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I was wondering if anybody can tell me what is wrong with this logic. Assume that you have a walkway that is adjacent to the house. When you install that you dug the recommended depth you add gravel and sand. Aren't you practically creating a French Drain this way? If the soil that is next to it is flooded by a heavy rain wouldn't the water take the path of minimal resistance and go under your walkway which is now like a french drain? Wouldn't this affect the wall in the end ?

Edit: What I am trying to understand here is if a walk way adjacent to a house is a good idea when it can actually suck water in due to high resemblance with a french drain, (actually worse because the bedding is similar to the french drain but it misses the pipe to evacuate the water

  • French drain? Does that mean a sidewalk? – Tyler Durden May 22 '15 at 22:18
  • Not exactly. What I am trying to understand here is if a walk way adjacent to a house is a good idea when it can actually suck water in due to high resemblance with a french drain, (actually worse because the bedding is similar to the french drain but it misses the pipe to evacuate the water) – MiniMe May 23 '15 at 0:29
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    I think everyone is getting caught up on the French Drain analogy. I don't think you're suggesting that the sidewalk would act as an actual french drain, you are only using that to illustrate the concern that water will sit under the sidewalk and potentially damage the foundation of the house. Is that a correct understanding of this question? – Joel Keene May 25 '15 at 20:25
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I'm confused on the intended purpose of the french drain in your scenario. Is the intent to help drain the outside of a foundation wall of a house? Or to help dry out a soggy part of a yard? Or divert water away from the top or bottom areas of a retaining wall? The exact reason this line-of-thought would be wrong depends on these specifics.

None-the-less, it is likely wrong.

French drains work based on the permeability of the surface and immediate sub-surface. In a typical french drain, a trench is created that is typically deep and somewhat narrow. This provides an area of easier flow for water to drain from surrounding areas to the drainage pipe at the bottom. French Drain example photo

Your walkway example would not have the drain pipe to eliminate the water from the area and allow drainage to flow away from the area.

Further, french drains are typically covered by a very thin layer of soil (2-4") to allow the surface water a quick entry into the pipe below. Because in most instances the french drain is located in a low area, this area naturally holds water and needs a quick escape to prevent waterlogged terrain.

Of course, if not that much water needs to be removed, french drains are possible without the drain pipe. These hold only as much water as the volume available of the trench minus the rock. In permeable subsoils, these function well for quickly holding smaller amount of water and then allowing them to slowly seep into the subsoil. If your subsoil is less permeable, you'll want a pipe to remove the water.

In a typical sidewalk, the area of gravel is relatively shallow and wide. If your sidewalk had 3" of gravel, they would need to be 4' wide to have the same volume as the narrow, deeper trench, but this gravel will be compacted for even less water holding capacity. Further, this would only be adequate for surface runoff. If you have any concerns of subsoil saturation, the deeper trench would be needed to help the deeper soils to drain as well.

If this is for a wall or foundation, the drains must be at the lowest level of those features and drain the water well away from them. Even with a walk out basement, your foundation is likely to be 1-3' below the depth of the flooring of the lowest house level. The sidewalk would not be deep enough to drain the to the extent needed.

There is a possibility of combining the two features, of course. Dig a deeper trench as would be typical to whatever depth was needed for the footer of the wall, and construct the drain as typical for your need. However, instead of filling the trench to grade, stop 4-8" below grade to the level of the sidewalk base material and construct the sidewalk as normal, however, with the base gravel 3-6" wider than the walk. After the concrete sets, replace the sod around the sidewalk, building up the gravel thickness if needed to keep sod depth minimal with good draining soils (especially if you poured thicker than average walks) This will allow water to quickly reach the drain on either side of the sidewalk.

So - it is possible if you have very limited requirements, but for almost all cases, you'll need to create both the drain and the walk, but perhaps they can have the same footprint. Hope this helps!

Edit: If you are not concerned about draining the foundation but instead just the surface flow, then I think this would work best with a ridged pipe included under the base for the concrete (or pavers) and a quick migration path from the surface to the base and flushing from beneath. Depending on the climate, I'd be worried about freeze damage to the walk way at the end of run without providing extra runout to ensure the water can travel well off the walkway areas prior to freezing. Finally, usual french drains will hold and transport more per ft3 due to the compacted nature of the fill - 1" gravel provides a lot of space between the aggregate that sand and other paver base would simply not provide. Of course, the paver base would, to a lesser extent, move water better than the soil alone. It entirely depends on how much water you want/need to move.

  • Hi Ben. I know very well how the french drain works, if you check my posts around here I have commented a lot about that. In this case I am just discussing the slight resemblance between the structure of a french drain and the bedding needed for a walkway. Although to a smaller degree, the walkway seem to potentially have the effect to attract water by lateral migration (the water will pick the path of lesser resistance, the permeability is higher for the layer of sand/gravel under the walkway than the permeability of the soil next to the walkway) and it might accumulate under the walkway – MiniMe May 21 '15 at 11:37
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    In my case I am trying to drive the water away from the house, the water will drain along the pavers following their slope but when it reaches the outer end of the pavers it might as well go back under the pavers due to the above mentioned effect – MiniMe May 21 '15 at 11:39
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The short answer is no.

A french drain is designed to divert ground water away via a pipe to another location. Your walkway just has a very thin layer of gravel and bedding sand. It is missing the most important component which is the drain pipe.

It is true that some of the surface water may get beneath your pavers and go into your bedding material until it is water logged, but the water will basically just sit there and slowly percolate through the soil.

A true french drain would be much deeper than your walkway. Ideally it should extend to the bottom of your footings. The drain pipe then would be setup to divert water away from your foundation. There should also be a layer of gravel below the pipe as well as a cover on the pipe to prevent it from getting clogged up with soil. Then a layer of gravel would be added to completely cover the pipe, and then finally the hole will be backfilled.

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I don't think that the walkway will "suck" water in and hold it against the side of the house; however, you have essentially made a low point against the side of the house in the gravel under the walkway. Your intuition is correct, in that, if water is flooding on the lawn, it will sink to the lowest spot the easiest way it can. If you had the same dirt against the house that you had in the rest of the yard, the water would just as likely roll away from the house before soaking in to the ground, but by giving it an easy place to pool, it will fall to the bottom of your sidewalk and sit there. It's not so different from if you actually dug a 6" trench all the way around the house against the wall, except, you have the walkway covering it which will help to shed some of the rain.

Whether this will cause a problem will depend on the weather and soil in your area and a lot of other things. If it tends to flash flood a lot, that would certainly be worse than if it is always steady slow rain that is less likely to be in a position to move back under the walkway.

If the ground adjacent to the walkway is graded properly, and the walkway itself is graded properly, that might be sufficient to prevent the water from collecting under it, but again, this will depend on the type of rain you have.

If there is a problem with collection of water, one possible mitigation would be to put an actual french drain under the sidewalk so as to not allow the water to stagnate there.

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I just realized that the correct way to build the walkway adjacent to a house is to actually build the bedding ABOVE the ground level. You put the edging first and within the perimeter formed by the edging and the wall of the house you put the usual layers of gravel and lime stone crush and the pavers on top of it.

Of course you will need to grade the soil next to your house first.
Digging the soil is a wrong way to do it!

  • Please vote this answer or speak against it so I can close this question – MiniMe May 26 '15 at 11:17

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