What does "Show Support Forces" do?

When using this option, the distribution you are seeing is not the force in the nail.  It is the nail force diagram which gives you the support force distribution.  This is the stabilizing force added to the limit equilibrium equations due to the support.

So for each point along the support, the force at this point is equal to the capacity of the nail if the failure surface went through this point.  A description of nail force diagrams can be found in the Overview of Support Implementation topic of the online help.

To get the load distribution within the support, you'd need to use a finite-element program that computes deformation, like RS2.  The other option, which is not as accurate, is to look at the mobilized force in the nail by looking at the Slice view (see image below).  This is the force in the support (per unit distance out of plane), if you were to reduce the strength in the soil mass by the factor of safety.  A quick measure is to take this force and divide it by the factor of safety.  If the factor of safety is one, then the support is in a state of equilibrium.

 

nail_force.jpg

Slice view in Slide

 

The units are Force in the nail force dialog (kN metric, lbf imperial), and are per unit distance out of plane.  So they will change depending on the out of plane spacing of the support.  To get the force in an individual bolt, you have to multiply the force number from the slice data by the spacing.  So in figure above, the bolt force is 9.2kN, and the spacing is 2m.  So the actual force in the bolt is 18.4kN.  If the FS=1.5, then the true force in the support is 18.4/1.5=12.3kN.