Retaining Wall Soil Shear Key
Retaining Wall Soil Shear Key
(OP)
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.
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.






RE: Retaining Wall Soil Shear Key
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.
RE: Retaining Wall Soil Shear Key
Structural Engineering
http://structuraldesignbs.blogspot.com/
http://eurocode-2.blogspot.com/
RE: Retaining Wall Soil Shear Key
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
RE: Retaining Wall Soil Shear Key
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.