Plain Strand Cable
The Plain Strand Cable bolt model in RS2 works as follows:
- The entire bolt behaves as a single element. For purposes of the algorithm, the bolt is discretized according to the intersections with the finite elements. However the behaviour of each segment of the bolt has a direct effect on adjacent segments, therefore the bolt can be considered a single element.
- The stiffness of the grout, and the strength and stiffness of the bolt/grout interface is taken into account.
- The effect of confining stress on the bolt due to excavation induced stress change is taken into account.
Failure Mechanisms
- At present, the only failure mechanism of a Plain Strand Cable bolt is by tensile rupture of the cable.
- Failure of the cable/grout interface also occurs, but it is not a failure mechanism as such, since this interface is always assumed to be in a plastic state as the rock moves. The amount of relative slip at this interface, and the stiffness of the interface, determines how much shear force is generated at the cable.
- Failure of the grout/rock interface is NOT currently considered, but may be in future.
For more information regarding the bolt models and their numerical implementation in RS2, see the Bolt Formulation document in the Theory section.
For detailed information about the development of the Plain Strand Cable model, see the Plain Strand Cable references. A more extensive list of references is given here.
The following properties define a Plain Strand Cable bolt in RS2.
Borehole Diameter
The diameter of the borehole used to install the bolts.
Cable Diameter
The diameter of the cable.
Cable Modulus
The effective Young’s Modulus of the cable.
Cable Peak
The peak tensile capacity of the cable.
Water-Cement Ratio
The Water to Cement ratio of the grout used to install the cable.
Out-of-Plane Spacing
The spacing between bolts in the out-of-plane direction (i.e. normal to the analysis plane).
Face Plates
The Face Plates option allows the user to simulate the effect of face plates used on bolts.
- If the Face Plates check box is selected, then the first vertex of each bolt will be "fixed" to the rock mass, allowing the bolt to develop load starting at the face plate.
- If the Face Plates check box is NOT selected, then the load at the beginning of the bolt will be zero.
Display of Face Plates
Bolts with face plates are drawn with a small rectangular icon at the beginning of the bolt, to indicate the presence of the face plate.
Add Bolt Option and Face Plates
If you are using the Add Bolt option to add individual bolts to a model, and the bolts have face plates, it is important to note that the face plate is always added to the FIRST vertex of the bolt, when you add bolts to the model.
Add Pull Out Force
The Add Pull Out Force option allows you to simulate the effect of a pull-out test on a bolt, by applying a tensile force to the beginning of the bolt. To use this option, select the Add Pull Out Force check box, and enter the desired value of pull out force. The force will be applied as a tensile load to the first vertex of the bolt.
Constant Shear Stiffness
In the Plain Strand Cable model in RS2, the shear stress vs. shear displacement graph for a bolt, will in general be non-linear. Therefore the shear stiffness will be variable, and will change according to the shear stress. The user may impose a Constant Shear Stiffness, by selecting the Constant Shear Stiffness check box, and entering a value. The shear stiffness will then be independent of the shear stress on the bolt. Note that the units of Shear Stiffness are FORCE / LENGTH / LENGTH.
Add Bulges
Cables with bulges (Garford Bulb or Nut Case) can be modeled as follows:
- Select the Add Bulges check box in the Define Bolt Properties dialog.
- Select the Define Bulges button and you will see the Define Bulges dialog.
- In the dialog, select the Bulge Type. You may choose from:
- 25 mm Garford Bulb
- 21 mm Nut Case
- Select the Add Bulge button until the required number of bulges (rows) appears in the dialog. The location of each bulge is specified as a percentage of the bolt length, where the percentage is measured from the excavation face (i.e. the boundary on which the bolts will be applied). Enter the percentage (location) for each bulge and select OK.
Display of Bulges
If bulges are defined for a Plain Strand Cable, the bulges will be displayed on each bolt as a small circle icon with a square inside. They will be drawn at the actual locations along the bolt length as specified in Step 4.
Joint Shear
See the Joint Shear topic for information.