Retaining Wall Activated by Trigger Lines

Hello, I am designing a template that involves a standard two directional curbed road-way (split between two templates (one for left and one for right)) with sidewalks (shown as a vertical line with no horizontal dimension) adjacent to the curbside, and several back slope end conditions tying into existing ground (see below):

   

With a parametric controlled switch display rule showing curb, or open shoulder. 

Now, what I want to do is create a trigger line on the curbed section that will put a retaining wall when ever the back slope becomes greater than 4:1, and if the back slope is equal to or less than 4:1, show no retaining wall.

I have this so far:

but I'm not sure how to get it to do what I want it to. Any guidance would be very appreciated.

Thank you.