We are working on a FLAC3D model where a blast is being simulated with the model. The blast is applied internally (not on the boundaries of the model) and is applied as a stress. All gridpoint fixities are removed before the blast is applied and replaced with quiet boundaries. No free field boundaries are applied as the blast stress is applied internally within the model.
We are finding that the entire model is ‘drifting’ (only a few millimetres, but still a concern) during the application of the blast.
In your model do you change any static loading conditions after applying the quiet boundaries, such as excavating zones? Here is a good description for using Quiet Boundaries
No static loadings are changed, the only alteration made after applying the quiet boundaries is the application of an internal acceleration or stress signal (we have tried both) which results in the drifting of the model.
Without seeing the model I don’t have any additional insights. You can send a tech support email to FLAC3Dsupport@itascacg.com, perhaps with the model attached and we can provide further insight into your problem.