Drainage Board Crushing at BasementWall and Slab Joint

[RFreund]

I'm curious to know if there is a concern for drainage board crushing at the joint of a foundation wall and basement slab. This detail seems fairly reasonable as a way to mitigate water intrusion through basement walls, but many times the basement wall is relying on the concrete slab to resist lateral forces. Especially in old basements which seem to typically have these type of water intrusion issues. See sketch below:

EIT

http://www.HowToEngineer.com

 

[JAE]

Why isn't the drain board on the dirt side of the wall?

This doesn't make sense to put it on the inside face of a basement wall.

 

[dauwerda]

JAE, I believe that this is a retrofit that is added to existing basement foundations that have ground water issues. I have seen this done before (I don't believe there was any type of engineer involved) without the drain board.

 

[JAE]

OK - makes sense - I've seen them done below the slab - never extended above.

If so - the concern stated is valid that the basement slab is used to resist lateral earth pressures and the drainboard would have to be designed to resist this compression.

 

[RFreund]

Correct, retro fit.
I have also seen this without the drainage board on the wall, but apparently water tends to come in through the joints in the wall. Especially old masonry or stone basement walls.

EIT

http://www.HowToEngineer.com

 

[dauwerda]

I'm not familiar with what the drainage board looks like, but could you have spots every couple of feet where the floor continues all the way to the wall and the drainage board directs any moisture around these spots? It may be that the drainage board won't have any issues either - do a quick calc on what the pressure on the drainage board would be due to sliding and you may find that it results in a low enough pressure its not a concern.

 

[oldestguy]

Slabs for the floor usually shrink away and wall stays put. I'd not worry about it. I'm curious that the wall drainage board is sealed that no moisture gets in the room from it. Before using it, I'd want to see similar installations to see how they perform.

 

[EdStainless]

I have seen jobs here in WI where the trenched around the inside and put in drains, and then punched holes through the wall below the floor to let the water run down the outside of the wall and into the drains. Then they sealed the inside of the wall.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[KootK]

Some googling turned up compression strengths in the range of 25 psi - 250 psi. I suppose you either spec a particular strength or, alternately, maybe thicken the SOG against the wall so get the compression down to the lower end of things.

 

[RFreund]

Thanks for the replies. Where'd you find the drainage board compression values? I tried to contact a couple companies but didn't hear back from therm.

EIT

http://www.HowToEngineer.com

 

[HouseBoy]

A few observations I've made over the years:

A common "leaky wall" repair strategy is to use an interior perimeter drain system.
8 to 12" of slab is cut out at the perimeter and a perforated pipe and gravel is installed. Sometimes a "drainage board" is used. That is often a stiff plastic sheet that runs up the wall to provide a barrier so that water that does leak thru will trickle down to the drainage pipe.
I wonder how moldy it gets behind that plastic?
I also wonder how a system like that is supposed to reduce the soil pressure on the wall. (Spoiler alert - I don't think it does reduce the pressure substantially because I think the pressure is coming from expansive clay which, with interior systems, is not affected by the system).

Anyway, I usually find that the thickness of the slab that is replaced is not as great as the original slab.

KK - Most of the drain products I see do not provide much contact between the drain chamber and the foundation wall so I don't expect that compression strength is a meaningful measure of the ability of a given product to resist the pressure.

 

[KootK]

Where'd you find the drainage board compression values?

I just googled [drainage board compressive strength] and a bunch of product info came up.

KK - Most of the drain products I see do not provide much contact between the drain chamber and the foundation wall so I don't expect that compression strength is a meaningful measure of the ability of a given product to resist the pressure.

Well, I am by no means a drainage board expert. What do you take the quoted compression strength values as meaning then? This stuff obviously has some crushing strength or we wouldn't be able to compact mounds of dirt behind it.

 

[r13]

Even the drainage board is crushed, what damage will be done? If the basement water problem was due to the intrusion of the ground water, which now is taking care by the drainage pipe, if designed/works properly.

 

[PropertyGuy67]

For anyone landing at this thread from a basement drainage search, the sketch the OP provided is for an interior drain system. It doesn't solve the water intrusion problem, it merely manages it. Sometimes that's the most cost-effective thing to do. Sometimes it's like putting lipstick on a pig. If money were no object, the right thing to do is trench the exterior foundation wall and seal/drain it from the outside. The choice boils down to a value decision. With that said, we've all seen properly designed and installed systems work like a charm, and we've all seen shoddy work fail.

As for crushing of the sheet drain or hydro channel or whatever the manufacturer calls their edging, if it starts crushing, you have bigger problems than water. If it starts crushing, it means your footings aren't being restrained and they're moving inward.

 

[r13]

The drainage pipe must have an outlet, usually through sump and pump. Sized and works properly, the ground water shall be kept below the footing. I prefer to seal the slab to wall edge than use the drainage board.

 

[HouseBoy]

PG67 is right about "managing" the water intrusion. Too elaborate.... the issue I have with the trade-off is that it does very little too reduce the soil pressure when that (the soil pressure) is the result of clay swelling when it gets wet.

Water on the basement floor is a nuisance and naturally it is undesirable. The pickle that the structural engineer might get drawn into has to do with responding to how much or how well an interior drainage strategy will help a structural problem which, as I wrote, is very little. Around here ( Greater Cincinnati) we have a lot of fat clay. Not like in Texas/Oklahoma or even St. Louis and we also have a lot of 1905's basements that were built with concrete block. We don't get as much volume change as some of the other places and maybe that is a little part of the problem since the result is that the problem is just slight enough that folks don't appreciate it until it's too late. Over time though, the pressure on a basement wall is a problem and an interior drain doesn't help.

KK - Compressive strength values SOUND great don't they? You know, is't there one of those infomercials that touts something being "stronger than steel"? Now, doesn't THAT sound impressive? Here's a pic of a common product. I don't see much contact area....

 

[PropertyGuy67]

HouseBoy that's probably one of the more insidious interior drain products out there. If you're really trying to manage a water issue, you should be placing your drain lower than the underside of the slab. At the level shown in the photo, water can still wick up through the concrete. But here's the kicker: if you go down to the bottom of the footing on the inside the basement, you've now disturbed the restraint (fixity) holding the footing in place in the face of lateral pressure from the exterior. This is why you really have to think long and hard about installing these interior systems. You really are changing the performance of everything inboard of the footing. I wonder if 10 years from now, we're going to be addressing footing movement due to interior drain system installations. Be on the lookout for >-|-< crack patterns in foundation walls.

 

[BridgeSmith]

Houseboy, I think what was being proposed was what is commonly referred to as a sheet drain. The core compressive strength of that particular one is listed as 11,000 psf, which is 75psi, or 5,500 lbs/ft for a 6" slab. As KootK suggested, if more is needed, thicken the slab at the edge.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[HouseBoy]

PG you'll get no argument from me about the correctness of your position however, that is distinctly NOT what is done in the "business" of basement repairs. I don't do it, but the franchised services are installing stuff that is essentially like the product I showed.

BS - You're probably right about the nominclature. I didn't mean to cause confusion. Thanks for the clarification.
Regarding 5,500 lbs/ft.... I think that might be a generous interpretation of that information.
Maybe I'm wrong but it doesn't seem realistic to me. At the same time, 5,500 lbs/ft doesn't seem like it will develop to be that high at the bottom of most residential basement walls.
As far as thickening the slab at the edge - it's the right idea but I don't remember ever seeing that done.

 

[bones206]

This detail seems fairly reasonable as a way to mitigate water intrusion through basement walls, but many times the basement wall is relying on the concrete slab to resist lateral forces.

If you are demolishing the edges of the basement slab, how can it continue to provide lateral support?