Effectivness of retaining wall key?

[WmacG]

When checking sliding for a cantilever retaining wall with a footing key,

1) Is it correct to include active pressure behind the wall down to the bottom of the key or only down to the base of the footing.

2) Does this depend on if the key is at the front, middle, or back of the footing?

In "Foundation Design and Analysis" by Bowels, it states that the recommended location of a key is at the back (retained soil side) of the footing. Then it says, "The increase in "H" by the depth of the key may null its effect.

I am not sure if the increase in "H" is necessary for key locations other than at the back of the footing.

 

[fattdad]

I'd say make a free-body diagram and use both the active and passive to the bottom of the shear key. When the shear key is on the retained soil side, you have the bearing pressure acting to increase the passive reaction. If you take the angle 45-phi/2 (from the horizontal) and starting from the toe of the shear key, you will see that the foundation bearing surface is acting on the triangle. I'm sure this is why Bowles provides this guidance.

f-d

¡papá gordo ain’t no madre flaca!

 

[GeoPaveTraffic]

The pressure should be taken to the bottom of the key. The location of the key (front, middle, back) doesn't matter.

Keys are not a very effective way of increasing the sliding resistence. That was the point Bowles' was trying to make.

 

[PEinc]

The RetainPro 2007 design example #2 for a keyed, cantilevered retaining wall shows the active pressure extending only to the bottom of the footing, not to the bottom of the key. If the active failure plane which extends up from the bottom of the key doesn't extend beyond the heel of the footing, then the active pressure probably shouldn't extend from OG to bottom of key.

 

[fattdad]

I'm not sure of PEInc's last point. While the active plane may not "see" the bottom of the key, there are active pressures acting on the soil behind the key.

I'm also not sure of GeoTrafficPave's contention that it wouldn't matter where the shear key is located. To me there is some fundamental difference, whether it amounts to an effect on the design would require that I work up the problem. That said, there may be some theoretical difference (at the risk of repeating myself) on the calculated passive resistance.

f-d

¡papá gordo ain’t no madre flaca!

 

[MSEMan]

I think the point with a shear key is this:

You design the wall with a conventional spread footing (i.e. active pressures to the underside of the base) and if you have a sliding problem you add a shear key. The wall doesn't expereince any additional disturbing forces due to the addition of the key, so why change the analysis.

 

[MSEMan]

Oh sorry, forgot, unless you want to include the weight of the key in your eccentricity analysis (hair splitting really), the position of the key only matters if the wall is founded in a slope.