retaining wall with soil key - key placement and coefficient of friction soil-soil vs soil-concrete

[engjg]

When you add a soil key to increase sliding resistance my understanding is that you can consider soil on soil tan(phi) friction as opposed to soil on concrete tan(2/3*phi) since the failure plane will be through the soil. Can anyone provide a reference that supports this? Also under this understanding it seems to me the most efficient placement of the key would be at the rear of the heel, any thoughts?

 

[r13]

To me, a soil key is effective for earth dam on rock using clayey material at the base. I'll leave it to the geotechnical guys to address other types. IMO, I agree the concrete shear key is more efficient when placed at the heel.

 

[Settingsun]

Hong Kong Geoguide 1 uses different friction values like you describe. Figures 41 and 42. Free download. It gives different values for the respective friction coefficients though.

Key at back is good but check whether the wider excavation to dig it causes a problem. The excavator will usually straddle the trench so the excavation at base level needs to be wide enough for half the machine to be behind the shear key trench.