Active and Passive Force Application
When a support force is applied, the force may be applied as either an Active or Passive force in the Limit Equilibrium analysis.
- Bolts can applied as either Active or Passive
- Shotcrete is always considered as Passive
- Support Pressure can applied as either Active or Passive
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 a block accelerated through gravity, seismic forces, external loading, and water pressure. Resisting forces arise from the shear strength of the block sliding planes.
Active support force is included in the analysis as in Eqn.1.
Where:
TN = normal component of the support force
TS = shear component of the support force
Active Support is assumed to act in such a manner as to DECREASE the DRIVING FORCE in the Factor of Safety calculation.
If you are modeling supports such as tensioned cables or rock bolts, which exert a force on the block before any movement has taken place, then this could be considered as Active support.
Passive support force is included in the analysis as in Eqn.2.
Where:
TN = normal component of the support force
TS = shear component of the support force
Passive Support is assumed to INCREASE the RESISTING FORCE provided by shear restraint, in the Factor of Safety equation.
If you are modeling supports such as shotcrete, grouted dowels, wire mesh or steel sets, which only develop a resisting force after some movement of the block has taken place, then this could be considered as Passive support.