Hi everyone,
I am currently running a dynamic effective stress analysis on a embankment dam model using FLAC2D, utilizing PM4Sand and PM4Silt constitutive models for the liquefiable/sensitive foundation and deposit layers.
Since I am relatively new to advanced dynamic modeling in FLAC, I would love to get the community’s perspective on my workflow setup, troubleshooting steps, and how to interpret the contrasting results I am getting between large-strain and small-strain modes. Most of the .dat files are heavily inspired from this Itasca tutorial.
Overview of Modeling Workflow
My modeling sequence is structured across the following steps:
-
Static Initialization & Seepage: Initial stress state generation under gravity (
1_Static_Analysis.dat), followed by a steady-state phreatic surface seepage analysis (2a_Seepage_Analysis_Part_1.dat&2b_Seepage_Analysis_Part_2.dat). -
K0 Adjustments: Custom FISH functions check and adjust lateral earth pressure coefficients (K0) in key material groups (
3_Static_Check_K0_1.dat). -
Model Conversion and K0 Check: Conversion of static/elastic models to
pm4sand2dandpm4silt2dfor liquefiable or prone to cyclic mobility units, and Mohr-Coulomb for zones non-susceptible to strength loss (4_Dynamic_Convert_Soil_Models.dat&5_Static_Check_K0_2.dat). -
Dynamic Setup: Application of Rayleigh damping (0.5% at 4.0 Hz based on motion frequency content) and resetting initial kinematics (
6_Dynamic_Setup.dat). -
Dynamic Run: Execution via a compliant base using velocity time histories converted to shear stress amplitudes, paired with free-field and quiet boundaries (
7_Dynamic_Compliant_Base.dat).
I have verified that all units are consistent, and the input ground motions have been properly baseline-corrected.
Issues & Observations
-
Large Strain ON (
model large-strain on): When running with large-strain mode enabled, the calculation eventually runs indefinitely. Specific zones within theDepot de sable(sand deposit) experience such severe deformation that the dynamic timestep plummets down to around 10−15. I have to manually abort the analysis. Everything points toward a numerical indicator of a major shear failure or liquefaction-induced collapse mechanism. -
Large Strain OFF (
model large-strain off): To get through the full earthquake duration, I re-ran the model with large-strain mode turned off. The analysis successfully completed. However, looking at the results:-
The sliding mass located above the shear bands (clearly outlined by the maximum shear strain increments) exhibits increasing velocities until the end of the analysis, and no stabilization.
-
Displacements within these zones reach up to ~100 m, which obviously looks physically unrealistic and strongly suggests a flow-type failure.
-
My Questions for the Community:
-
How should I interpret the timestep collapse in Large Strain ON? Is this a typical sign of localized numerical instability/mesh distortion in PM4Sand elements under extreme pore pressure ratios, or is it a valid indicator that the system has reached a point of general instability? I am considering using the remeshing option, but holding on due to attachments being present in the model, which would need to be removed.
-
Dealing with Massive Displacements: If large-strain off yields 100m+ displacements, is it safe to treat this qualitatively as an indicator of flow failure, or are these numbers completely uninterpretable due to the small-strain assumption missing the geometric softening/large-displacement kinematics?
-
Recommendations: Are there specific model controls, mesh refinement strategies, or structural damping tweaks typically recommended when dealing with severe liquefaction flow failures using PM4Sand in FLAC2D?
Any feedback or guidance on my modeling procedure would be greatly appreciated!
Thanks in advance,
Jérôme
Project and data files attached here:
Original Files.zip (1.3 MB)
Save files for the dynamic analyses are here: