Cantilever RW sliding resistance with shear key

[apriley]

I'm designing cantilever retaining walls per Caltrans BDS, which gives two different methods for calculating sliding resistance when shear keys are present (see attached). The first assumes a horizontal failure plane at the bottom of the key. For this, my sliding FS is OK.

The second method assumes an angled failure plane that extends from the base of the key to the toe of the footing. The larger the angle, the lower the resistance, which means that a deeper key results in lower resistance for this method, so my FS is NG.

My question is this: has anyone used this method, or have any guidance as to whether it is a typical failure mode that is considered for retaining walls? I haven't been able to find the 2nd method in any reference.

Thanks!

 

[apriley]

Upon further examination of the code, I realized that the equation they have for the factor of safety is incorrect. I believe when summing forces they considered the weight of the wall as acting upwards instead of downwards, which resulted in the weight of the wall CONTRIBUTING to sliding. I guess normal forces are tricky.

The moral of this story is to never take anything at face value unless you truly understand where it's coming from.

Thanks.

 

[miecz]

Yeah, it looks like the -Qsin [α] term should be +Qsin [α]

 

[cvg]

the corps recommends checking both horizontal and sloped failure planes for walls with keys. See attached example from EM 1110-2-2502 Retaining and Flood Walls