7/28/2019 8582

1/5

Developers Tip:

Active or Passive Support Modeling in Slide

Introduction

A variety ofsupport typescan be modeled in Slide including soil nails, tiebacks, geotextiles, micropiles

and user-defined support. For each support type, the user may choose the method of force application:

Active or Passive, as shown in the Support Properties dialog below.

A common question we are asked is, Should I use Active or Passive, and what capacities should be

used? This developers tip aims to clarify the definition of Active or Passive support in Slide and provide

some guidance on the values of support capacity that are recommended for each method.

Slide Support Properties input dialog

http://www.rocscience.com/downloads/slide/webhelp/slide_model/support/Define_Support.htmhttp://www.rocscience.com/downloads/slide/webhelp/slide_model/support/Define_Support.htmhttp://www.rocscience.com/downloads/slide/webhelp/slide_model/support/Define_Support.htmhttp://www.rocscience.com/downloads/slide/webhelp/slide_model/support/Define_Support.htm

7/28/2019 8582

2/5

Definition of Active and Passive Support

In the most general terms, the Factor of Safety in a limit equilibrium analysis is defined as the ratio of the

forces resisting motion, to the driving forces (Eqn.1).

Eqn. 1

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.

The primary difference between the active and passive support implementation in Slide is as follows.

Active Support is assumed to act in such a manner as to DECREASE the DRIVING FORCE in the Factor

of Safety calculations (Eqn.2).

Eqn. 2

Passive Support is assumed to INCREASE the RESISTING FORCE provided by shear restraint, in the

Factor of Safety calculations (Eqn.3).

Eqn.3

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.

Soil nails or geotextiles, 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 (see Appendix 3 of Ref. 1). The user may

decide which of the two methods is more appropriate for the slope and support system being analyzed. In

general, for a given support capacity, Passive support will give a lower Factor of Safety than Active

support. The difference can be quite significant as shown in the following figures.

7/28/2019 8582

3/5

(A)

(B)

Active (A) and Passive (B) support results for equal support capacity (10,000 lbs / ft).

Note: critical slip circles are different for each method.

7/28/2019 8582

4/5

Recommended Capacities for Active and Passive Support

Another important question is, What capacity should I use for my support?

Duncan and Wright (Ref. 2) offer the following guidelines for active and passive support capacities used

in limit equilibrium slope stability analysis.

Note that the Active and Passive support options in Slide correspond to the Method A and Method B

options described by Duncan and Wright.

For Active support (Method A), because the support forces are included in the denominator of the safety

factor equation (Eqn.2) 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.3) the support forces ARE divided by the factor of safety calculated during theanalysis (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 (Ref. 2):

Method A is preferable, because the soil strength and the reinforcement forces have different sou rces 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 (Ref.2).

7/28/2019 8582

5/5

Active or Passive Support in other Rocscience Software

In the wedge analysis programsSwedgeandRocPlane, rock bolt support may be applied using either

active or passive methods of force application. The same issues described in this paper forSlide, are

equally applicable to Swedge andRocPlane which are also limit equilibrium programs.

Swedge support dialog

It is worth noting that in our finite element stress analysis programPhase2, support is automatically

assumed to be active. Therefore, when comparingSSR slope stability analysisresults fromPhase2 with

limit equilibrium slope stability results from Slide, the active support option should be used in Slide.

Summary

We conclude this developers tip by stating that there is no clear right or wrong answer with respect to

the choice of active or passive force application for support forces in Slide. Our current thinking is to

recommend the use of Active support application and an allowable support capacity. In our experience

you generally get better convergence properties with active support. However the topic is open for

discussion and we welcome any feedback on this subject.

References

1. Hoek, E. and Bray, J. W. (1981), Rock Slope Engineering (revised 3rd edition), The Institute ofMining and Metallurgy, Chapman and Hall, London.

2. Duncan, J.M. and Wright, S.G. (2005), Soil Strength and Slope Stability (chapter 8), John Wileyand Sons Inc.

http://www.rocscience.com/products/9/Swedgehttp://www.rocscience.com/products/9/Swedgehttp://www.rocscience.com/products/9/Swedgehttp://www.rocscience.com/products/5/RocPlanehttp://www.rocscience.com/products/5/RocPlanehttp://www.rocscience.com/products/5/RocPlanehttp://www.rocscience.com/products/3/Phase2http://www.rocscience.com/products/3/Phase2http://www.rocscience.com/products/3/Phase2http://www.rocscience.com/downloads/phase2/webhelp/phase2_model/Shear_Strength_Reduction_Overview.htmhttp://www.rocscience.com/downloads/phase2/webhelp/phase2_model/Shear_Strength_Reduction_Overview.htmhttp://www.rocscience.com/downloads/phase2/webhelp/phase2_model/Shear_Strength_Reduction_Overview.htmhttp://www.rocscience.com/downloads/phase2/webhelp/phase2_model/Shear_Strength_Reduction_Overview.htmhttp://www.rocscience.com/products/3/Phase2http://www.rocscience.com/products/5/RocPlanehttp://www.rocscience.com/products/9/Swedge