Hello ITASCA Support Team,
I am developing a 6.4 m high MSE wall model in FLAC3D and would greatly appreciate your guidance to confirm that my workflow and commands are correct.
Model summary (current setup)
- Geometry is created with two soil layers.
- 10 layers of Geogrid reinforcement installed, and material parameters assigned.
- Created rigid connections with free rotational behavior between the concrete panel and the geogrid.
- Water table is set at mid-height of the wall.
- I initialized under gravity using a No Flow — Mechanical Generation of Pore Pressure approach and ran the model to equilibrium.
After completing the staged construction and geostatic stress initialization, I extracted stress, pore pressure, and geogrid tensile histories from several depths along the mid-height of the wall (z = 0 to 6.38 m). After equilibrium I plan to compare numerical results with my hand calculations for:
- Horizontal and vertical soil stresses,
- Geogrid tensile force and tensile stress,
- Pore pressure and effective stress.
Later I will apply wave (cyclic) loading and external loads, but for now I want to validate the geostatic initialization.
I would be grateful for help with the following points:
- Model procedure and syntax
- Is the sequence — mechanical initialize-stress → model solve → initialize pore pressure with zone water or zone gridpoint initialize pore-pressure — correct, for geostatic loading with water table set at mid height of the MSE wall?
- After creating the geometry, I understand that I need to initialize the geostatic stresses and then define the water table using the
zone water plane origin (0,0,3.2) normal (0,0,1)command. However, I am unsure whether I also need to assign a fluid constitutive model. Specifically:- Do I need to configure a fluid model using
zone cmodel configure fluidand assign fluid properties such as permeability, porosity, fluid density, etc.? - If I am not performing a coupled flow–mechanical analysis (only “no-flow” mechanical pore-pressure generation), should I activate the fluid model initially and then turn it off using
model mechanical active onandmodel fluid active off? - I want to confirm whether the fluid model is necessary just to define initial pore pressures for the geostatic step.
- After setting history commands (for example, to track stress, pore pressure, or displacement), I noticed that if I do not run additional steps, the histories remain empty. Could you please confirm the correct procedure?
- After defining history records, do I need to run something like:
- Do I need to configure a fluid model using
> model step 1000
> model solve
> model save
or any similar command to ensure that the histories actually record data?
- If this approach is correct for the initial geostatic state, then what additional steps are required during the external loading phase when excess pore pressure will develop?
- Commands for geogrid forces
- Please advise the recommended commands or procedure to extract geogrid tensile force and tensile stress (and any preferred history/export variables) so I can compare them to my hand calculations.
- Interpretation of exported data
- When exporting histories to Excel I obtain many values across steps/cycles. I also noticed that the results I exported from FLAC appear as negative values, and the history plots show many values over the cycles/steps. At the moment, I have been taking the absolute maximum values from these histories. For comparing with hand calculations, should I use the final equilibrium value or the absolute maximum value seen during subsequent cycles? I am currently using the absolute maximum values, but I am unsure if that is the correct approach for geostatic validation.
** 4. Reversed FLAC3D Stress Profile Compared to Hand Calculations**
- I am seeing a strange discrepancy between my FLAC results and my hand calculations. I extracted data from 13 locations in the soil at depths from 0 to 6.38 m. According to my hand calculations the horizontal stress should start near 0 at 0 m depth, increase to about 10 at 0.58 m, and continue increasing until about 106 at 6.38 m. However, the FLAC results show the exact opposite: at 0 m FLAC gives 123, at 0.58 m it gives 116, and at 6.38 m it gives 2.47. This reversed pattern appears not only for horizontal stress but also for vertical stress, pore pressure, geogrid tensile force, and tensile stress. I need to know why this might be happening. Is it a logical outcome of the model, or is FLAC (or my FLAC script) producing incorrect numbers? Could I have made a mistake in the script, or are the coordinates I gave to FLAC wrong? Please explain the possible reasons and how to check them.
** 5. All extracted values are negative (stresses, pore pressure, and geogrid forces)**
- The values for σxx, σzz, pore pressure, geogrid resultant-nx, and geogrid axial stresses are all negative throughout the profile. I understand that FLAC3D adopts a compression-positive sign convention, which means compressive stresses will appear negative when interpreted in a tension-positive sign convention.
However, I wanted to confirm whether the negative signs for:
-
σxx, σzz,
-
pore pressure, and
-
geogrid tensile force / stress
are consistent with expected behavior in FLAC3D, given my mechanical-only geostatic solution and the applied gravity loading.
Request
Could you please review my workflow, commands, and interpretation, and guide me on the correct geostatic initialization process and the probable causes of the reversed stress profile?
I would also be grateful if you could confirm the correct way to extract geogrid forces and stresses for comparison with hand calculations.
Thank you very much for your time and support.
