Slope – 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.