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dgreene (Structural) (OP)
9 Oct 06 22:11
I am building a house on a lot that slopes from front to back. I have an engineered retaining wall running down the entire side of the house (approx. 90 ft.) and across the back (approx 100 ft.). There is a french drain set in gravel and cloth running along the retaining wall. There is plenty of fall for good drainage. I have my gutter drains running into a catch basin in the front. My question is this ... Can/Should I tie my catch basin into my french drain and let the water run down the side of the house and out the back of the retaining wall? Can a french drain leak water out even if gravity is moving the water through the pipe sufficiently?
moogbro (Civil/Environmental)
9 Oct 06 23:27
I definitely wouldn't combine the two.  The french drain is sitting in drain rock.  Water can't get into the pipe until it builds up in the drainrock to the level of holes in the perforated pipe.  If you connect the two, you will be filling up the drain rock to what ever level the pipe is running.  In my first home, I tied the gutters into the standard ADS black 3" flex pipe, then ran it to the curb near the street.  I recall watching this during one of the big rain events and it was pushing water up about 2 ft into the air (through the small holes in the rodent screen). Imagine how much water pressure might build up behind your expensive engineered retaining wall if the french drain ever plugged with debris or rodents. You likely have a rodent screen on the outlet end of the french drain.  Additionally, water permeating through the wall could lead to unsightly effervecent staining of the wall (i.e. calcium leach stains on the face of the wall).
emmgjld (Geotechnical)
9 Oct 06 23:56
dgreene

Look over this  thread194-166822
CSLufkin (Mechanical)
11 Oct 06 13:01
The best reason I have heard NOT to do that is the fact that you are introducing the Environment (Bugs, Snakes, Leaves, Mice, Mildew) into a system that would otherwise be protected from it, except the outlet end. A buildup of any of these things could plug up the system, even if it were sized large enough to accomodate the extra water, and a backup in this system has very expensive consequences. I recommend installing a small dry well (Perforated Culvert Pipe with a solid cover, placed vertically with stone around it) about 15 feet from the foundation near each downspout. This will get the water into the ground and if it plugs up you will be able to get in there and clean it out.
emmgjld (Geotechnical)
11 Oct 06 14:51
A Caution!!!
Always consult local practice AND local Geotechs before using 'Drywells".  ALWAYS!!!

In my area of both expansive clays and collapsible soils, a drywell introducing large amounts of water into such soils can have devastating effects.  And yet, I use drywells quite a bit, if the conditions are right.
cvg (Civil/Environmental)
11 Oct 06 19:50
using drywells in clay soils is not a very efficient way of percolating water into the ground and really is not recommended unless you have drilled through the impermeable clay layer down to a more permeable layer (above the groundwater table).  This is probably not feasible for the ordinary homeowner, however is frequently done for larger developments around here. Discharging the water 15 feet away from the foundation is a good idea, but it should be discharged either on the surface, into a drainage swale or into a storm drain with an outfall at the edge of the property.
oldestguy (Geotechnical)
12 Oct 06 21:09
I think everyone needs more info.

Why is there a French Drain at the wall?   Is it inside for keeping backfill saturation from happening, or is it outside for some reason.

What soil type do you have?

For the heck of it do a search of these forums using "French Drain" as a key word.  There are many that may help.

What is your climate?  Southern, Northern?
oldestguy (Geotechnical)
12 Oct 06 21:13
Matter of fact there is one down this forum a ways with that subject title.
dgreene (Structural) (OP)
13 Oct 06 9:55
The french drain is "inside" the wall to alleviate backfill saturation. I am in Dallas. Soil type has quite a bit of clay.
oldestguy (Geotechnical)
14 Oct 06 19:17
Thanks for info.

On that basis,why dump water into a place where you do not want any water?  I trust the top of this porous "drain" is sealed to keep out surface water.  If not do so.

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