Cable model grout-rock interface

FLAC3D
1

Hello Itascans! We are seeking your help with the performance results of bolts installed in a shaft model.

The cable model fails predominantly along the grout–rock interface, showing a pull-out type failure, while the bolts themselves do not fail at any point. However the shear strength of the grout–rock interface is higher than the tensile capacity of the bolts.

Each excavation bench is 16 ft high.

State Grout
State Grout887×636 174 KB

We would appreciate any recommendations or potential solutions, since we have already tested larger borehole diameters and even improved the ground properties, but we have not been able to prevent the problem.

Thanks,

Carlos.

2

Dear Carlos,
By bolt, do you mean a rock bolt or a cable without bending resistance?
If you model a rock bolt, it is better to use a pile than a bolt and keyword flag-rockbolt on.
Or use a segment value more than you defined before for the node link in all zones.
Khezr

3

Hi Khezr, the model uses a cable element without bending stiffness, as it is intended only to capture the tensile forces in the bolts. I have also tried using pile elements with the rockbolt option, but the same problem persists. In my opinion the grout-rock interface is not given real results.
Thanks

4

Hello @Carlos_Mora

It could be a variety of reasons that the cables have failed. Note, none of them are in active failure as indicated in the plot. Some reasons may be properties

  • Did you assign both cohesion and friction to the grout strength.

  • How is the excavation being conducted? Are you using zone relax or just removing/nulling the zones. Using zone relax excavate is always the preferred approach when doing an excavation in FLAC3D.

  • Check the number of segments along the cable. Ideally you want one cable element per zone

These are just some suggestions. I hope they help!

1 Like
5

Thank you very much, Derrick, for your valuable comments. I have solved the problem using the zone relax excavate option. Now the interface no longer fails predominantly along the grout–rock interface.

Now the question is regarding how many relaxation steps should be used to remove the excavation benches, since this is a drill-and-blast process. I notice that the number of the relaxation steps affect the axial normal forces in the cable elements.

Thanks!

Carlos.