With or without 'FastFlow' - which is more trustable

Hi, I am looking at the manual on the consolidation assessment. I found the results in “file:///C:/Program%20Files/Itasca/FLAC3D700/exe64/doc/flac3d/docproject/source/theory/fluid-mechanical/solving/solving.html?node2319” strange, and disagree they are similar. Obviously there is some difference in the reasults, which make me wonder which solution is more reliable, with or without ‘fast flow’? Could any Itascan give some insights?
Also, for the case without ‘fast flow’, I found the adopted fluid modulus is too low, so I have run a case with Kw = 2e9 Pa (see bottom figure below), which increase the pore pressure slightly.

In addtion, as a FLAC user, I am surprised to see “zone gridpoint fix pore-pressure 0 range position-z 10”, i.e. permeable boundary conditions when setting up the initial distribution of pore pressure. I remember FLAC manual recommends impermeable boudnary conditions when setting up the initial distriubtion. Then make it permeable when ‘flow on’. Is it different for FLAC3D?

The pore-pressure contours do have some notable difference. The difference of maximum values, however, is less than 5%, which can be considered “similar”, although it is arguable.

Fast-flow is alternative for faster fluid calculation. When the Biot modulus (or fluid bulk modulus) is much bigger than the bulk modulus of the solid matrix, the result difference between with or without fast-flow is small. However, the difference becomes bigger when Biot modulus (or fluid bulk modulus) is relatively smaller; for this case, the result without fast-flow is more reliable but with the cost of longer calculation time.

For boundaries without fixed pore-pressure, it is impermeable by default. However, if the boundary has fixed pore-pressure, it is a permeable boundary. The command “zone gridpoint fix pore-pressure 0 range position-z 10” means the surface keeps zero pore-pressure, or there is drainage there to ensure zero pore-pressure. There is no difference on this between FLAC and FLAC3D. This is quite “standard” fluid boundary for similar software frameworks, including FEM ones.

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