# Questions About Fully Coupled Flow–Mechanical Analysis and Solution-Control Parameters in FLAC3D

**URL:** <https://forum.itascainternational.com/t/questions-about-fully-coupled-flow-mechanical-analysis-and-solution-control-parameters-in-flac3d/2389>\
**Category:** FLAC3D\
**Tags:** tips, suggestions, training, flac3d, flac2d\
**Created:** [June 1, 2026, 3:48pm UTC](https://forum.itascainternational.com/t/questions-about-fully-coupled-flow-mechanical-analysis-and-solution-control-parameters-in-flac3d/2389 "2026-06-01T15:48:49Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![Sudipta](https://avatars.discourse-cdn.com/v4/letter/s/f475e1/32.png) [@Sudipta](https://forum.itascainternational.com/u/Sudipta)\
**Post date:** [June 1, 2026, 3:48pm UTC](https://forum.itascainternational.com/t/questions-about-fully-coupled-flow-mechanical-analysis-and-solution-control-parameters-in-flac3d/2389/1 "2026-06-01T15:48:49Z")

</div>

Hello [@everyone](https://forum.itascainternational.com/groups/everyone), @itascaYang @isaac @itsmoho_sen @ITE @Inwoo @Inu @itasca @dafo407_itasca [@itascan](https://forum.itascainternational.com/groups/itascan)

I am planning to perform a fully coupled flow–mechanical analysis in FLAC3D. My current understanding of the command sequence is as follows:

```auto
model configure fluid

```

Then build the geometry, assign mechanical models/properties, interfaces, geogrids, and construction stages.

Next, assign the fluid model and properties:

```auto
zone fluid cmodel assign isotropic range group 'X1' or 'X2'
zone fluid property permeability 1.02e-9 porosity 0.365 range group 'X1'
zone fluid property permeability 1.0e-12 porosity 0.365 range group 'X2'
zone gridpoint initialize fluid-modulus 2.2e9 range group 'X1' or 'X2'

zone fluid biot on
zone fluid property biot 0.9931 range group 'X1'
zone gridpoint initialize biot-modulus 5.46e9 range group 'X1'

```

Then define water density and water table:

```auto
zone water density 1000
zone water plane origin (x,y,z_water) normal (0,0,1)

```

Then initialize saturation:

```auto
zone gridpoint initialize saturation 1.0 range group 'X1' or 'X2'

```

Then bring the model to coupled equilibrium:

```auto
model fluid active on
zone mechanical active on

model mechanical substep 100
model fluid substep 10
model mechanical follower on

model solve fluid time-total 1.0e4 or mechanical convergence 10

```

After that, external loading will be applied and the coupled analysis will continue using:

```auto
model fluid active on
zone mechanical active on

model mechanical substep 100
model fluid substep 10
model mechanical follower on

model history fluid time-total

zone history pore-pressure position x y z
zone history displacement-z position x y z

model solve fluid time-total 3.0e6 or mechanical convergence 10

```

I would like to ask whether this overall approach for fully coupled analysis is reasonable or if there are any important corrections I should make.

I also want to better understand the selection of the following commands and values:

```auto
model mechanical substep 100
model fluid substep 10
model mechanical follower on
model solve fluid time-total ...

```

Specifically:

1. How are the values for `model mechanical substep` and `model fluid substep` selected?

2. Is there any physical or numerical basis for choosing values like 100 and 10, or are they mainly trial-and-error control parameters?

3. What is the exact purpose of `model mechanical follower on` in fully coupled analysis?

4. In fully coupled flow–mechanical simulations, should `model mechanical follower on` generally always remain on?

5. How can `model solve fluid time-total` be estimated or calculated for a coupled consolidation-type problem?

6. Is there a recommended procedure to determine appropriate fluid time-total values for a model?

If possible, could anyone also please provide example problems, documentation links, or references related to fully coupled flow–mechanical analysis and the selection of these solution-control parameters?

Any guidance would be greatly appreciated.

Thanks in advance.