CANTILEVER RETAINING WALL KEYS

[tbone73]

Does anyone still use keys below cantilevered retaining wall footings to resist sliding? I'm debating using one, and I've getting conflicting ideas as to where best locate it (i.e under the stem, or under the heel at the footing edge).
Thoughts/Comments? Appreciated.

 

[geoman110]

The key is better to be placed where you have the highest vertical contact pressure. If the base is considered rigid, under the heel of the footing (back side) may not be the highest vertical pressure (stress reduction due to overturning moment). In this case, under the stem is better. If the base is flexible, under the heel may be where is the highest vertical pressure. If you put the key under the heel, you have too consider the tension along the base (between the key and stem)during the design.

 

[tumbleleaves]

I'd usually put it directly under the stem--and extend the main reinforcing down into it.

I believe the current trend is not to place it here for stability considerations and to place it more toward the retaining side of the wall (the heel). Then to check the stability for a (sliding) shear failure along a plain from the base of the toe to the bottom of the key, as well as a horizontal shear failure plane at the base of the key. In addition to the usual checks.

In future I will likely continue to place the key directly under the stem, use my retaining wall program to run the conventional checks and size the reinforcing and members; then run a stability program for additional checks of failure surfaces and overall stability.

However, per some other post I just made in some other forum 90% of the time it's best just to use the State Standard plan, or if the local agency has a standard retaining wall just use that one. No point on reinventing the wheel.

 

[VoyageofDiscovery]

Check with the geotech wrt the failure mode when using a key.

 

[msquared48]

Personally, if needed, I position the key directly below the stem too, but provide separate vertical steel for the stem extending into the stem. I always bend the stem steel to the toe, making both the same for ease of construction.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask