Active / Passive Force Application

For each Support Type in the Define Support Properties dialog, the user may choose the method of Force Application – Active or Passive. The significance of the Force Application method is as follows.

Active versus Passive Support

In general terms, the Factor of Safety is defined as the ratio of the forces resisting motion, to the driving forces. Driving forces include the mass of each slice accelerated through gravity, seismic forces, and water in a tension crack. Resisting forces arise from the cohesion and frictional strength of the slip surface.

Active Support is included in the SLIDE analysis as in Eqn.1.

               Eqn.1

where is the normal component and is the shear component of the force applied to the base of a slice, by the support.

Active Support is assumed to act in such a manner as to DECREASE the DRIVING FORCE in the Factor of Safety calculation. Grouted Tiebacks, tensioned cables or rockbolts, which exert a force on the sliding mass before any movement has taken place, could be considered as Active support.

Passive Support is included in the SLIDE analysis as in Eqn.2.

         Eqn.2

By this definition, Passive Support is assumed to INCREASE the RESISTING FORCE provided by shear restraint, in the Factor of Safety equation.

Soil nails or geo-textiles, which only develop a resisting force after some movement within the slope has taken place, could be considered as Passive support.

Since the exact sequence of loading and movement in a slope is never known in advance, the choice of Active or Passive Force Application is somewhat arbitrary. The user may decide which of the two methods is more appropriate for the slope and support system being analyzed. In general, Passive support will always give a lower Factor of Safety than Active support.