anybody tell you it’s simple or they did it in a week (other than maybe a straight 100’ shot with no roots or rocks. You will encounter all sorts of challenges and delays, let alone working harder than you could imagine. But if you do your research, take your time and work carefully you’ll be solving problems. The pride of watching that water flow and walking though what was once a swamp is very satisfying.
I’ll never do it again though... way too much work :D