Basement Retaining Wall - Sliding 1

[CLTEng]

I am designing for a restrained basement retaining wall with an isolated basement slab on grade and am having difficulty designing for the sliding with the 1.5 factor of safety code requirement. My understanding of the typical detailing(attached)this company uses is that the slab on grade would eliminate the sliding concern but I am having trouble rationalizing this due to the 1/2" expansion joint that is installed between the slab on grade and the retaining wall.

Does anyone have any insight on this?

 

[Ron247]

I am not familiar with using a floor slab to resist lateral load when using an expansion material. Can't take load until it compresses the material. Most basements use 1/4" to 1/2" expansion material.

I have had to set the foundation elevation lower, widen the heel or use keys when I have a sliding problem.

Not related to the structural issues, the following comments relate to your drainage and waterproofing. I am assuming you are responsible for the waterproofing. In my geographic area:
[li]The French Drain pipe is usually much lower than the floor slab since it has to gravity drain around the basement to daylight. I have seen projects with 2 drain pipes, one at the elevation you show and one beside the foundation. The highest point is still well below basement floor level.[/li]
[li]The ground at the top should be shown to "slope to drain per code". I do not tend to show it flat. Many waterproofing systems have that requirement in their warranties.[/li]

It is one of those situations where as long as the basement does not leak, no problem but when it does, the contractor points out they installed it per the drawing. I have see them set the drain pipe on the top of foundation all the way around the house with no slope.

 

[r13]

I suggest to eliminate the expansion joint, and instead use control joint to minimize the potential of cracks.

 

[XR250]

Use 6 mil poly as a bond breaker. An enclosed basement slab will more than likely get smaller, not bigger due to shrinkage.

 

[jayrod12]

I've never specified a bond breaker for a residential basement slab on grade. Generally speaking the slab and wall are going to move the same for at least the first two feet regardless of bond breaker or not since it sits over the footing for the first bit (around here the footings are typically 30" to 36" wide therefore the first 11-15" of slab is directly over the footing).

 

[Celt83]

This is one of those things that is pretty much hand waived away by any person I've every attempted to talk to. Primary issue I have with all the hand waiving is today:
-Earth pressures for some reason seem to be coming it at 1.5-2x what used to be specified
-Thicker and Thicker vapor barriers being utilized beneath the slab on grade essential reducing the friction coefficient
-Insulation layers being added or thickened around the wall base due to new energy requirements

Getting sliding to come in around 1.5 I've found requires dropping the foundation to get more or any passive pressure and utilizing every bit of self weight from the wall,fnd, floor framing, finishes to contribute for friction resistance at the base of the foundation. In my area almost no one seems to be checking this so lots of times get hamstrung into rationalizing it and accepting the joint will just compress and the slab will need to act as a compression member between opposing basement walls, when it's an open basement on one side I put up more of a fight but still constantly hear "we've never had to do this before"

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[jayrod12]

I put up the fight on walk-out style basements as well. Otherwise yes, I've just considered the slab on grade to be a diaphragm of sorts acting in pure compression between the two opposed foundation walls. I do the same for wood basement floors.

 

[XR250]

In my neck of the woods, the poly is run over the footing so it will break the bond. Also, sometimes the 4" gravel base runs over the footing.
I never worry about slab sliding on residential projects. Code considers it as braced by the slab and the guy down the street does too. In my 29 years of residential, i have never seen or heard of any case where this occurred.

 

[Celt83]

XR250:
Would you mind pointing me to the code section that indicates the slab on grade braces the wall, I don't recall every seeing this in IRC or IBC and would be nice to have for future backup.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

See footnote C. I assume if they are measuring the backfill height from the top of the slab, they are using the slab as a bracing point.

 

[Celt83]

Would you consider the slab on grade in OP's detail as being in contact with the wall provided there is a 1/2" isolation joint filled with compressible material. Throughout my admittedly shorter carrier I have never seen a detail showing the slab on grade in direct contact with the foundation wall.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[XR250]

Would you consider the slab on grade in OP's detail as being in contact with the wall provided there is a 1/2" isolation joint filled with compressible material.

I wouldn't but most or my peers do. That is why I call out a bond breaker in lieu of filler.
Honestly, i have never seen an issue in the field with this so maybe it is not something to worry about with limited backfill heights.

 

[dauwerda]

So, if the wall/foundation does actually slide in and engage the slab/expansion joint material is that really an issue? At the typical pressures, what will this result in - an extra 1/8" of movement, maybe? I don't see this ever being an issue.

 

[r13]

IMO, the expansion joint is not required, as long as the slab is free to move (relatively speaking), the cracks due to drying shrinkage will be minimized by the reinforcing mat, and we know that the slab won't expand. However, casting slab directly over the footing raises red flags, as the variation in support rigidity may cause structural cracks, and the tendency of the slab locked onto the footing concrete, thus becomes a restraint.

 

[dflva]

I agree with XR250. I provide one layer of building paper (30 lb roofing felt) on vertical surface (face of wall, etc) as a bond breaker. I have seen a slab stick to the wall (over about a 3 ft length) and cause a shrinkage crack (also 3 ft long) parallel to the wall about 1 ft out from the face. I also try to have gravel between the footing and underside of slab, rather than casting the slab directly on the footing.