The vertical reaction on the horizontal component of the foundation is one calculation (i.e., bearing capacity and eccentricity).  The passive resistance from the shear key is another.  Pertaining to a toe slope, there are calculations (or chart takeoffs) to show the passive pressure that you can develop on the key.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

Generally speaking you design the resistance of the shear key as if it were retaining the loads and soil above it with stiffness of the soil = Kp.  So use a pressure that corresponds to your geometry.  However, it takes a fair movement in a soil to develop Kp so make sure that you are prepared for your wall to move a bit before relying on a shear key.  

My wall has a relatively toe, so I haven't looked at the passive resistance for overturning.  The only information I found on sliding was to take the weight of the structure and the vertical earth pressure and multiply by the tangent of the angle of internal friction.  In my case this is way short of the horizontal earth force on the wall.  I'm wondering if I look at the weight of the soil in front of the key as a part of the force balance and compare it to the slope behind the wall?  The other part to the question is that I'm only mid-way down a slope, so I have a set horizontal distance and I'm guessing that compressive passive resistance doesn't occur?

I appreciate the info!

Duck,
How tall of a wall are you looking at?  I will suggest you consult a local geotech.  Anyway, you seem to have a base sliding problem which is common with retaining walls.  You might want to consult NAVFAC DM 7.1, 7.2 for determining passive pressure on slopes.  You might also want to try increasing the width (B) of the wall footing.  And be careful in just multing the vertical load (dead load only), by tan(phi). That tan(phi) is an estimate of the coefficient of friction.  Similar to basic physics- friction force is equal to normal force(reaction force) x coefficient of friction. However, it also depends of the type of soil.  NAVFAC DM also have a table showing the coefficient between different soils and concrete.  A shear key is also a good idea, maybe as a redundancy if you are not feeling to comfortable. (By the way, you can download the NAVFAC online).  But again I advise you to consuld a geotechnical engineer  

• http://www.wbdg.org/ccb/NAVFAC/DMMHNAV/engineering_criteria.pdf