Lateral resistance to strip footing movement along a basement wall

[PropertyGuy67]

A thread came up in the general discussion section about interior drainage systems along the footing. These systems are ubiquitous in homes in neighborhoods with poor drainage and wet soils, and owners who don't want to dig out around the exterior of the foundation and correct the issue from the outside. With interior systems, you basically jackhammer out an 18-inch wide strip of the basement slab along the foundation wall all the way around the basement, hog out the fill down to the bottom of the footing, install a drainage pipe that leads to a sump, backfill with gravel, cap with concrete. As part of this scheme, dimple plate material is typically placed at the face of the foundation prior to re-pouring concrete, to allow for water to drain out of the base of the foundation and over the top and edge of the footing down towards the new drain pipe.

My question: doesn't the act of hogging out the fill on the inboard side of the footing leave less lateral restraint for the footing to move inward, especially in an older home without grade beams or rebar in the footing? Is it a mistake for these companies to assume it's ok to remove the fill material, and give no thought to the loss of lateral restraint? I know the concrete will get re-poured, providing restraint, but some of the systems I've seen have dimple plate devices that are an inch thick, made of plastic.

Here's a link to what I'm talking about:

https://www.compmold.com/portals/0/Images/Drain Tile Design.jpg?ver=2017-12-15-111001-667

 

[oldestguy]

Well, my comment on this subject brought up the question "How many times have we seen this excavation causing a slippage of the footing? I'd say I have never heard of this slippage happening cases and have heard of many such drain operations done. If it does, maybe they didn't advertise it here. The fact these basement walls hold likely can be blamed on the various factors that affect the walls, such as down loads from the structure "clamping" it there.

 

[r13]

The soil behind the wall is not smart enough to detect the structural change in a short duration. If the removed slab is remained unreplaced, then problems can occur.

 

[geotechguy1]

This is an interesting question since it could become quite complicated if you let it (since it's essentially a 3d soil-structure interaction problem).

1. The building weight is still holding the footing down and thus you have friction along the base of the footing resisting sliding
2. If the footing continues around the corner then in order for the footing to 'slide' inwards in a way that causes failure, the footing would actually have to fail in shear (by which I mean, the concrete itself would have to shear through). I suppose you could still have some elastic movement inwards but I imagine the weight of the wall is enough to hold the footing down and prevent this from happening.
3. If the foundation wall is a big concrete thing like we build in Canada (see picture below), then is the footing really taking much lateral load? I guess it would still take the lateral load over the height of the footing itself.

http://stonesprestigehomes.com/imag..._Is_Right_For_You/8ft_concrete_foundation.jpg

In the end since it's a residential house you'll never get the chance to prove any of this via calculations, and you'll just have to rely on experience as Oldestguy says. This exact scenario has been done hundreds of thousands of times and nothing has ever happened as far as I know.

 

[MTNClimber]

We're talking about maybe a 1' excavation. If the structure was designed with that little of a factor of safety then I'd be concerned with more than some movement and cracking in the foundation wall.