French drain? 1

[Lion06]

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?

 

[msucog]

effectiveness of a french drain will depend on it's location and soil type. essentially, you've got to cut the water off before it hits the back side of the wall. in a well draining sandy soil, it's pretty easy. in some silty, clayey or plastic soils, you can't just stick it at the bottom and expect it to do it's job. also, if the wall is unreinforced, i suspect it could be difficult to adequately compact the wall backfill without busting/cracking the wall. and if you've got very loose backfill, then it's more susceptible to saturation which doesn't help things. if it's well compacted, the surface water will hopefully drain away (if the grades are sloped) from the wall then the french drain will pick up the groundwater that does make it near the wall. if necessary, backfill the wall with select material or densified crushed stone with seperation fabric to minimize soil migration in to the stone.

others here can probably provide much greater detail than myself since i don't touch residential but that's my thoughts.

 

[jechols]

IMO a french drain system is free draining stone (#57 maybe) in wedge behind basement wall with a perforated pvc drain pipe next to footing or at base of wall. If that is installed the Geotech usually gives me 50 pcf equivalent fluid pressure for an at-rest condition like a basement wall.

J

 

[msquared48]

I've always thought of a French Drain as similar to a sump system where water is infiltrated into the soil, whether it's from the roof downspouts or surface water runoff. Generally the sump is placed in infiltratable soil well away from the foundation of the structure.

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

 

[idecharlotte]

If the basement is leaking a French Drain alone usually will not fix the problem. Waterproofing the wall is the only sure way of preventing the wall from leaking. If the water leaks up through the slab, you have a bigger problem like a spring. But that's pretty rear. If cost is an issue with waterproofing the outside of the wall, you can put a french drain on the inside of the basement against the wall, (drained to daylight). Of course you have to allow the wall to leak, but at least the floor won't get flooded.

http://www.idecharlotte.com

 

[Lion06]

My initial thought was that to have the soils checked. If the soils are poor (clays, or something else that just retains water), then dig out soil for about 2'-3' all around the foundation wall and replace with stone (which would allow that water to drain down), going into a pipe which takes the water away.
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.

 

[msquared48]

StructuralEIT:

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

 

[cvg]

this subject has been discussed numerous times in various forums. do a search for french drain or sump pump and you should find quite a bit of information

http://www.google.com/custom?domain...000000;GFNT:0000FF;GIMP:0000FF;FORID:1;&hl=en

 

[Lion06]

Mike-
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.

 

[jechols]

Definitions:

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

 

[msquared48]

StructuralEIT:

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

 

[msquared48]

Also, depending on the amount of fines in the surrounding soil mass, the entire drain rock mass may or may not be be enclosed by Mirafi Filter Fabric or equal to prevent the drain rock from becoming clogged with silt. As the filter fabric tends to become clogged too, this is rather problematic.

Mike McCann
MMC Engineering

 

[Lion06]

Ok, so the stone needs to be full height (not just at the base of the wall) and this will drain to somewhere else. I'm not concerned if it goes to a sump or just goes somewhere else that won't be a problem as long as it truly alleviates the lateral pressure on the wall and doesn't just stop water infiltration into the basement. It sounds like this will accomplish that goal.

Thanks!

 

[jechols]

StructuralEIT:

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

 

[Lion06]

The homeowner has a basement waterproofing company wanting to install a "french drain" around the perimeter of the basement wall ON THE INSIDE, without doing anything on the outside. While I believe this may alleviate the problem of having water in the basement, I don't believe it will correct the real problem of pore pressure against the wall. The problem is that I'm not positive it's an issue as there are no signs of distress on the wall. That being said, I don't believe it's a good situation whether the wall is currently showing signs of distress or not.

 

[msquared48]

French drain systems can be installed under the slab or in the crawlspace to de-water areas. However, unless there is flow of groundwater from the outside to the inside, which could undermine the foundation (bad idea), I would be skeptical here. I share your concern about the porewater pressure seen by the wall. Personally, I would not be comfortable with water flowing under the foundation to my house, unless it was in a piped system that was protected from damage.

Have the contractor explain to you how his system works and see if it makes sense hydraulically.

Mike McCann
MMC Engineering

 

[DWHA]

I had a similiar situation with the house I currently own. While the previously owner promised there was never a water issue, we had 2" of standing water the first spring we where in there. I hired a "Basement Technologies" installer and they came out and installed a under floor tile system and it took care of the problem. I have a poured foundation, but when I was talking to the owner of the company he told me that they have a system similiar for block foundations.

After it was installed, the following year was the wettest in a decade and my basement was bone dry.

 

[HouseBoy]

StructuralEIT is correct to be concerned about the lateral pressure on the wall. Lateral pressure against the wall is most commonly from wet clay soils causing the clay to swell (if it is sensitive to moisture which is common). An interior drain system will not improve that part of the problem.

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.

 

[dik]

A French drain can be used to intercept water or to draw down the water table locally depending on the ground/surface water conditions and the soil. Wrap the drain in geotextile to keep fines from rendering it useless. Drain to free drainage or a sump.

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

 

[MiketheEngineer]

See all above - good info.

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.