Best backfill materials to keep new perimeter drain clean
After a few years of issues with water entering our basement (mostly at the slab/foundation wall cove) and before we embark on finishing our basement space, we decided it was time to fix the water problems the right way by addressing it from the outside. The house is essentially a 1947 brick square colonial in northern Virigina where the soil is clay (haven’t had a soil test). Basement is about 4-5′ under grade. Currently, we have excavated everything to remove the clay-tiles that were silted completely full, cleaned and dampproofed the walls, and are ready to install the new drainage mats, perimeter drains and backfill.
The original plan was to install drainage mats (j-drain) over the damproofing that go over the edge of the footing,dig down the the bottom of the footings, put down filter fabric, a couple inches of gravel, the pipe (with weep holes facing down), then more gravel to cover up the footing with 6″ or so, complted by wrapping rest of the filter fabric over the gravel. We would then simply backfill using the same clay we took out, tamping every 6″. The drain would terminate into an external sump pit with pump located at the bottom of an exterior stairwell, so none of the water would be brought inside.
However, in talking with folks around here and in reading online, there seems to be no agreement on whether this is the best way. It meets minimum code, but there is a contingent which is completely against using filter fabric as they assert it will clog over time from the silt in the clay, rendering the entire system useless. Instead, they recommend using only coarse sand as the backfill around the drain. Of course, after all this work, the last thing I want is for it to fail…I’m hoping (perhaps naively) that this will last decades, and I’m adding a few cleanouts as a backup precaution.
I ran across this recommendation on the GBA’s site (http://www.greenbuildingadvisor.com/green-basics/foundation-drains-0), which essentially recommends doing the same thing we originally planned with the addition of adding coarse sand around the outside of the filter fabric to keep it clean. Does anyone have experience with this? If I had to backfill right now (I’m stalling until I get this figured out), that seems to make the most sense to me, but I’m still uneasy about the filter fabric clogging… A lower-risk option seems like gravel around the pipe, with sand top of the gravel then soil backfill (essentially removing the fabric), but I’m not sure.
Two other small side questions:
(1) if we use gravel, the stone yard said around here all the builders backfill using #57 bluestone because it’s the cheapest. But most recommendtions I see call for a “3/4 washed stone” which I assume would be preferred as its rounded edges (as compared to the 57’s sharp edges) would not puncture the fabric? (The yard also said none of the builders’ around here use fabric when backfilling, but who knows…plus I know a lot of builders’ make the most cost-effective chioce, which isn’t necessarily the best).
(2) we’re planning to use 4-inch “Hancor 3000 triple-wall plastic pipe” (http://www.hancor.com/product/triple_wall.html) for the drain from a big box store, which is smooth, solid pipe but nowhere near as heavy duty as schedule 40. In addition, the fittings aren’t glued…they’re snapped into place, which makes me wonder if/when we need to have it cleanout out whether the plumber’s snake would pull the joints apart. Is it worth the extra money to buy schedule 40 and drill my own weep holes in it?
Replies
Drain rock
The #57 is probably an AASHTO #57 coarse concrete aggregate gradation, with a maximum size of 1-1/2-inch, and 95 to 100% passing a 1-inch screen. It will be free draining, but is probably coarser than you need. Check the pricing for an AASHTO #67 Aggregate which has a maximum size of 1-inch, with 95 to 100% passing a 3/4-inch screen. You want it washed to minimize the amount of fines that are there to start with.
The sand outside the filter fabric is there only to increase the area of filtration. I'd recommend a larger diameter filter fabric blanket filled with more coarse aggregate. The sand around the filter will work, but is a lot more tedious to install.
The thinner wall pipe should be fine, if you bed it properly, the soil will keep it as round as it was when you bedded it. And reallistically, if the pipe collapses slightly, it won't effect the drainage capability at all.
drew
Here's what has worked for 22yrs in NW Oh. with some of the least perk'ing clay around.
Pipe next to footer, waterproofing down wall, over footing and along side.....
Over that we use what was called Warm and Dri-a 2" rigid fibreglas panel, that wicks the water over the outer layer, down and insulates as well.
I put a tent of 30 lb felt over the pipe and then backfilled with 57's all the way up. Filter fabric on top graded away from foundation and topped with more stone. No soil whatsoever, you shouldn't be planting that close to the foundation anyway.
No gutters on the house-the gutters are in the ground. This is hillside, so pipe runs to daylight.
I most certainly would not backfill with clay. It shrinks and expands, leaving a void below grade between it and the foundation-easily fills with water that could freeze in our climate.
Here in New England, we use crushed rock, not gravel with any fines, to bed and cover the drain pipe. We use a continuous roll of plastic pipe that has a silt sock on it. The pipe is perforated with small holes all around. An additional layer of silt fabric is placed over the pipe before the top layer of crushed rock goes down, backfill around the foundation would be gravel, never clay or other impermeable type soils. Inspectors want to see the drain pipe before it is covered.
Appreciate all the replies. From the last two posts, it sounds like just putting the filter fabric over the top seems to be the common practice (i.e., not completely wrapping the pipe/gravel with the fabric as I had planned), as well as backfilling with some sort of rock/gravel all the way up to grade. On the backfill...my understanding of using a drainage mat like the J-drain is that when soil pushes against the filter fabric that covers the dimpled area, the pressure pushes the water through the fabric, which then falls down the dimpled wall to the drain while the soil stays in place. Looks like this: http://www.j-drain.com/images/400web.jpg. So I'm assuming using that would be equivalent to backfilling with gravel all the way to grade. So am I correct in assuming both techniques could be used interchangleably to relieve the hydrostatic pressure from building up against your house, but that both of them are not necessary?
@jigs-n-fixtures...thanks for the tip about #67 gravel. My local stone yard doesn't carry it (the next lowest size they have is what they call #8...quite a bit smaller), but I might call around and see what delivery charges would be for another yard that does carry it. On the sand...on the GBA website link I referenced above, it says: "Finish with coarse sand. A 6-inch layer of coarse sand spread on top of the fabric will prevent soil from washing into the fabric and clogging its pores." I've never heard this before, but it seems like they're claiming that the sand would prevent the fabric from clogging. I've asked other people their opinion on this and got all sorts of answers, including that the sand would immediately clog the fabric and to stay far away from it. Any thoughts? Other than that, the gravel area will cover roughly two feet away from the foundation wall including a good 6-12" or so on top of the footing. Hopefully that provide enough surface area to let the water in.
@calvin...warm-n-dri looks like a great product. I actually considered adding 2" XPS insulation on the outside since everything is opened up right now, but my impression from reading about it is that it attracts insects like crazy, esp. ants, and I'm not sure how to prevent that. When you backfill with 57 all the way to grade, do you put a 6" or so "cap" or clay or other impervious soil to keep water running away from the house as much as possible, or do you leave it gravel at the top so your perimeter drain handles all the water? One difference between our situations may be that I have redirected our downspouts out to the street and the backend of the property, so this perimeter drain won't have to handle that added water.
@Hammer1...thanks for sharing your experience in New England. A few years back I helped my brother (who lives a few blocks away) install a perforated pipe covered in a filter sock (a cheap one from a big box store) to drain water from a window well. It worked great for a couple of years until the water started backing up again. When we checked out the pipe, we found the sock was silted completely full and would not allow any water into the pipe anymore. Since then, I've been very hesistant to use the filter socks, but maybe I just bought the wrong kind. Also, I notice you put the gravel on top of the additional filter fabric...is this meant to keep the fabric clean or does it serve a different purpose than the gravel underneath the fabric?
All filters clog eventually. The coarse sand is also a filter, and will eventually clog, unless there is ehough flow through the system flush it through then you will end up with subsidence around the filter as the fines get carried away.
The only solution is to increase the surface area of the filter, to extend the time required before enough fines migrate in and clog it.
Coase sand, then filter fabric, then coarse gravel surrounding the perfed pipe will work. As will just lots of coarse gravel around the perf pipe. If you are going to have a daylight drain, it might work to have just coarse gravel around the perf pipe, and have cleanouts so you could sitck the hose in once a year and flush out any fines that have migrated in. Then cover the top with a coarse wood mulch to hide the subsidence.
@calvin...warm-n-dri looks like a great product. I actually considered adding 2" XPS insulation on the outside since everything is opened up right now, but my impression from reading about it is that it attracts insects like crazy, esp. ants, and I'm not sure how to prevent that. When you backfill with 57 all the way to grade, do you put a 6" or so "cap" or clay or other impervious soil to keep water running away from the house as much as possible, or do you leave it gravel at the top so your perimeter drain handles all the water? One difference between our situations may be that I have redirected our downspouts out to the street and the backend of the property, so this perimeter drain won't have to handle that added water.
The warm and dri being rigid fibreglass-2" does not attract ants like foam bd. does. You are sure right with your thoughts on the foam, however-there is a borate product in foam that evidently mitigates that problem. (boracare)
I took the stone the full width of the trench all the way up to grade-did not cap it with anything and yes, we look at stone. I didn't add pretty stone either-I live in the woods-walnut and locust-that kind of leaf matter would ruin the view.
2' overhangs on the house so the dripline is out that far. No gutters. The perimeter drain has handled the water for 23 yrs.
masonary sand
I'm doing a perimeter drain exactly what the original poster is doing. I was planning to: (a) lay 6' wide filter fabric in the trench and pin it to the outside dirt wall, (b) add 3" of gravel, (c) lay the drain pipe, (d) add 18" of gravel, (e) wrap the filter fabric around the gravel, (f) add surplus masonry sand over the filter fabric, and (g) add about 3' of clay backfill, and (h) add about 1' of top soil.
The problem is the masonry sand is very fine. (My mason didn't like it because it was so fine, he said it was for brick.) Would it be worse than clay?
Also, I was surprised to read recommendations to add gravel all the way to the top. Is that commonly done? That would be a lot of gravel. (My trench is 3' wide).
You'll always here backfill with stone from me.........
I'm built in a non percolating ugly clay and have seen what it does to house foundations from many years back all the way up to new builds.
Clay here shrinks and eventually expands in this area of NW Ohio.
Shrinks and area up against the foundation fills with water-or expands and pushes against the block or poured walls.
If it was dry b/4 winter and it has shrunk back, then a wet winter with deep frost-can actually push in and buckle walls.
Even with filter cloth around a stone fill, I'd go up to the top with stone. A three foot trench beyond the foundation is quite large, but I'd do it with stone. What to do with the clay you dug out? On a hillside you can create some more flat ground or interesting ridges and swales.
Clay doesn't perc here, so a cap of clay would be just that-a lid over your drainage system.
As far as fine sand as fill-you're asking for the fabric to clog up with what is essentially silt.
drainage pipe with silt screen.
Do I put rock in with it . It's just going along a sidewalk to catch run off?
It's 4" pipe.