Retaining Wall Soil Shear Key

[StructuralEd]

What would be the requirements and construction difficulties generated by having a soil shear key on the bottom of a retaining wall footing?
In designing a retaining wall with substantial lateral loading it is difficult to get a design with a reasonable footing width due to the sliding safety factor requirements and the soil data.
Add a key seems to cut the footing size down so the OT and Sliding SFs are about balanced. Without a key the OT safety factory is almost twice what the sliding control is.
Is there another way?
Constructability is another concern. Soil is silty sand.

 

[JedClampett]

I don't think they're too hard too build as long as you don't expect a lot of precision. They dig a trench, the sides are rough and unformed, the reinforcing is dropped in and the key is poured. Don't expect vertical smooth sides. It might be smaller at the bottom than at the top.
As far as the engineering you're depending on passive pressure. It requires some movement to activate. I've also seen a lot of rules from the geotechnical engineers regarding the use of it. Some of these include; ignore the top 12 inches, limit the passive resistance to some maximum value, reduce the friction resistance if passive is used.

 

[prasadrmb]

If there is a considerable shear force, we need to provide a shear key. We can not avoid that. If the shear key is provided directly bellow the wall, there will not be much affection on over turning. When we move the shear key toward the land side, factor of safety for shear increases.



Structural Engineering

http://structuraldesignbs.blogspot.com/

http://eurocode-2.blogspot.com/

 

[GeoPaveTraffic]

Agree with Jed.

Also, if you use a shear key you should check back with the original geotechnical engineer since sliding failure modes can change depending on the size of the shear key. For example, the critical slide plane may not be along the base of the wall, but instead on an angled plane through the base of the shear key or some combination. Depending on the soil conditions, it can get complicated very quickly.

Mike Lambert

 

[Teguci]

Requirements for shear key - check moment and 1-way shear. Size for bucket width (1 to 2 ft wide) use 3" cover since it will be cast directly to the earth. If it rains, they will need to clean out the muck so that the key has a clean surface to act against. Resisting force on shear key will add to the overturning moment.
Other ways - Increase heal length to increase soil weight to increase sliding friction. Call the Geotech and suggest that he can sharpen his pencil on this one (passive pressure and soil shear friction). Resist the shear with a poured slab. Increase footing depth. Reinforce the local soil by ramming stone aggregate into it (geopiers). Provide a battered deep foundation system to resist the lateral loading (piles or piers - On one project with decent soil bearing, I used a single row of drilled piers and on another with poor soil, I used 2 rows of piles - In both cases, the retaining wall supported portions of the building which required a deep foundation system for settlement control.

When I was designing retaining walls, my company had a standard of not using shear keys. I put sizing together showing how effective adding a shear key would be and they still instructed me to proceed based on the company standard of not using a shear key.