Force Diagrams Overview

In RocSlope2, a Force Diagram represents the available tensile force which a bolt can apply to a block at any point along the length of the bolt. The Force Diagram assumes that the bolt force mobilized is in pure tension.

• For Mechanically Anchored bolts, the Force Diagram is simply a horizontal line since a constant support force will be applied, regardless of where a block plane intersects the bolt.
• For Grouted Dowel, Cable, Split Set or Swellex bolts, the Force diagram is determined by considering each possible tensile failure mode along the length of the support (Tensile, Pullout and Stripping, if applicable). The Failure Mode which generates the MINIMUM force determines the support force which can be generated, at each point along the support.
• For User-Defined Support, the Force Diagram is defined explicitly by the user, tensile properties (e.g. tensile strength, plate capacity, bond strength) are not entered.

For example, a possible Bolt Force Diagram for a grouted dowel, with a Bond Length less than 100% and a plate capacity which is less than the tensile capacity is illustrated below.

To toggle visibility for the Force Diagram:

1. Select Support > Spot Bolts > Add
2. Apply the necessary spot bolts to the analysis models.
3. Select Results > Force Diagrams > Toggle Visibility to make the force diagram visible.
   1. This will also display the Support Force legend.

For more examples of typical Bolt Force diagrams, see the following topics:

• Mechanically Anchored Bolt
• Grouted Dowel
• Cable Bolt
• Split Set / Swellex
• User-Defined

In addition to the Force Diagram, there are two other factors which ultimately determine the support force which can be supplied by a bolt in RocSlope2 - Bolt Orientation Efficiency and Shear Strength. See these topics for more information.

For an overview of the bolt support implementation in RocSlope2, see the Bolt Support Force topic. This discusses all of the parameters which affect the bolt support capacity and summarizes all possible cases.

Interpretation of Force Diagrams

The user should appreciate that a Force Diagram, as implemented in RocSlope2, does not necessarily represent the true state of stress or loading mobilized by a given bolt. The Force diagram simply represents the maximum available tensile force, which a given bolt can theoretically apply to a block, at any point along its length. This is based on the assumptions described for each bolt type in RocSlope2.

Remember that the RocSlope2 analysis is a limit equilibrium analysis. Displacements and strains are not considered in such an analysis. In order to realistically model the interaction of support and excavation, and to calculate actual strains and loading mobilized within the support, more sophisticated numerical analysis methods are required, such as finite element or finite difference methods.

The modelling of support in RocSlope2 is a simple but useful method, of accounting for the effect of the support system on the block, in a limit equilibrium analysis. It is not intended to model the effect of the block on the support system, and the Force Diagrams should not be interpreted in this way.