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I'm having issues with a leaky basement in the Portland area. There's dirt, mud, and vegetation outside of my foundation wall, and it's absorbing lots of rainwater. 4 feet of it is inside my property line, but my neighbor has a wide grassy driveway that's also contributing. This water trickles through mini cracks in the foundation, and it's leading to lots of puddles inside:

enter image description here enter image description here

The solution I'm going with is a french drain. I'm digging a 12"-wide trench 2ft away from the foundation wall, and 2ft deep. I'm planning on using some non-woven Geo-textile fabric to line the walls and bottom of the trench, and fill it up with river-rocks. I'm using a 4" perforated PVC pipe to carry away the water at a slope of 1 vertical inch for every 10 horizontal feet. (I think this is all best practice, but feel free to correct me if I'm wrong)

enter image description here

My question is: I've read several articles and seen some YouTube videos that suggest adding a 3-inch bedding of rock underneath the PVC pipe, like step 4 in this article.

Pour and compact about 3 inches of gravel or landscaping stone along the bottom of the trench. This will act as bedding for the drainage piping.

But why? Wouldn't this mean that the water has to climb higher before entering the pipe? This would allow for more pooling and absorption by the soil, thus higher likelihood that the water will still reach my foundation walls. If I set the pipe directly on the non-woven fabric, the water would get carried away sooner, no? Why is that extra 3-inches necessary?

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    Additionally, based on the pic of the inside of the wall, you might want to consider excavating around the foundation and installing/fixing waterproofing on the outside of the wall then adding in the French Drain. It seems that there are several weak points in the wall and any waterproofing added to the exterior will only help.
    – FreeMan
    Jan 20 at 17:18
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    @Marquizzo perhaps checking with your foundation guy if temporary bracing would help, and whether you could dig a short section at a time, say a metre or 4 feet, enough to get in and clean the outer face, then apply an appropriate paint-on sealant. You might choose a roll-out cloth barrier on top/beside the painted layer, like horizontal wallpaper to help reject water.
    – Criggie
    Jan 21 at 1:01
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    Because that's (one of the steps of) how to bed a pipe. And yes, leave as much soil as you can that's against your foundation undisturbed. Out of the pipe and down into the earth is where you want it to go; it's gotta go somewhere, otherwise it fills up and is moot. 4' deep at the house would be better, leading to somewhere else deeper than that - or at least at 2' doing so. - With "suitable compacted granular material"
    – Mazura
    Jan 21 at 2:10
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    Before proceeding with underground drains you should determine whether the water is moving on the surface or underground. You think your neighbor's pervious driveway is a recharge zone, but is this what is happening? Maybe with a post hole digger you could dig a vertical hole 3 ft deep near to the property line and see if it fills up with water to some level. Or consult a local expert on what is happening. Jan 21 at 12:12
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    Agreed with @FreeMan. The long-term fix here will be to excavate down to the foundation and waterproof the concrete. I'd be wary of spending a lot of money on halfway mitigation when you can just save up and fix it for good.
    – J...
    Jan 21 at 15:31

4 Answers 4

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If you put the pipe right on the fabric, you'll end up filtering the inflow through the 1/2" circles of fabric pressed up against the holes in the pipe, as opposed to filtering through all the fabric supported by stones and then flowing into the pipe holes. You can probably use less than 3".

You can always dig several inches deeper to compensate, of course.

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The water will still flow through the rocks with the pipe assuming your ditch slopes as well. If any sediment makes its way through or around the fabric, the extra 3" will help prevent the sediment from going into the pipe. Hopefully it will be a long time before that 3" fills in.

I bought a home that had a french drain installed across the property 20-25 years ago using 2 4" pipes on top of each other. I could only see the exit and the water flow was very poor. After digging it up, both pipes were completely full of sediment despite having fabric and gravel around it.

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  • What's the benefit of stacking two pipes like that? Does the top pipe do any work? I'd expect the bottom one to do all the heavy-lifting, since ideally it wouldn't accumulate up to 4" to reach the top one.
    – Marquizzo
    Jan 20 at 21:54
  • After 100ft they went 2 different directions, and they both came out wedged into an 8" steel culvert in the middle of my driveway. I think the intent was to collect at one end of the property and the 100ft was for transport, but that was all french drain as well. It's now 100ft of 12" to transport and ditches to collect, and I've seen the entrance 2/3 full...there's no way the old drainage even if clear could handle all that water.
    – rtaft
    Jan 20 at 22:21
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    @Marquizzo - the benefit of stacking the pipes is that gravel is expensive while plastic piping is relatively cheap. In addition, pipes take longer to plug up than gravel although both will eventually be filled by sediment as noted by above. Jan 22 at 14:21
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I'm doing similar right now,at my home.

The problem is twofold.

  1. Silt and sediment will flow with the water and ultimately, may either clog the geotextile, or bypass it (through it if its a broad enough pores, or round the open end if left open). Let alone weed roots. You don't want that.
  2. The geotextile greatly impedes water flow. You want it to have as much free area and possible pathways to allow water flow through it. As well as excess area, as safety margin, to ensure water can still flow when the inevitable silting happens.

Both of these have the same end result. You separate the geotextiles from silt, as best you can, externally, and from the pipe, internally. Digging a foot deeper is easy. You don't want to have to redo the job.

So I'd go further than you propose. I'd do it like this:

  1. Dig out 3 feet deep, not 2, and 18 not 12 inches wide.
  2. Using plywood sheets cut lengthways, divide the trench above this layer into 3 sections widthwise - 4 inches wide on each side, 10 inches wide in the middle.
  3. Hammer into the soil or stake them lightly, so the bottoms and tops stay where they're put while you are working. Not too heavily, though, you want to be able to remove the plywood and stakes easily after. An alternative to.hammering or staking would be shallow battens (up to 1 inch top-to-bottom deep) at the bottom of the trench, acting as spacers. That's shallow enough to not be an issue and can be left there when the plywood is removed.
  4. In all 3 sections lay 4 inches of 10mm (half inch) gravel or other coarse aggregate.
  5. Put your geotextile in the centre section.
  6. Put another 3 inches aggregate in each section.
  7. Lay your pipe in the middle of the centre section
  8. Fill all 3 sections to just above pipe level
  9. Remove all plywood/stakes
  10. Fill trench with rest of aggregate to a few inches below ground level.
  11. Roll geotextiles around over itself and lay flagstones on it to hold it closed against silt from near surface level, and to reduce soil movement over time at the top and edges of the trench.

enter image description here

  • black dots - plywood sheets (removed after)
  • brown stripes - aggregate
  • blue line - geotextile
  • grey - flagstones

The aim is that your geotextiles has space on all sides externally for maximal water entry and to keep silt away as best possible, and space on all sides internally to ensure any water that enters past the geotextile isn't blocked or obstructed by the plastic of the pipe, and that if it is, there's a chance it can percolate into a different perforated hole. Silting will happen and we want to minimise the obstructions to water flow, maximise possible water pathways, and delay the time it takes, as much as we reasonably can. Everything is geared to that goal.

The use of small aggregate means there is maximum chance of small passageways remaining open as silt tries to percolate. The use of a decent thickness of aggregate inside and out (3 - 4 inches all directions, internal and external) maximises the chance that despite inevitable silting, there's enough space for many pathways that water will still percolate. The rolled top and flagstones aim to prevent anything affecting the top, and specifically silt entering from above, or as the ground changes or is trodden over time.

One thing, if your perforated pipe has slots not holes, its better. You can use 1/2 inch gravel and it won't get into the pipe, whereas with holes you need much larger aggregate between the geotextile and pipe. This kind of thing (you can cut extra slots with a saw if needed as well):

enter image description here

Update; Last, add an upward 45 degree bend at the start of the pipe, and add a rodding point at ground level, so you can jetspray or rod the pipe if it does block, or at least check it isn't blocked.

Other update: Consider a waterproof coating (tanking product its called here, don't know the US name). Something you coat on the inside wall + floor, to block or at least reduce water intrusion, before optionally overcoating. This sort of thing.

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  • Thank you for the detailed advice! I hadn’t considered using a slotted pipe, it does seem better than simple holes. I found a corrugated pipe with 8 slots here, do you think this would perform better than a solid PVC pipe?
    – Marquizzo
    Jan 22 at 21:28
  • Sorry, website doesn't let UK visitors see that page. But solid is probably better from sound of it. If its flexible, how can you be sure of correct slope at all points, and as the ground settles over the years it won't sag in places. So i wouldn't use a flexible pipe for this, at all. Buy a solid pipe and cut slots yourself if needed,if you can't find a precut one. (For reference if cutting yourself, slots are maybe 3-5mm (1/8 - 3/16 inch) wide)
    – Stilez
    Jan 23 at 10:42
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If that is a poured foundation, and the water is coming through shrinkage cracks, an epoxy/expanding foam solution might work from inside the house. I have had great success with a few cracks on my home, and have plans to finish the rest of them this coming summer.

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  • How big do the cracks have to be for you to be able to inject the expanding foam? Mine are less than 1mm cracks, so I don't know if a nozzle would fit.
    – Marquizzo
    Jan 25 at 19:17
  • I am not entirely sure on the rules about me posting to actual products. The company I used as been great on over the phone on their two options. You can use low pressure which involves attaching ports with epoxy, then using a caulk gun. The high pressure is what I am about to do in summer which can handle hairline cracks. Amazon sells some kits under several different names.
    – Evil Elf
    Jan 25 at 23:35

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