French drain into a wet clay mess

25516 Views 13 Replies 6 Participants Last post by Dugger52, Apr 19, 2009

geckokarma

Discussion Starter · Apr 4, 2009

We're in one unit of a triple decker, and I made the bright suggestion of tackling the drainage problem in the common backyard. Its a little 40' x 20' patch, and the neighbors' retaining walls abut the back portion, along the 40' side.

We have a swamp. In no small part b/c one neighbor built a spiffy new impermeable wall that dumps water out its side onto our yard.

Grade runs diagonally from the highest part near that new wall in one corner towards the lowest in the other corner. French drain along the 40' side seems just the ticket, terminating in a rain garden.

The problem: the "soil" is built of 1-4" topsoil and almost solid clay beneath that, and entirely saturated. I started digging the trench and the walls just collapse.

Any thoughts? I'd wait for a week of dry weather, but then the neighbors will start watering! I'd cut a temporary trench upslope to dry out my work area but I've got a foot or less to work with.

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Daniel Holzman

· #2 · Apr 4, 2009

First, since you live in the Peoples Republic of Cambridge, you had better check with the local Conservation Commission before you do anything. They tend to take drainage projects near a wetland (swamp) very seriously.

Second, if the soil is really clay, and is saturated, the drain you are proposing will do absolutely nothing during the wet season, since the water table will not permit drainage. So before spending time and effort, you should get a level and verify that there is available drop between the water table at your backyard and the water table where you propose to drain to. If you have the pitch, you could theoretically drain into a barrel full of stone, which might work. Then again, if the soil is as bad as you say, the barrel will simply fill up, and not drain. Best to find a spot with permeable soil to drain to. But as I said, check with Conscom first, they will blow a cork if you are messing with a regulated wetland.

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geckokarma

Discussion Starter · #3 · Apr 5, 2009

Thanks Dan. I should have put quotes around "swamp" -- its just a tiny backyard in a super-dense residential area. Before the new retaining wall, it wasn't nearly so wet...

I'm hoping to just divert the surface run-off to a rain garden and reduce its impact on the foundation. You are right to question the water table height... But my impression is that its saturated due to the run-off, not the table -- or so I hope!:boat:

Even for a project of this ridiculously small scope, I wouldn't be surprised if the Commission expects some alms or Frankinsense or something. Shh, don't tell em.

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Dugger52

· #4 · Apr 5, 2009 (Edited)

I've wroked in local government for over 35 years. Universally, it is illegal for one neighbor to discharge their rain/storm water run off onto anothers property without the propety owners permission. You need to go and talk with the building code folks. A permit would have been required for that "hard wall" too and I'll bet you money they don't have it. Take some time and ask the city to send an inspector out, politely explain and show them the situation, ask for relief and I believe you'll find it resolved at your neighbors expense.

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geckokarma

Discussion Starter · #5 · Apr 5, 2009

Dugger -- I wish that were true... We've been down that route already. We had a city engineer come out and he was sympathetic to our case but encouraged a neighborly solution, next thing we know the lawyer/owner got a retroactive structural permit then sold the house. At this point our only avenue is court, and life is too short, you know? Excepting that I'm not sure building this drain is any easier!

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Dugger52

· #6 · Apr 5, 2009

OK The permit is for the wall. Did you talk to zoning or stormwater folks? The engineer is being lazy. It's not a court issue it's an enforcement issue and bascially he didn't want to get involved. I'd give it one more poke through the combination of zoning and stormwater. It's going to be very important that the permit that was issued was after the fact. This means that the normal group of folks that review this type of permit didn't see it.

In the mean time.... What's under the ground? Any utilities? What you need to do is dig a hole about three foot deep (roughly waist deep). You could do it with a post hole digger but unless you have one of the Amish ones you cannot get that deep. This will probably be shovel work. Block it off so no one falls in. Fill it about half to two thirds full of water and record how long it takes for the water to drain out. Check it every 15-30 minutes. If you've got kids and a timer give them the job and you confirm when they think it's empty. 10 year olds think this is great fun.

The French drain is just a place to store the water so it'll seep out. If the soils don't perc (drain) all the French drains in the world won't help.

In the mean time part 2... Is the area flat? is there an area where you could redirect the water to? Two or three pictures would be good at this point. Can you take the out put from the weep hole(s?) in the wall and redirect it?

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AllanJ

· #7 · Apr 5, 2009 (Edited)

But can't you block where the neighbor's wall dumps water onto your yard?

Before they build the retaining wall, where did the water on their property go?

The faster you act, possibly needing court, the less the chance the situation as it stands today will become status quo and you will be stuck with all that water.

Normally a French drain has a pit at its lowest point. Where will the water ending up in the pit be pumped to?

(copied from another post) A French drain is supposed to collect ground water and bring it to a pit for frequent and prompt removal, not to take water from a pit and disperse it along the various perforated pipes or tiles.

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geckokarma

Discussion Starter · #8 · Apr 6, 2009

Thanks for your help everyone.

Some photos, then.

Here's a shot looking up the grade, from the near left corner of the backyard. The new retaining wall is on the far right -- it actually terminates a few feet from our property. We have built a bit of a berm to keep that water out (our neighbor to the right has given up fighting the water -- not surprising since he's renting out the property). Note the considerable slope -- roughly two timbers or about a foot over 30'. I was hoping not to complicate this story terribly but yes, I imagine a good fraction of water is coming thru the timber wall. Note also the soggy state of the wet cardboard in the foreground.

The remaining portion of the yard to the left of the photo above:

And here's a shot of some clay in the soggy, shallow trench I've already dug:

Now I appreciate your collective sense of fairness and urging me to pursue justice, but its been much too long since the new retaining wall went in. And you may have concerns about the timbers -- we do too and are just starting to pursue that.

So, again, the goal was a French drain that exits into daylight in the back left corner. A dry well wouldn't do me much or any good since I'll bet the perc rate is anemic. The hope is that a new rain garden would respire a good fraction of the water, and the rest -- well, I dunno. At least the topsoil layer there is a bit deeper than elsewhere so maybe we've got a prayer. Even if a good fraction got past the garden as surface water, at least we've diverted it away from the foundation, and along the side of the house (with any luck, we won't impact our left neighbor!).

I am not expecting the French drain to do much leaching. It would just be a collector/diverter in my mind. It should dry up the majority of the backyard. That right side neighbor (abutting the new wall) did hire an engineer to comment on the situation and he reported that the water table was in fact not very close to the surface (sorry, I don't have a figure on its depth). So there might yet be potential for a dry well or some such, if in the right spot and of the right size...

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Dugger52

· #9 · Apr 7, 2009 (Edited)

OK picture worth about a million words in this case. You've got the right idea collect and run to daylight. What you're going to do is basically build a reverse drainfield. A fairly wide trench, say 3' to 4' minimum in this case that is fairly shallow, say 18" is what you're looking for. Put it in the lowest spot (where the water collects). You're just going to have to be smart on the length. I'd think 8'-10' would be the minimum. Use the natural slope to create your flow if possible. You're going to need about a 3-4% slope minimum to carry the water away and if you can get the while thing up to 7-10% you'll be amazed how much water comes out. You're going to need to line the trench with geo-tex material, if you don't it'll fill up with soil. and have enought to the sides so you can wrap it like a package over the top when you're done.

Lay perforated pipe in a 3" bed of of # 57 gravel (about 1" diameter) explain to the hardware or plumbing supply guy what you're doing and they'll fix you up. They will probably give you 4" black perforated (with holes) flexible pipe. Get the stuff that has as many holes as possible. Lay the pipe on the gravel and run it to a collector box (plumbing house) or make your own and GENTLY back fill with #57 to an overall depth of 12". Put a 2x4 in the pipe when filling and pull it out when done. this will keep the pipe from collapsing. Wrap the geotex over the top and pin it with some galvenized nails.

Tie everything into the collection box and run the solid pipe to daylight. You need to block the open end, most folks use fine grid hardware cloth inserted in the pipe, or you'l have every type of critter known to man living in that pipe.

Fill the rest of the hole (5") with a mixture of 2/3's sand and 1/3 top soil. Spring for some sod and install. Water the sod and it will grow yet permit percolation to you're french drain. Another thing wiould be to plant perennials and other such flowers but you do not want to do anything that will mess with the geotex.

You have just built a rain garden and it would have cost you about, oh, anywhere between $5-$10K, probably the $10K end in your neck of the woods, maybe more depending on plants.

By the looks of the pictures you dealing with some type of marine clay. That's the stuff they use to line farm and irrigation ponds to prevent leaching. Works well!

Hope this helps.

Almost forgot, get your rock from a quarry and have it delivered. You're going to need a bunch and Uncle Joe's puckup ain't gonna cut the mustard.

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geckokarma

Discussion Starter · #10 · Apr 8, 2009

Wow. Great stuff Dugger, many thanks. Makes my day.

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handy man88

· #11 · Apr 8, 2009

Dugger52 said:
OK picture worth about a million words in this case. You've got the right idea collect and run to daylight. What you're going to do is basically build a reverse drainfield. A fairly wide trench, say 3' to 4' minimum in this case that is fairly shallow, say 18" is what you're looking for. Put it in the lowest spot (where the water collects). You're just going to have to be smart on the length. I'd think 8'-10' would be the minimum. Use the natural slope to create your flow if possible. You're going to need about a 3-4% slope minimum to carry the water away and if you can get the while thing up to 7-10% you'll be amazed how much water comes out. You're going to need to line the trench with geo-tex material, if you don't it'll fill up with soil. and have enought to the sides so you can wrap it like a package over the top when you're done.

Lay perforated pipe in a 3" bed of of # 57 gravel (about 1" diameter) explain to the hardware or plumbing supply guy what you're doing and they'll fix you up. They will probably give you 4" black perforated (with holes) flexible pipe. Get the stuff that has as many holes as possible. Lay the pipe on the gravel and run it to a collector box (plumbing house) or make your own and GENTLY back fill with #57 to an overall depth of 12". Put a 2x4 in the pipe when filling and pull it out when done. this will keep the pipe from collapsing. Wrap the geotex over the top and pin it with some galvenized nails.

Tie everything into the collection box and run the solid pipe to daylight. You need to block the open end, most folks use fine grid hardware cloth inserted in the pipe, or you'l have every type of critter known to man living in that pipe.

Fill the rest of the hole (5") with a mixture of 2/3's sand and 1/3 top soil. Spring for some sod and install. Water the sod and it will grow yet permit percolation to you're french drain. Another thing wiould be to plant perennials and other such flowers but you do not want to do anything that will mess with the geotex.

You have just built a rain garden and it would have cost you about, oh, anywhere between $5-$10K, probably the $10K end in your neck of the woods, maybe more depending on plants.

By the looks of the pictures you dealing with some type of marine clay. That's the stuff they use to line farm and irrigation ponds to prevent leaching. Works well!

Hope this helps.

Almost forgot, get your rock from a quarry and have it delivered. You're going to need a bunch and Uncle Joe's puckup ain't gonna cut the mustard.

Before laying down the gravel, do you first lay down some landscape fabric to prevent mud/dirt from seeping into the gravel?

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Dugger52

· #12 · Apr 13, 2009 (Edited)

Read the last sentace of the first paragraph. :whistling2: You don't want to use landscape fabric use geo-tex material it's designed for this application and will still be there in 50 years.

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secretspy711

· #13 · Apr 19, 2009

In one of these posts, someone said that if the clay doesn't drain, then a French drain won't help. Can someone direct me to some helpful hints on how to put in a Trench drain instead?
Thanks

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Dugger52

· #14 · Apr 19, 2009 (Edited)

They are basically the same thing. The secret with heavy or marine clay is that you use the drain as the collector and run the water to another area. A "normal" french drain simply collects and lets the water seep into the ground whihc will not occur in heavy clay soils. There are clay soils that drain fine but only slowly. That's why you dig a test hole fill it up halfway and see what happens before you do anything.

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