Many users ask whether Active or Passive support should be used, and what capacities should be used. First it is important to identify the main difference between Active and Passive support implementation in Slide. Basically, active support forces work to reduce the driving forces in the factor of safety calculations, while passive support forces work to increase the resisting forces.
In terms of convergence properties, it has been found that active support is generally better. However, in the end, it is up to the user to determine what type of support makes the most sense in each situation.
For a more detailed discussion comparing the two support types, one can refer to the "Active or Passive Support Modeling in Slide" developer's tip. There is also an Active/Passive Force Application section in the Online Help. Another good reference for support in slope stability analyses can be found in Duncan and Wrights Soil Strength and Slope Stability Text, the reference for which is provided below:
Duncan, J.M. and Wright, S.G. (2005). Soil Strength and Slope Stability, John Wiley and Sons Inc.
In particular, Chapter 8 deals with support. In Chapter 8 of the Duncan and Wright text, Method A is the same as Slide’s Active support model, while their Method B is the same as Slide’s Passive support model.
For Active support (Method A), because the support forces are included in the denominator of the safety factor equation (Eqn.1) the support force is NOT divided by the factor of safety calculated during the analysis. Only the soil strength is divided by the factor of safety. Hence the support capacity input by the user for Active (Method A) should be the ALLOWABLE support force.
For Passive support (Method B), because the support forces are included in the numerator of the safety factor equation (Eqn.2) the support forces ARE divided by the factor of safety calculated during the analysis (i.e. both soil strength and support forces are divided by the factor of safety). Hence the support capacity input by the user for Passive (Method B) should be the ULTIMATE support force.
According to Duncan and Wright (2005), Method A is preferable, because the soil strength and the reinforcement forces have different sources of uncertainty, factoring them separately makes it possible to reflect these differences. The allowable support force (Method A) allows the user to choose an acceptable safety factor for the support capacity in advance, and this determines the value (e.g. tensile strength) which is input into the Support Properties dialog.
For either active or passive support methods, the long-term capacity of the support should be considered if long term slope stability is important. The long-term capacity of reinforcement may depend on several factors including creep characteristics, installation damage, durability and other factors.
The active force due to support is applied as a force to the slice base it intersects in the same way as any external force is applied. The magnitude of the force comes from the nail force diagram as described in the Online Help and the position of the force is the point of intersection with the base of the slip surface. This force is then resolved into normal and shear forces for input into the limit-equilibrium equations. The force is not distributed in any way through the soil or adjoining slices.
There is, however, an advanced option (see image below) for spreading the support force over the slices in which it intersects.
Project Settings: Advanced Tab