Why does applying a soil nail force as a surface load obtain a different Factor of Safety?

With the default option of unchecked "anchor force is applied as surface load"� box (BS8006 calculation method)

  • For a given slip circle, Slope looks for the minimum anchor capacity based on pullout, tensile failure or stripping;
  • Resolves it parallel to the slip surface at the point at which the anchor crosses the slip surface in order to derive a simple restoring moment;
  • Adds this moment to the other restoring moment derived from slope stability theory to give a total restoring moment.

 

With the "anchor force is applied as surface load" box checked

  • Slope again calculates the minimum anchor capacity but applies it as a surface load;
  • This means that it is worked through the method of slices calc, ultimately resulting in an increased normal force on the failure surface, and hence a greater restoring moment by virtue of increased friction;
  • There is no explicit calculation of moment due to the reinforcement.

This means that the surface load option will tend to obtain a higher factor of safety and the BS8006 calculation is acknowledged to be conservative.

In essence, the two solutions give the following results:

Default option: MSu/F + Mn/F = Mw

Surface load option: MSu/F + Mn = Mw

where Msu is soil restoring moment, Mw is soil disturbing moment, Mn is moment due to nail forces.

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