Hi Roozbeh,

I'm trying to numerically reproduce the Convergence?Confinement Curve (CCC) for a circular tunnel in rock mass, based on the formulation by Carranza-Torres & Fairhurst (2000), specifically the case for GSI = 40, using the standard Hoek?Brown failure criterion in ADONIS.

Data of the example:

R = 1 m (tunnel radius)
σ₀ = 7.5 MPa
σci = 20 MPa
mi = 15
ν = 0.25
ψ = 30?
GSI= 40   
mb= 1.8          
s= 1.3?10⁻?
Gm (GPa)= 1.0

The simulation reduces internal pressure on the tunnel boundary from 7.5 MPa to 0 MPa in 10% increments. Radial displacement is tracked at the crown.

Despite following this procedure, I get a completely linear displacement curve with no yielding. This contradicts the analytical CCC, which shows plasticity initiating at internal pressure ≈ 2.6 MPa for this GSI.

I suspect I'm missing something. Could you help me identify what's wrong?

Thanks a lot in advance, and congratulations on this powerful software.

Santiago

Hi Cope, this is excellent?thanks for sharing the script! I?m sure other users will find it helpful as well. I?ll go ahead and move the entire topic to the "Useful Scripts" section.

Hi Roozbeh,

Thanks for moving the topic and for your feedback!!!!! Hope someone finds it useful.

One follow‑up question:

Could this behavior be related to how the Hoek?Brown constitutive model is implemented in ADONIS??

Or is there something I should adjust in my script?

I?d really appreciate your guidance on this point.

Thanks again!!!

Please note that you can use the Solve Relax command to replicate the same behavior for comparison purposes. The Sequential Excavation tutorial explains this in detail.

Regarding your script, I typically avoid using uniform stress to reduce internal forces. Instead, I monitor the unbalanced force at the tunnel boundary and gradually apply an opposing force proportional to that unbalance. The approach you're using works well only under homogeneous stress conditions.

I briefly reviewed your code, and one point that stood out is how you're reducing the normal stress. Keep in mind that applybc is a permanent command?each time you use it, you're accumulating pressure on the boundary. I recommend first removing any previously applied pressure before applying the new one. Give that a try, and when I have more time, I'll review your script in more detail.

As mentioned, Solve Relax achieves this behavior with a single line?you just need to specify the relaxation factor.

Hi Roozbeh,

Thanks for the detailed reply!

Just to clarify: I tried both approaches ? using applybc("nstress", ?) as in my script, and also the solve("relax") command as explained in the Sequential Excavation tutorial ? and in both cases I still get the same issue: the Ground Reaction Curve remains essentially a straight line with no sign of yielding.

My reason for initially using internal pressure steps instead of solve("relax") was that I wasn?t sure if applying, for example, a relaxation factor of 0.1 followed by 0.2 in the next step is relative to the initial condition or to the updated state. So I chose the explicit pressure approach to have full control over the boundary condition and check the behavior (that was exactly the same).

But in practice, both methods give me the same (linear) result, which makes me think the issue may not be with the loading procedure but perhaps with how the constitutive model behaves in this scenario.

Thanks again for your time and I look forward to your review.

Best Regards,

Santiago

Santiago,

There doesn?t appear to be any issue with the constitutive model. I ran the model in FLAC, and since it?s purely elastic with no internal pressure applied, a linear response is expected.

-Roozbeh

Thanks for checking this so quickly!

Please find attached a fragment from the analytical solution by Carranza‑Torres & Fairhurst (1999) for the same case (GSI = 40), as well as a numerical analysis that was run in PLAXIS, which matches the analytical Ground Reaction Curve quite well.

This is why I believe the input parameters (σci, mb, s, a, s3cv, etc.) are correct, and the model should exhibit yielding once the internal pressure drops below approximately 2.6 MPa, as shown in the analytical curve as well as in the numerical.

Best regards,
Santiago

I'll look into it. If you have access to the FLAC3D model referenced in Figure 1, please send it to me.

You're right?there's nothing wrong with the input. I believe I?ve identified the source of the error. I?ll fix it and release a new version on Friday.

That?s great news Roozbeh, thanks a lot for taking the time to review this and for identifying the issue

I really appreciate your support and look forward to testing the new version.

Best regards,

Santiago

Hi Santiago,

I believe the issue with the Hoek-Brown constitutive model has been resolved. Please give it a try and let me know if the problem still persists.

-Roozbeh