Local Damping - Dynamic module of FLAC3D

In the dynamic module of FLAC3D, how can I know if a reflection of the seismic waves with respect to the boundaries is occurring in my model? Is there any way to view it?

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Hello Cristian,
A seismic wave would be reflected in your model if you used regular fixed boundaries. Typically, you want to avoid this situation and use quiet or free-field boundary conditions. More info on these boundary conditions can be found here - Dynamic Modeling Considerations â€” FLAC3D 7.0 documentation

If you want to verify or view that boundary conditions are reflecting or absorbing I think the best way would be to monitor the velocities on the boundary. A great example that shows how a quiet boundary condition absorbs shear wave is the â€śShear Wave Propagation in a Vertical Barâ€ť example. You can find the example here - Shear Wave Propagation in a Vertical Bar â€” FLAC3D 7.0 documentation

I hope this helps. Let me know if you have any additional questions.

Thank you very much for your reply. What I am trying to simulate is the kinetic energy released in the construction of a circular tunnel with internal radius equal to 1 (m). I am currently using a local damped model to simulate the propagation of waves in the medium and in the contour of the model I have not installed any constraints for the dynamic calculation (ie free edges). The profile of the velocities that I obtain at different points of the contour of the model is the following.

It is observed that the magnitude of the velocities is damped but towards a value other than zero (approximately 0.09). I canâ€™t understand why the value isnâ€™t closer to zero.

Using local damping in dynamic analysis is not recommended. Try using Rayleigh Damping, or using Combined Damping if we accept a bigger tolerance.

Make sure the model is in sufficiently static equilibrium before performing dynamic analysis.

Another possibility is that the model itself is physically unstable.

I have reviewed in the literature that the use of local damping is not recommended for seismic cases and for cases where the waveform is complicated. Since my case is quite simple, there shouldnâ€™t be any problems. In fact the author Y. Gunzburger performs several simulations to determine the kinetic energy in the room and pillars method in underground mining occupying local damping.

In terms of equilibrium I have made sure that the model is sufficiently static before the dynamic analysis.

Then I would recommend you send your datafile to FLAC3D support team for a review.

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Have you used the Dynamic Input Wizard to finish the baseline correction before you input the dynamic wave?
Dynamic Analysis in FLAC3D | US Minneapolis - Itasca Consulting Group, Inc. (itascacg.com)

What happens is that I am not manually entering any wave in the model, I am studying the kinetic energy that is released in the form of a wave when excavating a section of a circular tunnel. Therefore I think that the dynamic input wizard does not apply to my case.

Again, the best way is to send your datafile to Itasca FLAc3D support team for a review.

ok, Iâ€™ll send the file to the itasca support team.

I may be incorrect here, but another possibility is if changes in static loading during the dynamic phase occur, this should be avoided. For example, if a tunnel is excavated after quiet boundaries have been specified on the bottom boundary, the whole model will start to move upward. This is because the total gravity force no longer balances the total reaction force at the bottom that was calculated when the boundary was changed to a quiet one.

More information regarding this concept can be found in the Dynamic documentation for FLAC3D - Dynamic Modeling Considerations â€” FLAC3D 7.0 documentation. Scroll to the section Quiet Boundaries, 5th paragraph.

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