Preventing Landscape Filter Fabric From Clogging
We’re trying to put in a foundation drain that will be trouble-free for years. Pretty much following the suggestions on this site – using rigid pipe, wrapping the filter fabric around the gravel like a burrito, etc. Now we come to the question of how to protect the filter fabric from getting clogged. We asked our builder how much it would cost to get a load of coarse sand to cover the filter fabric. He came back with a price ($500) but as I asked more questions I found out that what he’s really trying to get us to use is #10 gravel. I have some concerns about that … would that really protect the fabric from clogging? For one, it wouldn’t seem to have the same filtering capacity as coarse sand. Also, I’m wondering if it might have too much fine stone dust in it, since it’s a crushed stone product.
Are my concerns justified or does the builder have a good idea here?
If coarse sand is preferable, how exactly should I specify this, meaning what lingo should I use to be assured I get the right material?
Any advice would be greatly appreciated.
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Replies
William,
If you have installed crushed stone and filter fabric around the footing drain, you can install any type of free-draining material on the outside of the filter fabric. You will be depending on the filter fabric to keep your footing drain from getting clogged with silt.
Crushed stone, gravel, or coarse sand are all acceptable backfill materials.
So Martin, do you put no stock in the idea that the filter fabric itself can become clogged with silt, and that a protective layer of filtering sand can prevent or at least delay that outcome?
William,
Sure it would be better, but you have to decide when is enough. Using filter fabric at all is a regional practice. Not all builders in all areas use it. You're going to protect the drain rock with filter fabric, then want to protect the fabric with sand. Why not protect the sand with more fabric?
If water infiltration is a particular worry, you might be better to put the effort and money into more robust water-resistant foundation treatments.
I have heard people use the phrase "no fines." I'm not sure if that will help in this situation, but it might.
William,
Practices are all over the map, as Malcolm correctly notes. If you (a) have a functioning footing drain pipe, and (b) surround the drain pipe with crushed stone, and (c) wrap the crushed-stone-plus-pipe burrito in filter fabric, and (d) backfill with some type of free-draining material (almost anything other than soil from the site), you are way ahead of your neighbors. So stop worrying.
Yes, Malcom, trying to decide when enough is enough, and to sort out some seemingly contradictory information. I'm a grower, very little experience in building things ... Part of my concern is having worked in landscaping and seen how landscape fabric can really lose water permeability over time. Builder says the fabric he's using is designed for backfill applications, so hopefully that's not going to be as much as a problem.
Martin, I was feeling good about our plan until I got to your point (d) ... we're using a fiberglass drainage board (Tremco's Warm n Dri), but were planning on backfilling with the native fill. I'd be looking at several thousands of dollars more for filling with gravel to grade.
I think maybe I'll worry less once we get above ground. Something about work that get's buried ...
William,
Free-draining backfill material (crushed stone, gravel, or coarse sand) does not need to extend to grade. In fact, a cap of high-clay soil is better than a cap of free-draining material.
The idea is to begin the backfilling process with good free-draining material hauled in from a gravel quarry. Then you can use soil.
That said, your fiberglass drainage board performs a similar function to the free-draining material I mentioned. So I think you can stop worrying.
What Martin said. The way to conceptualize your backfill is this: free draining material immediately above the pipe, combined with largely impervious material above to a finish grade which slopes away from the house. The impervious cap should do 99% of the work of keeping your foundation dry. The perforated pipe should be considered to function as a vent rather than a drain and a backstop in case the cap becomes compromised.
Most common causes of compromised cap material: ponding from localized grade uplift as a result of mulch and rootball growth of foundation plantings over time, often combined with overflow from clogged gutters. Valley corners are especially vulnerable. In the long term, keep your gutters clean, maintain free flowing surface grades and keep shrub plantings well away from the house beyond the swale line.
Martin, Malcolm.
I agree with William. Apple Drains youtube channel has some great examples showing how filter fabric simply fails in some cases. You bluntly say Martin that William should stop worrying and that this is a theory all over the map. Where is the real proof on how these things work and what causes the failures I think is what William is getting at, I too am wondering more on the specifics of this. The whole purpose of asking if the landscape fabric would clog is to learn if it is a good idea to burrito wrap a french drain for example or similar system in filter fabric and rely on that fabric to work. I think the important thing here is that the free draining fill is surrounding the french drain system that is wrapped in filter fabric, without that I imagine the fabric would definitely get clogged and basically make the drain useless so in my mind it sounds like that's a critical part of this. Martin I would suggest that unless you know, perhaps not saying don't worry about it so bluntly lol. Where's the proof I think is what William is saying and so am I.
French Drain Man on Youtube (https://www.youtube.com/channel/UCs09Xm81fnqfuilUKJvKyPw) also seems to have some pretty good content.
I posed this same question on soils engineering forum awhile back and there was an old timer on there that said the only way to backfill a drainpipe was with "concrete sand." He said on the projects he used to run if he saw that a contractor backfilled with gravel he'd make them dig it back up. This would very much depend on local conditions. What the concrete sands do is prefilter ultra fines like clays and silts. If you are building on either of these I would think twice about filter fabric and gravel for the exact concerns the OP poses. The silt or clay could clog the fabric in a year in the right conditions. I'll add I had never heard of concrete sand and still don't really know what it is=-)