French drain?
French drain?
(OP)
I have a question regarding a residential application. I've never worked on a single family residence before, so I'm somewhat unfamiliar with what probably seems like standard terms/issues.
I have a house where there is pretty severe water penetration into a basement. The foundation walls are CMU (no grout). A contractor was called and said a french drain would alleviate the problem, but I'm not sure exactly what a french drain is and if that is true. When I think of a french drain, I think of my grandmother's basement with a stone trench around the exterior basement wall taking away the water that is coming into the basement.
While this might keep the basement dry, it is not addressing the issue of increased lateral pressure against the basement wall.
Can anyone offer any insight into this?
I have a house where there is pretty severe water penetration into a basement. The foundation walls are CMU (no grout). A contractor was called and said a french drain would alleviate the problem, but I'm not sure exactly what a french drain is and if that is true. When I think of a french drain, I think of my grandmother's basement with a stone trench around the exterior basement wall taking away the water that is coming into the basement.
While this might keep the basement dry, it is not addressing the issue of increased lateral pressure against the basement wall.
Can anyone offer any insight into this?






RE: French drain?
others here can probably provide much greater detail than myself since i don't touch residential but that's my thoughts.
RE: French drain?
J
RE: French drain?
I don't see that gravel backfill over a perf pipe to channel the water elsewhere, as behind a basement wall, qualifies. To me, that's just normal basement wall construction to control the water.
Mike McCann
MMC Engineering
RE: French drain?
www.idecharlotte.com
RE: French drain?
This seemed like it would be pretty expensive, and, as someone mentioned above, the idea of compacting gravel against an unreinforced masonry wall doesn't seem like a good idea. The wall is not currently showing any signs of distress, but it is severely discolored from years of water infiltration.
idecharlotte-
That still doesn't address the issue of increased lateral pressures on the wall, it's just making sure the water doesn't get into the basement.
RE: French drain?
If you are able to lower the porewater pressure, and the groundwater level, through the use of gravel and a lateral foundation drain, but do not compact the gravel as there is really no need, how is the lateral pressure on the wall increased? I do not see a structural concern here for the wall...
Mike McCann
MMC Engineering
RE: French drain?
http://www
RE: French drain?
I agree with you if the water can be drained from the soil immediately surrounding the wall. I didn't know if that is what a french drain is, or if it is only a localized stone at the bottom of the wall. I don't believe that latter will significantly reduce the lateral pressure since the soil above will likely remain saturated.
I was really trying to figure out exactly what a french drain is.
RE: French drain?
A shallow trench, perhaps lined with tile or containing a perforated pipe, backfilled with gravel and then covered over, used to collect and channel runoff water.
a drainage trench filled to ground level with fragments of brick, rock, etc.
Here is a link:
http://www.wisegeek.com/what-is-a-french-drain.htm
RE: French drain?
Normally, behind the wall full height, a minimum 1' wide vertical strip of drain rock is installed to within 1 foot of the surface with a drain pipe at the bottom. Sometimes, depending on the volume and type of runoff, an intermediate pipe may also be used. All the water is then tightlined away from the foundation via gravity to a point lower than the foundation. However, unless the water goes to an infiltration sump, this is not a French Drain system, just a standard foundation wall drain.
Mike McCann
MMC Engineering
RE: French drain?
Mike McCann
MMC Engineering
RE: French drain?
Thanks!
RE: French drain?
A sump is not required for it to be a French Drain. Sometimes a sump (which is just a pit) is located at the end of a French Drain System but not always. Some french drains just daylight and the water flows out at a grade lower than the elevation you directing water away from, e.g. if your house is on a sloping lot. In summary a sump is not required to qualify it as a French Drain.
J
RE: French drain?
RE: French drain?
Have the contractor explain to you how his system works and see if it makes sense hydraulically.
Mike McCann
MMC Engineering
RE: French drain?
After it was installed, the following year was the wettest in a decade and my basement was bone dry.
RE: French drain?
Msquared48: The notion that well draining gravel over 12 inches wide and nearly full height is placed against basement walls for backfill is far from reality in my experience. It is often needed but usually, the native soils are used predominantly for backfill. I realize this is not what the code says but that just doesn't happen very often. (I have noticed that it is a big deal for the "Superior Wall" systems...)
Idecharlotte: I think your notion that a French drain will "fix the problem" can (possibly) be misleading. The key concern I would have is not to overlook the potential lateral earth pressures against the wall that might result from the presence of water and clay. Yes, in this case it seems that there is not a problem but I think soil pressures build over time in clayey soils and around here (Cincinnati, Ohio) we don't see block basements being built anymore because of the prevalence of moisture sensitive clays. It would be important to verify the soil type that is likely to be present before concluding that "the problem" is fixed.
RE: French drain?
There are several other things that can be done at the house... a small swale can be constructed to divert surface water. Location of RWL conductor pipe can be reviewed, window wells, etc. The soil should slope away from the home and possibly a clay cap located 6" below the topsoil.
Dik
RE: French drain?
I have had good sucess by installing a sump pump or two only without the "French" drain or drainage pipes - spaced about 40' apart. The water finds its way to the deeper hole, reduces wall pressure and then is pumped out. A LOT cheaper than cutting up the basement floor and putting in pipes. Might work - might not. But you can always add drains later.
ALSO - MAKE sure all water from gutters is drained well away from house as well as any sump water.
MAKE sure the landscaping and soil is well sloped away from the house.
After doing all this - there is a paint product called Dry-Lok that will help somewhat. It is applied to the interior walls. You can buy it at any Big Box store. Make sure you ventilate WELL!! It is nasty. It really only works with minor water pressure - so the other aspects need to be addressed first.