Why does the bolt support on my tunnel seem to have little effect on the results?

With respect to using bolts for tunnel support in RS2, the following comments may help you to interpret the analysis results:

  • If the material in the vicinity of the bolts is elastic, there are small relative displacements between the two ends of the bolts.  This results in small strains and small stresses/forces in the bolts.  If you look at the axial forces in the bolts and compare these to the stress in the rock mass, it is possible that the load is insignificant, and therefore the bolts may be appearing to have no effect.  The bolts are simply not putting significant force back into the system.
  • In order to see changes in deformation due to bolts you need a couple of things.  First, strains in the bolt generating a significant force that is placed back into the system.  These forces, when compared to the stresses in the rock mass, must be significant.  For some tunnels, you may not see a significant influence on displacements due to bolts.  This is because of the large difference between the ground stresses and the limited amount of force a bolt can put back into the system.
  • Plasticity of the material around the excavation can create the displacement gradient and resulting strain necessary to induce the load in the bolts necessary to see an effect on displacement.  If the material is elastic you rarely see an effect on the displacements.